Stable Microaggregates Research Articles

Biochar rate-dependent regulation of extended nitrogen supply by modifying stable aggregates-N and microbial responses

Besides carbon sequestration, biochar amendment in soils can regulate the loss of applied fertilizer-nitrogen (N) into the environment. Soil aggregates are essential in controlling soil N stocks' stabilization and supply. However, unraveling the rate-dependent impact of biochar on stable soil aggregates and their associated N in fertilized soils over an extended period is a prerequisite to understanding its implications on soil-N dynamics. We unraveled how high and low biochar application rates combined with inorganic fertilizer (20- and 40-tons ha−1 [FB1 and FB2]) affected soil N fractions, stable soil aggregates, aggregates associated-N, and microbial responses to regulate N supply for Nageia nagi after one year. Results revealed that biochar amendment, especially at higher rate, increased the concentration of inorganic N and some amino acids compared to sole fertilizer (F). Available N increased by 16.5% (p = 0.057), 23.8% (p = 0.033), and 34.8% (p = 0.028) in F, FB1, and FB2, respectively, compared to the control (C). Also, 28.1% and 32.8% significant increases in the availability of NH4+-N were recorded in FB1 and FB2, respectively, compared to F. NO3−-N availability was significantly increased by 15.2%, 21.8%, and 20.8% in FB1, FB2, and C, compared to F. Biochar amendment, irrespective of rate, increased stable microaggregates (< 0.25 mm). However, FB2 significantly increased macro- and intermediate-aggregate-N, and urease activity, and hence higher N supply capacity to meet the N need of N. nagi even after one year. Hence, the N content of N. nagi was 41.3%, 28.8%, and 12.2% higher in FB2, FB1, and F, respectively, compared to the control. Biochar amendment decreased bacterial species diversity but increased the proportion of NH4+-oxidizers (especially the Betaproteobacteria) to maintain the mineralization and slow release of N. Although a low biochar rate was more beneficial than sole fertilization, higher biochar application rate could sustain higher N supply by stabilizing soil microaggregates and increasing macro- and intermediate-aggregates N, its mineralization, and slow-release over longer periods.

Effects of organic matter characteristics on soil aggregate turnover using rare earth oxides as tracers in a red clay soil

Organic materials input is remarkably essential for soil aggregates formation and breakdown processes. Which characteristics of organic materials control soil aggregate turnover is still largely unknown. Eleven organic materials were characterized in terms of nutrient stoichiometry, biochemical features and carbon (C) functional groups. The effects of organic matter characteristics on soil aggregate turnover were investigated by using rare earth oxides (REOs) as tracers. REOs concentrations in four aggregate fractions were measured on 0, 14, 28, and 56 d of incubation to calculate the aggregates transformation paths and turnover time. Our results exhibited that aggregate turnover time was reduced considerably with the addition of organic materials in order of easily decomposed residues (ED) < moderately decomposed residues (MD) < slowly decomposed manures (SD) but increased within aggregate fractions in sequence of silt and clay fractions < macroaggregates < microaggregates, such effects attenuated over time (P < 0.05). Nutrient stoichiometry had no impacts on relative changes and turnover time of aggregates. Soluble sugars increased the formation of large macroaggregates at early stage of incubation, but laid no impacts on aggregate turnover time. Lignin reduced soil aggregates formation but increased aggregate turnover time in the first four weeks. C functional groups showed short-term effects on relative changes of aggregates while these characteristics did not explain aggregate turnover time except aromatic carbon. Under ED treatments, the relative formation of 0.053–0.25 mm aggregates increased with the accelerating breakdown of macroaggregates, suggesting the formation of stable microaggregates in the mid-to-late incubation time. With MD and SD application, the relative formation was increased with the decrease of aggregate breakdown over time. We proposed the pathways of soil aggregates turnover, in which the stable microaggregates were released with the breakdown of stable macroaggregates in ED treatments, while such transformation was not observed in MD or SD treatments during the incubation time. Our results demonstrate that aggregate turnover depends on the initial characteristics of incorporated organic matters defined by biochemical features and C functional groups.

Changes in soil organic matter associated with afforestation affect erosion processes: The case of erodible volcanic soils from Patagonia

Large areas of the Patagonian Andean Region with high potential for planting fast-growing exotic conifers, based on the volcanic nature of its soils, are affected by erosion processes. This study aimed to analyse the effects of Pinus ponderosa afforestation on soil organic matter (OM) fractions in non-allophanic volcanic soils and to determine the relationship between organic matter, soil aggregates and erosion processes. The study was conducted along a forested hill slope showing different soil properties according to slope position. OM fractions and aggregate size fractions under different dispersion forces were analysed on 0–5 cm soil samples in rangelands and in 14- and 24-year old plantations on steep and gentle slopes. Simulated rainfall assays were performed to assess soil erodibility; OM and granulometry of sediments were also studied. Results showed that OM contents, mainly OM labile fractions (i.e., particulate OM, and OM associated with macro and large microaggregates), soil aggregation and the formation of very stable microaggregates were enhanced in the afforested soils. However, soil changes varied depending on the initial soil OM contents and on the age of the plantation, with more erratic and smaller changes in most fertile soils. Although potential erosion rates are lower in plantations than in rangeland soils, the high OM enrichment rates found in sediments imply a high OM loss when the soils remain uncovered. Erosion processes in afforestation involve the removal of microaggregates rich in OM and silt fractions, while in the rangelands, coarse and very coarse sand single particles are lost. Afforestation replacing degraded rangelands may be a way to control erosion in these highly erodible volcanic soils, as long as the soil remains covered. Otherwise, the loss of soil enriched in OM from the superficial soil could favour soil carbon depletion.

Clay behavior following macroaggregate breakdown in Ferralsols

An increase in organic matter content is one of the most important factors favoring stabilization of macroaggregates in soils. However, organic matter is also known to inhibit the flocculation of suspended clay, which implies that flocculation is unlikely to happen after clay is released by the breakdown of water-stable macroaggregates. The aim of this study was to investigate whether clay flocculation is possible following the mechanical breakdown of water-stable macroaggregates in Ferralsols. Profiles of three soils weathered from igneous rocks were studied to create a natural gradient of organic matter, macroaggregation, clay dispersion, and flocculation. Macroaggregates stabilized mainly by inorganic bonds showed high water re-flocculable clay content following breakdown. Organic bonds increased the proportion of water-stable macroaggregates, but, following breakdown, the released clay showed a high imbalance in charges, thereby inhibiting its flocculation. Flocculation can still prevail over dispersion in clay saturated with organic matter when the base saturation is low and Al3+ concentration is high, but the charges in surface horizons are too imbalanced to allow flocculation. Highly stable microaggregates (pseudo-silt and pseudo-sand) are favored under both very low and very high organic matter contents.

The Influence of Microbiology on Soil Aggregation Stability

In the present study, this work investigated the effects of bacteria (Bacillus sp.) decay and biological aggregating factors (organic matter, microbial calcium carbonate precipitation) at various steps on soil aggregate stability. Experiments carried out at different cultivation time 2, 5, and 8 weeks. The soil samples were compared the control (pure soil) versus soil treated with bacteria. The results showed that bacterial activity played an essential function in the macro-aggregate foundation. Nevertheless, the influence of organic matter (OM) on soil aggregate stability was important, which might be related to the microbial activity of bacteria to decay the organic material to an organic mineral that is connected with soil particles to form stable micro-aggregates. The CaCO3 is one of the most stable bonding agencies in nature as it is called natural cement, The soil aggregates stability increased as calcium carbonate increased. No change of soil pH levels observed due to microbial reactions with soil. Hence, the appropriate pH was 7.5. In addition, the higher pH decreases enzymatic activity and carbonate tends to dissolve at pH levels are very low. This study affords the helpful of soil stabilization in the presence of natural cementation of bacteria in contrast with the previous studies.

Particle-Size, Microaggregate-Size, and Aggregate-Size Distributions in Humus Horizons of the Zonal Sequence of Soils in European Russia

Experimental data on size distribution of micro- and macroaggregates and primary soil particles were obtained by laser diffraction analysis of the samples of humus horizons of the zonal soil sequence (from Retisols to Kastanozems) in European Russia. Both undisturbed native soils and agricultural soils were analyzed. The relationships between the parameters of the size distribution of aggregates, microaggregates, and primary particles were valuated. The parameters of the size distribution (content of the particles and their mean-weight diameter) were compared with data on the contents of water-stable aggregates, organic carbon, nitrogen, phosphorus, potassium, and pH. Size distribution of primary particles in the humus horizons of Endocalcic Chernozems had a bimodal pattern. A positive linear correlation between the contents of microaggregates and organic carbon was found to occur only when the latter exceeded 2–3%. An index of the size distribution of microaggregates—the minimum size of stable microaggregates—was proposed; it can be determined as an intersection point of differential curves of size distribution of microaggregates and primary particles. For this index, a negative correlation with the soil pH was found. It was supposed that the observed correlation reflects the coagulation of soil colloids, when their electric charge is compensated.

Micromorphology and Genesis of Soils from Topolitosequences in the Brazilian Central Plateau

ABSTRACT The micromorphology of deeply weathered soils (Ferralsols/ Latossolos ) from the Central Plateau of Brazil remains little studied, and its affiliation to different parent materials, poorly known. To clarify the processes of soil formation of these acric, gibbsitic, Fe-oxide rich Ferralsols, three lithotoposequences on local ultrabasic to basic intrusive rocks were studied. The influences of mixing and pedobioturbation are evident in all soils, and Ferralsols of the Central Plateau of Brazil are polygenetic, based on the coarse mineral composition, with ultrabasic and metapelitic rock contributions. The typical oxic microstructure with stable microaggregates encompasses a gibbsite/Fe rich micromass with random inclusions of charcoal, allochthonous quartz, Ti-magnetite, and perovskite grains. Shallow Cambisols ( Cambissolos ) on tuffites also display a similar “oxic” microstructure, but a much lower degree of weathering. These Cambisols possesses apatite, and mafic and pelitic minerals as residuals minerals grains, indicating the polygenetic colluvia mixture of different substrates. The common occurrence of perovskite (CaTiO3) as a residual grain in deep-weathered, acric Ferralsols on ultrabasic rocks, not yet reported in the literature, shows an unusual resistant Ca reserve in oxic soils, though nothing is known about its implications to soil fertility.

Nitrogen-rich microbial products provide new organo-mineral associations for the stabilization of soil organic matter.

Understanding the cycling of C and N in soils is important for maintaining soil fertility while also decreasing greenhouse gas emissions, but much remains unknown about how organic matter (OM) is stabilized in soils. We used nano-scale secondary ion mass spectrometry (NanoSIMS) to investigate the changes in C and N in a Vertisol and an Alfisol incubated for 365days with 13 C and 15 N pulse labeled lucerne (Medicago sativa L.) to discriminate new inputs of OM from the existing soil OM. We found that almost all OM within the free stable microaggregates of the soil was associated with mineral particles, emphasizing the importance of organo-mineral interactions for the stabilization of C. Of particular importance, it was also found that 15 N-rich microbial products originating from decomposition often sorbed directly to mineral surfaces not previously associated with OM. Thus, we have shown that N-rich microbial products preferentially attach to distinct areas of mineral surfaces compared to C-dominated moieties, demonstrating the ability of soils to store additional OM in newly formed organo-mineral associations on previously OM-free mineral surfaces. Furthermore, differences in 15 N enrichment were observed between the Vertisol and Alfisol presumably due to differences in mineralogy (smectite-dominated compared to kaolinite-dominated), demonstrating the importance of mineralogy in regulating the sorption of microbial products. Overall, our findings have important implications for the fundamental understanding of OM cycling in soils, including the immobilization and storage of N-rich compounds derived from microbial decomposition and subsequent N mineralization to sustain plant growth.

Influence of natural regeneration on fractal features of residue microaggregates in bauxite residue disposal areas

AbstractBauxite residue is often physically degraded, which limits vegetation establishment on the disposal areas. Microaggregate stability is an important physical property due to its significant effect on erosion and surface runoff; however, this is rarely reported for bauxite residue. Native plant encroachment on a bauxite residue disposal area in Central China has revealed that natural regeneration may ameliorate the residue and help to support plant growth. Residue samples from 5 different disposal ages were collected to determine microaggregate stability and to identify their fractal features. Following natural regeneration, the aggregate fraction 250–50 μm increased significantly from 27.4% to 40.3%, whilst the silt + clay size aggregate fraction decreased from 58.4% to 30.7%. With increasing disposal age, the residue clay dispersion ratio (CDR) ranged from 7.7% to 22.5%, whilst aggregated silt and clay ranged from 15.3% to 19.0%, indicating a stable microaggregate structure. The single‐fractal dimension (D) of the residues for different disposal ages varied from 2.2 to 2.4. The high pH and salinity of bauxite residue indicated a high value of single‐fractal dimension. The multifractal parameters of residue microaggregates, including capacity dimension (D0), information dimension (D1), and information dimension/capacity dimension (D1/D0), decreased, which resulted in homogeneity following natural regeneration. Correlation analysis revealed that both single‐fractal and multifractal dimensions had significant correlations with residue microaggregate stability. Our results suggested that natural regeneration may improve microaggregate stability of bauxite residue, and fractal parameters of residue microaggregates may be used to describe residue microaggregate stability and the physical condition of bauxite residue.

The Effect of Tillage on Organic Carbon Stabilization in Microaggregates in Different Climatic Zones of European Russia

Tillage may affect the microstructural organization of soil, including the distribution of microaggregates with different mechanical strengths. We quantified the impact of tillage treatment on the amount and distribution of free organic matter, microaggregates (unstable and stable under low intensity sonification) and their components, in the upper horizons of zonal soils of the Center of the Russian Plain. Under plowing, the carbon content decreases, both in unstable and stable microaggregates. The loss of carbon in unstable microaggregates was ~24%, whereas in stable microaggregates, it was ~37%, relative to native soils. The carbon content of organic (LFoc) and organo-clay (Clayrd) fractions in unstable microaggregates (CLFoc/CClayrd) was almost identical in the upper horizons of native soils: the ratio of these components is for Albeluvisols (1.1), Phaeozem (0.8) and Chernozems (1.0). Under plowing, these decrease to: Albeluvisols and Chernozems (0.6) and Phaeozem (0.5). The shares of carbon accumulated within the unstable and stable microaggregates (Cunstable/Cstable) are constant under equilibrium conditions and show a tendency to decrease from north to south on the order of: Albeluvisols and Phaeozem (2.2) &gt; Chernozems (1.0). Under plowing, they increase to: Albeluvisols (3.0) and Phaeozem (3.2) &gt; Chernozems (1.5).

Soil health and carbon management

AbstractSoil, a natural four‐dimensional body at the atmosphere–lithosphere interface, is organic‐carbon‐mediated realm in which solid, liquid, and gaseous phases interact at a range of scales and generate numerous ecosystem goods and services. Soil organic carbon (SOC) strongly impacts soil quality, functionality and health. Terms soil quality and soil health should not be used interchangeable. Soil quality is related to what it does (functions), whereas soil health treats soil as a living biological entity that affects plant health. Through plant growth, soil health is also connected with the health of animals, humans, and ecosystems within its domain. Through supply of macro‐ and micronutrients, soil health, mediated by SOC dynamics is a strong determinant of global food and nutritional security. Soil C pool consists of two related but distinct components: SOC and soil inorganic C (SIC). The SIC pool comprises of primary and secondary carbonates, and the latter consists of calcitic (no net sequestration of atmospheric CO2) and silicatic (net sequestration). While SOC is highly dynamic, its mean residence time depends on the degree of protection (physical, chemical, biological, and ecological) within the soil matrix. Formation of stable microaggregates and of organo–mineral complexes can protect SOC against microbial processes for millennia. In addition to formation of silicatic type of secondary carbonates, leaching of bicarbonates into the subsoil or shallow water table is also an important mechanism of sequestration of CO2 as SIC. Numerous soil functions and ecosystem services depend on SOC and its dynamics. Improvements in soil health, along with increase in availability of water and nutrients, increases soil's resilience against extreme climate events (e.g., drought, heat wave) and imparts disease‐suppressive attributes. Enhancing and sustaining soil health is also pertinent to advancing Sustainable Development Goals of the U.N. such as alleviating poverty, reducing hunger, improving health, and promoting economic development.

THE STATE OF SOIL ORGANIC MATTER IN DIFFERENT PHYSICAL FRACTIONS DEPEND ON LAND USE TYPE

UDK 631.417(497.6)&#x0D; Known fact is that arable soils compared with correspondent native soils differ in the state of soil organic matter (SOM) and that agricultural practices halved content of soil organic carbon in last decades. Further, soil structure or aggregation mediates many biological and chemical processes in soil and influence on protecting of SOM. The objectives of this paper is to analyze how the different land use (forest, grassland and arable soil) effects on quality and quantity of SOM in different physical fraction. In particular, it is important to determine what pool (physically separated fraction) of SOM is most sensitive on cultivation and what pool is more resistant to mineralization.&#x0D; The research is carried out on two soil type with different texture, Pseudogley and Terra rossa, in condition of different climate and soil management. Two groups of SOM analysis are singled out: 1) quantify SOC and N storage, and δ13C by horizons; and 2) quantify the SOC and N in the following physically separated fractions of SOM: a) coarse and fine particulated organic matter (POM) density &lt;1.8 gcm-3; b) occluded POM in stable microaggregates 53-250 µm; and c) SOM in small microaggregates &lt;53 µm.

Redifferentiation of High-Throughput Generated Fibrochondrocyte Micro-Aggregates: Impact of Low Oxygen Tension

In meniscus tissue engineering strategies, enhancing the matrix quality of the neomeniscal tissue is important. When the differentiated phenotype of fibrochondrocytes is lost, the quality of the matrix becomes compromised. The objective of this study was to produce uniform fibrochondrocyte micro-aggregates with desirable phenotype and tissue homogeneity in large quantities using a simple and reproducible method. Furthermore, we investigated if hypoxia could enhance the matrix quality. Porcine fibrochondrocytes were expanded at 21% oxygen until passage 3 (P3) and a gene expression profile was determined. P3 fibrochondrocytes were cultivated in chondrogenic medium at 5 and 21% oxygen in high-throughput agarose chips containing 2,865 microwells 200 µm in diameter. Evaluation included live/dead staining, histological examination, immunohistochemistry, dimethylmethylene blue assay and real-time reverse transcriptase quantitative polymerase chain reaction of the micro-aggregates. Gene expression analysis showed a drastic decline in collagen II and high expression of collagen I during monolayer culture. After 4 days, uniform and stable micro-aggregates could be produced. The redifferentiation and matrix quality of the hypoxic cultured micro-aggregates were enhanced relative to the normoxic cultures. Sulfated glycosaminoglycan synthesis was significantly higher, and collagen II expression and the collagen II/collagen I ratio were significantly upregulated in the hypoxic cultures. High-throughput production of uniform microtissues holds promise for the generation of larger-scale tissue engineering constructs or optimization of redifferentiation mechanisms for clinical applications.

Effect of the endogeic earthworm Aporrectodea tuberculata on aggregation and carbon redistribution in uninvaded forest soil columns

The long-term impact of earthworm presence on soil carbon (C) dynamics of previously uninhabited northeastern forests is still largely unknown. Currently, earthworm presence is understood to both enhance soil respiration and create stable microaggregates, processes assumed to have conflicting effects on long term C storage. To date, studies investigating earthworm-created microaggregates and occluded C have rarely been done in undisturbed forest soils. A paired mesocosm study (n = 5) was conducted investigating the impact of the endogeic earthworm species Aporrectodea tuberculata on the physical proportion of microaggregates and the associated mineral soil C of a minimally disturbed forest soil. Pairs analyzed after 4 weeks of incubation demonstrated no significant aggregate effects. At 4 months, paired cores with earthworms (WW) showed a 67% increase in large macroaggregates (>2000 μm diameter, lgMA), compared to cores without earthworms (NW). While distribution shifted among various microaggregate pools (free and occluded within macroaggregates), the net proportion of microaggregates in the soil (dry weight basis) was unaltered. After 4 months, the mineral soil of WW cores had an average of 60% more C than the NW cores due to the relocation of the forest floor. The C associated with the microaggregate fractions increased an average of 56%. Of this increase in C, 95% was accounted for by the microaggregates occluded within the lgMA fraction, a fraction that was almost 4 times greater in the WW cores. Over 50% of the C relocated into the mineral soil was associated with the physically protected microaggregate fractions, indicating that though this species of earthworm did not alter the proportion of microaggregates in these soils, they occluded a substantial proportion of C within those physical fractions. In this particular forest soil, the actions of Aporrectodea tuberculata increased the physically protected C pool through microaggregate restructuring and C enrichment.

English

The undesirable legacies of iron mining activities lead to unintended consequences in soil subsystem with altered geomorphic system influencing ecosyste m processes. The textural and elemental composition of iron mine spoil with nutrient defici ency determines the characteristic soil microbial communities by providing habitat and nutrient resou rces. Extremities of iron mine overburden spoil allow the existence of specific group of microbial populations especially chemolithotrophs, which reduce Fe 3+ to Fe 2+ thereby influencing the biogeochemistry and minera lization of organic matter. In addition, microbes influence soil functioning du e to their involvement in nutrient cycling, biotransformation, formation of stable microaggrega tes and structural stability. Being the initial colonizers, their presence and activity is crucial for subsequent colonization of other microbial groups in iron mine spoil over time. Therefore, emp hasis has been given to isolate a chemolithotrophic bacterial strain and subsequently characterized and identified by 16S rDNA sequencing. The isolated bacterium was found to be gram-negative, obligatory and facultative chemolithotroph, which exhibited maximal growth in culture medium with pH 9 at 45 o C. The bacterium was observed to be alkalophilic and moder ately thermophilic exhibiting higher growth rate in heterotrophic condition as compared to chem olithotrophic culture condition. Molecular phylogenetic analysis based on 16S rDNA sequence ho mology of the isolated bacterium revealed its close resemblance with Methylobacterium organophilu m DSM 18172T (99% sequence identity), which possess heavy metal resistance genes, regulat ors and metal binding proteins having the ability to survive even in such adverse environmental condi tions.

Land use effects on soil organic carbon sequestration in calcareous Leptosols in former pastureland – a case study from the Tatra Mountains (Poland)

Abstract. The purpose of the paper is to describe soil organic carbon (SOC) sequestration rates in calcareous shallow soils in reforested areas in the Tatra Mountains with a particular focus on different forms of organic matter (OM) storage. Three plant communities creating a mosaic on the slopes of the studied valley were taken into account. Fifty years since the conversion of pastureland to unused grassland, dwarf pine shrub and larch forest have emerged in the study area, along with the development of genetic soil horizons as well as SOC sequestration in the soil despite the steepness of slopes. SOC stock was measured to be the highest in soils under larch forest (63.5 Mg ha−1), while in soil under grassland and under dwarf pine shrub, this value was found to be smaller (47.5 and 42.9 Mg ha−1, respectively). The highest amount of mineral-associated OM inside stable microaggregates (MOM FF3) was found in grassland soil (21.9–27.1 % of SOC) and less under dwarf pine shrub (16.3–19.3 % of SOC) and larch forest (15.3–17.7 % of SOC). A pool of mineral-associated OM inside transitional macroaggregates (MOM FF2) was found in soil under dwarf pine shrub (39.2–59.2 % of SOC), with less under larch forest (43.8–44.7 % of SOC) and the least in grassland soil (37.9–41.6 % of SOC). The highest amount of the free light particulate fraction (POM LF1) was found in soil under dwarf pine shrub (6.6–10.3 % of SOC), with less under larch forest (2.6–6.2 % of SOC) and the least in grassland soil (1.7–4.8 % of SOC).

Soil organic matter fractions as affected by tillage and soil texture under semiarid Mediterranean conditions

The inherent complexity of soil organic matter (SOM) and its stabilization processes make suitable the identification of SOM fractions that reflect the management-induced changes in soil organic carbon (SOC) dynamics. This is of special interest in semiarid regions where the capacity of soil for agricultural production is limited. This study aims to evaluate the effect of different tillage and soil management practices on the distribution of C among SOM fractions and determine the influence of soil texture on the protection of SOC in a semiarid Mediterranean region (Aragon, NE Spain). Under on-farm conditions, pairs of adjacent fields under long-term no tillage (NT) and conventional tillage (CT) were compared in five different cereal production areas. In all cases, a nearby undisturbed soil under native vegetation (NAT) was included. Results indicate that the two isolated mineral-associated OM (Min) fractions, d-Min and μagg-Min (outside and within stable microaggregates, respectively), constituted the main part of total SOC (mean contributions of 54 and 26%, respectively) and were not consistently affected by soil management. Soil clay was a determinant factor for d-Min-C and total SOC (r2=0.60–0.70; P<0.001), indicating that chemical stabilization, through clay–organic complexes, seems to be a main preservation mechanism in the studied soils. Physical protection seems to be another SOC stabilization process in these soils due to strong correlation found between μagg-Min-C and the mass of water-stable microaggregates (r=0.900; P<0.0001). With smaller contributions to total SOC, the two labile fractions, coarse and fine particulate OM (cPOM and fPOM) were sensitive to soil management and their concentrations decreased as soil disturbance increased (NAT>NT>CT). The highest differences between NT and CT corresponded to fPOM at the soil surface where this fraction was 1.2–3 times higher under NT. Higher soil stratification ratios in NT, always >2 for the POM fractions, indicate an improvement in soil quality with long-term NT adoption in this semiarid Mediterranean region.

Effects of mineral characteristics on content, composition, and stability of organic matter fractions separated from seven forest topsoils of different pedogenesis

Mineral topsoils possess large organic carbon (OC) contents but there is only limited knowledge on the mechanisms controlling the preservation of organic matter (OM) against microbial decay. Samples were taken from the uppermost mineral topsoil horizon (0 to 5cm) of seven sites under mature deciduous forest showing OC contents between 69 and 164gkg−1 and a wide range in mineral characteristics. At first, organic particles and the water-extractable OM were removed from the soil samples. Thereafter, Na-pyrophosphate extractable organic matter (OM(PY)), assumed to be indicative for OM bound via cation mediated interactions, and the OM remaining in the extraction residue (OM(ER)), supposed to be indicative for OM occluded in mechanically highly stable micro-aggregates, were sequentially separated and quantified. The composition of OM(PY) and OM(ER) was analyzed by FTIR and their stability by 14C measurements. The OC remaining in the extraction residues accounted for 38 to 59% of the bulk soil OC (SOC) suggesting a much larger relevance of OM(ER) for the OM dynamic in the analyzed soils as compared with OM(PY) that accounted for 1.6 to 7.5% of the SOC. The FTIR analyses revealed a lower relative proportion of CO groups in OM(ER) compared to OM(PY) indicating differences in the degree of microbial processing between these fractions. Correlation analyses suggest an increase in the stability of OM(PY) with the soil pH and contents of Na-pyrophosphate soluble Fe, Al, and Mg and an increase in the stability of OM(ER) with the soil pH and the contents of clay and oxalate-soluble Fe and Al. Despite the detected influence of soil mineral characteristics on the turnover of OM(PY) and OM(ER), the Δ14C signatures indicated mean residence times less than 100years. The presence of less stabilized OM in these fractions can be derived from methodological uncertainties and/or the fast cycling compartment of mineral-associated OM.

Impact of no-till and reduced tillage on aggregation and aggregate-associated carbon in Northern European agroecosystems

Minimum tillage practices have been shown to enhance soil aggregation and soil organic carbon (SOC) stabilization. Carbon turnover rate slows down when soil aggregation increases and SOC is protected within stable microaggregates (53–250μm). However, this has not been investigated in boreal soils. Therefore, the objective of this study was to quantify the long-term effects of no-till (NT) and reduced tillage (RT) on SOC stabilization in four soils typical for the boreal region. Distribution of SOC in different soil fractions in a 0–20cm soil layer was analyzed by wet sieving and further isolation of microaggregates (mM) from large (>2000μm, LM) and small (250–2000μm, sM) macroaggregates. Aggregate size decreased in the order of NT>RT>CT at all study sites. In addition to increased mean weight diameter (MWD) under NT, a general trend of redistribution of SOC into these formed macroaggregates was found at all study sites, i.e., the LM fraction gained SOC. However SOC was lost in other fractions under NT compared to CT at some sites and none of the sites showed any significant changes in bulk soil SOC content under NT or RT. Also our hypothesis that there would be more SOC incorporated in mM fraction in NT and RT compared to CT was corroborated only at site 4 under NT. Thus, although the potential to accumulate SOC under NT or RT compared to CT seems to be limited in boreal agroecosystems, the redistribution of SOC to the more stable conditions within the aggregates indicates positive impacts of no-till practice.

Long-term effect of no-tillage on soil organic matter fractions in rainfed Aragon (NE Spain) .

This paper assesses the long-term effect of no-tillage (NT) on soil organic carbon (OC) content and its distribution among different organic matter fractions in rainfed agrosystems of Aragón (NE Spain). Adjacent fields of NT, conventional tillage (CT) and natural soils (NAT) were compared in three different cereal production areas. In the soil surface, the higher OC content found in the NAT soils was due to the particulate organic matter. In the case of agricultural soils, in general, the fractions responsible for the OC increase under NT were the fine particulate organic matter and the mineral-associated organic matter occluded within stable microaggregates.