Soil Aggregate Size Distribution Research Articles

Impact of bare fallow management on soil carbon storage and aggregates across a rock fragment gradient

AbstractBackgroundOur understanding of C storage in soils lacks insights investigating organic matter (OM) depletion, often studied in bare fallow systems. The content of coarse rock fragments is often excluded, whereas it may affect C storage.AimsWe aim to contribute to a better understanding of the impact of bare fallow on C storage mechanisms in the soil as influenced by its coarse rock fragment contents. We investigated whether bare fallow induced a depletion of C in OM fractions and analyzed to which extent this affected soil aggregate size distribution and the C loading of the clay‐sized fraction.MethodsA comparison of 14 years bare fallow management with adjacent cropped soils located in Selhausen (Germany) provided a gradient of coarse rock fragments of 34%–71%, from which sites with three different fine earth (FE) contents were compared. Across the FE gradient, we isolated particulate OM and mineral‐associated OM fractions, obtained microaggregate and macroaggregate size fractions, and quantified the C loading.ResultsBare fallow management induced an OM depletion at lower contents of FE. There, the management influence was more concentrated onto less FE volume. The contribution of both particulate and mineral‐associated OM fractions to the C in the low‐FE soils decreased. The C loading increased under bare fallow, compared to cropped soil. In the low‐FE soil, we also found less macroaggregates, whereas the C content decreased in some microaggregate size fractions.ConclusionsA high content of coarse rock fragments can enhance OM depletion decreasing mineral‐associated and particulate C under bare fallow.

Biotic relationships to aggregation in a Vertisol under long-term no-till, stubble retention and nitrogen fertilisation

Soil aggregation is critical for high-clay Vertisols to facilitate water movement, aeration, nutrient cycling, microbial activity, and hence soil productivity. Both abiotic and biotic factors affect aggregation, although the effect of biotic factors on aggregation remains unclear in subtropical Vertisols managed under no-till (NT), stubble retained (SR) and N fertilisation practice. Using a 54-years old cereal cropped NT experiment in subtropical semi-arid region, we examined the biotic effects on soil aggregate-size distribution in a smectitic Vertisol (65 % clay) at 0–0.1 m depth. The experiment consisted of NT versus conventional till (CT), SR versus stubble burned (SB), and N fertiliser rates (0, 30, and 90 kg N ha−1; N0, N30, and N90, respectively) arranged in a randomised complete block design, with four replications (48 plots). NT generally had no significant effect on biotic factors and aggregate mean weight diameter (MWD). SR had significantly higher microbial activity (as assessed by cumulative CO2 production over 140 days), C-acquiring and N-acquiring enzyme activities (β-glucosidase, cellobiohydrolase, N-acetyl-β-D-glucosaminidase), easily extractable and total glomalin-related proteins, microbial biomass C, large- (>2 mm) and medium aggregates (1–2 mm), and MWD (1.23 ± 0.06 mm in SR and 1.02 ± 0.07 mm in SB (P < 0.02)). In addition, N fertiliser rates had significantly higher soil organic C and mineral associated organic N but lower C:N ratio. Thus, soil organic C and N, microbial biomass C, glomalin, and microbial and enzyme activities enhanced by SR, and lower C:N ratio of the organic matter following N fertilisation improved MWD in this subtropical Vertisol.

Evaluation of the effects of long‐term maize–peanut intercropping on soil aggregate stability based on different methods

AbstractIntercropping is an effective measure to increase crop yield and improve soil structure. The soil aggregate is the basic unit of soil structure, and its stability is affected by intercropping system. To study the effects of intercropping system on soil aggregate stability, the particle size distribution and stability characteristics of soil aggregates under three planting patterns of maize only (SM), peanut only (SP) and maize intercropped with peanut (M‐P) were evaluated by dry sieving (mainly used to determine the mechanical stability of soil aggregates), wet sieving (mainly used to determine the water stability of soil aggregates) and Le Bissonnais (mainly used to distinguish different decomposition mechanisms of soil aggregates) (including slow wetting (SW), wet stirring (WS) and fast wetting (FW) tests) methods. The results showed that the particle size distribution of soil aggregates was mainly the &gt;2 mm fraction in the dry sieving test, the 1–0.5 mm and &lt;0.053 mm fractions in the wet sieving test, the &gt;2 mm fraction in the SW test, the &gt;2 mm and 2–1 mm fractions in the WS test and the 1–0.5 mm and 0.5–0.25 mm fractions in the FW test. The mean weight diameter (MWD), geometric mean diameter (GMD) and R0.25 of soil aggregates in the three determination methods were all ranked as SP &lt; SM &lt; M‐P, indicating that intercropping could increase the proportion of large aggregates and improve the stability of soil aggregates. In the three tests using the Le Bissonnais method, the MWD order was FW &lt; WS &lt; SW, indicating that the soil aggregates in this area were the most sensitive to slaking effects and the least sensitive to mechanical breakdown, and intercropping could reduce the sensitivity of soil aggregates to slaking effects and mechanical breakdown. In addition, the MWD of dry sieving was significantly positively correlated with the MWD of SW and WS, and the MWD of wet sieving was significantly positively correlated with the MWD of FW. The results of this study could provide a reference for better understanding of farmland soil structure and aggregate stability under intercropping system in the North China Plain.

Effects of Substituting Synthetic Fertilizer with Organic Materials on Soil Organic Carbon Sequestration and Aggregate Size Distribution in Red Soil in South China

Effects of Substituting Synthetic Fertilizer with Organic Materials on Soil Organic Carbon Sequestration and Aggregate Size Distribution in Red Soil in South China

Effects of straw-returning combined with application of microbial inoculants on soil aggregates and related nutrients

We analyzed the particle size distribution of soil aggregates in 0-20 and 20-40 cm soil layers of rice-wheat rotation field based on a field plot test with two treatments, conventional straw returning (CK) and straw returning with the addition of straw decomposition promoting microbial inoculants (IT). We evaluated the water stability indices of soil aggregates (the number of soil water stable large aggregates R0.25, the average weight diameter MWD, and the geometric average diameter GMD), and measured the contents of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in the soil aggregates of ＜0.053, 0.053-0.25, 0.25-1, ＞1 mm. The results showed that: 1) The number of aggregates ＜0.053, 0.053-0.25, ＞0.25 mm in the 0-20 and 20-40 cm soil layers under IT decreased by 10.0% and 6.8%, increased by 3.0% and 5.7%, and 17.9% and 26.1% compared with CK, respectively. IT effectively increased R0.25, MWD, and GMD by 26.4%, 20.0%, 18.2% and 18.2%, 10.5%, 10.0% in 0-20 and 20-40 cm soil, respectively. 2) Compared to CK, the TP content of 0.25-1 mm aggregates in 0-20 and 20-40 cm soil under IT was significantly increased by 40.3% and 37.5%, respectively, without difference in TN and SOC contents. There was no significant difference in nutrient contents of the other aggregates between the treatments. The contents of SOC and TN in large aggregates (>0.25 mm) were higher than those in silty aggregates (<0.053 mm). Compared to CK, the cumulative contribution rates of SOC, TN and TP of <0.053 mm aggregates under IT were decreased in two soil layers. There was no significant difference in the nutrient cumulative contribution rates of 0.053-0.25 mm aggregates between treatments. The cumulative contribution rates of SOC, TN, and TP of large aggregates (>0.25 mm) under IT were 32.1%, 19.6%, 52.8% and 22.8%, 11.8%, 42.9% higher than those under CK in 0-20 and 20-40 cm soils, respectively. 3) The number of <0.053 mm aggregates was significantly negatively correlated with SOC and TP contents, while that of 0.053-0.25 mm aggregates was negatively correlated with nutrient content. The number of large aggregates (>0.25 mm) were significantly positively correlated with SOC, TN, and TP contents. In conclusion, straw returning with microbial-inoculant addition could promote the formation of soil macroaggregates (>0.25 mm), and improve the water stability of soil aggregates, increasing nutrient contents in soil macroaggregates, with the nutrients transferring from silty aggregates to macroaggregates.

Calcium carbonate promotes the formation and stability of soil macroaggregates in mining areas of China

We studied changes in the concentrations of aggregate-cementing agents after different reclamation times and with different fertilization regimes, as well as the formation mechanism of aggregates in reclaimed soil, to provide a theoretical basis for rapid reclamation of soil fertility in the subsidence area of coal mines in Shanxi Province, China. In this study, soil samples of 0–20 cm depth were collected from four fertilization treatments of a long-term experiment started in 2008: no fertilizer (CK), inorganic fertilizer (NPK), chicken manure compost (M), and 50% inorganic fertilizer plus 50% chicken manure compost (MNPK). The concentrations of cementing agents and changes in soil aggregate size distribution and stability were analysed. The results showed that the formation of >2 mm aggregates, the aggregate mean weight diameter (MWD), and the proportion of >0.25 mm water-stable aggregates (WR0.25) increased significantly after 6 and 11 years of reclamation. The concentration of organic cementing agents tended to increase with reclamation time, whereas free iron oxide (Fed) and free aluminium oxide (Ald) concentrations initially increased but then decreased. In general, the MNPK treatment significantly increased the concentrations of organic cementing agents and CaCO3, and CaCO3 increased by 60.4% at 11 years after reclamation. Additionally, CaCO3 had the greatest effect on the stability of aggregates, promoting the formation of >0.25 mm aggregates and accounting for 54.4% of the variance in the proportion and stability of the aggregates. It was concluded that long-term reclamation is beneficial for improving soil structure. The MNPK treatment was the most effective measure for increasing maize grain yield and concentration of organic cementing agents and CaCO3.

Nitrogen fertilizer and landscape position affect soil aggregate size distribution, and intra-aggregate carbon and nitrogen under switchgrass in a marginal cropland

AbstractDedicated bioenergy crops such as switchgrass (Panicum virgatum L.) can be grown on marginally productive lands and positively influence soil properties. However, nitrogen management, and landscape can alter soil structural attributes under bioenergy crop production. This study investigated the impacts of long-term nitrogen fertilization (0-N, 0 kg N/ha; 56-N, 56 kg N/ha and 112-N, 112 kg N/ha) and landscape positions (shoulder and footslope) on soil organic carbon (SOC) and structural attributes under switchgrass production. The 112-N rate enhanced the proportion of 2–4 mm water-stable aggregates by 49%, aggregate associated carbon in 2–4 mm and &gt;4 mm aggregates by 16 and 24%, respectively, aggregate associated nitrogen in &gt;4 mm aggregates by 33% and reduced soil bulk density by 19% compared to the 0-N rate. Footslope position increased the proportion of 2–4 mm water-stable aggregates by 26% and lowered bulk density by 8% compared to the shoulder position. Results showed a significant N-rate × landscape position interaction on SOC and glomalin related soil protein content in bulk soil. Overall, this study showed that nitrogen application to switchgrass planted at footslope on a marginally yielding cropland improved soil structure and physical conditions.

Afforestation Influences Soil Aggregate Stability by Regulating Aggregate Transformation in Karst Rocky Desertification Areas

Surface vegetation has a substantial impact on soil aggregate stability, which is an important indicator of soil quality. However, there is still limited research on the response of soil aggregate stability indicators and the organic carbon, total nitrogen, and total phosphorus content in soil aggregates for different vegetation patterns in rocky desertification fragile ecological areas. Therefore, in order to study the effects of different vegetation restoration models on soil aggregate stability and aggregate related nutrient content and their promoting relationships in the karst rocky desertification areas in southwest China, soil samples under three artificial restoration vegetation measures (Juglans regia L.-Rosa roxburghii Tratt., Rosa roxburghii Tratt.-Lolium perenne L., Juglans regia L.-Lolium perenne L.) were collected in 0–10 cm and 10–20 cm soil, and the traditional farmland (Zea mays L.) was used as the control, combined with dry and wet sieving experiments for the research and analysis. The results showed that there were significant differences in the distribution of aggregates and soil nutrients among the four types of plots. Compared with traditional agricultural land, artificial afforestation increased the content of soil large macroaggregates (LMAs) and decreased the proportion of microaggregates (MIAs) and silt+clay (SCA), which enhanced the soil aggregate stability and reduced the soil fragmentation and erodibility. The afforestation restoration increased the content of soil aggregate-related SOC, TN, and TP, and increased with the decrease in the aggregate particle size. Research has found that soil aggregate stability indicators are significantly influenced by the particle size distribution of soil aggregates. In the positive succession process of vegetation types, soil nutrient accumulation is controlled by changes in the soil aggregate particle size, which affects the soil aggregate stability and reduces soil erodibility, thereby protecting the soil nutrient loss. The composite management of forest and irrigation in degraded ecological areas has certain reference and indicative significance for ecological restoration in rocky desertification areas.

Influence of pulse–wheat crop rotations on aggregate size distribution dynamics in the brown soil zone in southern Alberta, Canada

Diversification of conventional cereal-based cropping systems with pulse crops may aid producers to grow crops in an appropriate sequence and frequency with environmental, social, and economic benefits. This study examined the effects of including three pulse crops with different rooting depths (shallow- and deep-rooted) in wheat-based crop rotations on soil aggregate size distribution under semi-arid and rain-fed conditions. A 4 year cycle rotational study was established in Brooks, AB, using five selected treatments: continuous wheat, wheat alternately grown with lentil, field pea, or chickpea, or lentil and chickpea alternately grown with wheat. Soils were collected from 0–5 cm depth and dry-sieved to produce eight aggregate size classes, &lt;0.053, 0.053–0.125, 0.125–0.149, 0.149–0.05, 0.05–1.0, 1.0–2.0, 2.0–6.35, and &gt;6.35 mm. The continuous wheat treatment improved the macro-aggregates (&gt;6.35 mm) development, whereas the rotations with pulse–wheat crops increased the micro- and meso-aggregates (0.50–1.0 and 0.15–0.5 mm) development. Soils sampled at 0–15 cm depth were used for soil organic matter and microbial analysis. The pulse–wheat rotations collectively had more light fraction organic matter (LFOM) than the continuous wheat, and chickpea alternated with wheat had the highest amount of LFOM in both years. All treatments had similar soil microbial biomass and microbial community composition. Our study underscores the contribution of pulse crops in cereal-based cropping systems in the formation of small aggregates.

The Impacts of the Hydrological Regime on the Soil Aggregate Size Distribution and Stability in the Riparian Zone of the Three Gorges Reservoir, China

The impoundment of the Three Gorges Reservoir (TGR) has greatly altered the hydrological regime and thus formed a distinctive riparian zone with anti-seasonal inundation and exposure, which may affect the soil aggregate properties in this riparian zone. Yet, the soil aggregate size distribution and stability influenced by the hydrological regime along the step-impounded elevation have rarely been documented. This study aimed to elucidate how the hydrological regime of the TGR affected the aggregate size distribution and stability in the riparian zone. Based on the step-impounded elevation, topsoil samples were collected from four elevation-dependent transects in a middle section of the TGR. Dry-sieving and wet-sieving methods were employed. The results showed that, with a decrease in the elevation gradient, the mass percentage of the &gt;5 mm aggregates significantly decreased, while the proportions of the other size classes presented an increasing trend. Additionally, the mean weight diameter (MWD), geometric mean diameter (GMD), aggregate stability rate (ASR), and percentage of aggregate destruction (PAD) of the fractal dimension showed a successive decrease with a decrease in the elevation gradient, whereas PADMWD, PADGMD, PADASR, and the fractal dimension demonstrated a reverse trend. It can thus be deduced that the hydrological regime of the TGR significantly modified the aggregate size distribution and dramatically reduced the aggregate stability, which may provide a crucial basis for assessing the soil erosion in similar riparian zones.

Soil glomalin-related protein affects aggregate N2O fluxes by modulating denitrifier communities in a fertilized soil

Agricultural ecosystems contribute significantly to atmospheric emissions of soil nitrous oxide (N2O), which exacerbate environmental pollution and contribute to global warming. Glomalin-related soil protein (GRSP) stabilizes soil aggregates and enhances soil carbon and nitrogen storage in agricultural ecosystems. However, the underlying mechanisms and relative importance of GRSP on N2O fluxes within soil aggregate fraction remain largely unclear. We examined the GRSP content, denitrifying bacterial community composition, and potential N2O fluxes across three aggregate-size fractions (2000–250 μm, 250–53 μm, and <53 μm) under a long-term fertilization agricultural ecosystem, subjected to mineral fertilizer or manure and their combination. Our findings indicated that various fertilization treatments have no discernible impact on the size distribution of soil aggregates, paving the way to further research into the impact of soil aggregates on GRSP content, the denitrifying bacterial community composition, and potential N2O fluxes. GRSP content increased with the increase in soil aggregate size. Potential N2O fluxes (including gross N2O production and N2O reduction and net N2O production) among aggregates were highest in microaggregates (250–53 μm), followed by macroaggregates (2000–250 μm) and lowest in silt + clay (<53 μm) fractions. Potential N2O fluxes had a positive response to soil aggregate GRSP fractions. The non-metric multidimensional scaling analysis revealed that soil aggregate size could drive the denitrifying functional microbial community composition, and deterministic processes play more critical roles than stochasticity processes in driving denitrifying functional composition under soil aggregate fractions. Procrustes analysis revealed a significant correlation between denitrifying microbial community, soil aggregate GRSP fractions, and potential N2O fluxes. Our study suggests that soil aggregate GRSP fractions influence potential nitrous oxide fluxes by affecting denitrifying microbial functional composition within soil aggregate.

Improved permafrost stability by revegetation in extremely degraded grassland of the Qinghai-Tibetan Plateau

Revegetation is an effective approach for restoring extremely degraded grassland (DG) in the Qinghai-Tibetan Plateau (QTP). However, little is known about its effects on permafrost stability. Our study investigated changes in the characteristics of DG and revegetated grassland (RG) in alpine permafrost regions of the QTP by means of in situ monitoring and sampling. Compared with DG, soil temperature was lower in warm months and slightly higher in cool months both at 2 and 10 cm depths after revegetation, while soil moisture generally decreased. Revegetation advanced the onset and increased the duration of completely frozen stage. The number of freeze–thaw days decreased at 2 cm but increased at 10 cm depth. The freeze–thaw strength weakened at 2 cm depth in spring and autumn, and at 10 cm depth in autumn, but increased at 10 cm depth in spring. The thawing index at the two depths and active layer thickness in RG were also significantly lower than those in DG. Revegetation significantly affected the particle size distribution and stability of soil aggregates by increasing the proportion of large macroaggregates. Thus, revegetation can effectively improve the permafrost stability of degraded grassland in the QTP and enhance the service functions of alpine grassland ecosystems.

Soil aggregate development and associated microbial metabolic limitations alter grassland carbon storage following livestock removal

Natural restoration of degraded grasslands following livestock removal changed soil nutrient and carbon concentrations, soil aggregate distributions, and patterns of extracellular enzyme activities. Here, we used an ecoenzymatic stoichiometry model to quantify microbial resource limitations in semiarid grassland soil aggregates at 0, 11, 26, and 36 years after livestock removal and linked these limitations to microbial carbon use efficiency (CUE), which was estimated from stoichiometry theory (CUEST). Overall, livestock removal altered the size distribution of soil aggregates, increased microbial resources and stimulated extracellular enzyme activities. Long-term livestock removal increased the proportion of small macroaggregates (2–0.25 mm). Microbial carbon (C) and phosphorus (P) limitations in soil aggregates declined to minimum levels at 26 years following livestock removal and then increased after 36 years, with an opposite trend for CUEST. Additionally, the greatest resource limitations to microorganisms were found in smaller aggregates (<2 mm) under long-term livestock removal. Random forest and structural equation models revealed that soil abiotic factors, especially total nitrogen and pH, were key determinants of microbial resource limitation in soil aggregates. Moreover, microbial C and P limitations had significant direct effects on the CUEST. Thus, increasing microbial metabolic limitations after long-term livestock removal could reduce microbial C turnover, potentially reducing soil C sequestration. Overall, this study revealed that livestock removal altered soil aggregate development as well as the allocation of resources in aggregates and consequent microbial resource limitations, providing information that may be useful for developing grassland management strategies to enhance C sequestration.

Soil Aggregate Size Distribution Alters CO2 but not N2O Emission Rates in Chinese Fir (Cunninghamia lanceolata) Plantations with N and P Additions

Soil Aggregate Size Distribution Alters CO2 but not N2O Emission Rates in Chinese Fir (Cunninghamia lanceolata) Plantations with N and P Additions

Effects of innovative long-term soil and crop management on topsoil properties of a Mediterranean soil based on detailed water retention curves

Abstract. The effectiveness of conservation agriculture (CA) and other soil management strategies implying a reduction of tillage has been shown to be site-dependent (crop, clime and soil), and thus any new soil and crop management should be rigorously evaluated before its implementation. Moreover, farmers are normally reluctant to abandon conventional practices if this means putting their production at risk. This study evaluates an innovative soil and crop management (including no-tillage, cover crops and organic amendments) as an alternative to conventional management for rainfed cereal cropping in a calcareous soil in a semi-arid Mediterranean climatic zone of Navarra (Spain), based on the analysis of soil water retention curves (SWRCs) and soil structure. The study was carried out in a small agricultural area in the municipality of Garínoain (Navarre, Spain) devoted to rainfed cereal cropping. No other agricultural area in the whole region of Navarre exists where soil and crop management as proposed herein is practiced. Climate is temperate Mediterranean, and the dominant soil is Fluventic Haploxerept. Within the study area there is a subarea devoted to the proposed soil and crop management (OPM treatment), while there is another subarea where the soil and crop management is conventional in the zone (CM treatment). OPM includes no-tillage (18 years continuous) after conventional tillage, crop rotation, use of cover crops and occasional application of organic amendments. CM involves continuous conventional tillage (chisel plow), mineral fertilization, no cover crops and a lower diversity of crops in the rotation. Undisturbed soil samples from the topsoil and disturbed samples from the tilled layer were collected for both systems. The undisturbed samples were used to obtain the detailed SWRCs in the low suction range using a HYPROP©device. From the SWRCs, different approaches found in the literature to evaluate soil physical quality were calculated. The pore-size distribution was also estimated from the SWRCs. Disturbed samples were used in the laboratory to assess soil structure by means of an aggregate-size fractionation and to perform complementary analysis from which other indicators related to soil functioning and agricultural sustainability were obtained. The approaches evaluated did not show clear differences between treatments. However, the differences in soil quality between the two forms of management were better observed in the pore size distributions and by the analysis of the size distribution and stability of soil aggregates. There was an overabundance of macropores under CM, while the amount of mesopores (available water) and micropores were similar in both treatments. Likewise, more stable macroaggregates were observed in OPM than in CM, as well as more organic C storage, greater microbial activity, and biomass. The proposed management system is providing good results regarding soil physical quality and contributing also to the enhancement of biodiversity, as well as to the improvement in water-use efficiency. Finally, our findings suggest that the adoption of the proposed practice would not result in a loss in yields compared to conventional management.

The effects of co-utilizing green manure and rice straw on soil aggregates and soil carbon stability in a paddy soil in southern China

The co-utilization of green manure (GM) and rice straw (RS) in paddy fields has been widely applied as an effective practice in southern China. However, its effects on soil aggregate and soil organic carbon (SOC) stability remain unclear. In the present study, the effect of GM, RS, and co-utilization of GM and RS on particle size distribution of soil aggregates and SOC density fractions were measured in a field experiment. The experiment included six treatments, i.e., winter fallow (WF) without RS return (Ctrl), WF with 50% RS return (1/2RS), WF with 100% RS return (RS), GM without RS return (GM), GM with 50% RS return (GM1/2RS) and GM with 100% RS return (GMRS). The results showed that the proportion of small macro-aggregates (0.25–2 mm) and the mean weight diameter (MWD) of aggregates in the GMRS treatment was greater (by 18.9 and 3.41%, respectively) than in the RS treatment, while the proportion of silt+clay particles (<0.053 mm) was lower (by 14.4%). The concentration of SOC in microaggregates (0.053–0.25 mm) and silt+clay particles was higher in the GMRS treatment than in GM and RS treatments individually. The concentration and proportion of free light organic carbon (fLOC) in aggregates of various particle sizes and bulk soil was greater in the GMRS treatment than the RS treatment, whereas the concentration and proportion of mineral-associated organic carbon in small macroaggregates, microaggregates, and bulk was lower in the GMRS treatment than in the RS treatment. The proportion of intra-aggregate particulate organic carbon (iPOC) was greater in the GMRS treatment than in GM treatment. The GMRS treatment had strong positive effects on iPOC in small macroaggregates, suggesting that SOC was transferred from fLOC to iPOC. In conclusion, co-utilizing green manure and rice straw cultivated the SOC pool by increasing the concentration of fLOC and improved soil carbon stability by promoting the sequestration of organic carbon in iPOC as a form of physical protection.

Effects of 10 Years of the Return of Corn Straw on Soil Aggregates and the Distribution of Organic Carbon in a Mollisol

The return of straw is a widely used agricultural practice for increasing the soil organic carbon (SOC) content and improving soil structure in Mollisols, owing to the decline caused by continuous high-intensity tillage. We conducted a field experiment where corn straw was continuously returned for 10 years to investigate effects of the straw on the size distribution and stability of soil aggregates and on SOC density fractions. The treatments were no straw return (CK) and four rates of straw return: 6000 kg hm−2 (S1), 9000 kg hm−2 (S2), 12,000 kg hm−2 (S3), and 15,000 kg hm−2 (S4). SOC contents after straw return for bulk soil, a free light fraction (F-LF), an occluded light fraction (O-LF), and a heavy fraction (HF) were significantly higher by 27.0, 644.3, 720.0, and 69.2%, respectively, in S4 than CK. The contents of F-LF, O-LF, and HF in aggregates &gt;2.00 mm were significantly higher by 194.2, 162.1, and 35.8%, respectively, in S4 than CK. Structural equation modeling indicated that SOC contents and aggregates &gt;0.25 mm were directly correlated with the amount of straw returned. We conclude that returning 15,000 kg m−2 of straw would be an effective agronomic practice to restore Mollisol fertility.

Effects of Biopesticides and Undersown Cover Crops on Soil Properties in the Organic Farming System

The inclusion of undersown cover crops in crop rotations and the use of biopesticides are essential for the long-term sustainability of the agroecosystem in organic farming. We hypothesized that biopesticides and undersown cover crops (crimson (incarnate) clover (Trifolium incarnatum Broth.), hairy (winter) vetch (Vicia villosa Roth.), perennial ryegrass (Lolium perenne L.), and winter rye (Secale cereale L.)) are likely to have a positive impact on soil agrophysical and biological properties. Soil shear strength, soil aggregate–size distribution, plant root dry biomass and number and biomass of earthworms were determined at the end of the plant growing season. We concluded that the application of biopesticides and growing of undersown cover crops decreased soil shear strength and increased the percentage of macro-aggregates (0.25–10 mm) in the plough layer. In the plots with cover crops, plant root biomass was found to be significantly higher by a factor of 1.9 to 3.5, compared to the plots without cover crops. The application of biopesticides and undersown cover crops did not significantly affect the number and biomass of earthworms in the soil. The abundance of earthworms was more influenced by meteorological conditions.

Assessing the Impacts of Fertilization Regimes on Soil Aggregate Dynamics in Northeast China

Determining the effects of fertilization regimes on soil aggregates, carbon (C) and nitrogen (N) distribution, and pH is essential for improving soil structure and soil organic carbon (SOC) accumulation to help in proper soil fertility management. Based on a 41-year field fertilization experiment conducted on dark brown soil in northeast China, we examined the soil aggregate size distribution and associated C, N, and pH to provide a scientific basis for elucidation of the mechanisms underlying the effects of fertilization treatments on soil structure and fertility. Six different fertilization treatments included no fertilizer (CK), low-dose chemical fertilizer (NP), moderate-dose chemical fertilizer (2NP), high-dose chemical fertilizer (4NP), normal-dose organic fertilizer (M), and normal-dose organic fertilizer plus moderate-dose chemical fertilizer (M+2NP). Our findings showed that compared to CK, M and M+2NP significantly increased the proportion of macroaggregates by 40% and 28%, respectively, whereas 4NP significantly decreased it by 19%. The mean weight diameter (MWD) and geometric mean diameter (GMD) under M and M+2NP were significantly higher than that under CK, at 12–21% and 24–36%, respectively. The fractal dimension (D) value of M+2NP was significantly lower than those of 2NP and 4NP by 4% and 5%, respectively. Soil pH under the M treatment was highest, followed by M+2NP. Soil pH under 2NP and 4NP more significantly decreased, by 0.1 and 0.2 units, than under M treatment. Soil pH values were correlated with the proportion of soil macroaggregates, MWD, and GWD, respectively (p &lt; 0.05). Relative to CK, M and M+2NP increased the contents and stocks of SOC (by 40–49% and 89–93%, respectively) and total N (59–68% and 119–123%, respectively). Furthermore, the contents and stocks of aggregate-associated SOC and total N decreased following the order: NP &gt; 2NP &gt; 4NP. Overall, the long-term application of organic fertilization regimes (M and M+2NP) effectively improved soil aggregation as well as SOC accumulation and decreased soil acidification in dark brown soil in northeast China.

Linking the chemical nature of soil organic carbon and biological binding agent in aggregates to soil aggregate stability following biochar amendment in a rice paddy

Changes in soil aggregation with biochar amendment have been investigated extensively, but how biochar affects the chemical composition of organic carbon (C) and biological binding agents in aggregates and their linkage with soil aggregate stability remains unclear. Soil samples were collected in a rice paddy treated with 0 (C0, control), 10 t ha−1 (C10), 20 t ha−1 (C20) and 40 t ha−1 (C40) biochar for twenty months. The amount and chemical composition of soil organic C (SOC), microbial abundances and glomalin-related soil protein (GRSP) were determined in bulk soil and four fractions: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm), and silt + clay (<53 μm). Our results showed that the proportion of >250 μm water-stable aggregates and mean weight diameter were gradually increased with increasing biochar addition rate. The concentrations of SOC, readily oxidizable C and microbial biomass C increased most in the small macroaggregates, followed by microaggregates under biochar amendment. Increasing biochar addition rate gradually decreased the relative contents of alkyl C, O-alkyl C and carbonyl C, but increased those of aromatic C across the aggregates, resulting in a higher aromaticity and hydrophobicity of SOC with respect to the control. The abundances of bacteria, fungi and archaea and the content of GRSP were significantly enhanced in the large and small macroaggregates under the C40 treatment. The proportion of >250 μm aggregates was significantly correlated with the contents of soil organic C fractions, GRSP and microbial abundance. Structural equation modeling further revealed that changes in SOC hydrophobicity and GRSP content under biochar amendment had significant and direct effects on the soil aggregate size distribution. In summary, our findings suggest that biochar amendment in rice paddy could improve soil aggregation through altering the chemical composition of soil organic C and the abundance of biological binding agents.