Water-stable Aggregate Fractions Research Articles

Compost amendment maintains soil structure and carbon storage by increasing available carbon and microbial biomass in agricultural soil – A six-year field study

Soil organic amendments in agricultural production can benefit crop production and a wide range of soil properties, including soil aggregation. Soil aggregate formation is largely driven by microbial activities, and can in-turn influence microbial communities by generating distinct microbial habitats, as well as associated impacts on water and nutrient dynamics. We investigated the long-term effects of two fertilizer management strategies (poultry manure compost vs. mineral fertilizer) and biochar amendment (0 vs. 10 t ha−1 walnut shell biochar, 900 °C pyrolysis temperature, by-product of gasification) on soil aggregation, soil organic C, and microbial community dynamics in water-stable aggregate fractions in corn-tomato rotations. Using wet-sieving, soils (0–15 cm) were divided into four size fractions: large macroaggregates (2000–8000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and silt and clay (<53 μm) for calculation of mean weight diameter in both 2014 and 2018. The total C and microbial community composition and abundance within each fraction were evaluated in 2018. Across all treatments, six years of continuous compost application maintained soil aggregate stability and C storage by increasing soil microbial biomass and associated dissolved organic C. Bacterial and fungal populations under compost treatments were significantly higher than under mineral fertilizer treatments based on 16S rRNA gene copy number and internal transcribed spacer (ITS) abundance, which likely contributed to the formation and maintenance of macroaggregates in compost treatments. Interestingly, continuous application of manure compost may increase microbial available C sources by increasing the abundance of bacteria with the potential to degrade aromatic C as predicted from 16S sequences. Soil under the mineral fertilizer treatment showed decreases in the proportion of large macroaggregates, bulk soil C, and aggregate-associated C storage compared to the compost treatment. The application of highly recalcitrant walnut shell biochar had limited long-term impacts on soil aggregation and C dynamics, likely due to its lack of microbially-available C and limited interaction with the soil environment. Our results indicate that continuous compost inputs maintained soil structure and associated physical stabilization of SOM by enlarging soil microbial available C pool, higher soil microbial biomass, and increasing aggregate formation. The soil aggregate structure, in-turn, generated diverse habitats and altered soil microbial communities. Compost inputs, in addition to or in partial replacement of mineral fertilizer inputs, can provide valuable microbial-driven ecosystem services, such as carbon storage and soil structure, while still providing fertility for crop growth.

Effects of long-term (70 years) nitrogen fertilization and liming on carbon storage in water-stable aggregates of a semi-arid grassland soil

Grasslands cover up to 40.5% of the world's landmass and store 30% terrestrial carbon (C). Various practices, including mineral fertilization and liming, are used to manage these ecosystems with potential long-term effects on the size and distribution of soil aggregates and inevitably carbon dynamics. The objective of this study was to examine the long-term effects of nitrogen fertilization and liming on soil carbon storage and its dynamics in water-stable aggregates of a semi-arid grassland. Soil samples (0–10 cm) were collected from Ukulinga long-term grassland trial in Pietermaritzburg, South Africa where nitrogen fertilizers have been applied annually and lime every five years for 70 years. Ten treatments were studied: the control (0 kgN/ha and unlimited), lime at 2250 kg/ha (L), ammonium sulphate at 70 kg/ha (AS70) and 211 kg/ha (AS211); ammonium nitrate at 70 kg/ha (AN70) and 211 kg/ha (AN211); AS70 + lime (AS70L); AS211 + lime (AS211L); AN70 + lime (AN70L) and AN211 + lime (AN211L).Nitrogen fertilizers significantly reduced soil pH and increased total soil N. Liming increased soil pH with no effect on total soil N. Lime and lime + N fertilizer treatments had no effect on mean weight diameter (MWD) while separate N application decreased MWD and large macro-aggregates (LMA). Lime only treatment had no effect on water stable aggregate (WSA) fractions. Nitrogen fertilization and liming (separately or in combination) did not affect total C concentration and stocks. Overall, soils had very high total soil organic carbon ranging from 49.7 – 57.6 g/kg across treatments. Nitrogen fertilization decreased organic carbon in LMA in AS70 (1.52%) and AN211 (1.67%) treatments compared to the control (3.40%) which was in concert with increases in C associated with small macro-aggregates (SMA) and micro-aggregates (MiA and SCA). Organic carbon in SMA was 2.67 % (AS70); AS211 (2.62 %); AN70 (2.02 %); AN211 (2.49 %) compared to 1.26 % in the control. Lime + N fertilizer treatments increased C storage in all aggregate fractions compared to N fertilizer only treatments. The lack of response in total SOC to 70 years of N fertilization and liming suggests possible C saturation given the high soil C concentration. Changes in C associated with WSA fractions suggests their importance as diagnostic indicators of N fertilization and liming induced changes in SOC. Findings also show that ammonium-based N fertilization is associated with soil acidification, dispersion of LMA resulting in an increase of microaggregates and C stored in them. Liming can counteracts acidifying and the dispersive effect on NH4+ associated with ammonium-based fertilizers thus restoring macro-aggregation in N fertilized grasslands. These findings suggests that long-term N addition may result in poor soil physical condition and possible stabilization of C in stable fractions.

Winter cover crops and no-till promote soil macrofauna communities in irrigated, Mediterranean cropland in California, USA

Cover crops and reduced tillage are two elements of conservation agriculture with potential to support larger and more diverse soil biological communities. Soil macrofauna can be indicators of soil quality and can regulate multiple soil functions, but their response to cover crop adoption in temperate regions is not well studied. We evaluated soil macrofauna communities and related soil quality metrics in response to a 16-year winter cover crop and tillage trial located in the San Joaquin Valley of California, USA. The most abundant taxa were Coleoptera (beetles) and Lumbricidae (earthworms), with average densities ranging between 5 and 318 beetles m−2 and 65–480 earthworms m−2 depending on sample date and treatments. Overall macrofauna abundance increased by 93% with the addition of cover crops in rotation, and 50% with the elimination of tillage. Taxonomic richness increased by 48% with cover crops in treatments managed without tillage, though there was no effect of cover crops under standard tillage. Macrofauna abundance was positively related to total soil carbon, water infiltration, and soil aggregate stability. Macrofauna community composition was best explained by soil carbon, water stable aggregate fraction, and cover crop usage. Agricultural management practices that foster soil biological communities may support biodiversity and improve environmental outcomes critical for sustainable food production.

Combined Application of Flue Gas Desulfurization Gypsum and Straw Pellets to Ameliorate Sodicity, Nutrient Content, and Aggregate Stability of Sodic Soil

Flue gas desulfurization (FGD) gypsum and straw return have been recognized for their roles in reducing soil sodicity and improving soil organic matter. However, there are few studies on the effect of their combination on improving and fertilizing sodic soil. FGD gypsum at a rate of 15 t ha−1 and three types of straw (sunflower seed shell, corn stover, and corncob crushed into straw pellets) at a rate of 7.5 t ha−1 were applied to sodic soil from 2017 to 2019, and their effects on soil salinity and sodicity, nutrient content, and water-stable aggregate fractions were observed. FGD gypsum application significantly decreased soil pH, sodium adsorption ratio, and exchangeable sodium percentage, with significant changes in NH4+-N content. After FGD gypsum treatment, the 250–2000 μm aggregate fraction increased, and the <53 μm fraction decreased, resulting in an increase in the mean weight diameter of the soil aggregates. Except for corncob straw, straw application had no significant effect on soil salinity or sodicity. Straw application increased the soil organic carbon content in 2017 and increased the inorganic nitrogen content in 2017 and 2018. Moreover, FGD gypsum and straw applications interactively affected the soil NH4+-N and NO3−-N contents. Water-stable aggregates, excluding the <53 μm fraction, were not affected by straw application. The combined application of FGD gypsum and corn stover or sunflower seed shell is a feasible option for sodic soil amelioration and can control salinity and sodicity and improve soil fertility.

Large macroaggregate properties are sensitive to the conversion of pure plantation to uneven-aged mixed plantations

In forestry, stand conversion influences soil physicochemical and biological properties, yet the effects of conversion of pure plantation to uneven-aged mixed plantation on soil aggregate distribution and stability, C/N/P stoichiometry and enzyme activity remain unclear. We studied the distribution of water-stable aggregate fractions, mean weight diameter (MWD), stoichiometric ratios (C/N, C/P, and N/P), and urease, invertase, and acid phosphatase activity in the soils of pure and uneven-aged mixed plantations (including artificially and naturally established mixed plantations). The mixed plantations showed improved bulk soil and aggregate physiochemical and biological properties, including higher soil organic C (OC), total N (TN), total P (TP), and enzyme activity in the artificially mixed plantations than in the naturally established mixed plantation. Additionally, in contrast to pure plantation soil, mixed plantation soil showed a significant decrease in the microaggregate (MA) (<0.25 mm) mass fractions, and a marked increase in the proportion of large macroaggregate (LMA) (>2 mm), MWD, and other soil properties (especially OC and TN content, stoichiometric ratios, and enzyme activity in LMA). Principal component analysis indicated that compared to other aggregate sizes, LMA had a significant effect on soil properties, and LMA mass fractions were strongly positively correlated with OC, TN, C/N, and MWD in the bulk soils. Moreover, OC and nutrient content, stoichiometric ratios, and enzyme activity in LMA were significantly positively correlated with the corresponding variables in the bulk soils. These findings indicate that LMA properties can be used as sensitive indicators to changes in soil properties after stand conversion from pure to uneven-aged mixed plantation.

Molecular characterization of soil organic carbon in water-stable aggregate fractions during the early pedogenesis from parent material of Mollisols

The aims of this study were to investigate how short-term agricultural practices varying in vegetation cover, tillage, and organic amendments affected the chemistry of aggregate-associated C and particulate organic matter (POM) after 8-year soil restoration from parent material (PM) of a Mollisol. The research allowed indicating the effects of perennialization and organic amendments on the chemical compositions of soil organic carbon (SOC) in both aggregates and POM in short-term and thereof helped to unveil the mechanisms of C sequestration and stabilization. The field surface soils (0–20 cm) under 8-year natural perennials and an arable soil with organic amendments were collected. Soil aggregate fractionation method was applied to obtain three aggregate-sized fractions: macroaggregate (> 250 μm), microaggregate (53–250 μm), and silt-clay unit ( 250 μm), microaggregate (53–250 μm), and silt-clay units within macroaggregate, the fine POM (fPOM, 53–250 μm), and silt-clay units within microaggregate were further separated. In all, 10 subgroups of aggregates were obtained to determine organic carbon content and chemical structure using 13C-NMR spectroscopy technique. The PM for the establishment of the experiment was also used. The chemical composition of SOC differed among aggregate sizes, representing different organic compositions with different decomposition stages. Compared with PM, soil restoration under natural perennial and organic amendments increased SOC in both POM and aggregate fractions; higher proportion of POM with larger proportion of plant-derived O-alkyl C was observed in arable soil than in perennials. Meanwhile, the silt-clay units, sequestrating the most of the organic C, contained larger proportions of alkyl C, aromatic C, and carbonyl C, probably due to the microbial-derived by-products. The data partially supported our hypothesis that continuous organic amendments to PM likely promoted aggregate formation and subsequently affected the chemical composition of C therein. Perennialization and organic amendments increased not only the total SOC stocks in bulk soils and aggregate fractions but also changed the SOM chemistry by varying quantity and quality of plant residues. The silt-clay units contributed largely to the stock and stability of SOC during the soil development stage from PM of a Mollisol. The main mechanisms of plant-derived C sequestrated and SOC accrued were controlled probably not only by the physiochemical protection of soil aggregates but also by the microbial C sequestration capacity in silt-clay fraction at the initial stage of soil formation.

Effects of different vegetation restoration measures on soil aggregate stability and erodibility on the Loess Plateau, China

Vegetation restoration may affect soil aggregate stability and the ability of soil to resist erosion. To evaluate the influence of vegetation restoration measures on the stability of soil aggregate and soil erodibility, we chose 7 types of vegetation restoration measures, which included artificial forest (AF), artificial mixed forest (AMF), economic forest (EF), artificial shrub (AS), natural shrub (NS), artificial grass (AG) and natural grass (NG). Then, we analyzed the distribution of water-stable aggregate fractions, mean weight diameter (MWD), geometric mean diameter (GMD), soil erodibility (K value) and other soil properties in the 0–20 cm and 20–40 cm soil layers, as well as aboveground and underground biomass (AGB and UGB). The results showed that under 7 kinds of vegetation restoration measures, the aggregate fraction <0.25 mm was the main component (40.40–77.86%) and the proportion of the >5 mm aggregates fluctuated greatly and ranged from 1.87% to 32.50%. And for 7 different vegetation restoration measures, the percentage of aggregate <0.25 mm was lower than that of CK (abandoned land), however, the proportion of aggregate >5 mm was higher than the CK. Overall, compared with CK, the MWD (2.22 and 1.93 mm) and GMD (2.86 and 2.66 mm) were both highest in two soil layers under the NS but lowest under the EF (MWD 0.68 and 0.49 mm, GMD 1.08 and 0.93 mm). The trend of the K value was opposite to these values. These results indicated that the stability of soil aggregate and the ability of soil to resist erosion under NS were strongest. The soil organic carbon (SOC), total nitrogen (TN) and UGB had significantly positive correlations with the proportion of >1 mm aggregates and MWD but had negative correlations with the percentage of <0.25 mm aggregates (p < 0.05), which indicated that SOC, TN and UGB were involved in the formation of macroaggregates and increased the stability of soil aggregates. These results suggested that natural shrub restoration measures could improve the soil aggregate stability and ability to resist erosion better than forest and grass restoration measures, which can provide a reference for the assessment of vegetation restoration measures.

Effect of biochar on soil structure and storage of soil organic carbon and nitrogen in the aggregate fractions of an Albic soil

ABSTRACTField experiment was conducted to evaluate the effect of corn straw derived-biochar (700 °C) applied at 0 (control), 10 (B1), 20 (B2) and 30 t ha−1 (B3) on water stable aggregate (WSA), mean weight diameter (MWD), total organic carbon (TOC) and total nitrogen (TN) in WSA fractions of Albic soil. Compared with control, WSA in > 2 mm fraction increased, by 40.8% and 51.5% (0–10 cm depth) in B1 and B3, respectively. B1, B2 and B3 (10–20 cm depth) increased by 55.2%, 69.6% and 62.4%, respectively. MWD increased by 34.4%, 21.6%, and 17.6% with B3 at 0–10 cm, 10–20 cm and 20–30 cm depths, respectively. TOC in the > 2 mm fraction increased by 28.6%, 22.1%, and 23.2% (0–10 cm depth) in B1, B2, and B3, respectively, TN in 2–0.5 mm fractions increased by 32.4%, 23.4% and 33.6% (0–10 cm depth); and in the 0.25–0.05 mm fractions increased by 14.8%, 19.8% and 18.7% (10–20 cm depth), in B1, B2 and B3, respectively. Our findings suggest biochar application at 30 t ha−1 could improve structural stability and sequestration of TOC and TN in Albic soils.

Effects of maize straw and its biochar application on organic and humic carbon in water-stable aggregates of a Mollisol in Northeast China: A five-year field experiment

In agricultural ecosystems, one optimized crop residue management practice, i.e., incorporating biochar produced from crop residues via pyrolysis to the soil, is considered a promising technology for mitigating climate change and improving soil quality. However, the potential mechanisms are still not well known. In this study, a five-year (2011–2015) field experiment in a Mollisol of Northeast China that included four treatments, i.e., CK (no fertilizer), NPK (mineral nitrogen, phosphorus and potassium fertilizers), NPKS (NPK plus maize straw) and NPKB (NPK plus maize straw-derived biochar), was conducted to evaluate the effects of maize straw and its derived biochar application on the following: 1) the size distribution and water stability of soil aggregates; 2) the concentrations of organic and humic carbon (C) in bulk soil and water-stable aggregate fractions; and 3) the chemical compositions of organic and humic C in water-stable aggregate fractions. Compared with the CK and NPK treatments, the proportion of macroaggregates (>0.25 mm) [mainly large macroaggregates (>2 mm)] increased whereas the proportions of microaggregates (0.25–0.053 mm) and silt + clay fractions (<0.053 mm) decreased in NPKS and NPKB treatments, resulting in the mean weight diameter (MWD) of soil aggregates being higher under NPKS and NPKB treatments than under CK and NPK treatments. The concentrations of organic and humic C in bulk soil and aggregate fractions were also generally higher under NPKS and NPKB treatments than under CK and NPK treatments. The treatment effects were more apparent for NPKB than for NPKS. With respect to other treatments, the NPKB treatment significantly increased the ratios of macroaggregate- to microaggregate-associated organic C as well as humin C. The solid-state 13C cross-polarization magic-angle-spinning (CPMAS) NMR spectra of small macroaggregates (2–0.25 mm) and its humic acid fraction showed that the NPKS treatment did not alter the proportions of the C functional groups. In contrast, the NPKB treatment clearly decreased the proportions of alkyl C and O-alkyl C but increased the proportion of aromatic C, resulting in the alkyl C/O-alkyl C and hydrophobic C/hydrophilic C ratios being higher whereas the aliphatic C/aromatic C ratio was lower under NPKB than under other treatments. These findings indicate that the application of maize straw biochar enhanced soil aggregate stability and aggregate-associated organic and humic C concentrations. Biochar C was preferentially incorporated into macroaggregates and its humin fraction with respect to microaggregates. Moreover, the molecular structure of small macroaggregates and its HA fraction became more stable, hydrophobic and aromatic. Therefore, physical protection through organic C in aggregate fractions, physical-chemical protection through the encapsulation of humic C within aggregate fractions, and physical-biochemical protection through recalcitrant alkyl C and aromatic C within aggregate fractions may be the primary mechanisms for C preservation in biochar-amended soil. Our results suggested that maize straw biochar application was an effective strategy based on the comprehensive goal of mitigating climate change and improving soil quality in Mollisols of Northeast China.

Sewage sludge amendment improved soil properties and sweet sorghum yield and quality in a newly reclaimed mudflat land

Growing bioenergy crop on marginal lands has the potential to minimize land use conflicts and meet global energy demand. The newly reclaimed mudflats through sewage sludge amendment can be used as a potential marginal land for bioenergy production. This paper was one of first studies to investigate the persistent impact of sewage sludge application at the rates of 0, 25, 50, 125, and 250 t ha−1 on selected soil physicochemical properties, yields and quality of sweet sorghum (Sorghum bicolor L.) cultivated in newly reclaimed mudflat saline-alkaline soil. The results showed that sewage sludge amendment (SSA) improved physicochemical properties by decreasing bulk density, electric conductivity, pH, and increasing soil organic carbon, >0.25 mm water-stable aggregate fraction, cation exchange capacity, nitrogen, and phosphorus contents in mudflat soil. Consequently, the sweet sorghum biomass and gross energy content were significantly elevated by SSA. The sweet sorghum achieved the maximum biomasses of 4.73 and 6.62 t ha−1 at 250 t ha−1 SSA rate in 2016 and 2017, respectively. The gross energy content of sweet sorghum significantly increased with the SSA rates, although SSA slightly reduced the calorific values in stem and leaf of sweet sorghum. The maximum gross energy contents of sweet sorghum were 79.62 and 104.47 GJ ha−1 at SSA rate of 250 t ha−1 in 2016 and 2017, respectively. Although SSA led to accumulation of heavy metals in sweet sorghum, the growth of sorghum was not inhibited even at the highest SSA. In summary, cultivation of bioenergy crop in mudflat amended by sewage sludge is feasible and can be an innovative solution for mudflat reclamation, safe disposal of solid waste, resource reuse and recycle, and bioenergy production.

The stability mechanism for organic carbon of aggregate fractions in the irrigated desert soil based on the long-term fertilizer experiment of China

Soil samples from the Hexi Corridor located in the arid regions of Northwestern China were collected from a 24–year–old field experiment. The distribution of organic carbon (OC) in the different size water stable aggregate (WSA) fractions and the stability mechanism of OC in WSA size fractions were studied. The results showed that long-term application of solo organic manures or in combination with inorganic N fertilizer significantly increased the OC concentrations of different size WSA fractions compared to the CK, while long-term application of solo N fertilizer had no significant effect. After fertilization for 24 years, the OC concentrations of 0.25–2 mm WSA fraction was significantly positively correlated with the percentages of this WSA fraction, while the OC concentrations of >2 mm, 0.053–0.25 mm and <0.053 mm were not significantly correlated with their corresponding percentages of WSA fractions (P > 0.05). In addition, the OC concentrations of loosely, stably and tightly bound humus in 0.25–2 mm WSA fraction were significantly higher than those in other size WSA fractions, which were also significantly correlated with the corresponding chemical components, e.g. different forms of calcium, in 0.25–2 mm WSA fraction (P < 0.05). It could be explained that affinities of OC to chemical components in 0.25–2 mm WSA fraction were stronger than those in other size WSA fractions. The results of NMR analysis also showed that the aromaticity and alkyl/O-alkyl ratio for the 0.25–2 mm WSA fractions were significantly lower than those for other size WSA fractions (P < 0.05). These results demonstrated that the 0.25–2 mm WSA fraction played an important role in stabilizing soil OC and protecting OC from degradation in the irrigated desert soil.

Improvement of soil structure through organic crop management, conservation tillage and grass-clover ley

Conventional intensive tillage is a widespread soil management practice that controls weeds and promotes nutrient mineralization at the expense of a degraded soil structure and soil carbon (C) loss. Alternative soil management practices such as no tillage, reduced tillage and temporary leys, however, can minimize the negative effects of intensive tillage on soil structure. To improve understanding of these management practices on soil structure, we sampled a field trial combining organic and conventional crop management with different intensity levels of tillage, resulting in four cropping systems: conventional intensive tillage (C-IT), conventional no tillage (C-NT), organic intensive tillage (O-IT) and organic reduced tillage (O-RT). A ley period was added following a 4-year arable crop rotation. We measured mean weight diameter (MWD), total C and total nitrogen (N) in whole unfractionated soil and water-stable aggregate fractions after the 4-year arable crop rotation and again after a following 2-year grass-clover ley period, to assess the impact of the cropping system and the ley, respectively. Results showed that 4 years of organic crop management including the application of cattle manure slurry combined with reduced tillage led to significantly better soil structure (i.e. aggregate MWD) at the 0–6 cm soil depth, compared to the other cropping systems tested. After the ley period, the proportion of large macroaggregates increased by 65% for C-IT and 47% for O-IT at 0–6 cm depth. Total C increased significantly for only O-IT and O-RT after the ley, both of which also showed a high C stratification between 0–6 cm and 6-20 cm depth. In conclusion, soil structure is most improved during continuous cropping when combining organic crop management with reduced tillage, while a ley period was effective in improving the soil structure in intensive tillage plots to the level of O-RT.

Early structural stability of fine dam sediment in soil construction

PURPOSE: Fine sediments accumulate upstream of hydroelectric dams. To ensure that dams can operate properly, most sediments are returned downstream reaches, but a portion of them have to be dredged and land managed. In parallel, using topsoil from agricultural parcels for urban greening is currently controversial because arable surface areas are decreasing. An alternative idea for protecting these natural resources consists in reusing fine dredged sediment to construct functional soils. The agronomical use of fine dredged sediment raises the question of its ability to provide acceptable physical properties for plant growth. MATERIALS AND METHODS: Six dredged sediments with contrasted initial agronomical properties were mixed or not with green waste compost and submitted or not to drying-wetting cycles in a 105-day-long experiment under greenhouse conditions. The sediments were analyzed for their capacity to generate stable aggregates. We measured the proportion of macro aggregates > 2 mm with dry-sieved aggregate (DSA) fraction and the > 0.25 mm water-stable aggregate (WSA) fraction for each treatment over time. We also assessed the main sediment physicochemical properties, microbial biomass carbon, and carbon mineralization potential. RESULTS AND DISCUSSION: Compost addition (40% in volume) enhanced the WSA fraction of the low-organic-matter-content sediments (< 30 g kg⁻¹), whereas it had a non-significant effect in the other sediments. Multiple linear regressions highlighted a determining role of organic matter in the WSA fraction. At this scale of aggregation, organic matter seemed to mainly interact with clay in an acidic context. Compost addition had either no significant effect or a negative effect on the DSA fraction. Multiple linear regressions suggested that organic matter was not clearly involved in DSA formation in the sediment, whereas macro-porosity was. The first drying-wetting cycles enhanced the DSA proportion of the sediments, but the following cycles had a negative effect. We suggest that disaggregating mechanisms gradually predominated over aggregating mechanisms over time. CONCLUSIONS: All sediments showed acceptable physicochemical properties for plant growth. WSA formation in low-organic-matter-content sediments was improved by adding compost. Drying-wetting cycles had a controversial effect on DSA formation, and only the very first cycles had a beneficial effect on macro-aggregation. The mechanisms involved in aggregation and aggregate stability in soil constructed from sediments seem to be similar to those involved in natural soils. That is why we propose that the soil hierarchical aggregation model initially developed for natural and cultivated soils could be adapted to sediments.

Effects of rubber-based agroforestry systems on soil aggregation and associated soil organic carbon: Implications for land use

Rubber-based agroforestry (Hevea brasiliensis) systems are considered the best way to improve soil properties and the overall environmental quality of rubber monoculture, but few reports have examined soil aggregate stability in such systems. The objective of this study was to examine the management and landscape effects on water stable soil aggregates, soil aggregate-associated carbon, nitrogen content and soil carbon, and nitrogen accumulation in Xishuangbanna, southwestern China. Treatments were rubber monoculture (Rm) and four rubber-based agroforestry systems: H. brasiliensis–C. arabica (CAAs), H. brasiliensis–T. cacao (TCAs), H. brasiliensis–F. macrophylla (FMAs) and H. brasiliensis–D. cochinchinensis (DCAs). The results showed that, with the exception of CAAs, the rubber-based agroforestry treatments significantly increased total soil organic carbon (SOC) and N contents and enhanced the formation of macroaggregates compared to the rubber monoculture treatment. SOC and N contents in all water-stable aggregate fractions were significantly higher in rubber-based agroforestry systems (except CAAs) compared to rubber monoculture. The macroaggregate fractions contained more organic carbon and nitrogen than the microaggregate fractions. The proportions of C and N loss from slaking and sieving were shown to have significantly negative correlations with the mean weight diameter and the SOC and N concentrations in bulk soil. The results suggest that soil surface cover with constant leaf litter fall and extensive root systems in the rubber-based agroforestry systems increased soil organic carbon and nitrogen, helped improve soil aggregation, reduced soil erosion, decreased carbon and nitrogen loss, and ultimately improved the carbon and nitrogen accumulation rates. Given that the soil physical-chemical properties improvement and the patterns of the intercropping system played key roles in managing artificial forests, we recommend that local governments and farmers should prefer T. cacao, F. macrophylla and D. cochinchinensis and not C. arabica as the alternative interplanted tree species within rubber plantations.

Distribution of persistent organic pollutants in aggregate fractions of a temperate forest and semi-rural soil

The fate of persistent organic pollutants (POPs) and their interactions with aggregates of forest soils are not completely understood. Our objectives here were to quantify the distribution of different POPs in water-stable aggregate fractions and to study their influence on soil organic carbon (Corg) content. Soil samples were taken from a forest-site, Gogerddan (G) and a semi-rural site, Hazelrigg (H) in Great Britain, from 0–2 and 2–5 cm and 0–4 and 8–12 cm soil depth, respectively. POPs analyzed were PAHs, PCBs, total DDT, PBDEs and HCB. The bulk soil analysis showed that the concentration of POPs was significantly higher (p ≤ 0.05) in forest site G than in semi-rural site H, particularly at the surface soil levels compared to the subsurface soil depths in both sites. Total concentrations of PCBs and PAHs of both sites were positively correlated with Corg contents. POPs concentrations and Corg, Nt contents of forest site G were significantly higher (p ≤ 0.05) in water-stable macro aggregates (>0.25, >1, >2 mm) than the micro aggregates (>0.053 mm). The POP concentrations of all aggregate fractions after normalizing to their respective Corg content were increased due higher contamination and strong sorption by Corg. These results showed a strong effect of Corg on the partitioning of organic pollutants to soil aggregate size fractions. The present study affirms the ecological significance of forest soils act as a potential sink of POPs. In summary, our results suggest that aggregate fractions may promote soil C storage and act as a potential POP sink in surface soil without increasing their concentration in the aggregate fraction of subsoil.

Long-term cultivation and landscape position effects on aggregate size and organic carbon fractionation on surface soil properties in semi-arid region of Iran

ABSTRACTThis study was carried out to investigate the effects of long-term cultivation and landscape position on organic carbon content and soil aggregation. Sampling sites were determined based upon land use at the end of 50 years soil use and management, cultivated/annual wheat cropping and grazed pasture, and landscape position in Chaharmahal-va-Bakhtiary province, southwest Iran. Soil samples were collected from the 0–5 cm and 5–15 cm depths in two adjacent fields that have the same slope and aspect. The soil was silty clay at the summit and footslope positions, and was a silty clay loam at the backslope. Wet-sieving analysis and aggregate-size fractionation methods were used to separate the samples into three aggregate fractions (i.e., 2–4.75, 0.25–2, and 0.053–0.25 mm). The treatments were arranged in a factorial design. Land use significantly affected the water-stable aggregate fractions, so that the wet soil stability of the macroaggregates (i.e., 2–4.75 mm) was higher in the pasture, whereas it was greater for the meso-aggregates (i.e., 0.25–2 mm) in the cultivated soils. Cultivation decreased both the wet-aggregate stability and percent of macroaggregates whereas long-term pasture enhanced aggregation. Soil organic carbon (SOC) content within aggregates and primary particles was also significantly influenced by landscape position, land use, and the depth of sampling. The SOC content was higher in clay than those in silt and sand contents. The SOC content decreased as depth increased in all fractions. In general, the highest and lowest wet-stable aggregates were observed on the footslope and backslope positions, respectively.

Response of Soil Organic Carbon and Its Aggregate Fractions to Long-Term Fertilization in Irrigated Desert Soil of China

Irrigated desert soil samples in the Hexi Corridor of China were collected over a period of 23 years from a site where different fertilization methods had been used. Changes of soil organic carbon (SOC) and its water stable aggregate (WSA) size fractions were studied. The effects of various fertilization methods on the distribution of added organic carbon (OC) in different WSA size fractions were also analyzed. The results showed that the applied fertilizations for 23 years improved SOC concentrations and OC concentrations in all WSA size fractions compared to the non-fertilized treatment (CK). In addition, fertilization obviously increased the OC stocks of <2 mm WSA size fractions compared to the CK. The average OC stock of <0.053 mm WSA fraction was 1.7, 1.6 and 3.2 times higher than those of >2 mm, 0.25-2 mm and 0.053-0.25 mm WSA fractions, respectively. A significant positive correlation was found between soil C gains and OC inputs (r=0.92, P<0.05), indicating that SOC may have not reached the saturation point yet at the site. The C sequestration rate was estimated by 14.02% at the site. The OC stocks in all of the <2 mm WSA fractions increased with the increase of OC input amounts; and the conversion rate of the input fresh OC to the OC stock of <0.053 mm WSA fraction was 1.2 and 2.6 times higher than those of the 0.25-2 mm and 0.053-0.25 mm WSA fractions, respectively. Therefore, the <0.053 mm WSA fraction was the most important component for soil C sequestration in the irrigated desert soil.

Effects of freeze–thaw on aggregate stability and the organic carbon and nitrogen enrichment ratios in aggregate fractions

AbstractSoil samples from the Hexi Corridor located in the arid regions of Northwestern China were collected from a site that had received fertilizer applications for 23 years. Effects of freeze–thaw on aggregate stability and the organic carbon (OC) and nitrogen (N) enrichment ratios in water stable aggregate (WSA) fractions were investigated. In treatments combining the application of N fertilizer with green manure (GN) or straw (SN), the percentage of &gt;0.25 mm WSA fraction was not significantly different from the control soil that received no fertilizer or organic amendment. After a freeze–thaw cycle, the percentages of the &gt;0.25 mm WSA fraction in the GN and SN treatments showed no change, but the size of this fraction in the other treatments decreased. In addition, the organic carbon (OC) and N enrichment ratios in the &gt;0.25 mm WSA fraction in GN and SN treatments increased after a freeze–thaw cycle. However, in this size fraction, the OC and N enrichment ratios decreased in other treatments. Both the changes of the percentages of the &gt;0.25 mm WSA fraction and the OC and N enrichment ratios in this fraction under freeze–thaw in the GN and SN treatments exhibited the most significant increases compared with other treatments (P &lt; 0.05). The results indicated that the GN and SN treatments could prevent the damaging effects of freeze–thaw on aggregate stability and protect soil from erosion.

Effects of Long-Term Winter Planted Green Manure on Distribution and Storage of Organic Carbon and Nitrogen in Water-Stable Aggregates of Reddish Paddy Soil Under a Double-Rice Cropping System

In agricultural systems, maintenance of soil organic matter has long been recognized as a strategy to reduce soil degradation. Manure amendments and green manures are management practices that can increase some nutrient contents and improve soil aggregation. We investigated the effects of 28 yr of winter planted green manure on soil aggregate-size distribution and aggregate-associated carbon (C) and nitrogen (N). The study was a randomized completed block design with three replicates. The treatments included rice-rice-fallow, rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass. The experiment was established in 1982 on a silty light clayey paddy soil derived from Quaternary red clay (classified as Fe-Accumuli-Stagnic Anthrosols) with continuous early and late rice. In 2009, soil samples were collected (0-15 cm depth) from the field treatment plots and separated into water-stable aggregates of different sizes (i.e., >5, 2-5, 1-2, 0.5-1, 0.25-0.5 and <0.25 mm) by wet sieving. The long-term winter planted green manure significantly increased total C and N, and the formation of the 2-5-mm water-stable aggregate fraction. Compared with rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass, the rice-rice-fallow significantly reduced 2-5-mm water-stable aggregates, with a significant redistribution of aggregates into micro-aggregates. Long-term winter planted green manure obviously improved C/N ratio and macro-aggregate-associated C and N. The highest contribution to soil fertility was from macro-aggregates of 2-5 mm in most cases.

Fate of Carbon in Water-Stable Aggregates during Decomposition of 13C-Labeled Corn Straw

Fundamental knowledge about decomposition, fate of crop residue, and allocation of residue-derived carbon (C) in soil aggregates is essential to understand the C dynamics in soil. The incorporation of C derived from corn residue in water-stable aggregate fractions, particulate organic C (POC), and mineral-associated C (MAC) in soil were examined using the 13C tracer technique. Soil was treated with corn straw at the rate of 1% dry mass of soil brought to 66% of field capacity and incubated for 70 days at 25 °C. Samples were taken at 20, 35, and 70 days and analyzed for water-stable aggregates. Values for POC and MAC were analyzed for total C and 13C enrichment. The addition of corn straw caused a shift in the distribution of recoverable particles with significant decreases in <53-μm silts and clays, microaggregrates (53–250 μm), and smaller macroaggegates (250–2000 μm); however, the large macroaggegates (>2000 μm) increased significantly. Macroaggregates contained greater amount of C than microaggregates. The proportion of 13C recovered in the fractions <53 μm (silt and clay), 53–250 μm, and 250–2000 μm increased during decomposition of corn straw, whereas there was no significant change in >2000-μm fraction. Most (70–76%) of the soil organic C was affiliated with MAC (<53 μm). Carbon (13C) derived from corn straw decreased in POC but increased in MAC as decomposition proceeded. In the long term, microaggregate fraction appears to be involved in storage and stabilization of the C derived from corn straw and is important for soil quality and soil C sequestration point of view.