Nitrous oxide and methane emissions from soil are reduced following afforestation of pasture lands in three contrasting climatic zones.
Land use change from agriculture to forestry offers potential
opportunities for carbon (C) sequestration and thus partial mitigation
of increasing levels of carbon dioxide (C[O.sub.2]) in the atmosphere.
The effects of land use change of grazed pastures on in situ fluxes of
nitrous oxide ([N.sub.2]O) and methane (C[H.sub.4]) from soil were
examined across 3 forest types in Australian temperate, Mediterranean,
and subtropical regions, using a network of paired pasture--forest
sites, representing 3 key stages of forest stand development:
establishment, canopy-closure, and mid to late rotation. During the
12-month study, soil temperature ranged from -6[degrees] to 40[degrees]C
and total rainfall from 487 to 676 mm. Rates of [N.sub.2]O flux ranged
between 1 and 100 [micro]g/[m.sup.2]x h in pasture soils and from -5 to
50 [micro]g/[m.sup.2]x h in forest soils; magnitudes were generally
similar across the 3 climate zones. Rates of C[H.sub.4] flux varied from
-1 to -50 [micro]g/[m.sup.2]x h in forest soil and from + 10 to -30
[micro]g/[m.sup.2].in pasture soils; C[H.sub.4] flux was highest at the
subtropics sites and lowest at the Mediterranean sites.
In general, [N.sub.2]O emissions were lower, and C[H.sub.4] consumption was higher, under forest than pasture soils, suggesting that land use change from pasture to forest can have a positive effect on mitigation of non-C[O.sub.2] greenhouse gas (GHG) emissions from soil as stands become established. The information derived from this study can be used to improve the capacity of models for GHG accounting (e.g. FullCAM, which underpins Australia's National Carbon Accounting System) to estimate [N.sub.2]O and C[H.sub.4] fluxes resulting from land use change from pasture to forest in Australia. There is still, however, a need to test model outputs against continuous [N.sub.2]O and C[H.sub.4] measurements over extended periods of time and across a range of sites with similar land use, to increase confidence in spatial and temporal estimates at regional levels.
Additional keywords: [N.sub.2]O, C[H.sub.4], GHG balance, afforestation, forest, paired sites, grassland, pasture, plantation, temperate, tropics, Mediterranean.
Land use (Environmental aspects)
Afforestation (Environmental aspects)
Methane (Environmental aspects)
Nitrous oxide (Measurement)
Nitrous oxide (Environmental aspects)
Soil chemistry (Research)
|Publication:||Name: Australian Journal of Soil Research Publisher: CSIRO Publishing Audience: Academic Format: Magazine/Journal Subject: Agricultural industry; Earth sciences Copyright: COPYRIGHT 2009 CSIRO Publishing ISSN: 0004-9573|
|Issue:||Date: August, 2009 Source Volume: 47 Source Issue: 5|
|Topic:||Event Code: 310 Science & research|
|Product:||Product Code: 2813771 Nitrous Oxide NAICS Code: 32512 Industrial Gas Manufacturing SIC Code: 2813 Industrial gases|
|Geographic:||Geographic Scope: Australia Geographic Code: 8AUST Australia|
Afforestation of agricultural land offers a potential opportunity for partial mitigation of increasing carbon dioxide (C[O.sub.2]) levels in the atmosphere (Davis and Condron 2002). The assessment of mitigation effectiveness, however, requires greater understanding of spatial and temporal variability of biogenic greenhouse gases (GHG) such as nitrous oxide ([N.sub.2]O) and methane (C[H.sub.4]) at local and regional scales (Povellato et al. 2007), because small changes in [N.sub.2]O and C[H.sub.4] may substantially reduce C sequestration benefits due to their high global warming potentials of 298 and 25 times, respectively, compared with C[O.sub.2] (Conant et al. 2005; Forster et al. 2007).
There is potential to reduce [N.sub.2]O emissions and increase C[H.sub.4] consumption in soils when cropping or pasture land is afforested, because this land use change affects soil conditions (such as moisture content and mineral N concentration) that influence biological activity (O'Connell et al. 2003). Lower soil [N.sub.2]O emissions and higher soil C[H.sub.4] consumption in forest soils compared to adjacent pastures have been observed in temperate regions (Merino et al. 2004; Tate et al. 2007). Similarly, in humid tropical regions, Mutuo et al. (2005) observed that well-managed agroforestry practices can mitigate soil [N.sub.2]O and C[O.sub.2] emissions while maintaining strong soil C[H.sub.4] consumption, compared with high-input cropping systems.
Net C sequestration associated with afforestation appears to offer a promising reduction in GHG emissions; however, studies on non-C[O.sub.2] GHG mitigation benefits to date are few and highlight varied responses associated with plant community structure and species composition (Silver et al. 2004; Mutuo et al. 2005) and forest management including stand age and harvest cycle (Tare et al. 2006; Ball et al. 2007). Emissions at site and regional levels may be affected by local environmental and edaphic conditions, by influencing the availability of microbial substrates and diffusion of gases which in turn affect non-C[O.sub.2] GHG fluxes (Smith et al. 2003; Ball et al. 2007). Accurate quantification of biophysical interactions on [N.sub.2]O and C[H.sub.4] fluxes from soil is important not only for understanding N and C transformations in soil, but also to improve process-based models of soil [N.sub.2]O and C[H.sub.4] fluxes.
Plantation forestry is a significant land use in Australia, with around 1.7million ha under both softwood (56.9%) and hardwood (42.6%) forest, and this resource base is expanding rapidly at 80000 to 100000ha annually (Bureau of Rural Sciences 2006). Since 1990, new forests are mostly being established on farmland or other previously cleared sites, utilising a wide range of native and exotic hardwood species. While C sequestration benefits of afforestation are recognised, there is little available information on the changes in [N.sub.2]O and C[H.sub.4] fluxes after afforestation of ex-pasture land for a range of forest types, stand stages of development, and environments across Australia. This information is required to improve predictions of GHG emissions in national GHG accounting models such as FullCAM (Waterworth and Richards 2008).
We measured in situ soil [N.sub.2]O and C[H.sub.4] fluxes, local environmental parameters, and soil attributes associated with the afforestation of pastures. Our objectives were to: (i) identify the magnitude and variation of [N.sub.2]O and C[H.sub.4] soil fluxes seasonally over 1 year, and (ii) determine how these fluxes may be affected by land use change, forest species, age, soil type, and site-specific environmental conditions within Australian temperate, Mediterranean, and subtropical regions.
Materials and methods
Nine sites representing plantation forestry and adjacent grazed pastures were selected to fill a matrix of 3 climatic regions of Australia and 3 stages of forest development. Stages of development were: (i) at establishment, when the tree canopies were very small, (ii) the trees at or near canopy closure, and (iii) towards the middle to late part of the rotation. Information regarding site location, climate, soil types, and management is given in Table 1. Within each region, the 3 paired sites were located on similar soil type, and were previously managed as grazed pasture, or as first-rotation forest (23-year P. radiata). Each paired site consisted of land under pasture, a subsection of which was divided and converted to forest. Management of the forest types varied with respect to soil preparation, planting species, stocking density, grazing activity, and herbicide application, which are given in Table 1.
In situ sampling and measurements
Sampling of in situ gas fluxes and soil properties was undertaken intermittently at each of the sites during field campaigns between September 2006 and September 2007 (25 campaigns in total). Measurements were taken across a range of temperature conditions and rainfall events (e.g. immediately after rainfall and during extended dry periods) during different seasons (Fig. 1). Rainfall was measured on-site where possible and verified with daily reports from Bureau of Meteorology SILO database, based on site coordinates.
Gas samples were collected from static closed chambers positioned evenly along transects of approximately similar length in pasture and forest sites (generally 100-300m depending on forest type and stand age). Transects were established parallel to the boundary between the pasture and forest, and at least 20 m from this boundary to minimise edge effects. Chambers were positioned in mounded row (n = 9) and unmounded (n = 9) forest soil and in adjacent pasture soil (n = 9) for each of the paired sites (n = 81 per State). Data representing each land use were expressed as a mean of all measurements (pasture); at forest sites, data for mound and unmound positions were aggregated based upon the proportion of surface area coverage of forest mounds, calculated for each region as 33% (temperate), 45% (Mediterranean), and 40% (subtropics). Chamber design was as described by Saggar et al. (2004). In brief, modified PVC chamber bases were inserted ~50-100 mm into the soil to act as 'collars', which were permanently positioned along each transect throughout the sampling campaign. Internal chamber heights were measured and the volume (~5-7 L) inside each chamber calculated.
Wire cages were placed around chambers, where necessary, to protect them from animal damage. Where chambers were protected from grazing by wire cages, pasture height within the chamber collar was maintained at a height similar to adjacent grazed pasture using clippers. Gas samples were collected between 0900 and 1600 hours. At time of gas sampling, lids were placed on the chambers to form a gas-tight seal. During the closure period, ~25-mL gas samples were taken from closed chambers through a sampling tube fitted with 3-way stopcock and transferred to 12-mL pre-evacuated exetainers (Labco, UK) and transported to the laboratory for analysis.
Gas samples were analysed as soon as possible after collection by gas chromatography (3800, Varian, The Netherlands) as described in Allen et al. (2007), using commercial standards (BOC Gases, Australia) with [+ or -] 1% or better calibration accuracy to calculate sample gas concentrations. Standards were injected every 10 samples to monitor instrument precision. [N.sub.2]O, C[H.sub.4], and C[O.sub.2] gas fluxes were calculated using linear rate of change over time as per Saggar et al. (2007).
Gas fluxes were calculated for pasture, forest mound, and forest unmound positions using linear least-squared fit of the time series as described in Saggar et al. (2004). Tests for linearity of gas fluxes were performed at forest and pasture sites in all 3 regions during contrasting dry and wet conditions. Samples were initially taken upon chamber closure and then monitored at 30-min intervals over a closure period of 1-1.5 h. In general, fluxes of C[O.sub.2], [N.sub.2]O, and C[H.sub.4] were linearly related to time after closure for periods of up to 1.5 h (data not shown). A subset of chambers at the subtropical sites was monitored throughout the sampling campaign to ensure that linear flux rate was maintained ([R.sup.2] > 0.95).
For comparison of fluxes according to land use within each region, forest mound and unmound positions were aggregated, based on the proportional area of the forest mound. Seasonal mean fluxes were calculated from data collected within the following representative months: summer (December-February), autumn (March-May), winter (June-August), spring (September November). Annual flux was determined as a product of cumulative seasonal mean and number of days within season.
Soil temperatures at 0.05 and 0.20 m depths were measured at the same time as each gas sampling near chambers 2, 5, and 8 along each transect using a digital probe. Soil samples were collected at 0-0.10 and 0.10-0.30m depths adjacent to the chambers and 3 replicate samples were bulked sequentially along the transect (i.e. bulked sample 1 represented soil adjacent to chambers 1-3 along transect, and so on) at each site in pasture, forest mound, and unmound positions for each of the paired sites (9 soil samples at each depth for each paired site). Soil properties for forest land use were determined as per gas sampling, based on aggregation of mound and unmound area. Soil samples were placed in sealed plastic bags and transported to the laboratory and stored at 4[degrees]C until processing for analysis.
[FIGURE 1 OMITTED]
Gravimetric soil moisture content ([[theta].sub.g]) and soil mineral N (N[H.sub.4.sup.+]-N and N[O.sub.3.sup.-] -N) concentration were determined on soils collected at the same time as each gas sampling. A subset of field-moist soil samples was oven-dried at 105[degrees]C for 48h to determine [[theta].sub.g] and another subset of field moist soil (>5 mm sieved) was used to determine soil N[H.sub.4.sup.+]-N and N[O.sub.3.sup.-] -N in either 1 or 2 M KCl extracts using colourimetric methods (Bremner 1965; Searle 1974; Best 1976). Soil samples for bulk density measurements were obtained from pits using rings driven horizontally into pit face (temperate and subtropics sites) and from vertical soil cores (Mediterranean and temperate sites). Bulk density ([[rho].sub.b], g/[cm.sup.3]) was calculated from the weight of dry soil at 105[degrees]C contained in the total volume of soil sample. Water-filled pore space (WFPS) was calculated as ([[theta].sub.g.sup.*] [[rho].sub.b])/total soil porosity, where total soil porosity is taken as (1 - [[rho].sub.b]/particle density) with particle density of the soil assumed to be 2.65.
Soil pH (1 : 5 soil : solution) in (i) water and (ii) Ca[Cl.sub.2] and electrical conductivity (1:5 soil:water) were measured using robotic pH/EC instrument (AS-3000). Soil total carbon and total nitrogen (TN) concentrations were determined by direct combustion (LECO CNS 2000). Total organic carbon (TOC) concentration was taken as total C concentration, since soil pH was <7 in all soils sampled. Soil texture, particulate organic carbon (POC), and charcoal-C concentrations were derived using partial-least-squares prediction of mid infrared spectral data (Janik et al. 1995).
All statistical analyses were carried out using GENSTSAT V.8 (VSN International Ltd, UK) and Statistica (Carver, Brooks/ Cole, Canada) Software. Differences at P = 0.05 between means of [N.sub.2]O (quarter power transformed) and C[H.sub.4] gas fluxes, and between means of soil properties for comparison of pasture sites, were assessed using GLM-ANOVA. The ridge-regression technique was used to assess colinearity between soil variables at 0-0.10 and 0.10-0.30 m depths; highly correlated variables and those with high variance inflation factors were omitted from further analysis (see Dang et al. 2008 for full detail of this technique). A general linear regression model (all possible subsets, including interaction between temperature and water-filled pore space) was then applied to explore the significance of combinations of soil variables for describing gas emissions.
Between August 2006 and September 2007, air temperature at the study sites ranged between -4.5[degrees]C and 37.5[degrees]C (temperate), 2.5[degrees]C and 40[degrees]C (Mediterranean), and -5[degrees]C and 38[degrees]C (subtropics) and total rainfall was 612 mm (temperate), 709 mm (Mediterranean), and 501 mm (subtropics) (Fig. 1). The distribution of rainfall was winter-dominated at the Mediterranean sites and summer-dominated at the subtropics sites, while the temperate sites had an intermediate rainfall pattern with large rainfall events during spring and autumn (Fig. 1).
Differences in soil properties at 0-0.10 m depth for pasture sites within each region are given in Table 2. Significant differences in TOC were observed between pasture sites for all regions. Also, significant differences in soil properties between pastures were observed for temperate (total N, clay, bulk density, POC, and charcoal-C) and subtropical (total N, EC, bulk density) regions. Therefore, there was the need for adjacent paired pasture and forest sites to minimise the site effect but maximise the land use, climate, and species age effects on [N.sub.2]O and C[H.sub.4] fluxes from soil.
Few significant differences in static (once-only measured) soil properties pH, TOC and total N concentrations, clay content, C/N ratio, EC, bulk density, and POC and charcoal-C concentrations were observed between 0-0.10 and 0.10-0.30m depths in forest mound, forest unmound, and pasture sites; therefore, the values for 0-0.10m depth only are presented in Table 3 but significant differences at 0.10-0.30 m depth are presented in the text. Clay content varied from 2% to 50%; however, it was similar between the paired pasture and adjacent forest soils, except for the 23-year P. radiata site, which had significant differences between land use as well as between forest mound and unmound positions. Bulk density ranged between 0.8 and 1.4 g/[cm.sup.3]; however, significant differences were found in 3-year temperate paired site only. Soil pH ranged from 3.9 to 6.6 at the sites, with significant differences between pasture and forest land use observed in 6 of the 9 paired sites (Table 3). At the youngest Mediterranean and subtropical sites, pH was significantly higher in pasture than forest soil; however, this trend was reversed in 23-year Mediterranean and 2.5-year subtropical paired sites (Table 3).
Concentration of TOC ranged from 1.3% to 6.8% in the top 0.10m depth (Table 3), and from 0.8% to 4.4% in the 0.10-0.30 m depth. TOC concentrations were similar in pasture and forest soils, except for the 23-year P. radiata site, where the forest soil had higher C concentration than the adjacent pasture soil. Also, TOC concentrations in the 0.10-0.30m layer in the 8.5-year E. globulus forest soil were higher in the forest mound than forest unmound and pasture soils (data not shown), possibly due to incorporation of surface soil and fine litter during the mounding operation. Similar trends were evident at the other 2 E. globulus sites. Total N concentration was similar in pasture and adjacent forest soil. C/N ratio ranged from 10 to 23, reflecting variable quality of soil organic matter. The surface soil under 23-year P. radiata forest had higher C/N ratio than the adjacent pasture soil, possibly due to some mixing from partially and recently decomposed P. radiata litter and longer growth duration as a second-rotation forest. Electrical conductivity was low (<0.2 mS/m) at all sites.
Land-use had a significant effect on soil moisture in both 0-0.10 and 0.10-0.30m depths across all regions, with greater range in soil moisture content at 0-0.10 m (Fig. 2, left panel) than 0.10-0.30m (Fig. 3, left panel). In general, significantly higher soil moisture was observed in the pasture than the forest soils. Greatest differences in soil moisture contents between pasture and forest sites were observed during spring and summer at the Mediterranean sites (Fig. 3), except the 0.5-year Mediterranean sites, which had higher moisture content in forest soil than pasture soil, possibly due to initial slow vegetation growth in the latter to utilise available water.
[FIGURE 2 OMITTED]
Soil N[O.sub.3.sup.-]-N concentrations at 0-0.10m depth were up to 170kgN/ha (Fig. 4, left panel) and up to 95kgN/ha at 0.10-0.30 m depth (Fig. 5, left panel). N[H.sub.4.sup.+]-N oncentration was generally <40 kg N/ha at both 0-0.10 m (Fig. 4, fight panel) and 0.10-0.30m (Fig. 5, fight panel). Younger forest sites generally had higher soil mineral (N[H.sub.4.sup.+]-N plus N[O.sub.3.sup.-]-N) N concentrations than adjacent pasture sites, with significant differences observed in the 0-0.10 and 0.10-0.30 m depths at most sampling events at the tropical and temperate sites and during winter months at the Mediterranean sites (Figs 4 and 5). However, this trend was reversed at the oldest temperate and Mediterranean sites, where significantly higher mineral N concentrations were found in pasture soil than forest soil; in some instances this trend extending to 0.10-0.30m depth (Figs 4 and 5).
[N.sub.2]O and C[H.sub.4] fluxes
The mound and unmound areas of the forest land use showed some differences in soil fluxes of [N.sub.2]O and C[H.sub.4] (Fig. 6). At the temperate and Mediterranean sites, however, the soil under older forests showed little difference between mound and unmound positions, except in soil under the oldest subtropical forest, suggesting that the disturbance effect of forest establishment and management activities in this region may persist for several years.
When mound and unmound forest positions were combined to represent forest land use, [N.sub.2]O emission rates varied from -5 to 60 [micro]g/[m.sup.2]x h, compared with [N.sub.2]O emissions in adjacent pasture soils, which ranged from 1 to 100[micro]g/[m.sup.2]xh (Fig. 7). Overall, significantly lower soil [N.sub.2]O emission rates were observed under forest than pasture (P<0.05; Fig. 7). However, during establishment and up to canopy closure stage (2-3 years), the soil under forest generally had slightly higher [N.sub.2]O emission rates than adjacent pasture soil, although this trend was reversed during most sampling occasions in the mid to late rotation forest (5-23 years) and adjacent pasture sites. Highest [N.sub.2]O emissions were recorded from soil under pasture adjacent to the 8.5-year E. globulus forest, which was an order of magnitude greater than from forests in the temperate and subtropical regions.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The C[H.sub.4] consumption rate was greater than the C[H.sub.4] emission rate on most sampling occasions (Fig. 8). For the forest soils, C[H.sub.4] flux varied from -1 to -50[micro]g/[m.sup.2] x h, whereas C[H.sub.4] flux in pasture soils varied from 10 to 30[micro]/[m.sup.2] x h. In general, C[H.sub.4] consumption rate was significantly greater for forest soil than pasture soil; in all regions there was a trend for consumption under forest to increase relative to pasture with increasing forest age (Fig. 8). Regional differences in C[H.sub.4] consumption rates were also apparent; the general trend was subtropical > temperate > Mediterranean sites.
Estimates of mean seasonal and annual soil fluxes of [N.sub.2]O and C[H.sub.4] are shown in Table 4. Mean seasonal [N.sub.2]O emissions were generally <25 [micro]g [N.sub.2]O/[m.sup.2] x h; few seasonal trends in [N.sub.2]O emission were apparent, with the exception of 0.5-year E. globulus forest, which had an estimated mean spring value of 65 [micro]g [N.sub.2]O/[m.sup.2] x.h and the 5-year Mediterranean pasture site, where mean [N.sub.2]O emissions were estimated around 75 [micro]g [N.sub.2]O/[m.sup.2] x h in all seasons except summer. Mean seasonal C[H.sub.4] fluxes were generally <-20[micro]g C[H.sub.4] /[m.sup.2] x h at all sites except the oldest temperate and subtropical forest sites, where fluxes of around -40 [micro]g C[H.sub.4]/me x h were estimated. C[H.sub.4] uptake appeared greatest at temperate sites during spring and summer; in contrast, C[H.sub.4] uptake appeared least during summer and greatest during autumn at Mediterranean sites. No seasonal trend in estimated mean C[H.sub.4] emissions was apparent at subtropical sites.
Annual soil [N.sub.2]O emissions were <6 mg [N.sub.2]O/me x year, with much higher [N.sub.2]O emissions from soil under the 8.5-year Mediterranean pasture, which was dominated by legumes (Table 4). Both temperate and Mediterranean forests showed a general trend of decreasing annual [N.sub.2]O flux with forest age; however, no trend with forest age was apparent in the subtropics. Highest C[H.sub.4] uptake was also observed in the oldest forest plantations in all regions; annual estimates of C[H.sub.4] flux were generally <-200 mg C[H.sub.4]/[m.sup.2] x year, except in the oldest temperate and subtropical forests, which had estimated annual C[H.sub.4] flux of -250 and -340 mg C[H.sub.4]/[m.sup.2] x year, respectively (Table 4).
Drivers of [N.sub.2]O and C[H.sub.4] fluxes
The ridge regression technique was used to assess colinearity between measured soil properties at 0-0.10 and 0.10-0.30m depths and to identify variables with stable coefficients and variance inflation factors for inclusion in multiple regression analysis. Relationships between soil properties water-filled pore space (WFPS), soil temperature, N[H.sub.4.sup.+], N[O.sub.3.sup.-] -N, pH, TOC, total N, C/N, EC, clay, and POC at 0-0.10m depth are given in Table 5. A high degree of co-linearity (R >0.9) was found between soil properties as well as between soil depths. Therefore, soil properties total N and POC concentrations, as well as those from 0.10-0.30m depth, were omitted from subsequent multiple regression analyses.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
Relationships of key soil variables at 0-0.10m depth are shown in Fig. 9. Significant positive relationships between soil temperature and soil [N.sub.2]O emission and C[H.sub.4] uptake were found; soil nitrate and ammonium had a significant positive relationship with soil [N.sub.2]O emission, although soil nitrate and ammonium were significantly but negatively associated with soil C[H.sub.4] uptake. Highest [N.sub.2]O emissions and C[H.sub.4] uptake were generally observed where WFPS was <40%.
Multiple regression (all subsets) analysis of a subset of soil variables (bulk density, C/N, char, clay, EC, soil temperature, TOC, WFPS, N[H.sub.4.sup.+], N[O.sub.3.sup.-] -N, pH) showed greatest, but still moderate, explanatory power of C[H.sub.4] fluxes and [N.sub.2]O emissions at 0-0.10 m depth and accounted for only 20.2% and 27.5% of the variation in [N.sub.2]O emission rates and C[H.sub.4] flux, respectively (Table 6). These low partial correlations suggest interactive effects of the measured factors, or other factors not measured in this study, which may have affected [N.sub.2]O and C[H.sub.4] fluxes from soil.
[N.sub.2]O and C[H.sub.4] fluxes
The range of [N.sub.2]O emission rates from forest soils observed in this study (-5 to 60 [micro]g/[m.sup.2] x h) are similar to those from plantation forests in other regions (e.g. Tate et al. 2006; Ball et al. 2007) and are within the range reported for natural and managed forests in Australia and elsewhere (Breuer et al. 2000; Erickson et al. 2001; Kiese and Butterbach-Bahl 2002; Kiese et al. 2003; Merino et al. 2004; Veldkamp et al. 2008). Few studies have addressed land-use change effects on [N.sub.2]O and C[H.sub.4] fluxes in Australian pasture and forest systems, with most in situ studies focusing upon agriculture or grazed temperate pastures (Dalal et al. 2003; Eckard et al. 2003; Phillips et al. 2007) or controlled-environment studies of microbial processes, e.g. denitrification (Barton et al. 1999; Pu et al. 1999). [N.sub.2]O emissions from the lightly grazed pasture sites recorded in this study (1-100[micro]g/[m.sup.2] x h) are lower than those reported from heavily grazed and legume-dominated pastures (e.g. Saggar et al. 2004; Li and Kelliher 2005; Phillips et al. 2007). This may be due to higher N levels and high N mineralisation rates following nitrogen enrichment by the legumes in these pastures and/or from manure addition by animals in heavily grazed pastures. [N.sub.2]O emission rates from pasture soils under similar climate varied in this study, possibly due to local edaphic conditions and species composition.
[FIGURE 9 OMITTED]
Highest [N.sub.2]O emission rates were observed in pasture adjacent to the 8.5-year E. globulus forest at the Mediterranean site, most likely due to legume dominance and high soil mineral N turnover. In temperate New Zealand pastures, Niklaus et al. (2006) found that C[H.sub.4] and [N.sub.2]O fluxes strongly depended on plant community composition and its interaction with soil type. Dalal et al. (2008) note that recent reports of greenhouse gas production and emission by plants are inconsistent and suggest further examination is required. Estimates for C[H.sub.4] emission by vegetation itself range between nil and 30% (Keppler et al. 2006; Kirschbaum et al. 2006; Sanhueza and Donoso 2006; Dueck et al. 2007; Ferretti et al. 2007). Others suggest that [N.sub.2]O could be moved up through the plant (Chang et al. 1998; Rusch and Rennenberg 1998), although studies regarding this mechanism are few. In our study, the forest understorey varied between forest species. For example, the understorey of P. radiata consisted of pine needle leaf litter, whereas the understorey of E. globulus and C. citriodora forests consisted of pasture and leaf litter. It may be that flux estimates in the forest treatments are underestimated; however, at the present state of knowledge it is uncertain whether significant amounts of C[H.sub.4] emissions occur from plants.
Net C[H.sub.4] flux observed during this study in forest (-1 to -50 [micro]g/[m.sup.2] x h) and pasture (10 to -30 [micro]g/[m.sup.2] x h) soils was within the range of values reported from other afforested and grassland sites (Merino et al. 2004; Tate et al. 2006), and from Australian natural forest systems (Butterbach-Bahl et al. 2004; Fest et al. 2007). The reported rates of C[H.sub.4] consumption in the present study are, however, lower than those observed (C[H.sub.4] flux up to -120/[micro]g/[m.sup.2] x h) in undisturbed native forests in temperate regions (Khalil et al. 1990; Galbally et al. 1996; Ball et al. 1997; Meyer et al. 1997; Price et al. 2003). Suppression of soil C[H.sub.4] consumption rate has been associated with land use change from forest to agricultural systems (Prieme et al. 1997) and associated higher soil mineral N concentrations (Reay et al. 2005; Suwanwaree and Robertson 2005). The timeframe for recovery from agricultural system to original land use may be up to 200 years, depending on the original C[H.sub.4] consumption capacity of the soil (Prieme et al. 1997). Further study of native vegetation in these climatic regions and soil types would be useful to establish a benchmark to determine their capacity for C[H.sub.4] consumption, and to provide context for the rate of recovery in the afforested sites.
Soil properties and drivers of [N.sub.2]O and C[H.sub.4] emissions
Pasture sites within a climatic region had significant differences in soil properties, including TOC, POC, total N, and bulk density (Table 3). However, to minimise the site effects and maximise the effect of land use change and soil properties likely to be affected by it, paired sites of adjacent pasture-forest plantations were selected. The basic soil property clay content was essentially similar between the paired sites except for the 23-year P. Radiata site. Although paired sites were carefully selected according to similar soil type and land use history within each region, significant variability in soil properties, including TOC, POC, total N, and bulk density were observed under between multiple pasture sites within a region (Table 3), suggesting that caution is necessary regarding the inference of age-effects discussed below.
Bulk density differed significantly between land uses at only one of the paired sites in this study. Davis and Condron (2002), in a review of afforested and pasture paired-site studies, found similar mean bulk densities between grassland and forest soils, although values were somewhat higher in improved grassland sites than unimproved sites. All EC values reported in this study were lower (<2mS/m) than concentrations likely to affect microbial activity influencing [N.sub.2]O and C[H.sub.4] emissions (Denier van der Gon and Neue 1995; Adviento-Borbe et al. 2006).
Soil temperature was positively correlated with [N.sub.2]O emission rates (P<0.05; Fig. 9), in agreement with reports elsewhere (Dalal et al. 2003). Pilegaard et al. (2006) ascribed this to a general increase in enzymatic processes with temperature as long as other factors (substrate or moisture) are not limiting. The significant negative correlation between C/N ratio, as well as charcoal-C, and [N.sub.2]O emission rates, was possibly associated with reduced net soil organic N mineralisation, likely due to enhanced N immobilisation and thus reducing [N.sub.2]O production via nitrification. In our study, C/N ratios at the afforested sites were <20, except for the oldest P. radiata site. Significant negative relationships between C/N ratios and [N.sub.2]O emissions have been reported in temperate European forest soils, with C/N ratio generally >20 (Klemedtsson et al. 2005; Pilegaard et al. 2006). Klemedtsson et al. (2005) proposed that other interactive factors were more important as predictors of [N.sub.2]O emission at threshold C/N ratios below 15-20. The negative relationship between C[H.sub.4] consumption rate and clay content (data not shown) may have been due to reduced diffusion rate of atmospheric C[H.sub.4] into the soil, since bulk density and clay content of these soils were positively correlated (Table 5). Boeckx and VanCleemput (1996) found lower C[H.sub.4] consumption rate in fine-textured soils than coarse-textured soils; presumably, increased soil tortuosity reduced C[H.sub.4] diffusion in fine-textured soils.
Total organic C (TOC) concentration was similar between pasture and forest land use, as well as between forest mound and unmound soil. However, Lima et al. (2006) found substantial accretion of TOC stocks in the 0-0.10 m layer of more recently established Eucalyptus stands on ex-pasture in the tropics, although values were affected by cultivation and varied significantly between regions and at different depths. Mendham et al. (2003) found that there were no significant effects of afforestation on soil carbon pools down to 1 m after afforestation with E. globulus at 10 Mediterranean sites across south-western Australia, and this was supported by another study showing no changes in soil carbon in the top 0.20m depth across 30 sites in the same region (Grove et al. 2001). in P. radiata forests, Paul et al. (2002) suggested that afforestation leads to an initial decrease in soil C in the top 0.30 m, before gradually increasing after around 30 years to levels that arc often higher than previous agricultural land use. This is consistent with results from our study at the first-rotation 23-year P. radiata site. However, Davis et al. (2007) found that the response of soil C stocks to afforestation of a subhumid pasture varied according to rainfall, with little effect in low-rainfall areas, but significantly reduced stocks in higher rainfall areas. Dalal et al. (2003) note that soil denitrification capacity increases with increasing organic C concentration (Burford and Bremner 1975); significant positive relationships between [N.sub.2]O emissions and increasing TOC and N[O.sub.3.sup.-]-N concentrations were also observed in this study (Burford and Bremner 1975) and by others (see Li et al. 2005 review).
Increasing N[H.sub.4.sup.+] and N[O.sub.3.sup.-] -N concentrations were associated with decreasing C[H.sub.4] consumption rates (Fig. 9). N[H.sub.4.sup.+] production appears to have an inhibitory effect on C[H.sub.4] oxidation because ammonium mono-oxidase and methane mono-oxidase enzymes are linked in methanotrophs (Bedard and Knowles 1989). However, N[O.sub.3.sup.-] -N inhibition on C[H.sub.4] oxidation is less likely, except at very high N[O.sub.3.sup.-] -N concentrations (Bodelier and Laanbroek 2004).
Significant relationships between several soil properties and non-C[O.sub.2] GHG fluxes were identified in this study (Table 6). However, their combined strength for explaining in situ emissions remained relatively low. Le Mer and Roger (2001) suggested that identification of the factors that control emission rates is difficult, and there are uncertainties in determining how many different environmental combinations have to be studied to characterise the source. Findings from the these studies, including the current study, suggest that additional multi-year flux measurements from multiple sites, which encompass a range of ecosystem- and landscape-scale controls (temperature, rainfall, C and N patterns) and their interactive effects on [N.sub.2]O emissions and C[H.sub.4] fluxes are greatly needed to assist empirical modelling and for computation of national GHG inventories.
Our observations of paired-site studies of pasture and forests in 3 climate regions of Australia suggest afforestation of pasture sites generally enhanced soil consumption of C[H.sub.4], whereas [N.sub.2]O emissions were influenced by stage of forest development. [N.sub.2]O emissions were slightly higher in young forests (before canopy closure) than pasture. However, [N.sub.2]O emissions in the mid to late stages (5-23 years) of forest development were generally lower than those from adjacent pasture. Much of the forest life cycle is in the post-canopy-closure phase, so overall, afforestation is expected to reduce emissions of non-C[O.sub.2] GHGs from pasture soils. Fluxes of [N.sub.2]O and C[H.sub.4] were seasonally variable in all regions studied; high spatial and temporal variability in gas fluxes and soil mineral N concentration within each region may reflect local site management, including grazing activity, pasture species composition, and forest understorey management at these sites.
Biophysical drivers including soil moisture, temperature, and soil C/N ratio were found to be significantly associated with [N.sub.2]O and C[H.sub.4] fluxes, although these variables provided limited explanatory power to explain [N.sub.2]O and C[H.sub.4] fluxes measured in situ. Nevertheless, these variables could be used to simulate the effects of land use change from pasture to forest on [N.sub.2]O and C[H.sub.4] fluxes using FulICAM or similar models at different locations in Australia. There is still, however, the requirement to validate the model outputs with continual [N.sub.2]O and C[H.sub.4] flux measurements over extended periods, preferably using automated chamber methods or similar continuous gas measurement techniques. Complementing studies in natural forest systems on similar soil types are valuable to determine maximum C[H.sub.4] uptake capacity, and 'background' [N.sub.2]O emissions.
This work was funded by the Commonwealth Department of Climate Change, Qld DNR&W, NSW DPI, and CSIRO. We thank Neil Halpin from DPI Forestry for Qld site selection, JeffBaldock and Kris Broos from CSIRO for project advice, and David Mayer for statistical advice. We thank Forests NSW, Qld DPI Forestry and Timbercorp Limited for site access, Sawinder Bawa, David Giles, and Kamaljeet Kaur (Forestry NSW DPI), Steven Reeves, lain Gibson, Judy Brady, and Linda Chrabaszcz (Qld DNRW), and Jessie Rutter and Scott Walker (CSIRO) for skilled technical support. We also thank the several farmers who allowed us access to their properties, including lain Mckean and Graham Gilmore (NSW), Colin Marshall, Libby Lieu, Craig Jones and Loft McCarthy (Qld), Todd and Jane Stan-Bishop, Michael and Phillipa Murphy, and Peter Drygan (WA).
Manuscript received 2 July 2008, accepted 26 March 2009
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D. E. Allen (A,E), D. S. Mendham, (B) Bhupinderpal-Singh (C), A. Cowie (C), W. Wang (A), R. C. Dalal (A), and R. J. Raison (D)
(A) Department of Natural Resources and Water, 80 Meiers Rd, Indooroopilly, Qld 4068, Australia.
(B) CSIRO Sustainable Ecosystems, Private Bag 5, Wembley, WA 6913, Australia.
(C) NSW Department of Primary Industries, PO Box 100, Beecroft, NSW 2119, Australia.
(D) CSIRO Forest Biosciences, PO Box E4008, Kingston, ACT 2604, Australia.
(E) Corresponding author. Email: Diane.Allen@nrw.qld.gov.au
Table 1. Paired pasture--forest sites in three climatic regions in Australia Paired sites denoted by forest age (years) Climate: Temperate Forest species Radiata pine (Pinus radiata) Region Central New South Wales Lat/long. 33[degrees]S, 149[degrees]E Main soil type (A) Kandosols Mean annual temperature ([degrees]C) 10.5 Annual rainfall range (mm) (B) 860-864 Age (years) and stems/ha (in parentheses) at start of plantation forest phase: Establishment 1.0 (1200) Canopy closure 3.0 (1200) Mid-late rotation 23.0 (400) Dominant pasture species 1.0: Trifoliumrepens, Trifolium subterraneum, Cirsium vulgare, other perennial grasses 3.0 and 23.0: Phalaris minor, Trifolium sp., Lolium perenne Method of soil Cultivated mound (180 cm wide, preparation (land 20-30 cm high). use conversion; once-off activity) Weed control between mounds Grazing activity (no. 1: forest (nil) pasture of cattle per ha) (7.5-10) 3: forest (2.5-5) pasture (2.5-5) 23: forest (nil), pasture (2.5; 5 sheep) Grazed January 2007-September 2007 Fertiliser management Nil Climate: Mediterranean Forest species Blue gum (Eucalyptus globulus) Region South-western Australia Lat/long. 35[degrees]S, 118[degrees]E Main soil type (A) Brown Chromosols Mean an nual temperature ([degrees]C) 15.7 Annual rainfall range (mm) (B) 696-812 Age (years) and stems/ha (in parentheses) at start of plantation forest phase: Establishment 0.5 (1200) Canopy closure 2.5 (1200) Mid-late rotation 8.5 (1200) Dominant pasture species 0.5: improved pasture 2.5: improved pasture, seasonal coverage of grasses 8.5: improved pasture with seasonal Trifolium subterraneum Cultivated mound (180 cm wide, 20 cm high). Method of soil Weed control between mounds preparation (land use conversion; once-off activity) Grazing activity (no. of cattle per ha) Not available 50 kg urea/ha applied once-off Fertiliser management before any forest planting Climate: Subtropical Forest species Spotted gum (Corymbia citriodora) Region South-east Queensland Lat/long. 26[degrees]S, 151[degrees]E Main soil type (A) Red Ferrosols (Oxisol) Mean annual temperature ([degrees]C) 18.3 Annual rainfall range (mm) (B) 722-738 Age (years) and stems/ha (in parentheses) at start of plantation forest phase: Establishment 0.3 (700) Canopy closure 2.5 (700) Mid-late rotation 5.0 (300) Dominant pasture species 0.3: Chamaecrista rotundifolia, Panicum maxium, Pennisteum clandestinum, Chloris gayana 2.5 and 5.0: Aristida ramose, Eragrostis curvulata, Chloris gayana, Bothriochloa insculpta, Ceratro sp., Panicum maximum, Pennistum clandestinum, Chamaecrista sp. Method of soil Flat planting line 1.5-2 m preparation (land wide, small mound cultivated use conversion; either side of the planting once-off activity) line (60 cm wide, 10 cm high). Weed control between mounds (0.3 year) Grazing activity (no. of cattle per ha) 0.3: forest (nil) pasture (nil) 2.5: forest (1.75) pasture (2.5) 5.0: forest (~2.5) pasture (~2.4) Grazed February 2007- September 2007 Fertiliser management 0.3 and 2.5: 60 kg/ha of mono-ammonium phosphate applied once-off before any forest planting: applied as spray 5.0: 60 kg/ha mono-ammonium phosphate applied once-off before any forest planting; applied as a granular MAP continuum along planting line (A) Classified according to Isbell (1996). (B) Source: SILO 1970-2007 database, Commonwealth of Australia 2008, Bureau of Meteorology. Table 2. Soil properties at 0-0.10 m depth across pasture sites in temperate, Mediterranean, and subtropical regions Means followed by different letters within each paired site are significantly different (t-test, * P<0.05, ** P<0.01, 2-tailed, assuming equal variance). Paired site denoted by plantation age. [pH.sub.w], pH of 1: 5 soil: water suspension; TOC, total organic carbon; Total N, total nitrogen; [EC.sub.se], electrical conductivity of saturated extract calculated from [EC.sub.1:5] and clay; [rho]b, soil bulk density; POC, particulate organic carbon; Char, charcoal, n.s., not significant Region Paired [pH.sub.w] TOC Total N C/N Clay site (%) ratio (%) Temperate 1 n.s. a ** a * n.s. a ** 3 b ** b * b ** 23 c ** b * b ** Mediterranean 0.5 n.s. a * n.s. n.s. n.s. 5 a * 8.5 b * Subtropical 0.3 n.s. a * a * a * n.s. 2.5 ab * ab * b * 5 b * b * ab * Region [EC.sub.se] [rho]b POC Char (mS/m) (g/[cm.sup.3]) (%) Temperate n.s. a * a * a * b * b * b * c * b * a * Mediterranean n.s. n.s. n.s. n.s. Subtropical a * a * n.s. n.s. ab * ab * b * b * Table 3. Soil properties at 0-0.10 m depth across paired pasture-forest sites Means followed by different letters within each paired site are significantly different (t-test, P< 0.05, 2-tailed, assuming equal variance). Paired site denoted by forest age. F, Forest; UM, unmound; M, mound; [pH.sub.w], pH of 1:5 soil: water suspension; TOC, total organic carbon; Total N, total nitrogen; [EC.sub.se], electrical conductivity of saturated extract calculated from [EC.sub.1:5] and clay; [rho]b, soil bulk density; POC, particulate organic carbon; Char, charcoal Paired Land [pH.sub.w] TOC Total N C/N Clay site use (%) ratio (%) Temperate 1 Pasture 5.8a 2.2 0.2 12.8a 16.1 F-UM 5.5b 2.5 0.2 11.0b 11.9 F-M 5.2c 3.0 0.3 11.2b 13.2 3 Pasture 5.5 3.9 0.3 12.4 23.1 F-UM 5.2 3.8 0.3 12.5 24.0 F-M 5.0 4.0 0.3 12.6 22.4 23 Pasture 5.3a 3.0a 0.2 12.1a 23.8a F-UM 5.8a 5.1b 0.2 25.1b 13.5b F-M 6.1b 4.2b 0.2 21.4b 18.1c Mediterranean 0.5 Pasture 5.3a 3.7 0.3 12.6 11.6 F-UM 4.9b 3.9 0.3 12.5 12.8 F-M 5.0b 3.0 0.2 12.8 14.6 5 Pasture 5.5a 5.2 0.4 14.5 2.2 F-UM 5.0b 5.6 0.4 14.4 3.8 F-M 4.9b 4.7 0.3 15.3 2.5 8.5 Pasture 5.0 5.6 0.4 14.7 8.8 F-UM 4.7 7.0 0.5 15.2 13.1 F-M 4.8 6.5 0.5 12.7 9.6 Subtropical 0.3 Pasture 5.6a 2.0 0.2 10.7 50.0 F-UM 5.4b 1.4 O.1 10.0 45.3 F-M 5.3b 1.3 0.1 9.7 44.9 2.5 Pasture 5.2a 2.5 0.2 11.5 41.0 F-UM 6.3b 2.3 0.2 1.2 39.3 F-M 5.7c 2.1 0.2 1.3 45.4 5 Pasture 5.3 2.9 0.3 11.0 43.2 F-UM 5.0 3.3 0.3 11.3 40.1 F-M 4.9 3.0 0.3 11.6 41.9 Paired Land [EC.sub.se] [rho]b POC Char site use (mS/m) (g/[cm.sup.3]) (%) Temperate 1 Pasture 0.2 1.3 0.5 0.2 F-UM 0.1 1.1 0.5 O.1 F-M 0.1 1.1 0.6 0.1 3 Pasture 0.1 1.0a 1.1 0.4 F-UM 0.1 1.1b 1.3 0.3 F-M 0.1 0.9c 1.2 0.3 23 Pasture 0.1 1.1 0.9 0.2 F-UM 0.1 1.0 1.8 0.4 F-M 0.1 1.0 1.5 0.2 Mediterranean 0.5 Pasture 0.1 1.1 1.8 0.1 F-UM 0.2 1.1 1.8 0.2 F-M 0.1 1.1 1.2 0.1 5 Pasture 0.1 1.1 3.1 0.2 F-UM 0.1 1.1 3.5 O.1 F-M 0.1 0.9 2.4 0.0 8.5 Pasture 0.1 1.1 2.4 0.2a F-UM 0.1 0.9 3.2 0.4b F-M 0.2 0.8 3.1 0.2a Subtropical 0.3 Pasture 0.1 1.1 0.6 0.1 F-UM 0.1 1.4 0.3 0.1 F-M 0.1 1.2 0.3 0.1 2.5 Pasture 0.1 1.2 0.6 0.1 F-UM 0.1 1.2 0.5 0.1 F-M 0.1 1.3 0.5 0.1 5 Pasture 0.1 1.3 0.7 0.1 F-UM 0.1 1.1 0.8 0.2 F-M 0.1 1.1 0.8 0.2 Table 4. Estimated seasonal mean and cumulative annual emission of [N.sub.2]O and C[H.sub.4] across paired pasture-forest sites Paired site denoted by plantation age Region Paired Land Seasonal [N.sub.2]O flux mean site use ([micro]g [N.sub.2]O/[m.sup.2] x h) Summer Autumn Temperate 1 Pasture 30 [+ or -] 1 11 [+ or -] 2 Forest 27 [+ or -] 4 16 [+ or -] 2 3 Pasture 9 [+ or -] 3 14 [+ or -] 3 Forest 11 [+ or -] 2 21 [+ or -] 3 23 Pasture 19 [+ or -] 2 36 [+ or -] 7 Forest 2 [+ or -] 0 4 [+ or -] 1 Mediterranean 0.5 Pasture 13 [+ or -] 5 28 [+ or -] 1 Forest 4 [+ or -] 1 23 [+ or -] 1 5 Pasture 10 [+ or -] 2 19 [+ or -] 1 Forest 20 [+ or -] 3 23 [+ or -] 2 8.5 Pasture 14 [+ or -] 3 17 [+ or -] 1 Forest 14 [+ or -] 5 15 [+ or -] 0 Subtropical 0.3 Pasture 8 [+ or -] 3 28 [+ or -] 4 Forest 10 [+ or -] 1 23 [+ or -] 7 2.5 Pasture 15 [+ or -] 6 11 [+ or -] 6 Forest 11 [+ or -] 1 20 [+ or -] 3 5 Pasture 36 [+ or -] 13 72 [+ or -] 26 Forest 1 [+ or -] 0 4 [+ or -] 1 Region Paired Land Seasonal [N.sub.2]O flux mean site use ([micro]g [N.sub.2]O/[m.sup.2] x h) Winter Spring Temperate 1 Pasture 2 [+ or -] 2 9 [+ or -] 1 Forest 6 [+ or -] 2 19 [+ or -] 4 3 Pasture 4 [+ or -] 2 8 [+ or -] 3 Forest 13 [+ or -] 3 11 [+ or -] 2 23 Pasture 9 [+ or -] 3 11 [+ or -] 2 Forest 1 [+ or -] 1 1 [+ or -] 0 Mediterranean 0.5 Pasture 7 [+ or -] 1 10 [+ or -] 3 Forest 6 [+ or -] 1 2 [+ or -] 0 5 Pasture 11 [+ or -] 2 15 [+ or -] 7 Forest 16 [+ or -] 3 17 [+ or -] 3 8.5 Pasture 9 [+ or -] 2 25 [+ or -] 4 Forest 5 [+ or -] 1 6 [+ or -] 1 Subtropical 0.3 Pasture 30 [+ or -] 5 14 [+ or -] 2 Forest 18 [+ or -] 3 61 [+ or -] 15 2.5 Pasture 3 [+ or -] 1 4 [+ or -] 1 Forest 47 [+ or -] 22 18 [+ or -] 4 5 Pasture 68 [+ or -] 20 76 [+ or -] 14 Forest 1 [+ or -] 1 5 [+ or -] 1 Region Paired Land Annual Seasonal site use [N.sub.2]O C[H.sub.4] flux (mg/ flux mean [m.sup.2] x ([micro]g year) C[H.sub.4]/ [m.sup.2] x h) Summer Temperate 1 Pasture 5 -14 [+ or -] 2 Forest 5 -11 [+ or -] 3 3 Pasture 2 -24 [+ or -] 2 Forest 3 -22 [+ or -] 1 23 Pasture 5 -17 [+ or -] 2 Forest 1 -33 [+ or -] 3 Mediterranean 0.5 Pasture 4 -13 [+ or -] 1 Forest 6 -5 [+ or -] 1 5 Pasture 3 -11 [+ or -] 1 Forest 6 -3 [+ or -] 2 8.5 Pasture 15 -6 [+ or -] 2 Forest 1 -6 [+ or -] 2 Subtropical 0.3 Pasture 4 0 [+ or -] 2 Forest 2 -17 [+ or -] 2 2.5 Pasture 3 -7 [+ or -] 8 Forest 5 -20 [+ or -] 3 5 Pasture 4 -15 [+ or -] 7 Forest 3 -48 [+ or -] 4 Region Paired Land Seasonal C[H.sub.4] flux mean site use ([micro]g C[H.sub.4]/[m.sup.2] x h) Autumn Winter Temperate 1 Pasture -12 [+ or -] 4 -9 [+ or -] 1 Forest -6 [+ or -] 4 -10 [+ or -] 1 3 Pasture -18 [+ or -] 5 14 [+ or -] 2 Forest -16 [+ or -] 2 -13 [+ or -] 1 23 Pasture -10 [+ or -] 5 -13 [+ or -] 3 Forest -30 [+ or -] 3 -22 [+ or -] 3 Mediterranean 0.5 Pasture -14 [+ or -] 5 -10 [+ or -] 3 Forest -10 [+ or -] 3 -1 [+ or -] 2 5 Pasture -15 [+ or -] 3 -1 [+ or -] 3 Forest -15 [+ or -] 3 0 [+ or -] 1 8.5 Pasture -6 [+ or -] 3 -5 [+ or -] 7 Forest -8 [+ or -] 3 -12 [+ or -] 2 Subtropical 0.3 Pasture -7 [+ or -] 1 -3 [+ or -] 1 Forest -24 [+ or -] 1 -17 [+ or -] 2 2.5 Pasture -3 [+ or -] 12 -8 [+ or -] 4 Forest -21 [+ or -] 3 -17 [+ or -] 3 5 Pasture -6 [+ or -] 6 -4 [+ or -] 2 Forest -38 [+ or -] 3 -40 [+ or -] 4 Region Paired Land Seasonal Annual site use C[H.sub.4] C[H.sub.4] flux mean flux (mg/ ([micro]g [m.sup.2] x C[H.sub.4]/ year) [m.sup.2] x h) Spring Temperate 1 Pasture -13 [+ or -] 2 -107 Forest -9 [+ or -] 3 -80 3 Pasture -30 [+ or -] 2 -187 Forest -19 [+ or -] 1 -153 23 Pasture -14 [+ or -] 2 -119 Forest -31 [+ or -] 3 -253 Mediterranean 0.5 Pasture -7 [+ or -] 3 -96 Forest -11 [+ or -] 1 -61 5 Pasture -3 [+ or -] 2 -52 Forest -6 [+ or -] 2 -53 8.5 Pasture -5 [+ or -] 1 -47 Forest -17 [+ or -] 2 -96 Subtropical 0.3 Pasture -1 [+ or -] 1 -24 Forest -16 [+ or -] 2 -162 2.5 Pasture -14 [+ or -] 4 -71 Forest -21 [+ or -] 3 -172 5 Pasture -12 [+ or -] 4 -81 Forest -30 [+ or -] 5 -343 Table 5. Correlation matrix of soil properties measured at 0-0.10 m depth across paired pasture-forest sites Values in bold are significant at P < 0.05. [[rho].sub.b], Soil bulk density; WFPS, water-filled pore space; [pH.sub.w], pH of 1:5 soil: water suspension; TOC, total organic carbon; Tot. N, total nitrogen; [EC.sub.se], electrical conductivity of saturated extract calculated from [EC.sub.1:5] and clay; POC, particulate organic carbon; Char, charcoal [[rho].sub.b] WFPS Temp. [[rho].sub.b] WFPS 0.5 * Temp. 0.2 * -0.3 * N[O.sub.3] - [sup.-]N -0.1 -0.2 * 0.1 * N[H.sub.4] + [sup.-]N -0.1 * -0.2 * 0.1 pHw 0.4 * 0.4 * 0.1 * TOC -0.7 * -0.5 * -0.2 * Tot. N -0.7 * -0.5 * -0.2 * C/N -0.5 * -0.4 * -0.2 * EC -0.2 * -0.1 * 0.1 Clay 0.5 * 0.4 * 0.3 * POC -0.6 * -0.5 * -0.2 * Char -0.3 * -0.1 -0.1 * N[O.sub.3] - N[H.sub.4] + pHw [sup.-]N [sup.-]N [[rho].sub.b] WFPS Temp. N[O.sub.3] - [sup.-]N N[H.sub.4] + [sup.-]N 0.5 * pHw -0.2 * -0.2 * TOC 0.0 0.2 * -0.5 * Tot. N 0.0 0.1 * -0.5 * C/N 0.0 0.2 * -0.4 * EC 0.1 0.0 -0.1 * Clay -0.2 * -0.3 * 0.3 * POC 0.0 0.2 * -0.4 * Char 0.0 0.0 -0.1 * TOC Tot. N C/N [[rho].sub.b] WFPS Temp. N[O.sub.3] - [sup.-]N N[H.sub.4] + [sup.-]N pHw TOC Tot. N 1.0 * C/N 0.8 * 0.6 * EC 0.3 * 0.3 * 0.2 * Clay -0.6 * -0.5 * -0.7 * POC 0.9 * 0.9 * 0.9 * Char 0.5 * 0.5 * 0.3 * EC Clay POC [[rho].sub.b] WFPS Temp. N[O.sub.3] - [sup.-]N N[H.sub.4] + [sup.-]N pHw TOC Tot. N C/N EC Clay -0.2 * POC 0.2 * -0.7 * Char 0.4 * -0.2 * 0.3 * Note: Values in bold are significant at P < 0.05. [[rho].sub.b] indicated by *. Table 6. Significant partial correlations of multiple regression (all subsets) analysis of [N.sub.2]O and C[H.sub.4] emission rates and soil properties at 0-0.10m depth TOC, Total organic carbon; [[rho].sub.b], soil bulk density; WFPS, water-filled pore space. Used to compare relative contribution of each independent variable. All values listed are significant at P = 0.05 Soil parameter Partial correlation [N.sub.2]O Temperature 0.34 [R.sup.2] = 0.202 C/N ratio -0.23 TOC 0.18 Charcoal -0.14 N[O.sub.3] - [sup.-]N 0.11 [[rho].sub.b] 0.11 C[H.sub.4] WFPS* temperature 0.27 [R.sup.2] = 0.275 C/N ratio -0.23 N[O.sub.3] - [sup.-]N 0.13 Charcoal -0.12 pH -0.12 EC -0.06
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