The effects of rock fragmentation and / or deep tillage on soil skeletal material and chemical properties in a Mediterranean climate

In Puglia, Italy, deep tillage and rock fragmentation are common agricultural practices to prepare land for vineyards or orchards. Unfortunately, little is known about how these practices influence soil structure and quality. There is a lack of information on the consequences of these practices on the soil fractions coarser than 2 mm, which are known as rock fragments or skeletal material, the focus of this study. Soil samples were obtained from depths of 0‐20 and 20–40 cm and analysed for pH, electrical conductivity, total organic carbon, total nitrogen, available phosphorus, and total and active calcium carbonate. For each soil depth, we determined the amount of fine earth and skeletal material by volume. The results indicate that rock fragmentation and/or deep ploughing cause a major change in soils, leading to the progressive reduction in total organic carbon and nitrogen, and to an increase in total and active calcium carbonate. In addition, there was a marked increase in skeletal material compared to undisturbed soil. The results confirm that rock fragmentation causes significant changes in soil physical properties and increases greatly the amount of skeletal material.     

The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C

Rock fragments in soil can contain significant amounts of organic carbon. We investigated the nature and dynamics of organic matter in rock fragments in the upper horizons of a forest soil derived from sandstone and compared them with the fine earth fraction (<2 mm). The organic C content and its distribution among humic, humin and non‐humic fractions, as well as the isotopic signatures (Δ¹⁴C and δ¹³C) of organic carbon and of CO₂ produced during incubation of samples, all show that altered rock fragments contain a dynamic component of the carbon cycle. Rock fragments, especially the highly altered ones, contributed 4.5% to the total organic C content in the soil. The bulk organic matter in both fine earth and highly altered rock fragments in the A1 horizon contained significant amounts of recent C (bomb ¹⁴C), indicating that most of this C is cycled quickly in both fractions. In the A horizons, the mean residence times of humic substances from highly altered rock fragments were shorter than those of the humic substances isolated in the fine earth. Values of Δ¹⁴C of the CO₂ produced during basal respiration confirmed the heterogeneity, complexity and dynamic nature of the organic matter of these rock fragments. The weak ¹⁴C signatures of humic substances from the slightly altered rock fragments confirmed the importance of weathering in establishing and improving the interactions between rock fragments and surrounding soil. The progressive enrichment in ¹³C from components with high‐¹⁴C (more recent) to low‐¹⁴C (older) indicated that biological activity occurred in both the fine and the coarse fractions. Hence the microflora utilizes energy sources contained in all the soil compartments, and rock fragments are chemically and biologically active in soil, where they form a continuum with the fine earth.     

Exchangeable cations in rock fractions and fine earth in soil profiles of different genesis

Data on accumulated exchangeable H, Al, Fe and Mn (M_a) cations in rock fractions in German soil profiles are scarce. The objective of this study was to describe the sum of accumulated M_a cations of fine earth and rock fragments in 11 deep soil profiles of varying genesis. Soil profiles were laid out at the sites Solling, Eifel, Harz mountains and the Erzgebirge and the parent materials included sandstones, siltstones, quartzite, slate, greywacke, diabase, gneiss and quartz porphyry. Exchangeable cations in the fine earth and rock fragments were measured in depths down to 6 m. Additionally, effective porosity and specific surface of rock fragments were determined. The effective porosity of the different rock fragments ranged from 4 to 28% (v/v), indicating that the rocks were accessible to solutions. For most samples, the cation exchange capacities (CEC) of the fine earth fractions were larger than those of the rock fragments, and the CEC (fine earth)/CEC (rock) ratios decreased with depth. All 11 profiles had small (<40%) amounts of exchangeable Na, K, Mg and Ca (M_b) cations in the fine earth fraction. Exchangeable M_a and M_b cations in the rock fragments changed similarly with depth as in the fine earth fractions for all profiles. Cumulative (rock + fine earth) M_a cations from 0—200 cm ranged from 474 to 1592 kmol_c ha⁻¹. The contribution of the rock fraction to the cumulative exchangeable M_a cations accounted for 13 to 85% of the total. The sum of exchangeable M_a cations was much higher than the cumulative acid deposition in western Germany since the beginning of industrialization, suggesting that carbonic acid and organic acids contributed largely to soil acidification. The rocks contribute significantly to buffering the acidity of the seepage water by silicate weathering and cation exchange. Therefore, acidification models which consider the fine earth fraction only, may lead to an overestimation of the rate of soil and groundwater acidification.     

江西省土壤可溶态稀土元素含量和分布规律的研究

对江西 1 1种母质发育的有代表性的水稻土、旱作地、自然土壤 ,61个剖面 ,4 1 2个土样的可溶态稀土元素分析测定 ,统计结果表明 :江西土壤稀土元素的强度因素处于较高水平 ,平均值为 1 9 0mg/kg ,有 4 2 80 %的样点处于丰富级 ;有由南向北 ,由东向西逐渐降低的趋势 .其含量最高的地区是赣南由花岗岩、泥质岩、第四纪红粘土、酸性紫色土、碳质岩类风化物等母质发育的土壤 ,较低的为赣西北由石英岩 ,第三纪红砂岩、石灰岩、下蜀系黄土等母质发育的土壤 ;在土壤剖面分布中 ,旱作地表层低于底层 ,水稻土表层有生物富集作用高于底层 ;影响土壤中可溶态稀土元素含量的主要因子有 :成土母质 ,气候条件 (温度 ,降水量 ) ,土壤 pH ,土壤质地和耕作类型     

Rock fragments in soil support a different microbial community from the fine earth

Two sandstone-derived soils under pure stands of silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.) were studied to determine if the fine earth (<2 mm material) and two size-classes of porous rock fragments (>2 mm material) supported different microbial communities. Samples from three soil horizons (A, Bw, and BC) were analysed under both optical and scanning electron microscopes. Small stones (2-10 mm in average diameter) appeared more altered than larger ones (40-60 mm) and the effects of weathering became more obvious with shallower depth. In both soils, numerous hyphae and other living forms were observed on the surface of the stones from the A and Bw horizons; this contrasted with the stones from the BC horizon, which showed little or no colonisation. The microbial community of each fraction was characterised using Biolog-Community Level Physiological Profiles (CLPP) and phospholipid fatty acid analyses (PLFA) for samples in the A and B horizons. Significant potential microbial activity (C source utilisation) was associated with rock fragments, from the A horizon and, to a lesser extent, the B, although this was lower than for the equivalent fine earth fraction. The microbial colonisation of the stones appeared inversely related with their size and sampling depth. The PLFA analysis showed not only quantitative differences in the microbial biomass between horizons and size-fractions but also highlighted that the communities differed between soils, horizons (for the sole beech soil) and fractions. These findings demonstrate that by considering rock fragments as a microbiologically inert fraction and discarding them before analysis, as usually is done, can lead to an incomplete picture of both the total amount and, perhaps more importantly, the structure of soil microbial community.     

Influence of drying vs. freezing of archived soil samples on soil organic matter fractions

Archived soil samples are a valuable tool for any long‐term soil research. We analysed total carbon (C) and nitrogen (N) content and soil organic matter fractions in 38 archived soil samples that were stored for up to 21 years and compared air‐dried storage to frozen storage conditions. Samples include top‐ and upper subsoils, different soil texture and land use with C contents between 4.3 and 174 mg g^?1. The results from this study reveal no changes in total C and N contents with storage time up to 21 years or type of storage (freezing vs. air drying). The analyses of soil physical fractions also revealed no significant differences between air‐dried stored and frozen stored samples for most samples. However, we found indications, that freezing of soil material might lead to changes in the mineral fractions for soils containing high amounts of water. Therefore, and as archiving soils in a frozen state is more expensive than storing air‐dried samples, we recommend the use of air‐dried samples for C quality analyses of archived soil samples.     

Organic matter stabilization in soil aggregates and rock fragments as revealed by low-temperature ashing (LTA) oxidation

Low-temperature ashing (LTA), which is able to remove the organic material from mineral grains or aggregates without any disturbance to their physical structure, was used to oxidize the organic matter present into soil aggregates and sandstone- and siltstone-derived rock fragments. The three fractions were characterized for their mineralogy and pores distribution, and treated with LTA apparatus for 24 and 96 h. The losses of C and N were evaluated and the chemical modifications produced by LTA in the extractable organic matter and humin were investigated by chemical and spectroscopic methods. During the LTA treatment, the simplest organic molecules were lost, and the remaining material acquired a higher degree of oxidation, making this heterogeneous mix of fresh, partially and well-humified substances more homogeneous. In the aggregates and in the sandstone rock fragments, the composition of extractable material changed and was enriched in the aromatic component, due also to the contribution of fragments deriving from humin, as the oxidation produced compounds enriched in carboxyl groups and characterized by a higher solubility. This could mean the occurrence of a continuum between humin and extractable organic matter mediated by oxidation processes. In the siltstone rock fragments extractable organic matter and humin underwent lesser severe transformation than those in the sandstone rock fragments and aggregates; the mineralized C and N derived almost entirely from the extractable fraction, whereas humin appeared to be virtually indifferent to the LTA treatment. In fact, although the amount of C lost during the treatment was similar for the three fractions, the siltstone rock fragments lost only 11% of initial humin-C pool, against 30% of the aggregates and 60% of sandstone rock fragments. As the three fractions showed similar mineralogy, the obtained results indicated that the soil fraction richest in micropores exerts a better protection on the organic matter.     

贵阳市乌当区不同母质发育的土壤理化性质和重金属含量差异研究

在贵阳市乌当区采集了9种母质（岩）上发育的土壤样品763个，分别测定了土壤的主要理化性质和主要重金属元素含量，通过对比分析表明不同母质（岩）发育的土壤理化性质和重金属含量都具有很大的差异。土壤理化性质的差异性主要表现为：石灰岩、红色粘土、老风化壳和页岩发育的土壤质地粘重。砂岩发育的土壤质地较轻，钙质紫色砂页岩、白云岩、砂页岩和河流冲积物发育的土壤质地适中；石灰岩、白云岩和钙质紫色砂页岩发育的土壤pH值为中性至微碱性。河流冲积物发育的土壤pH值为中性，其余5种母质（岩）发育的土壤pH值为酸性至强酸性；白云岩、钙质紫色砂页岩和河流冲积物发育的土壤有机质含量较高，而砂岩发育的土壤有机质含量较低，其余5种母质（岩）发育的土壤有机质含量介于二者之间；河流冲积物、石灰岩、白云岩和钙质紫色砂页岩等发育的土壤CEC较高，而砂岩发育的土壤CEC较低，其余4种母质（岩）发育的土壤CEC介于它们之间。土壤重金属含量差异性表现为：钙质紫色砂页岩、石灰岩、河流冲积物等发育的土壤中锅、铬、汞的含量较高。而红色粘土、河流冲积物和石灰岩等发育的土壤中铅和砷含量较高，砂岩发育的土壤中5种重金属元素的含量均为最低，其余母质（岩）发育的土壤中5种重金属元素的含量均介于上述之间。通过相关性分析结果表明：研究区域内土壤重金属镉、铬、汞、铅、砷的含量差异主要是由于成土母质（岩）的差异性所致，土壤理化性质的变化则是影响土壤重金属含量的次要因素。     

利用方式和土壤肥力对土壤团聚体和养分的影响

The size distribution of water-stable aggregates and the variability of organic C, N and P contents over aggregate size fractions were studied for orchard, upland, paddy, and grassland soils with high, medium, and low fertility levels. The results showed that > 5 mm aggregates in the cultivated upland and paddy soils were 44.0% and 32.0%, respectively, less than those in the un-tilled orchard soil. Organic C and soil N in different size aggregate fractions in orchard soil with high fertility were significantly higher than those of other land uses. However, the contents of soil P in different size aggregates were significantly greater in the paddy soil as compared to the other land uses. Soil organic C, N and P contents were higher in larger aggregates than those in smaller ones. The amount of water-stable aggregates was positively correlated to their contribution to soil organic C, N and P. For orchard and grassland soils, the > 5 mm aggregates made the greatest contribution to soil nutrients, while for upland soil, the 0.25-0.053 mm aggregates contributed the most to soil nutrients. Therefore, the land use with minimum disturbance was beneficial for the formation of a better soil structure. The dominant soil aggregates in different land use types determined the distribution of soil nutrients. Utilization efficiency of soil P could be improved by converting other land uses to the paddy soil.     

Zur pedochemie des berylliums — untersuchungen einer bodengesellschaft im gebiet des bärhaldegranits (südschwarzwald)

The profile distribution of Be is given for a toposequence of podzols, brown earths, stagnogley, forest bog and “Ockererde” (slope gley with infiltrated Al, Mn and Fe). Analysed were the parent rock, fine earth, four fractions of mineral soil (fine skeleton, sand, silt, clay) and the needles of spruce trees.Beryllium is involved in selective weathering processes of elements. Accordingly, the Be-content increases from the fine skeleton to sand, rock, silt and clay, progressively. Through pedogenetic enrichment, Be accumulates in the mica-rich silt fraction.Uptake of Be by spruce is very low even from these acid soils. The accumulation in the needles becomes greater with increasing age.The pedochemical behaviour of Be is very similar to that of Al.Be is incorporated very little in the organic matter cycle, and is therefore not enriched in the soil humus.The brown earth has lost Be, especially in the A_h-horizon.In the A_e-horizon of the podzol, loss of Be is even greater; the loss is partly offset by Be-enrichment in the B-horizon.In the profiles of depressions, the Be-content is high and is ascribed to lateral flow from the slopes. Be-enrichment is especially high in horizons with a pH of about 5.The Be-balance of the watershed indicates an export of about 10% of the original amount.     

土壤微量元素含量及其影响因素的研究

论述了临沂市土壤中硼、锌、锰、铜、铁等5种主要微量元素的状况与其它因素的关系。有效硼含量0.09~3.67 mg/kg,平均0.35 mg/kg;有效锰22~572 mg/kg,平均244 mg/kg;有效锌0.15~4.02 mg/kg,平均0.53 mg/kg;有效铜0.09~5.78 mg/kg,,平均1.03 mg/kg;有效铁3.2~162 mg/kg,平均21.5 mg/kg;有效锰2.0~131.4 mg/kg,平均23.4 mg/kg。不同土壤类型的以上5种微量元素有明显差异,其特征是砂姜黑土缺锌,棕壤、水稻土富含铁、铜、锰、锌;成土母质是影响土壤微量元素的重要因素之一,发育在基性岩上的土壤一般含量较高,而由红土母质发育的土壤则含量较低。土壤有机质含量与土壤微量元素有明显正相关关系,其中速效锌、速效硼和速效铜与有机质的关系尤为显著。     

Influence of fertilization and organic amendments on organic‐carbon fractions in Heilu soil on the loess plateau of China

The influence of fertilization on organic‐carbon fractions separated by density and particle size in Heilu soil (Calcic Kastanozems, FAO) was investigated in a 20‐year (1979–1999) long‐term experiment on the Loess Plateau of China. Compared to an unfertilized treatment, N application alone did not increase total organic carbon (TOC) and its fractions of density and particle size. However, the treatment of N + P fertilization significantly increased salty‐solution–soluble organic carbon (SSOC), microbial biomass C (MB‐C), and organic C associated with fine silt. When manure was applied alone and in combination with N and P fertilizer, the light fraction of organic C (LFOC), SSOC, and MB‐C were increased significantly, and the TOC was as high as that of a native Heilu soil. Organic C associated with different particle‐size fractions was also increased significantly, and the allocation of C among the fractions was altered: the proportions of C in sand (>50 μm), coarse‐silt (20–50 μm), and fine‐clay (<0.2 μm) fractions were increased whereas fine‐silt (2–20 μm) and coarse‐clay (0.2–2 μm) fractions were decreased. It is concluded that N fertilizer alone is not capable of restoring organic‐matter content in the Heilu soils of the Loess Plateau and that C‐containing material like manure and straw is necessary to produce significant increase in soil organic carbon in these soils.     

Nitrogen distribution and 15N natural abundances in particle size fractions of a long‐term agricultural field experiment

The present study combined a physical fractionation procedure with the determination of the natural abundance of ¹⁵N to investigate the impact of organic manure and mineral fertilizer application, and fallow on changes of N associated with different soil particle size fractions. The long‐term field experiment was conducted since 1956 in Ultuna, Sweden, on an Eutric Cambisol. Nitrogen in bulk soil and in particle size fractions changed significantly since 1956. The N_t concentrations in bulk soil decreased in all treatments not receiving organic materials. Comparing the N contribution of particle‐size fractions to the total N amount revealed the following ranking: silt > clay > fine clay > fine sand > coarse sand. The relative contribution of N in silt sized particles significantly increased from low to high bulk soil N contents, whereas N in clay and fine clay fractions decreased. The C : N ratios of particle size fractions differed considerably more between treatments than C : N ratios in bulk soils. Generally, the C : N ratios decreased from coarse to fine fractions emphasizing the tendency of smaller fractions being more significant as N sink than as C_org sink. ¹⁵N abundances varied more between particle size fractions of single treatments than between bulk soil from differently treated plots. Within treatments we observed differences of up to 7.1 ‰ between particle size fractions. In most cases δ ¹⁵N values increased with decreasing particle sizes. This pattern on average was similar to changes in δ ¹³ C. Our results suggest that silt sized particles acted as medium‐term sink of introduced N and that ¹⁵N abundances in particle size fractions sensitively reflect changes in N status in response to soil management.     

Classification schemes for the acidity,base saturation,and acidification status of forest soils in Switzerland

Soil chemical parameters related to soil acidity were determined for 1450 soil samples taken from individual mineral soil horizons in 257 forest soils in Switzerland, 196 developed from carbonate‐containing and 61 from carbonate‐free parent material. The distribution of pH values and exchangeable base cations in corresponding pH ranges were related to the capacity and rate of buffer reactions in the soil. Based on this, five acidity classes for individual soil samples were defined. To describe and classify the status of soil acidity and base saturation (BS) of an entire soil body, the pH and the BS of the total fine earth in the soil were calculated from the pH and BS, respectively, of the individual soil horizons and the estimated volumetric content of fine earth. The status of soil acidification of soil profiles was assessed primarily using the total amount of exchangeable acidic cations in percent of the CEC of the fine earth in the entire soil profile. As a second factor, the gradient between the acidity class of the most acidic soil horizon and the estimated acidity class at the beginning of soil formation was used. The application of these classification schemes to our collection of soil profiles revealed the great influence played by the type of parent material. The acidification status of most soils on carbonate‐containing parent material was classified as very weak to weak, whereas soils on carbonate‐free parent material were found to be strongly to very strongly acidified. In terms of parent rock material, microclimate, and natural vegetation, the results of this study and the proposed classification schemes can be considered appropriate for large parts of Europe.     

Properties of soil particle size separates after 40 years of continuous corn

Abstract

Changes in chemical and mineralogical characteristics associated with different particle size fractions in soil after 40 years of continuous production of corn by the conventional tillage method (CC) as compared with those of an adjacent native grassland site (NG) are investigated. Results indicate that corn cropping in a soil previously supporting native vegetation produces a decline in total and humified organic matter, phenolic compounds, enzymatic activities, cation exchange capacity (CEC), and hydrosoluble ions, both in the whole soil and in its particle‐size separates. The’ largest losses in organic carbon (C) and nitrogen (N) contents of the cultivated soil were observed in the sandy fractions, the lowest in the silt+clay separates. The humification index (HI) indicates a higher degree of humification of the organic matter in NG than in CC samples. For both NG and CC sites the finest fraction (silt+clay) resulted to be enriched in organic C, total N, humus, phenolic compounds, enzyme activity, CEC, and hydrosoluble ions with the only exception of mineral N forms and sulphates (SO₄). Slight differences were observed in the mineralogical composition of NG and CC soils. The sandy fractions of NG showed greater amounts of phyllosilicates while a lower content was found in the silt+clay fraction of CC as a consequence of a crumbling of parent rock into small pieces induced by repeated tillage practices.     

Beziehungen zwischen Klimafaktoren und C-, N-Pools in Partikelgrößen-Fraktionen zonaler Steppenböden Rußlands

Relationships between climatic factors and C, N pools in particle-size fractions of steppe soils, Russia Many soils of the Russian steppe are characterized by high soil organic matter contents and similar parent material. Thus, they are suitable for investigations of a climatic impact on C and N pools. We sampled 10 topsoils of the zonal Russian steppe at 0–10 and about 50–60 cm depth intervals. After particle-size fractionation into clay (<2 μm), silt (2–20 μm), fine sand (20–250 μm) organic C and N concentrations were determined in bulk soils and fractions. The results suggest that especially the older organic matter of the subsoil (in the silt fraction) is correlated with climatic factors. Topsoils show less evidence for climatic influences on C and N pools. As the ratio of mean annual precipitation to potential evaporation (=N/V) increases, C/N ratios decrease in all fractions and, thus, in the bulk subsoil. Obviously the degree of soil organic matter alteration was more pronounced in the order Greyzem (N/V = 1.0) > Chernozem, Phaeozem (N/V = 0.89) > Haplic Kastanozem (N/V = 0.6) > Calcic (N/V = 0.34), and Gypsic Kastanozem (N/V = 0.32). The organic carbon contents of the bulk subsoil are highest in the subsoil of the Chernozem and Phaeozem, and decrease with increasing N/V ratio (i.e., increasing heat input and dryness) to the Calcic Kastanozem. This is accompanied by an increasing enrichment of organic carbon in the silt fractions (r = ?0.99 for the correlation of the C enrichment in silt with N/V).     

不同利用方式旱地红壤水稳性团聚体及其碳、氮、磷分布特征

针对江西红壤地区不同利用方式引起的土壤质量和肥力的相应变化,研究了不同肥力水平、不同利用方式下红壤旱地水稳性团聚体含量及其养分分布规律。研究表明,荒地土壤中>5 mm水稳性团聚体含量显著高于其他利用方式,花生地和果园土壤则以0.25～0.053 mm的水稳性团聚体为主。各肥力水平下,菜地土壤中除>5 mm水稳性团聚体外,各粒级团聚体中有机碳、全氮和全磷含量均显著高于花生地、果园和荒地土壤。说明菜地土壤长期大量施肥,导致土壤碳、氮、磷养分含量均相对丰富。不同利用方式旱地红壤中,有机碳、全氮主要分布在>5 mm、5～2 mm和2～1 mm的较大粒径水稳性团聚体中。说明随着团聚体粒径增大,其有机碳含量增加,土壤全氮的消长趋势和有机碳一致。土壤全磷较均匀地分布在水稳性团聚体中,如高肥力菜地和荒地土壤各粒级团聚体中全磷含量间均无显著性差异。各利用方式旱地红壤中2～1 mm和1～0.5 mm的水稳性团聚体含量与土壤有机碳、全氮和全磷含量间均达到了极显著正相关。     

The effect of soil moisture on the vertical movement of rock fragments by tillage

Chiselling in air-dry soils can rapidly create inverse grading of the plough layer as field experiments showed, i.e., the largest particles (rock fragments) are brought to the surface and the smallest particles concentrate at the bottom of the plough layer. Since no information about the effect of soil moisture and fine earth characteristics on this process is available laboratory experiments were conducted to examine the effect of soil moisture and fine earth characteristics on the vertical movement (segregation) of rock fragments due to tillage. An experimental trough, 120×60×40 cm³, was filled with three layers (each 4 cm thick) of fine earth (sand or silt loam), and rock fragments (1.2–2.2 and 2.7–4.0 cm). Tillage was simulated by moving a hand-held cultivator through the mixture. The results for the sandy soil matrix showed that inter-particle percolation was slowed down by soil moisture, however, at the same rate for different moisture levels. This was attributed to water-films that surround the sand particles. In the silt-loam soil matrix inter-particle percolation was stronger than that occurring in the sandy matrix at similar volumetric moisture contents but vertical movement was impossible at higher moisture contents (0.17 m³ m⁻³) because of a strong increase in stickiness. The results imply that at low moisture contents farmers in areas threatened by desertification can use moderate tillage as a means to create a surface rich in rock fragments which helps to increase water infiltration and decrease erosion.     

Organic Fertilisation Increases C and N Stocks and Reduces Soil Organic Matter Stability in Mediterranean Vegetable Gardens

Given their organic matter (OM) depletion, agricultural soils can act as carbon (C) sinks if adequate management practices are implemented. OM stabilisation in highly OM‐depleted agricultural soils may depend upon the allocation of OM inputs among particle size fractions that differ in their capacity to stabilise OM. In a set of vegetable garden fields, we determined the magnitude of the differences in soil C and N content between organically and conventionally managed fields and the incorporation of the increased C and N pools to the fine fractions as an indication of the stability of the soil OM accrual. It was carried out in a stockless scenario in which exogenous OM was only used in organically managed fields for the last 20 years (as opposed to conventional management only using mineral fertilisers). Organic fertilisation caused a notable increase in soil organic C and N stocks compared with mineral‐fertilised soils. Such increase remained significant below the plough depth. C and N content increased at all fractions, but the relative contribution of the fine‐silt‐plus‐clay fraction to total C and N decreased at all depths. We concluded that organic management increases soil OM storage, but overall, the stability of the increased OM stocks decreases slightly. Copyright © 2016 John Wiley & Sons, Ltd.     

Accumulation of soil organic carbon in aggregates after afforestation on abandoned farmland

To understand how organic C (OC) accumulates in afforested soils and to quantify the contribution of aggregate-associated OC to OC accumulation, we investigated the changes in soil structure, total soil OC, and aggregate-associated OC from 0- to 10- and 10- to 20-cm depths in afforested forests and adjacent farmlands of northwestern China. We assessed the contribution of macroaggregate-associated OC increase to total soil OC accumulation. Afforestation increased macroaggregate amount, mean weight diameter, and mean geometric diameter but decreased the amount of microaggregate and silt + clay-sized fractions. The improvement of soil structure was greater in surface than subsurface soils and was greater in soils afforested with white birch than in soils afforested with other tree species. Fifty years after afforestation, total soil OC concentrations and stocks and aggregate-associated OC concentrations increased depending on soil depth and tree species. Afforestation increased macroaggregate-associated OC stocks but decreased microaggregate- and silt + clay-associated OC stocks. Soil OC stocks and changes in OC stocks after afforestation mainly depended on macroaggregate-associated OC stocks and their changes. The results from this study suggest that OC accumulation in afforested soils is due to the accumulation of OC in macroaggregates and the redistribution of OC from fine particles to coarser fractions.