Increased Soil Aggregate Stability Research Articles

Long-term manure and/or straw substitution mediates phosphorus species and the phosphorus-solubilizing microorganism community in soil aggregation

The partially substitution of chemical fertilizers with organic materials can improve soil phosphorus (P) availability; however, the response of aggregate-associated P species and the microbial community remains unclear. We investigated P speciation transformation and the related microbial community of aggregates in a 10-year field trial containing chemical fertilizer (CC), 50 % chemical N replaced by pig manure, corn straw, and manure combined with straw. Compared to CC, the organic substitute pattern increased soil aggregate stability indices, which were positively correlated with labile and moderately labile P. The organic substitution pattern improved labile and moderately labile P in > 2 mm large macroaggregates (LM) and 0.25–2 mm macroaggregates (MA), and decreased labile P in 0.053–0.25 mm microaggregates (MI) and < 0.053 mm silt + clay (SC). CaHPO4 and MgHPO4 of labile P were the primary components in MA, while Ca(H2PO4)2 was the primary component in MI. Straw addition enhanced the relative abundance of P-solubilizing microorganisms, primarily Bacillus, in different aggregates. The labile P content was higher, and the microbial network displayed higher complexity and connectivity in MI than in aggregates of other sizes. pH was the most critical factor influencing the distribution of P species in the LM. Organic carbon and aggregate stability index were the primary factors in MA, whereas phosphatase activity was the primary factor in MI and SC. Overall, the organic substitution pattern improved aggregate stability and labile P content in LM and MA and reduced labile P in MI and SC, thereby promoting P retention. These insights advance our understanding the effects of organic fertilization on P transformation in soil aggregates, with implications for developing eco-friendly P management strategies.

Influence of long-term ecological reclamation on carbon and nitrogen cycling in soil aggregates: The role of bacterial community structure and function

Understanding the influence of microbial taxa and functions on soil carbon (C) and nitrogen (N) cycling, particularly concerning soil aggregate sizes, is crucial for ecosystem management. This study examines the taxonomic and functional dynamics of soil bacterial communities within different aggregate sizes over time. Soil samples from a reclamation forest on the Loess Plateau in North China were collected across reclamation ages of 0, 3, 18, and 28 years. Soil aggregates were categorized into large macro-aggregates (>2000 μm), small macro-aggregates (250–2000 μm), and micro-aggregates (<250 μm) using a modified dry-sieving method. Soil aggregate stability, C and N concentrations, newly derived plant C, enzyme activities, bacterial communities, and functional genes in each aggregate fraction were systematically analyzed. There was a notable increase in soil aggregate stability and a higher proportion of large aggregates was found with increasing forest age. There were significant differences in bacterial community structures, particularly between micro-aggregates and large macro-aggregates and across different forest ages. Reclamation led to an increased abundance of copiotrophic bacterial taxa. Decreases in N-acquiring enzyme activity in micro-aggregates were contrasted by an increase in C, N, and phosphorus (P) acquisition activities in larger aggregates over time. Larger aggregates showed a faster recovery of C and N cycling genes accompanied by a significant enhancement in acetyl-CoA and ammonia oxidation processes, underscoring their importance in soil nutrient cycling. These results highlight the critical role of aggregate size in shaping microbial community structures and functions that influence soil C and N cycling during reclamation and provide new perspectives highlighting the significance of incorporating aggregate size considerations into soil management and reclamation strategies.

Land Use affects the Soil Aggregation Pattern during Restoration of Degraded Land under Tropical Dry Climate

Soil aggregate formation and stability are crucial in safeguarding soil carbon pools, as they form the basic unit of soil structure in terrestrial ecosystems. The impact of different land uses on soil aggregation pattern has widely been studied but is limited under tropical dry climate conditions. This study aimed to determine the soil aggregation pattern at two soil depths (0-15 cm and 15-30cm) in three different plantations (Peltophorum pterocarpum, Eucalyptus globulus, and Acacia nilotica) established in degraded land under tropical dry climate. The soil aggregates were categorized into microaggregate, mesoaggregate and macroaggregate based on their size fractions which were determined by the wet sieving method. The percent of macroaggregate, mesoaggregate, and microaggregate fractions at 0-15 cm depth ranged from 62.2 – 82.6%, 31.6 – 36.7%, and 1.1 – 4.9 % respectively whereas at 15-30 cm depth, it ranged from 69.7 - 81.7%, 17.1 - 31.6%, and 1.2 - 2.8%. The mean weight diameter values at 0-15 cm and 15-30 cm soil depth ranged from 2.27-3.37mm and 2.85-3.35 mm respectively. The higher percentage of macroaggregate fractions in Peltophorum pterocarpum plantation compared to Eucalyptus globulus, and Acacia nilotica plantations indicated that the Peltophorum pterocarpum plantation has slightly more potential to improve aggregate formation and soil structure in the degraded lands under tropical dry climate. In addition, appropriate land use practices can increase soil aggregate stability in the study area.

Effects of fall-spring cement applications on soil physico-mechanical quality of seed bed and seedling emergence.

The physico-mechanical quality of the seed bed is extremely important not only for the soil air-water dynamics, but also for the germination, seedling emergence, and development of the plants. The crust layer formed after precipitation and irrigation in soils with poor structural properties is among the most important indicators of the lower seedbed quality. While there are different applications to increase the physico-mechanical quality of the seedbed, there are also cement applications that provide rapid aggregate formation and stability. The effects of cement application on the soil mechanical quality and seedling emergence of clay textured soil with poor structural characteristics were investigated in the present study. For this purpose, a total of 36 small plots with a size of 2.4 m2 were created. Cement was applied at 0.25-0.5-1 and 2% doses to 18 plots in the fall period, and to the other 18 plots in the spring period. After the treatment, soil samples were taken for analysis before the planting period and the number of seedling emergence was determined by planting the bean plant during the planting period. According to the results of the study, it was found that cement applied in two periods caused an increase in soil aggregate stability, decreased penetration resistance, modulus of rupture, and dispersion rate, and increased water holding capacity to a limited extent. On the other hand, cement application also had positive impacts on the emergence of the bean plant. The 1% dose of the spring application of cement was more effective on the determined soil structural characteristics and seedling emergence.

Broadleaf Trees Increase Soil Aggregate Stability in Mixed Forest Stands of Southwest China

In soils, high aggregate stability often represents higher quality and anti-erosion ability; however, few studies have systematically analyzed how different forest stands affect soil aggregate stability. We selected five typical mixed forest stands on Jinyun Mountain in Chongqing, China, as research sites to evaluate soil aggregate stability. Within these sites, we analyzed the factors influencing soil aggregate stability in different stands by measuring soil characteristics and root traits. Soil aggregation stability, plant root traits, and soil properties varied among the mixed forest stands. The broadleaf tree mixed forest improved soil aggregate stability by 57%–103% over that of the Pinus massoniana mixed forest. The soil organic carbon, cation exchange capacity, Fe-Al oxides, and fine root proportion were positively correlated with soil aggregate stability. The specific root length and very fine root proportion were negatively correlated with soil aggregate stability, whereas the fine root proportion was positively correlated with this property. Specifically, we found that arbuscular mycorrhizal fungi did not affect soil aggregate stability in acid rain areas. Structural equation modeling indicated that soil aggregate stability was closely related to soil physicochemical properties and plant root characteristics. Predictive factors accounted for 69% of the variation in mean weight diameter, and plant root traits influenced soil aggregate stability by affecting soil organic matter, texture, and Fe-Al oxides. This study elucidated the impact of soil physicochemical properties and plant root characteristics on soil aggregate stability in different forest stand types, which has crucial implications for optimizing the management of various forest types.

The role of extracellular polysaccharide produced by Bradyrhizobium strain in root growth, improvement of soil aggregate stability and reduction of soil detachment capacity

It is well demonstrated that the production of extracellular polysaccharide by bacteria, as biological binding agent, can increase soil aggregate stability. Also, plant roots have significant effects on the soil erosion rates, since they can strongly change the soil detachment capacity (Dc). This study quantifies Dc at different flow rates in soils treated with biofertilizer under four species (Alnus subcordata, Brachythecium plumose, Gleditsia caspica and Sambucus ebulus species) in the Saravan forest park of Iran and develops regression equations for predicting Dc. Undisturbed samples collected from soils with the four tree species and subjected to five slopes (from 4.2 to 38.3%) and five water discharges (from 0.28 to 0.71 L m−1 s−1) using a hydraulic flume. The results showed that Dc was significantly lower in soils with Alnus subcordata species compared to Brachythecium plumose, Gleditsia caspica and Sambucus ebulus species, as the consequence of the changes in the root characteristics, mean weight diameter (MWD) and macronutrients, so that Dc was negatively correlated with root weight density, root length and root biomass. A linear regression model was found for MWD as a function of extracellular polysaccharide concentration (R2 = 0.74). The unit stream power had high accuracy for predicting Dc for all of the studied species. The lowest value of rill erodibility (Kr) was obtained in soils with Alnus subcordata species using regression relationship between the Dc and the shear stress of the soil. This experiment helped to show the importance of microorganisms in reducing the soil detachment rates and provided a contribution in understanding the choice of appropriate species for soil conservation.

Amendment of soils with metal-rich sludge: Potential water toxicity due to metal release via over-time slaking

Industrial metal-rich sludge can improve soil properties, but it is potentially toxic to soils and adjacent aquatic systems. The soil-sludge-water interactions influence metals bioavailability over time, a phenomenon mostly regulated by the still debatable “sludge physical protection” or “sludge delayed release” hypotheses. The present study aimed to investigate: (1) whether sludge increases soil aggregate stability against slaking, (2) which hypothesis mostly regulates metal release from soils to water and (3) the ecotoxicity of the metals released during soil slaking for aquatic organisms. Under a realistic field scale, soils amended with an industrial sludge or spiked with equivalent metal solutions (of Cr, Cu, Ni, Zn) were collected over three months to test soil aggregate stability, the ecotoxicity of the slaking water and metal contents in soil and water. The “sludge physical protection” was verified for all metals, though for Cu the “sludge delayed release” hypothesis appears plausible after three months. Soil amendment with sludge did not lead to effects on the growth of the microalga Raphidocelis subcapitata, contrarily to the observed for the metal-spiked soil. Criteria regulating soils sludge-amendment management should thus include doses not hazardous to biota, and not only metal threshold levels.

Biochar derived from papermill factories improves soil physical and hydraulic properties in no-till cotton fields

Whether biochar produced as a by-product of energy generation from the papermill industry, and often disposed in landfills, can be gainfully applied to commercial croplands has not been investigated. The objective of this study was to investigate the physical and hydraulic properties of soils in commercial cotton fields managed as no-till systems following repeated applications of biochar generated as a waste of a papermill plant. Undisturbed cores and disturbed soil samples were collected from 0–5 and 5–10 cm layers from five commercial no-till fields in Mississippi, USA that received 6.7 Mg ha−1 year−1 biochar for 0, 2, 3, 5 or 10 years. A number of physical, hydraulic, and chemical properties of these samples were measured in the lab. The results showed that biochar reduced the degree of soil compactness and increased soil aggregation and structural stability index. The findings were particularly apparent for the 10 years of consecutive application, which increased soil aggregate stability by up to 67%, reduced bulk density from 1.40 to 1.26 g cm−3, and reduced degree of compactness from 73.2% to 62.8%. Biochar increased soil porosity but much of this increase (55%) occurred for small pores (< 0.5 μm) with little effect on storage pores (0.5–50 μm) or transmission pores (> 50 μm). Consequently, biochar increased soil field capacity by up to 26%, but PAW increased by only 17%. Biochar significantly increased soil physical quality index score in the 0–5 cm layer from 0.16 to 0.26 and the increase was positively correlated with the number of years of application. The results suggest biochar generated as a byproduct of papermill could be land-applied in real-world crop production systems to improve soil health as an alternative to disposal in landfills.Graphical

Introducing N2-fixing tree species into Eucalyptus plantations promotes soil organic carbon sequestration in aggregates by increasing microbial carbon use efficiency

The conversion of a monoculture plantation into a mixed forest can improve soil properties and increase soil organic carbon (SOC) sequestration. However, the impacts of introducing N2-fixing tree species into Eucalyptus plantations on SOC composition and transformation, as well as the associated changes in microbial properties remain poorly understood. Here, we examined the SOC fractions (LPI-C, LPII-C, and RP-C) in different aggregates (>2.0 mm, 1–2 mm, 0.25–1 mm, and <0.25 mm) from 15-year-old plantations of pure Eucalyptus urophylla (PP) and mixed E. urophylla and Acacia mangium (an N2-fixing tree species) (MP). The results showed that the contents of SOC fractions and the proportion of recalcitrant C pool (fRP-C) in aggregates were significantly higher (P < 0.05) in MP than those in PP. Pearson correlation analysis indicated strong correlations among litter properties, soil properties, and microbial properties. N2-fixing species increased soil aggregate stability and N availability while altering litter quality and quantity; all of these factors strongly impacted microbial properties. Changes in nitrogen availability and microbial properties (e.g. carbon use efficiency) were the drivers of changes in SOC fractions. Our findings highlight the importance of introducing N2-fixing species in regulating soil microbial properties thereby increased SOC sequestration and stability in Eucalyptus plantations, which should be a relatively good option for future afforestation projects in the view of improving the carbon benefits of the ecosystem.

The increased soil aggregate stability and aggregate-associated carbon by farmland use change in a karst region of Southwest China

Land use change can significantly alter the distribution of different soil aggregates, thereby influencing aggregate stability and aggregate-associated organic carbon (OC). However, there is minimal research on the variations in the distribution of soil aggregates, aggregate stability, and aggregate-associated OC following land use change from farmland to afforested land in the karst regions of Southwest China. Undisturbed soil samples were collected from farmland (FL) and three afforestation patterns (bamboo forest: BA; landscape tree planting: LAT; and orange orchards: ORO) converted from farmland in a karst region of Southwest China. These four land uses have similar geographical characteristics (e.g., elevation and slope aspects) and previous framing practices. The distribution of macroaggregate (>0.25 mm), microaggregate (0.053–0.25 mm) and silt + clay fractions (<0.053 mm) and the aggregate-associated OC contents under the four land uses were measured at the 0–10 cm and 10–20 cm depths. The results showed that afforestation converted from FL significantly increased the macroaggregate and microaggregate contents, geometric mean diameter (GMD), and structural stability index (SSI) but decreased the silt + clay fraction content. Compared with FL, the values at the 0–20 cm depth under BA, LAT and ORO increased by 108.79%, 34.89% and 36.56%, respectively, for GMD and by 239.84%, 94.05%, and 123.41%, respectively, for SSI. Land use change and soil aggregate size had obvious influences on soil organic carbon (SOC) content. The SOC content in the bulk soil, macroaggregate, microaggregate, and silt + clay fraction at the 0–20 cm depth after afforestation increased by 26.83%, 45.57%, 27.14%, and 13.80%, respectively, compared to FL. The contributions of macro- and microaggregates to the increase in SOC content in bulk soil were 146% and 78%, respectively, while the silt + clay fractions had a negative effect with a value of 125% after afforestation. The GMD and SSI were positively correlated with the macroaggregate content and SOC content in the bulk soil and different soil aggregates. Although the increase in SOC content in bulk soil and soil aggregates under BA and ORO was significantly higher than that for LAT, the soil aggregate stability under ORO was lower than that under BA. These findings suggest that afforestation increases aggregate ability, SOC content in bulk soil and aggregate-associated OC and that BA might be the optimum choice among the three afforested lands compared for soil sustainability from the perspective of improving carbon sequestration and soil erosion.

Biochar Addition with Water and Fertilization Reduction Increases Soil Aggregate Stability of 0–60 cm Soil Layer on Greenhouse Eggplant in Mollisols

Biochar application affects the soil organic carbon (SOC) content and distribution, which is relevant to facility agriculture and soil aggregates. However, how the fertilization management of facility agriculture affects the SOC content and aggregate stability at different soil depths in Mollisols is unclear. Intended to provide a basis for developing a reasonable fertilizer amount when adding biochar, the facility vegetable eggplant in Northeast China was used to explore the effects of biochar addition on the distribution and SOC content of whole soils and the organic carbon (OC) content of aggregates of each size in the profile (0–100 cm) of Mollisols. Three treatments were set up: WF (conventional application amounts of water and fertilizer), WFB (conventional application amounts of water and fertilizer and added biochar), and 80%W80%FB (20% water reduction and 20% fertilizer reduction and added biochar). The results demonstrated that the 80%W80%FB treatment significantly increased the SOC content by 56.1% and 34.0% in whole soils at a 0–20 cm soil depth compared to WF and WFB treatments, respectively. Simultaneously, compared with WF and WFB treatments, the significant increase in the OC content of 1–0.25 mm sized aggregates of 81.4–130.2% and 4.3–10.1% and the enhanced proportion of &gt;2 mm sized aggregates of 0.22–16.15- and 0.33–0.83-fold both improved aggregate stability in the 0–20 cm soil layer under the 80%W80%FB treatment, which was proven to result in 32.6% and 30.6% increments in the weight diameter (MWD) value. Therefore, biochar addition with water and fertilizer reductions increases surface soil aggregate stability for greenhouse eggplants in Mollisols.

Pelletized Straw Incorporation in Sandy Soil Increases Soil Aggregate Stability, Soil Carbon, and Nitrogen Stocks

In China, increasing the quantity and quality of total carbon and nitrogen stocks in sandy soil used for crop production is an important research issue. Soil amendment with pelletized straw could improve both soil physical structure and fertility in sandy soils, but these aspects remain understudied. The present pot and field experiments examined the dynamic changes in sandy soil water holding capacity, soil bulk density, soil total carbon and nitrogen stocks, and the distribution of water-stable aggregates and soil total carbon stocks related to aggregates across the following treatments: no fertilization (i.e., study control (CK)), normal fertilizer rate (NM), soil amendment at 150 Mg ha−1 (S150), manure amendment at 150 Mg ha−1 (M150), pelletized straw amendment at 75 Mg ha−1 (PS75), and pelletized straw amendment at 150 Mg ha−1 (PS150). The results show that the pelletized straw incorporation significantly increased water holding capacity and decreased soil bulk density. PS150 notably increased the large macroaggregates (&gt;2000 μm) proportion and decreased the ratio of &lt;250 μm aggregate size fractions in comparison with CK, NM, S150, and M150 at 0–20 and 20–40 cm soil depths. Compared with the CK treatment, the bulk soil carbon and nitrogen stocks in the 0–20 cm layers under the PS150 treatment were significantly increased by 85.2% and 302.9%, and in the 20–40 cm layers those increased by 136.4% and 257.1%, respectively. The PS150 treatment resulted in higher soil organic carbon (SOC) and particulate organic carbon content than the CK and PS75 treatments, whereas the PS75 treatment achieved maximum soil inorganic carbon content. The pelletized straw treatment increased the large macroaggregate-associated soil total carbon content at 0–20 and 20–40 cm soil depths. The maximum soil total carbon stocks were in the small macroaggregates (250 &lt; WSA &lt; 2000 μm) rather than in the large macroaggregate and microaggregates under the PS75 and PS150 treatments. Additionally, the pelletized straw and manure amendments increased the yield of silage corn, which was dependent on the increase in soil total carbon and nitrogen content in the macroaggregates, whereas the soil and manure amendments did not facilitate sandy soil aggregation and soil total carbon stock increases. In conclusion, PS150 was found to be the optimal amendment for maintaining sandy soil profile physico-chemical properties through macroaggregate stabilization. These results will be beneficial for arid and semi-arid regions, thus contributing to soil carbon and nitrogen conservation.

Increased Soil Aggregate Stability by Altering Contents and Chemical Composition of Organic Carbon Fractions via Seven Years of Manure Addition in Mollisols

Mollisols include an abundance of soil organic carbon (SOC) which is easily influenced by fertilization management. Manure addition could enhance soil aggregate stability; however, the dominating factor affecting its stabilization remains controversial. The fertilization practices were initiated in 2012 to investigate the influences of different fertilization managements on the contents and molecular characterization of organic carbon (OC) fractions, and to clarify the underlying mechanism of soil aggregate stability change. NoF (non-fertilizer), CF (only chemical fertilizer), CF + DM (chemical fertilizer plus single dairy manure at 15 t ha−1), and CF + 2DM (chemical fertilizer plus double dairy manure at 30 t ha−1) treatments were established. This research was aimed at exploring the potential mechanism that affects aggregate stability in Mollisols through the variation of contents and chemical composition of OC fractions, and screening out the appropriate fertilization practice on promoting SOC stabilization and crop yield under 7-year manure addition. Compared to CF, 7-year manure addition significantly enhanced SOC content by 17.4–35.9% at 0–10 cm depth, which was evidenced from the contribution of increased aromatic compounds with 4.3–19.9%. Simultaneously, compared with CF, CF + DM and CF + 2DM both significantly enhanced dissolved organic carbon and easily oxidizable organic carbon contents by 12.5–37.7% at a 0–30 cm soil layer. In regard to soil aggregates, the increased OC content and mass percentage of macroaggregates, and the decreased mass percentage of free microaggregates both improved aggregate stability under manure addition at 0-30 cm soil layer, which was proven to be the increment in mean weight diameter (MWD) and geometric mean diameter (GMD) values by 17.6–22.1%. Moreover, CF + DM and CF + 2DM raised aromatic compound amounts of POM fractions within macroaggregates [M(c)POM] by 5.6–11.6% and within free microaggregates (Fm-POM) by 4.3–10%. Furthermore, CF + DM and CF + 2DM both significantly increased maize yield by 5.7% and 4.2% compared to CF, but no significant difference was observed between CF + DM and CF + 2DM treatments. Collectively, physical protection through the occlusion within aggregates of POM might be the central mechanism for soil aggregate stability of manure addition in Mollisols. The manure addition of 15 t ha−1 was the effective management method to enhance SOC stabilization and crop yield in Mollisols.

Effect of Mechanized Ridge Tillage with Rice-Rape Rotation on Paddy Soil Structure

Ridge tillage is one of the essential tillage methods in China. It affects the soil structure by altering the soil-water environment. With the rapid development of agricultural mechanization, the mechanized ridge tillage technology that combines modern mechanized agriculture with conservation tillage has become a pertinent agricultural production technology in the hilly regions. However, to date, the effects of mechanized ridge tillage on soil structure have garnered little attention. In this study, a field plot experiment involving the following four treatments: conventional tillage with rice and winter fallow (CK1), conventional tillage with rice-rape rotation (CK2), wide ridge tillage with rice-rape rotation (BT), and narrow ridge tillage with rice-rape rotation (RT) was conducted to study the effects of mechanized ridge tillage with rice-rape rotation on soil aggregate stability and soil porosity. The results indicated that mechanized ridge tillage could reduce the porosity of pores &gt; 30 μm in diameter and increase the porosity of pores &lt; 0.5 μm. Furthermore, mechanized ridge tillage could increase soil aggregate stability; the order of average mass diameter and geometric average diameter was RT &gt; BT &gt; CK2 &gt; CK1. The BT and RT treatments could increase the agglomeration degree of the soil particles, which followed the order of RT &gt; BT &gt; CK2 &gt; CK1. Therefore, mechanized ridge tillage with rice-rape rotation is effective in enhancing soil structure.

The formation of small macro‐aggregates induces soil organic carbon stocks in the restoration process used on cut slopes in alpine regions of China

AbstractRoad construction in mountainous areas of China has formed bare slopes on a large scale, which has aggravated soil erosion and carbon loss, and the restoration processes have changed soil structure and fertility. However, there are few reports regarding the differences of soil aggregate dynamics and SOC sequestration under different restoration process for cut slopes. In this study, three types of cut slopes [cut slope restored naturally (CSN); cut slope restored with three‐dimensional mesh (CST); cut slope restored with galvanized wire mesh (CSG)] and natural slope (NS) in alpine regions were selected as the research objects. SOC stocks and their key driving factors were then analyzed. The results showed that CSG was more beneficial to the formation of macro‐aggregates than CSN and CST. There were significant differences in wet‐sieved aggregate stability under different restoration process, where the sort by strength of stability was NS &gt; CSG &gt; CST &gt; CSN. Aggregate size was a more important factor for SOC stocks than restoration process. The increase of SOC stocks was mainly due to that within small macro‐aggregates. RandomForest analysis revealed that the mass proportion of micro‐aggregates was the most important factor affected SOC stocks, while SOC stocks within large macro‐aggregates and small macro‐aggregates and porosity significantly affected SOC stocks. Partial least squares path modelling analysis found the particle size and stability of aggregates had negative total effects on SOC stocks, and SOC content had positive total effect on that. Overall, for the ecological restoration of cut slope in alpine regions, CSG was most beneficial to increase soil aggregate stability and SOC stocks.

Fifteen years of conservation tillage increases soil aggregate stability by altering the contents and chemical composition of organic carbon fractions in Mollisols

AbstractMollisols contain a high amount of soil organic carbon (SOC) that is highly susceptible to tillage practices. Conservation tillage (CT) could improve soil aggregate stability, but the predominant factor controlling this stability remains debatable. This study reports on a tillage management experiment established in 2004 to explore the influences of tillage practices on the content and chemical composition of labile and recalcitrant OC fractions, plus with the aggregate stability change and potential mechanisms. CT without straw mulching, reduced tillage (RT), and no tillage (NT) (zero tillage) with straw mulching were set up. Compared with CT, 15‐year RT significantly increased SOC content (by 11.4%) in the surface (0–10 cm) by increasing the amount of aromatic compounds. Simultaneously, RT and NT promoted soil aggregate stability, as verified by the increase in mean weight diameter (MWD) and geometric mean diameter (GMD) of 7%–71.3% at 0–30 cm soil depth. Of importance, RT and NT increased aromatic compound amount of silt + clay fractions (by 12.6%–27.2%) within macroaggregates (&gt;0.25 mm) but decreased that (by 15.6%–32.9%) within free microaggregates (0.053–0.25 mm) than CT at 0–30 cm soil depth. Collectively, conservation tillage controls the SOC stability in Mollisols by regulating the MWD and GMD values and further affects the amount of aromatic compounds in silt + clay fractions within aggregates. Rather than physical and biochemical protection, chemical protection through a mineral association could be the crucial mechanism for aggregate stability under long‐term conservation tillage in Mollisols.

Straw residue incorporation and potassium fertilization enhances soil aggregate stability by altering soil content of iron oxide and organic carbon in a rice–rape cropping system

AbstractBoth iron (Fe) oxide and soil organic carbon (SOC) content, among other factors, drive the development and subsequent stability of soil aggregates. However, the mechanism of these drivers in paddy‐upland rotation systems with straw residue incorporation is not well understood. In this multiyear (2011–2017) trial, we sought to study the factors which drive the stability of soil aggregates in such a rice‐rape (RR) agronomic system. In order to examine the effect of straw residue incorporation and potassium fertilization on soil aggregation dynamics, we divided our trial into four treatments: inorganic phosphorus (P) and nitrogen (N) fertilizer; inorganic NPK fertilizer; inorganic NP fertilizer plus straw residue; and inorganic NPK fertilizer plus straw residue. Treatments which incorporated straw residue significantly increased both the geometric mean diameter (GMD) and the mean weight diameter (MWD) of soil aggregates. Soil aggregate stability was greater after rape harvest than after rice harvest, while SOC shows the opposite trend. Straw residue incorporation increased the SOC and, specifically, concentrations of aliphatic‐C and aromatic‐C, particularly in aggregates greater than 0.25 mm. Straw residue addition significantly increased both the amorphous (Feo) and complex iron oxide (Fep) contents of soil. In both bulk soil and in aggregates greater than 5 mm, SOC and Fep were positively correlated with MWD, while in aggregates less than 5 mm, Feo was positively correlated with MWD. Overall, we suggest that the increased concentrations of SOC, alkane‐C, Feo, and Fep in soils after incorporation of straw residue were responsible for increasing soil aggregate stability.

Interactive Effects of Honeysuckle Planting and Biochar Amendment on Soil Structure and Hydraulic Properties of Hillslope Farmland

Plant roots and biochar amendment cause changes in soil structure and hydraulic properties; however, their interactive influences are still inadequately understood. A six-year field study was conducted on hillslope farmland in the Sichuan basin, China, to evaluate how honeysuckle planting and biochar application affect soil structure and hydraulic properties. Various parameters related to soil structure (soil organic matter (SOM), soil aggregate stability, bulk density were obtained in the laboratory) and hydraulic (hydraulic conductivity, and soil water retention characteristics by single porosity of van Genuchten 1980 and dual porosity bi-exponential model) properties were determined. The results showed that honeysuckle planting alone increased (SOM) content, honeysuckle planting following biochar amendment could not only enhance SOM content to a greater magnitude in top 20 cm soil but also markedly increase the SOM content in deeper soil layers (20–30 and 30–40 cm), while the application of biochar alone enhanced the SOM content in top 20 cm soil. The combination of honeysuckle planting and biochar amendment could increase soil aggregate stability. Furthermore, It was found that soil pores with size r &gt; 125 µm were the dominant macropores in all treatments. Honeysuckle planting increased saturated soil hydraulic conductivity (Ks) significantly (p &lt; 0.05). Biochar amendment also significantly increased Ks directly or indirectly through enhancement of SOM content. Results also showed that honeysuckle planting and biochar amendment could lead to a greater increase in saturated soil water content than saturated soil hydraulic conductivity. However, SOM showed lower value in bare land plots suggesting that both honeysuckle planting and biochar could increase SOM in soil, hence improving soil quality. Therefore, our field study demonstrated that the practice of honeysuckle planting and biochar amendment jointly in sloping farmland of purple soil could effectively strengthen soil structure and improve soil water retention.

Crop Residue Return Rather Than Organic Manure Increases Soil Aggregate Stability under Corn–Soybean Rotation in Surface Mollisols

Fertilization practices change soil organic carbon content and distribution, which is relevant to crop rotation and soil aggregates. However, how fertilization management under corn–soybean rotation affects soil organic carbon and aggregate stability at different soil depths in Mollisols is unclear. The effects of 6–yr fertilization under corn–soybean rotation on aggregate stability, soil organic carbon content and storage, and size distribution in soil aggregates were investigated. Five different fertilization practices were carried out in 2013: corn and soybean without fertilizer; corn with chemical fertilizer, soybean without fertilizer; corn with chemical fertilizer, soybean without fertilizer, returning the corn and soybean residues; corn and soybean with chemical fertilizer; and corn with chemical fertilizer, soybean with farmyard manure. Compared with corn and soybean without fertilizer, returning the corn and soybean residues increased bulk SOC content, and enhanced mean weight diameter and geometric mean diameter values at 0–10 cm because of increased water–stable aggregates (WSA) larger than 2 mm proportion and decreased WSA&lt;0.053mm proportion. Simultaneously, corn with chemical fertilizer and soybean with farmyard manure increased bulk soil organic carbon content but reduced mean weight diameter and geometric mean diameter values at 0–20 cm due to increased WSA&lt;0.053mm proportion and decreased WSA&gt;2mm proportion. Altogether, the application of consecutive returning crop residues and chemical fertilizer in alternate years is the most favorable approach for soil organic carbon accumulation and aggregate stability at 0–10 cm under corn–soybean rotation in Mollisols.

Effect of Grazing Exclusion and Rotational Grazing on Soil Aggregate Stability in Typical Grasslands in Inner Mongolia, China

Soil aggregate is extremely important for soil health and sustainable land management. Overgrazing has caused serious degeneration of grassland in the past decades and how to restore the degraded soil through grazing management is urgently needed. In this research, we investigated effects of long-term grazing exclusion and short-term rotational grazing with different grazing intensities on aggregate stability in the upper 10 cm of soil at two grazing sites in Xilinhot, Inner Mongolia. Treatments included long-term (34 years) exclusion, moderate-term (21 years) exclusion, and continuous grazing at adjacent reference plots. In addition, effects of rotational grazing under different grazing intensity [i.e., no grazing (0 days/month), light grazing (3 days/month), moderate grazing (6 days/month) and high grazing (12 days/month)] were investigated after 5 years. Stability of aggregate fractions were determined using wet sieving. Our results showed that the stable aggregates fraction were significantly increased under grazing exclusion for both fine (0.25–1 mm) and coarse (1–2 mm) size fractions. At the rotational grazing site, stability of fine aggregates was significantly enhanced under grazing compared with no grazing, while there was little influence on stability of coarse aggregates. Our results showed that grazing exclusion significantly increased soil aggregate stability and the peak appeared in moderate-term exclusion, meanwhile, rotational grazing had little influence on aggregate stability. We suggest that rotational grazing rather than long-term grazing exclusion is a better way for soil aggregate stability and soil health, and current grazing prohibition policies may need to be adjusted.