Limited influence of tillage management on organic matter fractions in the surface layer of silt soils under cereal-root crop rotations.
Abstract: Reduced tillage (RT) management may increase surface soil organic carbon (SOC) and nitrogen (N), particularly due to accumulation of labile organic matter (OM). We investigated the effect of RT compared with conventional tillage (CT) on the distribution of SOC and N over different soil fractions from 7 pairs of fields with cereal-root crop rotations, in the Belgian loess belt. Surface soil samples (0-100 mm) were physically fractionated according to a sequential sieving and density separation method into stable microaggregates, silt and clay, and free and occluded particulate OM fractions. RT management was previously found effective in increasing the organic C and organic N content of the surface soil (0-100 mm) at these 7 sites. Here, physical fractionation showed that the difference in amount of organic C and N in free particulate OM (fPOM), intra-microaggregate particulate OM (iPOM), and silt and clay associated OM between the RT and CT soils contributed 34, 29, and 37% of the increase in SOC and 35, 32, and 33% of the increase in N. The contribution of OC and N in iPOM and fPOM increased significantly on a relative basis under RT management. Only a modest increase in iPOM and slight enhancement of microaggregation was observed in RT compared with CT soils. We suggest that the repeated disturbance of soil by harvest of root crops and repeated use of cultivators and harrows may limit the accumulation of physically protected POM under RT management of these Western European cereal-root crop rotations. Instead, most of the accumulated OC and N in the surface horizons under RT management is present as free unprotected POM, which could be prone to rapid loss after (temporary) abandonment of RT management.

Additional keywords: soil organic matter, arable loess soil, conservation tillage, physical fractionation, long-term field experiment.
Article Type: Report
Subject: Soils (Carbon content)
Cropping systems
Conservation tillage
Soil conservation
Air pollution
Authors: Kader, Mohammed Abdul
Sleutel, Steven
D'Haene, Karoline
De Neve, Stefaan
Pub Date: 02/01/2010
Publication: Name: Australian Journal of Soil Research Publisher: CSIRO Publishing Audience: Academic Format: Magazine/Journal Subject: Agricultural industry; Earth sciences Copyright: COPYRIGHT 2010 CSIRO Publishing ISSN: 0004-9573
Issue: Date: Feb, 2010 Source Volume: 48 Source Issue: 1
Topic: Event Code: 200 Management dynamics Computer Subject: Company business management
Organization: Organization: Soil Science Society of America
Accession Number: 222313803
Full Text: Introduction

Silty soils under arable crop rotations in the central Belgian loess belt, and by extension in north-western Europe, are prone to erosion and structural degradation because of their location on hill slopes, low content of soil organic matter (SOM) (Sleutel et al. 2006a), and susceptibility to sealing and crusting. Erosion forms fills and gullies and washes away fertile soil, leading to financial loss to farmers through decreased crop yields. The loss of fertile soil results in decreased plant rooting depth, removal of SOM and nutrients, and reduced infiltration rates and plant-available water (D'Haene 2008). Most water erosion in Belgium occurs on silt loam soils and yearly erosion varies from a few to 100 Mg soil/ha. year (Verstraeten et al. 2003). Next to erosion, soil compaction also seriously threatens agricultural production on these soils and in other areas in Europe.

Soil organic carbon (SOC) plays an important role in the formation of stable aggregates and in the improvement of soil structure and, hence, is important in reducing the risk of soil erosion (Holland 2004). There is an intimate link between tillage, soil aggregation, and SOM turnover (Sleutel et al. 2007b). In contrast to conventional tillage (CT) management, plant residues are accumulated on the soil surface in reduced tillage (RT) or no-tillage (NT) fields, and water and energy exchange between the soil surface and the atmosphere are reduced. This results in decreased soil temperatures and wetter conditions, thereby favouring SOM accumulation (Franzluebbers et al. 1995). In addition, decreased macroaggregate turnover under NT compared with CT stabilises particulate organic matter (POM) in microaggregates formed within water-stable macroaggregates (Six et al. 2002b). In this context, there is much information on NT or RT practices adopted for cereal-based crop rotations or monocultures of maize or soybean in the USA (Dou and Hons 2006; Tan et al. 2007), Canada (Plante et al. 2006; Carter et al. 2007), and Brazil (Amado et al. 2006; Bayer et al. 2006).

In large parts of Europe, especially north-western Europe and Canada, crop rotations are not solely grain-based; rather, the typical rotation contains at least one root crop such as sugar beet or potato. Seedbed preparation for the root crops in these rotations requires some tillage, and the harvest of root crops itself often entails significant disturbance of the topsoil. Therefore, the potential for NT management in these rotations is very limited, but it is still possible to minimise soil disturbance by adopting RT. When considering the 0-600 mm layer, several studies point to little or no accumulation of OC in soils managed with RT compared with CT (eastern Canada, Yang and Kay 2001; Minnesota, Dolan et al. 2006; Indiana, Gal et al. 2007; Belgium, D'Haene et al. 2008). Also, for NT management, sometimes low or no OC accumulations have been found for 0-600mm depth, as discussed by Baker et al. (2007), and accumulation of OC by RT or NT seems to be primarily confined to the surface layer.

Next to saving time and fuel, soil structure improvement (for erosion control, countering sealing and crusting, and uniform growth of sugar beets) is the farmer's principal motivation for adapting RT management in north-western Europe. The accumulation of OM specifically in the surface layer under RT plays a crucial role, but this accumulated OM may be primarily 'labile'. For example, OC in POM has been shown to have an intermediate turnover time (1-50 years) and to account for about two-thirds of the total gain in soil carbon in NT soils compared with CT (Oorts et al. 2007). The increased SOM loss following crushing of microaggregates, as observed by Mikha and Rice (2004), corroborates the theory that this occluded POM stored in less disturbed soils is relatively labile. Many farmers in the study area still occasionally perform inverting tillage operations in some years and there is additional disturbance related to growing of root crops. Hence, the surface soil OM temporarily accumulated in RT fields may be prone to loss in a short time, with adverse effects on the intended amelioration of surface soil structure.

The aim of the present study was therefore to investigate the quality of OM under RT and CT management under arable crop rotations typical of the Western European temperate region. We did so by physically fractionating surface soil (0-100mm) samples from 7 pairs of RT and CT fields in Belgium under typical arable cereal-root crop rotations, and we assessed the distribution of SOC and N over different SOM fractions. A fractionation scheme based on the methodology proposed by Six et al. (2002a), which results in the isolation of free particulate organic matter, intra-microaggregate OM, and silt- and clay-sized OM, was selected because it allowed us to assess the amounts of unprotected and physically protected uncomplexed OM.

Materials and methods

Site description and soils

Seven pairs of adjacent RT- and CT-managed cropland fields, all located in the loess belt in central Belgium, were selected for this study. The locations were: Heestert (Fields 1 and 2), Kluisbergen (Fields 3 and 4), Baugnies (Fields 5 and 6), Maulde (Fields 7 and 8), Villers-le-Bouillet (Fields 9 and 10), Kuttekoven (Fields 11 and 12), and Court-St-Etienne (Fields 13 and 14). Some basic properties of the soils are given in Table 1. This central loess belt is characterised by arable farming on silt or silt loam soils and has an annual mean temperature of 9.8[degrees]C and an annual mean precipitation of 780 mm (KMI 2007).

The 7 RT fields were paired with CT fields with comparable soil type and crop rotation, and particular care was taken that the texture of the paired RT-CT fields was similar, because of the major influence of texture on C and N dynamics in soil. The inputs of effective organic carbon (EOC), i.e. the amount of organic carbon that is still in the soil 1 year after the application (Vleeshouwers and Verhagen 2002; De Neve et al. 2003) of crop residues, green manure, and organic manure in the period 2002-2004 calculated based on the approach used by of Sleutel et al. (2007a), are given in Table 2. The amount of EOC and the tillage operations of the selected fields of 2002-2004 are explained in detail in D'Haene et al. (2008, 2009). In general, soils under CT were tilled to a depth of 250-300 mm by a mouldboard plough which inverted the soil and buried the crop residues. Depending on the crop rotation, types of crop residues, and the application of organic manure, ploughing was combined with tillage by a cultivator and/or harrow. All RT-managed fields involved only non-inversion tillage and 2 types of non-inversion tillage were practised: reduced tillage with a cultivator or soil loosener ([RT.sub.C]), and direct drilling ([RT.sub.DD]). The period of RT (in years) is indicated in subscript, e.g. [RT.sub.C5].means 5 years of reduced tillage with cultivator or soil loosener (i.e. 5 years since conversion from CT to RT).

Briefly, at Heestert (50[degrees]48'N, 3[degrees]25'E), there was a cereal-based crop rotation with maize (Zea mays ssp. mays L.) and winter wheat (Triticum aestivum L.), and the RT experiment was established 3 years before sampling. Before sowing the main crop, the soil of Fields 1 and 2 were first worked with a cultivator to a depth of 100-150mm. The cultivator had 3 rows with 5 small bend tines ending in a sweep. The seedbed of [RT.sub.C2] Field 1 was prepared by harrowing. CT Field 2 was conventionally ploughed to a depth of 250-300 mm, followed by a secondary tillage with a tine harrow.

At Kluisbergen (50[degrees]46'N, 3[degrees]29'E) and Baugnies (50[degrees]33'N, 3[degrees]33'E), RT experiments were running for 5 years at the time of sampling, with root-cereal based 2-year crop rotations. The main tillage operation of [RT.sub.C5] Field 3 was done in the spring with a soil loosener with 1 row of 4 tines ending in a chisel (to a depth of 300-350mm). Secondary tillage operations of [RT.sub.C5] Field 5 included rotary harrowing to 50 mm depth and tillage by cultivator to a depth of 100mm. The main yearly tillage operation of CT Field 6 was ploughing in the autumn to a depth of 300 mm.

At Maulde (50[degrees]37'N, 3[degrees]3'E), the crop rotation was similar to those at Kluisbergen and Baugnies; however, the period of RT was 10 years. The main tillage operations at [RT.sub.C10] Field 7 and CT Field 8 were done by a soil loosener and plough, respectively.

At Villers-le-Bouillet (50[degrees]34'N, 5[degrees]15'E) and Kuttekoven (50[degrees]47'N, 5[degrees]20'E), a 3-year cereal root crop rotation with maize (Zea mays ssp. mays L.), winter wheat (Triticum aestivum L.), and potato (Solanum tuberosum L.)/sugar beet (Beta vulgaris L.) and the RT experiments were established 10 years before the sampling on Fields 9 and 11. During the first 5 years, tillage on these fields was restricted to soil loosening by a cultivator (chisel) and subsequent harrowing only to a depth of 50 mm, i.e. direct drilling managed. The main tillage operation of the adjoining CT Fields 10 and 12 was ploughing, combined with cultivator and harrow operations for seedbed preparation.

At Court-Saint-Etienne (50[degrees]38'N, 4[degrees]34'E), there was a 2-year sugar beet (Beta vulgaris L.)-winter wheat (Triticum aestivum L.)/yellow mustard (Sinapis alba L.) crop rotation. The deepest tillage operation of [RT.sub.C20] Field 13 was done in the autumn with a soil loosener to a depth of 250 mm, and at the time of sampling the RT treatment had been adopted for 20 years. CT Field 14 was conventionally ploughed every 2 years in winter before the sugar beets to a depth of 250 mm.

Soil samples were collected from 3 plots of 150[m.sup.2] (10 by 15 m) per field, spaced 10m apart. To avoid edge effects, the plots were located more than 20 m from the edges of the fields. On sloping fields, plots of RT and CT fields were at the same altitude on the slope. Fifteen core samples were collected from the surface soil (0-100mm) of each plot in March 2005 by means of an auger ([empty set] 25 mm). These samples were bulked and part of the samples was air-dried, grounded, and sieved with 2-mm sieve before analysis of the basic soil properties. The rest of the field-moist samples were gently pushed through an 8-mm sieve, air-dried, and kept for physical fractionation of SOM. Soil bulk density was assessed from undisturbed soil cores that were collected with steel tings (length 50 ram, [empty set] 50 mm).

Physical fractionation of soil organic matter

The field-moist soil was gently broken apart by hand and passed through an 8-mm sieve to break down large macroaggregates. The soil was then dried at 50[degrees]C. From bulked whole soil samples, 3 replicate 10-g subsamples were used for physical fractionation (Fig. 1). Organic matter was physically fractionated into OM associated with clay and silt particles, into POM residing in stable microaggregates, and into the free particulate organic matter (fPOM) based on the microaggregate isolation method proposed by Six et al. (2002a) modified as described by Sleutel et al. (2006b). The wet sieving method used was designed to break up all macroaggregates into microaggregates (53-250 [micro]m) while minimising disruption of the released microaggregates. The 10-g soil sample was placed on a 250-[micro]m sieve and was gently shaken with 40 steel beads ([empty set] 5 mm) on top of a reciprocal shaker. A constant water flow through the sieving column (consisting of a lid, the 250 [micro]m sieve, and a pan with water outlet, Eijkelkamp Agrisearch Equipment) flushed the <250-[micro]m fraction directly onto a 53 [micro]m sieve, thus avoiding further disruption of the microaggregates by the beads. The water + soil fraction <53[micro]m passing through the 53-[micro]m sieve was collected in plastic buckets. The coarse, free POM (coarse fPOM) and sand fraction which had been retained on the 250-[micro]m sieve and the microaggregates, free inter-microaggregate POM (53-250 [micro]m) (fine fPOM), and fine sand which had been retained on the 53-[micro]m sieve were collected in pre-weighed aluminium cups. After sedimentation the silt- and clay-sized fraction was also collected after aspiration of the remaining water from the buckets. All fractions were dried, left to cool down in a desiccator, and were then stored in glass vials.


The inter-microaggregate POM was isolated from the fine sand, the intra-microaggregate POM (iPOM), and the silt + clay fraction within the microaggregates by density flotation with 1.85 g/[cm.sup.3] of sodium polytungstate (SPT) (Sometu Europe, Germany) following a procedure very similar to that used by Six et al. (1998). The replicates of the dried fractions 53-250 [micro]m were bulked, and 5 g was weighed and pre-wetted as described above. Each sample was rinsed into an 80-mL nalgene centrifuge tube using 50mL SPT (the SPT was adjusted to 1.87 g/[cm.sup.3] as the water inside the aggregates would lower the density to 1.85 g/[cm.sup.3]), as described by Gale et al. (2000). The samples were centrifuged at 1250G for 60min at 20[degrees]C. The floating material, i.e. the fine fPOM, was aspirated and filtered on a pre-weighed 20-[micro]m nylon filter. The material on the filter was rinsed with water to remove remains of SPT, and the filters were dried at 50[degrees]C. The pellet was rinsed into a 1000-mL measuring beaker and was left to sediment overnight, after which the water + the dissolved remains of SPT were aspirated. This procedure was repeated twice to ensure a near-complete removal of the SPT.

A subsample of [+ or -] 5 g of the isolated microaggregates was dispersed inside an 85-mL Nalgene centrifugation tube filled with 10 glass beads by shaking for 18 h on a reciprocal shaker. The dispersed samples were passed through a 53-[micro]m sieve and rinsed thoroughly with water. The material retained on the sieve (fine sand and iPOM) was collected and dried at 105[degrees]C before weighing.

Carbon and nitrogen analysis

Subsamples (0.2 g) of the iPOM fraction + finc sand and the silt + clay (<53[micro]m) fractions of the soil having CaC[O.sub.3] were analysed for both OC and inorganic carbon (IC) content with a Shimadzu TOC analyzer. The SON content was determined from all the fractions by elemental analysis (Variomax CNS-analyzer, Elementar Analysesysteme, Germany), assuming that the mineral N fractions (N[H.sub.4.sup.+] and N[O.sub.3.sup.-]) were dissolved with decanted water. The coarse and fine fPOM fractions were also analysed for C by elemental analysis (Variomax CNS-analyzer), assuming that these 2 fractions did not contain any CaC[O.sub.3]. Absolute amounts of C and N in the different fractions expressed in grams were calculated from the relative dry matter weight of each fraction and its percentage of C and N.

Statistical analysis

Data on soil mass distribution, soil OC, and soil N (SN) in whole soil and its fractions between RT and CT soils were subjected to analyses of variance (ANOVA) tests using SPSS 15.0 (SPSS Inc., USA).


OC and N concentration and C: N ratios of whole soil

The RT management was previously found to be effective in increasing OC and N content of the surface soil (0-100 mm) at these 7 sites, except for OC content at Heestert (Fields 1 and 2) (Table 1) (D'Haene et al. 2009). Over all sites, the difference was significant (P < 0.05) between CT and RT soils for both OC and N content. A higher C : N ratio was also observed in all the RT soils except in fields 1 and 5.

Mass distribution of the isolated size and density fractions

The >250 [micro]m fraction (coarse sand + coarse fPOM), the fine fPOM, and the fine sand + iPOM fractions accounted for 1-3%, 0.1-0.3%, and 5-30% of the dry matter (DM), respectively. There was a significant difference in DM distribution between the CT and RT fields for coarse sand + coarse fPOM and fine fPOM; however, there was a limited difference for fine sand + iPOM (Table 2). Amounts of DM of the coarse sand + coarse fPOM, the fine fPOM and the fine sand + iPOM were on average 2.21 [+ or -] 0.52, 0.22 [+ or -] 0.08, 17.69 [+ or -] 10.92% of the total DM in the RT fields and 1.50 [+ or -] 0.49, 0.15 [+ or -] 0.07, 13.77 [+ or -] 8.46% in the CT fields, respectively (Table 3). A limited difference also observed for the summed DM weight of the intra-microaggregate 53-250 [micro]m fraction (fine fPOM, fine sand + iPOM, and intra-microaggregate silt + clay fractions) between CT (47.7 [+ or -] 6.9) and RT (49.7 [+ or -] 7.2) fields and there were no indications for a considerable difference in the amount of microaggregates. Averaged over all fields, the DM amounts of the intra-microaggregate silt + clay fraction and the free silt and clay fraction were in the range 22-38% and 38-63%, respectively. The sum of the free and intra-microaggregate silt + clay fractions was smaller for the RT fields (average 79.88 [+ or -] 10.92%) than for the CT fields (average 84.58 [+ or -] 8.5%) due to the relative increase of the DM of POM containing fractions.

OC and N concentration and C : N ratios of the isolated fractions

The amount of OC of all 3 POM fractions was significantly higher in the RT fields than the CT fields both on an absolute (P < 0.01) (g OC/kg dry soil) and a relative (P < 0.05) basis (expressed as g C/g OC), except for the relative proportion of coarse fPOM (Table 4, Fig. 2). Exceptions were the relative proportions of coarse fPOM at Court-St-Etienne (Fields 13 and 14) and Villers-le-Bouillet (Fields 9 and 10) and iPOM at Baugnies (Fields 5 and 6). The lack of significant difference in coarse fPOM between RT and CT fields arises possibly from a large variability in these data (Fields 13 and 14 and Fields 9 and 10). The biggest differences in the OC distribution over soil fractions between the tillage treatments were found for the [RT.sub.DD] fields (Fields 9 and 10 and Fields 11 and 12), particularly for the iPOM and coarse fPOM fraction. The difference in amount of OC in fPOM (coarse fPOM + fine fPOM) and iPOM between the RT and CT soils explained, on average, 34 and 29%, respectively, of the difference in amounts of whole SOC. The amount of silt- and clay-associated OC on an absolute basis was slightly higher in the RT fields than the corresponding CT fields for all the sites, except for the RT field at Heestert (Field 1). As a consequence of the relatively higher contribution of the OC in the coarse and fine fPOM and iPOM in the RT soils, the relative proportion of silt and clay OC was lower in the RT fields than CT soils among all 7 sites (Fig. 2).

As was the case for OC, N present in all 3 POM fractions in general was mostly higher in the RT fields than in the CT fields, both on an absolute (Table 5) and a relative basis (Fig. 3) at all sites except for the Court-St-Etienne (Fields 13 and 14). The difference was significant (P < 0.01 and P < 0.05 for absolute and relative basis, respectively) between RT and CT fields on an absolute basis for all the 3 POM fractions and a relative basis for coarse fPOM and iPOM. The contribution of fPOM and iPOM in explaining the difference in amounts of total SN between CT and RT soils was 35 and 32%, respectively, on average. On an absolute basis, the amount of silt- and clay-associated N was higher in the RT fields than the CT fields except at Kluisbergen (Fields 3 and 4) and Baugnies (Fields 5 and 6) but slightly lower on a relative basis. A larger relative proportion of silt + clay associated N (88.8 [+ or -] 5.7% of total N) compared with silt + clay associated OC (67.9 [+ or -] 6.9% of the total OC) was observed for all sites (Figs 2 and 3), reflecting the often reported larger proportion of N with decreasing particle size (Table 5) (Leinweber and Schulten 1995). A decreasing trend of C:N ratios was observed among the isolated SOM fractions in the following order: coarse tPOM > fine fPOM > iPOM > silt + clay associated OM fraction (Fig. 4). There was no consistent trend of higher or lower C : N ratios for the different SOM fractions in the RT fields compared with the CT fields individually; however, when averaged over all 7 locations, higher C : N ratios were observed in the RT fields than the CT fields for all the isolated SOM fractions except coarse fPOM.



Free particulate organic matter

The increase in fPOM (coarse fPOM + fine fPOM) in the RT fields compared with the CT fields likely resulted from a limited mixing of the harvest residues in the soil profile under RT management. Additionally the often observed decreased soil temperatures and wetter conditions in the RT field surface soil, reducing decomposition process (Franzluebbers et al. 1995), may also have favoured this fPOM accumulation. The fPOM was also more sensitive to tillage practices than the total SOM and its fractions as it explained one-third of the difference in amounts of whole SOC and SN between RT and CT soils, while it constituted only one-fifth and one-tenth of the whole SOC and SN, respectively. The coarse fPOM accounted for most of the observed differences. Free POM has indeed often been considered to be labile and very sensitive to management practices (Bremer et al. 1994; Solomon et al. 2000). The coarse and fine fPOM, the iPOM and the intra-microaggregate, and free <53 [micro]m fractions represent unprotected, physically protected, and the silt- and clay-associated SOM pools, respectively. Therefore, the increase of coarse and fine fPOM in the RT fields represents an increase of the unprotected SOM pool.

Intra-microaggregate particulate organic matter

The lower C : N ratios of the iPOM (14.6 [+ or -] 1.4) compared with the fPOM fractions (24.1 [+ or -] 5.4 and 22.5 [+ or -] 5.0 for coarse and fine fPOM, respectively) for all fields (Fig. 4) indicate the iPOM to be an intermediately decomposed fraction of OM that has been stabilised inside the microaggregates. D'Haene et al. (2009) evaluated the physical protection of SOM in microaggregates in the 0-50 mm layer of these soils by measuring C mineralisation in undisturbed and disturbed conditions (as a simulation of ploughing) and found no consistent difference between disturbed and undisturbed soils. These and our findings of little difference in the amount of microaggregation suggest no substantial difference in physical protection of SOM between these RT and CT soils. This result contrasts with previous findings by Denef et al. (2004) and Elliott (1986), who reported that the distribution of OM in aggregate fractions was primarily controlled by the amount of soil in that fraction. The additional soil disturbance with the harvest of sugar beets or potatoes in the RT fields could explain the only very small differences in microaggregate DM between RT and CT management. However, it should be noted that Denef et al. (2004) isolated microaggregates specifically residing within water-stable macroaggregates in contrast to the fractionations carried out here. The preceding selection of stable macroaggregates, as carried out by Denef et al. (2004) among others, resulted in the separation of mainly newly formed mieroaggregates which are highly C enriched, and a better relation between this fraction's DM weight and C content may be expected. The methodology followed here combined newly formed and older microaggregates within and outside of macroaggregates which are less C-enriched. Del Galdo et al. (2003) and Pulleman et al. (2005) isolated microaggregates in the same manner, and in line with our data, these authors did not observe any significant variations in the total amount of microaggregates between even native grassland, pasture, and organically and conventionally managed arable soils, in spite of obvious differences in soil disturbance.

Significantly higher accumulation of SOC and SN in the iPOM fraction on an absolute as well as relative basis is in line with the concept that an increased formation of stable microaggregates (53-250 [micro]m) in the surface layer of RT soils physically protects intra-microaggregate POM (iPOM). However, the accumulation of OC and N in iPOM was not as large as previously observed in fields under purely cereal-based NT management as reported by Denefet al. (2004). Those authors found that ~50% of the total SOC under NT management of continuous corn in Lexington, USA, was associated with the microaggregates. The contribution of iPOM (29 and 32% for OC and N, respectively) in explaining the difference in amounts of total SOC and SN between CT and RT soils is smaller than the results reported by Denef et al. (2004), who found that iPOM explained 14-48% of the difference in total SOC in the plough layer (0-200 mm) between NT and CT soils under cereal-based crop rotations. Still it is remarkable that while the iPOM constitutes only one-tenth of the total SOC and SN in our soils, differences in iPOM explain nearly one-third of the differences in total SOC and SN.


Silt and clay size fractions

The higher amounts of silt- and clay-associated OC on an absolute basis in most of the RT fields compared with the CT fields (Table 4) show that OM accumulation due to RT management is not restricted to POM fractions only. The relative increase of OC and N in POM fractions (coarse fPOM, fine fPOM, and iPOM) in the RT soils resulted in a relative decrease of the proportion of silt- and clay-associated OC and N in the RT soils compared with the CT soils (Figs 2 and 3). The slight absolute increase of silt- and clay-associated OM fraction under RT management could be due to the accumulation of labile SOM in clay-sized fractions. Sleutel et al. (2007b) analysed the SOM composition in the RT and CT soils of Villers-le-Bouillet by means of pyrolysis-field ionisation mass spectroscopy. They found significant differences between the RT and CT spectra of the sand and clay fractions, while in the silt fractions the RT and CT spectra were found to be very similar. Tiessen and Stewart (1983) and Tiessen et al. (1983) also found that the cultivation of native prairie silt loam soil with a grain-fallow cropping sequence decreased the absolute SOM concentration of all size separates, while the relative proportion of fine silt OM remained unchanged even after 90 years of cultivation. These and our findings thus suggest that the silt-sized OM fraction is not strongly affected by tillage management. In contrast, for the clay-sized OM fraction, it is generally accepted that it contains a labile SOM fraction consisting of soil biomass and its metabolites as well as root exudates and lysates (Leinweber and Schulten 1995; Christensen 2001). The much larger microbial biomass in all the RT fields compared with the CT fields (D'Haene et al. 2009) and the consequent higher input of microbial metabolites and faster transformation of flesh crop residues to smaller sized OM may partly have driven the accumulation of clay-sized OM and hence storage of OC and N in the here isolated <53 [micro]m size fractions.



Reduced tillage increased OC and N content of the surface soil layer (0-100 mm) in 6 of 7 silt loess soils with typical Western European arable rotations under temperate conditions. A considerable amount of OM accumulation under RT management may be attributed to an increase in OC and N in POM. However, the contribution of iPOM in explaining the OM storage with RT management was not as big as often reported in cereal-based crop rotation. Contrary to true 'no till' farming as practiced in the USA, Canada, and Brazil, the type of RT management that is most often practiced in Western European agriculture does not seem to increase microaggregation substantially and the accumulation of physically protected iPOM is limited. This could be a consequence of the occasional soil disturbance by harvest of the root crops (potato and sugar beet) under typical Western European arable rotations, but differences in the applied physical fractionation with previous studies may have interfered as well. Instead, a large portion of the difference in SOC and SN between RT and CT fields derives from accumulation of labile unprotected POM, which is prone to decomposition if the RT management is interrupted for just a few years.



M. A. Kader wishes to acknowledge the Flemish Inter-University Council (VLIR) for providing him a PhD grant to carry out this research. This research was conducted under the project 'Conservation agriculture in Flanders: influence on soil compaction and structure, C and N dynamics and C sequestration' with funding from the Flemish Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT).

Manuscript received 21 March 2009, accepted 9 October 2009


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Mohammed Abdul Kader (A,B,C), Steven Sleutel (A), Karoline D'Haene (A), and Stefaan De Neve (A)

(A) Department of Soil Management, Ghent University, Coupure Links 653, 9000 Gent, Belgium.

(B) Department of Soil Science, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh.

(C) Corresponding author. Email:
Table 1. Selected soil properties of the sampled soils (0-100 mm
depth layer)

[RT.sub.C]: Reduced tillage with cultivator or soil loosener--numbers
in subscripts indicate the years under RT management. [RT.sub.DD]:
Reduced tillage with direct drilling--6 years [RT.sub.C] + last
4 years [RT.sub.DD]. CT: Conventional tillage

Field        Site                 Particles (g/100g soil)

                         >50 [micro]m   20-50 [micro]m   <2 [micro]m


1        [RT.sub.C2]         33.9            52.6           13.5
2             CT             32.9            54.4           12.7


3        [RT.sub.C5]         30.3            51.6           18.1
4             CT             27.6            56.0           16.4


5        [RT.sub.C5]         30.4            59.0           10.6
6             CT             29.3            59.6           11.1


7        [RT.sub.C10]        8.5             70.9           20.6
8             CT             8.5             77.6           13.9


9       [RT.sub.DD.10]       7.9             72.2           19.8
10            CT             9.4             74.6           16.2


11      [RT.sub.DD.10]       12.8            71.7           15.5
12            CT             11.1            71.5           17.4


13      [RT.sub.DD.20]       13.8            71.5           14.7
14            CT             8.2             75.7           16.0

Field          OC                   TN                 C : N

                     (g/kg soil)


1       10.0 [+ or ] 0.3    1.09 [+ or ] 0.11      9.7 [+ or ] 0.9
2       10.8 [+ or ] 1.2    1.01 [+ or ] 0.05     10.7 [+ or ] 1.35


3       12.2 [+ or ] 0.6    1.16 [+ or ] 0.02     10.6 [+ or ] 0.5
4        8.9 [+ or ] 1.1    0.91 [+ or ] 0.12      9.8 [+ or ] 0.3


5       11.5 [+ or ] 0.6    1.15 [+ or ] 0.07     10.1 [+ or ] 1.1
6       10.0 [+ or ] 0.3    0.95 [+ or ] 0.05     10.5 [+ or ] 0.6


7       12.5 [+ or ] 0.3    1.27 [+ or ] 0.10      9.9 [+ or ] 1.0
8       10.8 [+ or ] 0.3    1.12 [+ or ] 0.02      9.7 [+ or ] 0.3


9       16.1 [+ or ] 1.5    1.55 [+ or ] 0.11     10.4 [+ or ] 0.6
10       9.4 [+ or ] 0.2    0.97 [+ or ] 0.02      9.7 [+ or ] 0.1


11      13.0 [+ or ] 0.8    1.18 [+ or ] 0.10     11.1 [+ or ] 0.7
12       9.7 [+ or ] 0.2    1.10 [+ or ] 0.01      8.8 [+ or ] 0.1


13      11.3 [+ or ] 1.3    1.17 [+ or ] 0.07      9.7 [+ or ] 0.7
14       9.2 [+ or ] 0.4    0.99 [+ or ] 0.02      9.3 [+ or ] 0.5

Field   Slope      Bulk density        [pH.sub.KCI]
         (%)      (g/[cm.sup.3])


1         3     1.38 [+ or ] 0.10     7.0 [+ or ] 0.03
2         3     1.48 [+ or ] 0.08     6.5 [+ or ] 0.8


3        10     1.37 [+ or ] 0.04     6.5 [+ or ] 0.1
4        10     1.43 [+ or ] 0.04     5.5 [+ or ] 0.3


5         0     1.46 [+ or ] 0.01     7.3 [+ or ] 0.2
6         0     1.24 [+ or ] 0.07     6.7 [+ or ] 0.2


7         2     1.43 [+ or ] 0.04     6.1 [+ or ] 0.1
8         2     1.49 [+ or ] 0.04     5.7 [+ or ] 0.1


9         0     1.37 [+ or ] 0.04     6.5 [+ or ] 0.2
10        0     1.49 [+ or ] 0.04     5.8 [+ or ] 0.1


11        0     1.47 [+ or ] 0.01     5.8 [+ or ] 0.1
12        0     1.51 [+ or ] 0.02     6.4 [+ or ] 0.2


13        0     1.35 [+ or ] 0.08     6.3 [+ or ] 0.2
14        0     1.47 [+ or ] 0.08     6.0 [+ or ] 0.3

Table 2. Crop rotation, average manure, compost, and additional OM
application for 2002-2004 and calculated yearly effective organic
carbon (EOC) input to the sampled (Based on D'Haene et al. 2009)

FM/GM, Fodder/grain maize (Zea mays ssp. mays L.); WW, winter wheat
(Triticum aestivum L.); MU, mustard (Sinapis alba L.); PO, potatoes
(Solanum tuberosum L.); WR, winter rye (Hordeum vulgare L.); SB, sugar
beet (Beta vulgaris L.); T, triticale (X. triticosecale); WB, winter
barley (Hordeum vulgare L.); PE, peas (Pisum sativum L.); R, rapeseed
(Brassica rapa L.); PH, phacelia (Phacalia tanacetifolia L.). PS, Pig
slurry; CHC, cattle/horse compost; GC, green compost; SC, cattle
slurry; SCM, stabilised cattle manure. SBh + 1, sugar beets heads +
leaves; W Ws, winter wheat straw; W Bs, winter barley straw. EOC is
the organic carbon that remains in the soil 1 year after application
(Vleeshouwers and Verhagen 2002; De Neve et al. 2003); calculations of
EOC are based on Holman et al. (1995), Sleutel et al. (2007a), and
measurements of organic manure composition of the Soil Service of

Site                   Field                  Crop      Manure, compost
                                            rotation      (t/ha.year)

Heestert               1. [RT.sub.C2]       FM/FM/WW        PS(25)
                       2. CT                FM/FM/WW        PS(25)
Kluisbergen            3. [RT.sub.C5]       GM/GM/GM        PS(20)
                       4. CT                FM/FM/PO        PS(25)
Baugnies               5. [RT.sub.C5]       FM/WR/WW    CHC(15), GC(5)
                       6. CT                 WW/T/WB        SC(18)
Maulde                 7. [RT.sub.C10]      PE/WW/WW
                       8. CT                SB/FM/FM        SCM(15)
Villers-le-Bouillet    9. [RT.sub.DD10]      WW/WB/R       SCM(8.3)
                       10. CT               WW/PH/WW
Kuttekoven             11. [RT.sub.DD10]     R/WW/WB       SCM(8.3)
                       12. CT               MU/WW/WB       SCM(8.3)
Court-St-Etienne       13. [RT.sub.C20]     MU/WW/MU        SCM(10)
                       14. CT               MU/WW/MU        SCM(13)

Site                   Field                   Additional OM

Heestert               1. [RT.sub.C2]
                       2. CT
Kluisbergen            3. [RT.sub.C5]
                       4. CT
Baugnies               5. [RT.sub.C5]             ISBh+I
                       6. CT
Maulde                 7. [RT.sub.C10]             2WWs
                       8. CT
Villers-le-Bouillet    9. [RT.sub.DD10]        2WWs, ISBh+H
                       10. CT                     ISBh+l
Kuttekoven             11. [RT.sub.DD10]    ISBh+l, IWWs, 1WBs
                       12. CT                     ISBh+I
Court-St-Etienne       13. [RT.sub.C20]        2SBh+l, 1WWs
                       14. CT                     2SBh+l

Site                   Field                    EOC

Heestert               1. [RT.sub.C2]          0.82
                       2. CT                   0.82
Kluisbergen            3. [RT.sub.C5]          1.50
                       4. CT                   0.72
Baugnies               5. [RT.sub.C5]          1.55
                       6. CT                   1.46
Maulde                 7. [RT.sub.C10]         1.10
                       8. CT                   1.00
Villers-le-Bouillet    9. [RT.sub.DD10]        1.58
                       10. CT                  0.81
Kuttekoven             11. [RT.sub.DD10]       1.59
                       12. CT                  1.15
Court-St-Etienne       13. [RT.sub.C20]        1.52
                       14. CT                  1.43

Table 3. Mass distribution of soil fractions separated by size and
density fractionation (g/100 g soil)

fPOM, Free particulate organic matter. ** P<0.01, * P<0.05 based on
ANOVA mixed model F-test (tillage fixed factor, site random factor);
n.s., not significantly different

Field            Site         Coarse sand,         Microagg.,
                               coarse MOM          fine fPOM,
                             (>250 [micro]m)       fine sand
                                               (53-250 [mciro]m)

                                                 Light fraction
                                                   (fine WOM)


l             [RT.sub.C2]         1.51                0.19
2                 CT              1.40                0.18


3             [RT.sub.C5]         2.17                0.31
4                 CT              0.86                0.16


5             [RT.sub.C5]         3.06                0.32
6                 CT              1.89                0.22


7            [RT.sub.C10]         2.34                0.29
8                 CT              1.78                0.23


9            [RT.sub.DD10]        2.38                0.16
10                CT              1.32                0.05


11           [RT.sub.DD10]        1.65                0.14
12                CT              1.03                0.11


13           [RT.sub.C20]         2.33                0.16
14                CT              2.24                0.11
RT (mean)                         2.21                0.22
CT (mean)                         1.50                0.15
s.e.                              0.132               0.013

  Tillage                           **                 **
  Site                            n.s.                n.s.

Field           Microagg., fine fPOM,          Silt +
                  fine sand (53-250             clay
                       [micro]m)           (<53 [micro]m)

                    Heavy fraction

             >53 [micro]m   <53 [micro]m


l               30.90          29.04           38.36
2               23.69          31.94           42.79


3               26.28          29.18           42.06
4               13.20          37.64           48.13


5               29.46          22.96           44.20
6               26.94          26.63           44.32


7                9.50          37.76           50.10
8                8.27          36.39           53.33


9                6.13          32.46           58.88
10               5.67          29.73           63.23


11              14.65          34.69           48.87
12              12.52          36.32           50.02


13               6.92          36.65           53.94
14               6.07          37.87           53.71
RT (mean)       17.69          31.82           48.06
CT (mean)       13.77          33.79           50.79
s.e.             1.236          0.975           0.646

  Tillage        n.s.           n.s.             *
  Site           n.s.           n.s.            n.s.

Table 4. Amounts of organic C (g OC/kg soil) in the separated size and
density fractions

fPOM, iPOM: Free or intra-microaggregate particulate organic matter.
** P < 0.01, * P < 0.05 based on ANOVA mixed model F-test (tillage
fixed factor, site random factor); n.s., not significantly different

Field           Site                       fPOM

                                Coarse               Fine
                            (>250 [micro]m)   (53-250 [micro]m)


1            [RT.sub.C2]         2.01                0.71
2                CT              1.31                0.71


3            [RT.sub.C5]         1.40                0.72
4                CT              0.66                0.43


5            [RT.sub.C5]         2.43                0.92
6                CT              0.95                0.67


7           [RT.sub.C10]         3.24                1.09
8                CT              2.54                0.84


9           [RT.sub.DD10]        2.46                0.56
10               CT              2.41                0.27


11          [RT.sub.DD10]        2.42                0.58
12               CT              0.98                0.46


13          [RT.sub.C20]         1.04                0.57
14               CT              1.05                0.43
RT (mean)                        2.14                0.74
CT (mean)                        1.42                0.54
s.e.                             0.157               0.029
  Tillage                         **                  **
  Site                             *                  **

Field       iPOM    <53 [micro]m fraction

                    Intra-micro-     Free


1           1.57        3.24         4.30
2           1.09        3.53         4.73


3           1.67        3.20         4.61
4           0.90        2.65         3.39


5           1.75        2.79         5.39
6           1.43        2.66         4.42


7           1.82        3.56         4.73
8           1.33        2.94         4.30


9           3.45        3.87         6.98
10          1.26        2.93         5.01


11          2.39        3.58         5.06
12          1.48        3.65         4.83


13          1.28        4.09         5.94
14          1.01        3.85         5.46
RT (mean)   1.99        5.29         8.76
CT (mean)   1.22        4.59         7.76
s.e.        0.178       0.207        0.309
  Tillage    **         n.s.         n.s.
  Site      n.s.        n.s.         n.s.

Table 5. Amounts of N (g N/kg soil) in the separated size and density

fPOM, iPOM: Free or intra-microaggregate particulate organic matter.
** P<0.01, * P<0.05 based on ANOVA mixed model F-test (tillage fixed
factor, site random factor); n.s., not significantly different

Field        Site                             fPOM

                                   Coarse              Fine
                               (>250 [micro]m)   (53-250 [micro]m)


1              [RT.sub.C2]          0.066              0.027
2                  CT               0.048              0.028


3              [RT.sub.C5]          0.065              0.041
4                  CT               0.041              0.023


5              [RT.sub.C5]          0.104              0.057
6                  CT               0.031              0.028


7             [RT.sub.C10]          0.137              0.056
8                  CT               0.081              0.036


9            [RT.sub.DD10] *        0.128              0.019
10                 CT               0.073              0.013


11           [RT.sub.DD10] *        0.118              0.019
12                 CT               0.048              0.016


13            [RT.sub.C20]          0.053              0.030
14                 CT               0.050              0.027
RT (mean)                           0.096              0.036
CT (mean)                           0.053              0.024
S.C.                                0.007              0.003
  Tillage                            **                  *
  Site                              n.s.                n.s.

Field        iPOM    <53 [micro]m fraction

                     Intra-micro-     Free


1            0.095      0.413         0.540
2            0.070      0.385         0.513


3            0.116      0.370         0.532
4            0.067      0.451         0.560


5            0.127      0.241         0.465
6            0.093      0.448         0.748


7            0.124      0.693         0.923
8            0.097      0.409         0.697


9            0.236      0.349         0.637
10           0.103      0.340         0.582


11           0.179      0.461         0.646
12           0.106      0.419         0.548


13           0.071      0.573         0.596
14           0.068      0.405         0.574
RT (mean)    0.135      0.443         0.620
CT (mean)    0.086      0.408         0.603
S.C.         0.011      0.043         0.041
  Tillage      *         n.s.         n.s.
  Site       n.s.        n.s.         n.s.
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