Stable Macroaggregates Research Articles

Mechanical stability of newly-formed soil macroaggregates influenced by calcium concentration and the calcium counter-anion

Aggregate stability, an indicator of soil structural resistance to mechanical forces, depends upon the strength of binding substances within the aggregate. Adhesion of calcium (Ca2+) oxide-based compounds to soil primary particles is expected to create stable aggregates. This occurs because Ca2+ will flocculate clay and silicate minerals, and precipitate with carbonate or as Ca2+ hydrates (calcium-silica-hydrate, calcium-aluminium-silica hydrate), filling the air-filled pore space with solids that resist mechanical rupture. The objective of this work is to determine how the Ca2+ concentration, as well as the type of Ca2+ counter-anion (CO32−, OH−), affects the binding strength of newly formed macroaggregates (0.84–3 mm in diameter) that resist abrasive force. Soil from six arid desert locations was ground (<0.125 mm) and mixed with solutions containing 40 to 2560 mg Ca L−1 made with CaCO3 or Ca(OH)2. The stable aggregate diameter of newly-formed macroaggregates was based on their resistance to abrasion in an erosive chamber. Stable aggregate diameter tended to increase as the log of Ca2+ concentration increased and was similar in the presence of the CO32− and OH− counter-anions. Larger stable aggregate diameter formed as the Ca2+ solution concentration increased from 0.04 to 2.6 g Ca2+ L−1, using CaCO3 or Ca(OH)2 solution, and was enhanced when the soil contained 48–170 g clay kg−1 dominated by montmorillonite clay minerals. We observed Ca2+ accumulation on Al- and Si-rich surfaces, which may indicate formation of flocculants < 20μ m, as well as soil particle adhesion within newly-formed macroaggregates by calcium carbonate or calcium silicate hydrate precipitates. This work highlights the importance of calcium oxide binding for macroaggregate stabilization in arid desert soils.

Effects of different amendments on aggregate stability and microbial communities of coastal saline–alkali soil in the Yellow River Delta

AbstractOrganic amendments have been widely used in coastal saline–alkali soil remediation; however, the mechanisms involved and the interactions between organic and inorganic amendments are still unclear. In this work, furfural residue (particulate; C/N ratio: 51.87; O‐alkyl C + di‐O‐alkyl C: 42.35%, aromatic C: 40.89%) and black liquor (dissolved; C/N ratio: 3.11; O‐alkyl C + di‐O‐alkyl C: 32.20%, aromatic C: 28.32%) were tested to examine their effects on chemical properties, water‐stable aggregate fractions, chemical compositions of solid‐state soil organic matter (SOM), gloaming‐related soil protein contents, and microbial communities of coastal saline–alkali soil under a 400‐day incubation experiment. Furthermore, organic amendments mixed with mineral amendment (4:1) were employed to explore the interactions between organic and inorganic amendments. Furfural residue had stronger and longer effects on soil macroaggregate stability (~240 days, intense) than black liquor (~15 days, weak), and mineral amendment addition had a positive effect on the stability of microaggregates. Our results revealed that qualities (primary form, C/N ratio, and chemical composition) of organic amendment which can change microbial communities by increasing soil C/N ratio and effective chemical compositions of solid‐state SOM, are the key factors in promoting the rapid formation and longer stability of coastal saline–alkali soil aggregates. Moreover, inorganic amendment addition can further improve the formation and stability of microaggregates rather than those of macroaggregates. This study provided a much‐needed technical basis for remediation of coastal saline–alkali soil.

Functional stability: From soil aggregates to landscape scale in a region severely affected by gully erosion in semi-arid central Mexico

Soil aggregate stability is a vital indicator of soil structure and potential degradation and is essential to understand gully erosion in arid and semi-arid regions. Stability of soil aggregates depends on soil mass properties, local conditions (within the gully), and hillslope conditions. The objectives of this study were 1) to analyze soil aggregate stability at three gully wall positions: headscarps, sidewalls, and gully bottoms, and 2) to explore the relationships between soil aggregate stability and local and landscape conditions to know how stability at different scales is interrelated. Soil macro aggregate stability was estimated from soil samples using the soil aggregate stability kit. Conversely, Cornell infiltrometers were used to evaluate soil microaggregate stability by applying 5-min rainfall events with an intensity of 150 mm−1. Local (within gully) conditions were registered in the field using indicators from the ephemeral drainage line assessment method. Landscape conditions (outside gully), such as terrain attributes, anthropogenic features, and vegetation cover, were computed from topographic maps, digital elevation models, and satellite imagery. Relationships between soil aggregate stability and local and landscape conditions were explored using classification and regression trees. The highest soil macro and micro aggregate stability were found at the gully headscarps, while gully bottoms had the lowest soil aggregate stability. Soil macroaggregate stability was related to terrain attributes, ground cover, and gully dimensions. In contrast, microaggregate stability was associated with ground cover, terrain attributes, gully dimensions, and distance to roads and farm dams. Our study showed that soil aggregate stability differs along gully walls and that soil aggregate stability can be related to different variables at the local and hillslope scales. This knowledge is a valuable indicator for understanding gully erosion and the foundation for restoration strategies in semi-arid regions.

Process sequence of soil aggregate formation disentangled through multi-isotope labelling

Microaggregates (<250 µm) are key structural subunits of soils. However, their formation processes, rates, and transformation with time are poorly understood. We took advantage of multiple isotope labelling of potential organic gluing agents and inorganic building units to unravel their role in soil aggregation processes being initiated with and without plant growth. We added 13C-labelled extracellular polymeric substances (EPS), 15N-labelled bacteria, 57Fe-labelled goethite, and 29Si-labelled montmorillonite to fine soil <250 µm of an Ap horizon from a Stagnic Luvisol, which was planted with Festuca heteromalla or kept bare in a climate chamber. Samples were taken after 4, 12, and 30 weeks, and separated into free (f) and occluded (o) microaggregates of different sizes (<20 µm, 53–20 µm, 250–53 µm), and in stable macroaggregates (>250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (<20 µm) when plants were absent. The excess of bacterial 15N closely followed the pattern of EPS-derived 13C (R2 = 0.72). In contrast to the organic gluing agents, the goethite-57Fe and montmorillonite-29Si were relatively equally distributed across all size fractions. Overall, microaggregates were formed within weeks. Roots enforced this process by stabilizing microaggregates within stable macroaggregates. As time proceeded the labelled organic components decomposed, while the labelled secondary oxides and clay minerals increasingly contributed to aggregate stabilization and turnover at the scale of months and beyond. Consequently, the well-known hierarchical organization of aggregation follows a clear chronological sequence of stabilization and turnover processes.

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.

Soil Aggregate Construction: Contribution from Functional Soil Amendment Fertilizer Derived from Dolomite

Elastic and water stable macroaggregate are significant to soil structure. which is the base of the soil, to maintain sustainable agriculture. Whether and how functional amendment fertilizer is capable of construction of the macroaggregate is the main purpose of the study. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to investigate the effect of dolomite-based functional soil amendment fertilizers on soil structure. The fertilizers are beneficial to elastic-stable and water-stable aggregate construction. Calcined dolomite based soil amendment functional fertilizer (CDFF) was favorable to water-stable aggregates. The elastic-stable macroaggregate increased with lime, uncalcined dolomite based soil amendment functional fertilizer (UCDFF) and CDFF, and it was 3.0 to 4.2 times the microaggregate. The water-stable one of the CDFF was increased by 20.0%. The mean weight diameter (MWD) of the CDFF and the UCDFF increased by 0.05~0.19 mm, while that of lime only increased by 0.05 mm. The percentage of aggregate dispersion (PAD) of the CDFF was the least. SEM and EDS images revealed that Fe, Al, Si, Ca, Mg, C and O existed on the aggregates. The construction of stable aggregate lies in that the functional fertilizers can gradually neutralize soil H+ and prevent soil colloid dispersion. Soil particles are bounded together to construct micro-agglomerates and then macro-agglomerates through Ca2+, Mg2+ bond bridge and CaCO3, MgCO3 salt bridge and adhesion of SiO2, Fe2O3, Al2O3 as well as the other amorphous substances from the functional fertilizers.

Long-term manure applications to increase carbon sequestration and macroaggregate-stabilized carbon

Long-term manuring supplies organic substrates containing carbon and nitrogen, which are expected to contribute to the stabilization of soil organic carbon (SOC) according to microbial stoichiometry decomposition and nitrogen mining theories. This possibility was evaluated after 33 years of manure application (i.e., farmyard manure at agronomic and elevated rates) compared to an unfertilized control and chemical fertilizer, at an agricultural field site in a Calcic Cambisol on the Loess Plateau. Soil was collected from 0-20 cm (plow layer), 20–40 cm and 40–60 cm depths at the end of the winter fallow season to assess the SOC content in whole soil or in macroaggregate, microaggregate, silt and clay fractions (plow layer only). Long-term management of this site increased the SOC stock (0–60 cm) by 44% in the unfertilized control, up to 68% with chemical fertilizer and from 140% (agronomic rate of farmyard manure) to 189% (elevated rate of farmyard manure). Carbon sequestration efficiency was greater in the unfertilized control (35%) and with chemical fertilizer (up to 31%) than in manure-amended soils (18–23%). Manuring increased the mass of macroaggregates and the specific activity of N-acetyl-glucosaminidase on a per unit SOC basis, which was negatively (P < 0.001) correlated with the specific respiration rate (on a per unit SOC basis) in the whole soil (three depths) and aggregates (plow layer only). Stable macroaggregates were associated with larger SOC stocks and greater organic nitrogen acquisition by microorganisms in the manure-amended soil. This appears to support the microbial nitrogen mining theory. In conclusion, long-term manure application increased soil aggregation as well as stability of the macroaggregate-associated organic carbon, which contributes to SOC sequestration in the Calcic Cambisols on the Loess Plateau.

Soil Organic Matter, Aggregates, and Microbial Characteristics of Intercropping Soybean under Straw Incorporation and N Input

Soil organic matter (SOM), soil aggregates, and soil microbes play key roles in agriculture soil fertility. In intercropping systems, the influences of straw incorporation and N input on the dynamics of soil physicochemical and microbial properties and their relationships are still unclear. We explore the changes in soil physicochemical and microbial properties with two straw managements, i.e., wheat straw incorporation (SI) and straw removal (SR), and four N supply rates for intercropped soybean, i.e., 60 (N60), 30 (N30), 15 (N15), and 0 (N0) kg N ha−1, in the wheat–maize–soybean relay strip intercropping systems. The results showed that SOM and SOM fractions contents, soil macroaggregate stability, and microbial and fungal α-diversity, e.g., Chao1 and Shannon indices, increased through straw incorporation and N input. The α-diversity was significantly positively correlated with soil physicochemical characteristics. Compared with SR, the relative abundance of ActinobacteriaandMortierellomycota in SI increased, but the relative abundance of Proteobacteria, Acidobacteria, and Ascomycota in SI decreased. In SI treatment, soil physicochemical characteristics and microbial diversity improved through N input, but that difference was not significant between N60 and N30. In conclusion, SI+N30 was the most effective way to maintain soil fertility and reduce the N fertilizer input in the wheat–maize–soybean relay strip intercropping.

Nitrogen addition decreases soil aggregation but enhances soil organic carbon stability in a temperate forest

Soil aggregates and the stability of their associated soil organic carbon (SOC) are important factors mediating soil carbon (C) sequestration and soil functions. However, the response of SOC stability to nitrogen (N) deposition is highly divergent, and the combined influences of N deposition and soil aggregates on SOC stability are poorly understood. The mechanisms underlying these influences were explored in a six-year field N-addition experiment covering a wide range of soil aggregates, root morphologies, soil properties and several SOC stability indices (represented by heterotrophic respiration and δ13C or δ15N) in a deciduous broad-leaved forest in Northeast China. The results showed that N addition significantly decreased the proportion of large macroaggregates (2–8 mm), reducing the soil aggregate mean weight diameter (MWD) by 10.0–17.2 %, which was negatively correlated with root length density (RLD) and root weight density (RWD). Organic C stability in soil aggregates was enhanced by N addition, as indicated by the decrease in the decomposition rate of organic C and the increase in the δ13C values but not the C content in microaggregates within macroaggregates (mM) or δ15N values. Furthermore, the promotion of mineral-associated organic C formation after N addition was detected by stable isotopic mixed model analysis (SIMM), indicating increased C protection by minerals. With the use of a structural equation model (SEM), the variation in the C stability of large macroaggregates (2–8 mm) was explained by the changes in fine roots, MWD and N availability but not those in small macroaggregates (0.25–2 mm) due to the instability of large macroaggregates. These results demonstrate that N addition may enhance soil C stability in all the soil aggregate sizes by promoting mineral sorption of C, with the C stability in large macroaggregates being more vulnerable to multiple environmental changes than that in small macroaggregates. Therefore, the soil C stability response to N deposition in the temperate forests of Northeast China could be modulated by soil aggregate sizes and may cause negative feedbacks to global warming.

Contrasting rice management systems – Site-specific effects on soil parameters

Conventional rice production systems (CRPS) with continuous flooding demand much water. While population growth increases the demand for rice and, consequently, water consumption, agricultural production needs to reduce its water demand. The System of Rice Intensification (SRI) is promoted as an alternative cropland management strategy to sustainably maintain rice yields while optimizing water use. Here, we aimed at investigating whether different management translates into differences in soil parameters. To this end, the two contrasting rice production systems were compared on the same soil types, at four different study sites of D.I. Yogyakarta Province, Indonesia. Crop yields were estimated, and soils were analysed for soil total soil organic carbon (TOC), total nitrogen (TN), dissolved organic carbon (DOC), macro-aggregate stability, and a fungal biomarker (ergosterol) indicative of oxidative soil conditions. Rice yields in the study area were between 6.7 and 9 t ha-1. For TOC, the combined effect of management and site was significant; in particular, in Kulonprogo and Bantul, SRI significantly exceeded CRPS’ TOC values. However, a significant management effect was observed for ergosterol and DOC concentrations. Significantly higher ergosterol concentrations in SRI vs CRPS were found in Sleman and Bantul. DOC was significantly higher under SRI compared to CRPS only in Sleman. DOC and ergosterol were most responsive to management and were improved in SRI systems. The observed site-specific effects suggest the importance to consider the prevailing site conditions for adapting management strategies.

Soil matrix infiltration characteristics in differently aged eucalyptus plantations in a southern subtropical area in China

In this study, we improved the method for measuring soil matrix infiltration by using a surface-positioned double-ring infiltrometer. The soil matrix infiltration processes of four differently aged (1, 3, 5, and 7 years) eucalyptus plantations were measured in situ in a southern subtropical area in China. Soil matrix infiltration characteristics (SMICs), including initial infiltration rate (IIR), stable infiltration rate (SIR), and cumulative infiltration (CI), were calculated using the experimental data of the soil matrix infiltration process. The physical and chemical properties of layered soil (0–60 cm with an interval of 20 cm), including soil textures, buck density (BD), soil organic matter (SOM), and soil water stable macroaggregate (WSMA) closely related to infiltration, were analyzed. Results showed that the IIR (140.33–233.14 mm h−1), SIR (116.24–194.04 mm h−1), and CI (118.42–217.20 mm) decreased with increasing forest age. In addition, the SIR and CI of the 1-year-old eucalyptus plantation were significantly higher than those of the other plantations. The decrease trend of SMICs with forest age was attributed to the soil properties changing in the eucalyptus plantations. The clay content, BD, SOM, and WSMA increased and sand content decreased with increasing forest age, thus inhibiting the SMICs. The results of stepwise regression analysis showed that the sand content of topsoil (0–20 cm) was the main factor affecting IIR, and the clay content of deep subsoil (40–60 cm) was the dominant factor affecting SIR and CI. The in situ soil matrix infiltration measurement in eucalyptus plantations was achieved with the improved method, and the main influencing factors were clarified. The results enhance understanding of soil matrix infiltration and its effect on precipitation redistribution and are helpful in quantifying the water storage capacity of plantation soil.

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

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

Effect of Long-Term Fertilization on Aggregate Size Distribution and Nutrient Accumulation in Aeolian Sandy Soil.

Soil aggregates are the material basis of soil structure and important carriers of nutrients. Long-term application of organic and inorganic fertilizers can affect the composition of soil aggregates to varying degrees, which in turn affects the distribution and storage of soil nutrients. We report the results of a 15-year long-term field-based test of aeolian sandy soil and used the wet sieve method to analyze the stability of water-stable aggregates, as well as the distribution characteristics of nutrients in different particle size aggregates. Our results show that long-term application of organic fertilizer (M3) and combined organic–inorganic treatments (NPK1-M1, NPK1-M2, and NPK1-M3) help to increase the amount of organic carbon, inorganic carbon, and cation exchange in the macro-aggregates, and the improvement rates are 92–103%, 8–28%, and 74–85%, respectively. The organic content of the fertilizers also promotes the formation of macro-aggregates, and the stability of aggregates increase from 0.24 to 0.45. In contrast, the application of inorganic fertilizers (NPK1, NPK2, and NPK3) has no marked effect on the formation and stability of macro-aggregates; the application of inorganic fertilizers can merely maintain the organic carbon content of the soil. Correlation analysis shows that the application of organic fertilizers and chemical (inorganic) fertilizers containing phosphorus and potassium can markedly increase the content and reserves of available phosphorus and potassium across all aggregate sizes, and there is a significant positive correlation between these parameters and the amount of applied fertilizer (p < 0.05). Aggregates of various sizes in aeolian sandy soils in arid areas have the potential for greater nutrient storage. Therefore, organic fertilizers can be used in the agricultural production process to improve soil structure and fertility.

Biochar enhanced soil aggregation and C‐related enzyme activity in post‐mining land on the Loess Plateau, China

AbstractSoil organic carbon in aggregates of different size classes varies greatly in stabilization and resistance, and its mineralization depends highly on enzyme activity within aggregates. However, the dynamics of enzyme activities in response to aggregate turnover remain unclear. In this study, we explored four soil enzymes within different aggregates (&gt;2, 2–0.25, 0.25–0.053, and &lt;0.053 mm) in a biochar‐addition (1%, 3%, and 5%, respectively) experiment in post‐mining land. Soil aggregate size had significant effects on β‐glucosidase (BG) and urease (UR) (p &lt; 0.01), and the enzyme activities were significantly higher in macroaggregates (&gt;0.25 mm) than in microaggregates (&lt;0.25 mm). Biochar addition increased the proportion and stability of macroaggregates. Moreover, BG, UR, and peroxidase activities were significantly stimulated across all aggregate fractions in treatment of 5% addition, increasing by 134.6%, 163.6%, and 35.7%, respectively (p &lt; 0.01), while alkaline phosphatase activity in macroaggregates decreased by 45.8% compared with that in the control. In macroaggregates, higher enzyme activities were closely correlated with the increase of easily oxidized carbon (p &lt; 0.01) due to biochar addition, and the increase in nitrogen content was also beneficial to promote microbial growth and activity. Our study suggests that biochar addition may improve soil structure and nutrient status, which would accelerate the recovery of microbial function in degraded lands.

Diversified cropping systems benefit soil carbon and nitrogen stocks by increasing aggregate stability: Results of three fractionation methods

Understanding carbon (C) and nitrogen (N) sequestration in diversified cropping systems provides a pivotal insight for soil health management. Here, the soil was sampled from an ongoing field experiment (five years) with three cropping systems: i) winter wheat/summer maize, ii) winter wheat/summer maize-early soybean, and iii) fallow. We evaluated C and N stocks in aggregates for topsoil (0–20 cm) and subsoil (20–40 cm) depending on cropping systems by comparison of three aggregate fractionation methods (dry, optimal-moisture, and wet sieving). Although the fertilizer application rate for wheat/maize was twice as much as for wheat/maize-soybean, this resulted in similar C and N stocks in the topsoil. The N stock, however, was 13% higher under wheat/maize-soybean than under wheat/maize in the subsoil due to N2 fixation by soybean. The C and N stocks decreased by 22% and 12% under fallow compared to wheat/maize in the topsoil. The wheat/maize-soybean cropping system increased soil aggregates size when estimated by dry and optimal-moisture fractionations. The aggregate size distribution shifted from the dominance of large (> 2 mm) toward small macroaggregates (0.25–2 mm) with increasing moisture used by fractionation due to the low stability of large macroaggregates. Thus, the combination of dry and optimal-moisture sieving is the preferred method to characterize aggregate stability. Overall, diversified cropping systems increase soil aggregation and stability, thus have great potential to enhance soil C and N stocks.

Microscale heterogeneity of soil bacterial communities under long-term fertilizations in fluvo-aquic soils

Differently sized soil aggregates, with non-uniform distribution of space and nutrients, provide spatially heterogeneous microenvironments for microorganisms and are important for controlling microbial community ecology and biogeochemistry in soils. Here, we investigated the prokaryotic communities within different aggregate-size fractions: macroaggregate (>0.25 mm), microaggregate (0.053–0.25 mm) and silt + clay (< 0.053 mm). These were isolated from fluvo-aquic soils under 39-year fertilization strategies: no fertilizer (CK), chemical fertilizer (NPK), manure fertilizer (M), and combination of manure and chemical fertilizers (MNPK). The results showed that the proportion of macroaggregate, soil aggregate-associated organic carbon (SOC) content and aggregate stability were all significantly increased by both manure and chemical fertilizations. Organic fertilizations (M and MNPK) more effectively boosted formation and stability of macroaggregates and enhanced SOC concentration than NPK. The distribution patterns of microorganisms in aggregates were primarily shaped by fertilization and aggregate size. They explained 76.9% of the variance in bacterial community compositions. Fertilizations, especially with organic fertilizers primarily transitioned bacterial communities from slow-growing oligotrophic groups (e.g., Chloroflexi) dominance to fast-growing copiotrophic groups (e.g., Proteobacteria and Bacteroidetes) dominance across all aggregate sizes. Macroaggregates possessed a more stable bacterial community and efficiency of resource transfer, while smaller aggregates increased antagonism and weakened mutualism among bacterial communities. Overall, combination of manure and chemical fertilizers was crucial for increasing SOC content and aggregation, leading to a clear shift in bacterial community structures at aggregate scale.

Green manuring and crop residue management: Effect on soil organic carbon stock, aggregation, and system productivity in the foothills of Eastern Himalaya (India)

Improving soil quality, especially the organic carbon (C) accumulation and soil aggregation through economically and environmentally sustainable alternatives, is the need for hill agriculture. A five-year (2012–13 to 2016–17) field experiment was conducted with four green manure treatments [green gram (Vigna radiata); cowpea (Vigna unguiculata); sesbania (Sesbania aculeata) along with non-green manuring] and three cropping systems [groundnut (Arachis hypogaea) –pea (Pisum sativum); maize (Zea mays) –pea, and maize + groundnut – pea] involving two levels of residue management practices [residue removal and residue retention]. The experiment was laid out in a split–split plot design and replicated three times. After five years of experimentation, the results showed that among green manure treatments, Sesbania had a significant effect (p < 0.05) on total soil organic C, C pool, and surface (0–0.15 m) soil aggregation. The increase in C pool (p < 0.05) in Sesbania-treated soil was 43.2% in the very labile C, followed by 40.8% in the labile C relative to those without green-manure. In addition, the profile (0–0.45 m depth) weighted average organic C stocks in Sesbania-treated soils were higher in active (+29.3%), passive (+20.8%), and total (TOC: + 24.1%) forms than in non-green manure soils. Sesbania-treated soils also exhibited a higher C-Lability index, water stable macro-aggregates, and a higher aggregate ratio (AR) than non-green manure soils. Among the three cropping systems studied, inter-cropping of maize and groundnut followed by peas increased (p < 0.05) the stock of C pools, water-stable aggregates, and ARs in soil relative to the other two cropping systems. Residue retention also increased C pools, C-Lability Index, and microbial biomass C in soil compared to residue removal. Therefore, the incorporation of Sesbania into maize+groundnut-pea cropping systems with residue retention is a recommended option to improve soil quality and productivity of maize cropping systems in the Indian Himalayan region.

Microplastic effects on soil system parameters: a meta-analysis study.

Microplastics are generally considered as an emerging contaminant in the environment due to their toxic additives and transport of other contaminants. However, the potential threats of microplastics in soil should be concerned due to inconsistent research results. In this study, a meta-analysis based on 32 recent relevant studies was conducted to compare the response of soil system parameters including microbial community, aggregate structure, soil nutrient contents, and crop growth to the presence of microplastics. The results showed that microplastics presented no significant effects on soil dissolved organic carbon contents and the amounts of available phosphate, nitrate, and ammonium. Although microplastics would not significantly influence the diversity of soil microorganisms, they could significantly increase soil microorganism amounts with a standard mean difference at 19.32. We also found that microplastics tended to significantly decrease soil water stable macro-aggregate (> 0.25mm) contents with a significantly negative standard mean difference (- 0.90) in meta-analysis. Moreover, soil microplastics seemed not to affect crop growth by having non-significant effects on both crop under-ground and above-ground biomasses. These results indicate that up to date, the main negative impacts caused by microplastics on soil systems could be their negative functions on soil aggregation.

Soil organic matter fractions in an Oxisol under tillage systems and winter cover crops for 26 years in the Brazilian subtropics

The improvement of carbon (C) accumulation in soils has been one of the main purposes of the conservation systems in agricultural production. This study aimed to assess the long-term effect of conventional tillage (CT) and no-tillage (NT) combined with winter cover crops, black oat and oilseed radish, and fallow on C accumulation and stabilization in a very clayey Oxisol in Southern Brazil. Soil samples were collected in the 0-0.05, 0.05-0.10 and 0.10-0.20 m layers of a 26-year-old experiment. Distribution of size-class aggregates, C stock in aggregates, total C stock, and C stocks in the physical fractions, free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM) and mineral-associated organic matter (min-OM) were assessed. NT had a higher percentage of macroaggregates and C stock in this size-class, and also higher C stock in bulk soil, free-POM and occluded-POM fractions than CT in 0-0.05 m (Tukey’s test p < 0.05), due to higher input of biomass and minimum soil mobilization in NT. Oat and radish had higher C stock in macroaggregates than fallow in 0.05-0.10 m (Tukey’s test p < 0.05). Radish had the highest C stock in the free-POM (0-0.05 m). Fallow decreased the stabilization of macroaggregates and C accumulation in free-POM, due to the lower C input from aboveground biomass over the years. In conclusion, NT after 26 years improved C accumulation and stabilization, mainly in the superficial layer and in POM fractions, and winter cover crops favored the formation and stability of macroaggregates.

Effects of saline water mulched drip irrigation on cotton yield and soil quality in the North China Plain

The shortage of freshwater resources is a considerable challenge for agricultural production in the North China Plain (NCP). Safe and efficient use of saline water resources is thus urgently required. To reveal the effects of different salinities of irrigation water on the yield and soil quality of mulched drip-irrigated cotton, a field experiment was conducted from 2017 to 2019. Five salinity levels of irrigation water were included: 1.3 (T1, control), 5.4 (T2), 8.8 (T3), 12.4 (T4) and 15.9 (T5) dS·m−1. The results showed that the harvesting density and seed cotton yield increased first and then decreased with increasing salinity of irrigation water. Saline water irrigation with salinity ≤ 8.8 dS·m−1 did not lead to salt accumulation in the main root zone (0–60 cm) with each passing year but a decrease of salt in 2018. In the third year of saline water irrigation, with the increase in irrigation water salinity, the electrical conductivity of the saturated soil extract (ECe), soil sodium adsorption ratio (SAR), pH and bulk density (BD) in the plow layer gradually increased. However, the soil saturated hydraulic conductivity (Ks), water stable macroaggregate (> 0.25 mm) content, catalase (CAT) and urease (URE) activity decreased with increasing salinity. Moreover, the soil organic carbon (SOC) content and alkaline phosphatase (ALP) activity increased first and then decreased. Irrigation water salinity ≤ 5.4 dS·m−1 had no significant effect on most physicochemical properties, such as pH, SOC content, BD, Ks, water stable aggregate content and activities of CAT, URE and ALP. Compared with the T1 treatment, the soil quality index (SQI) of T2, T3, T4 and T5 treatments decreased by 1.2%, 10.5%, 16.5% and 23.7%, respectively. Considering cotton yield, soil salt accumulation and SQI, mulched drip irrigation is conducive to the sustainability of cotton with saline water levels below 5.4 dS·m−1.