Soil Aggregate Stability Research Articles

Interaction of pyrolysis conditions and soil texture on biochar mineralization and its effect on soil structure.

<p>The effectiveness of biochar as a soil conditioner is depended on the feedstock type and pyrolysis conditions as these two factors determine its physical and chemical properties. Wheat straw was heated at two temperatures: a) 2500C and b) 5000C for two time periods: i)20 min and ii) 60 min to produce four types of wheat biochar (WB) (WB250/20, WB250/60, WB500/20 and WB500/60) that were added at two different textured soils, a sandy and a loamy one. We studied C mineralization and changes of the structural quality of the two soils. Incomplete carbonization of WB250 resulted in higher C mineralization in both soils. WB250 decomposed more intensely in the sandy soil while decomposition of WB500 was not affected by soil texture or duration of pyrolysis. Biochar addition reduced the cohesiveness of the loamy soil. WB500 enhanced the formation of smaller aggregates while pyrolysis time had no effect. None of the four types of biochar altered the aggregate size distribution of the no cohesive sandy soil. Biochar with longer pyrolysis time enhanced aggregate stability of both soils because of its higher C contents and EC that promote aggregating mechanisms. WB500/60 resulted in reduced clay dispersion in both soils.</p>

Structure and stability characteristics of zonal soil aggregates in the Three Rivers Source of the Qinghai‐Tibetan Plateau

AbstractThis study aimed to explore the structure and stability characteristics of zonal soil aggregates in cold high‐altitude regions and reveal the variation patterns of alpine soil aggregates, using the Three Rivers Source of the Qinghai‐Tibetan Plateau as an example. Zonal soils representing the local vegetation types (alpine meadow soil, alpine grassland soil) were collected, and soil aggregates were separated using wet and dry sieving methods. Random forest modeling was used with climate data from 2011 to 2019 as variables in order to generate multifactor digital maps of water‐stable and mechanically stable aggregates. The composition and differences of zone‐specific soil aggregates were compared and analyzed using the evaluation indices of macroaggregate content (R > 0.25), mean weight diameter (MWD), geometric mean diameter, and fractal dimensions. Their controlling factors were also explored. The study results showed that the model's explanatory power for soil aggregates was over 68%. In the random forest model, elevation and sunshine duration contributed more to soil water‐stable aggregates, whereas precipitation contributed more to soil mechanically stable aggregates. The content of large aggregates with particle size greater than 0.5 mm was higher in alpine meadow soils than in alpine grassland soils. In contrast, the content of large aggregates with particle size less than 0.5 mm was lower than that of alpine grassland soils. There are also some differences in the distribution of water‐stable aggregates and mechanically stable aggregates between alpine meadow soils and alpine grassland soils in each particle size, and these differences are most pronounced in the particle sizes >2 and <0.25 mm. In addition, the stability of alpine meadow soil aggregates is higher than that of alpine grassland soil aggregates. Finally, the mapping results show that the stability of soil aggregates in the study area has similar zonal characteristics to the zonal variation of vegetation cover and climate and other factors.

Aggregate-Associated Carbon Fractions Under Forty-Eight Years of Fertilization in North-Western Himalayas

ABSTRACT The present investigation was carried out in an on-going long-term fertilizer experiment, initiated during 1972 at experimental farm of Department of Soil Science, CSK HPKV Palampur, to study the long-term effect of fertilizers and amendments on soil aggregate stability and carbon dynamics in an acid Alfisol under maize-wheat cropping system. The experiment comprised of eleven treatments. The soil of the experimental site was silt loam and classified taxonomically as “Typic Hapludalf” (subgroup). Soil samples were collected from surface (0–0.15 m) layer after the harvest of wheat (rabi, 2019–2020) and were analyzed for different aggerate size and their carbon fractions. The addition of FYM enhanced the macroaggregates (>0.25 mm) as compared to microaggregates (<0.25 mm). Incorporation of FYM alone or in combination with chemical fertilizers enhanced the accumulation of different C pools in all aggregate fractions over unfertilized control. Among the macroaggregates, 1–0.5 mm fraction had higher total organic carbon (TOC), organic carbon (OC), labile carbon (LC) and non-labile carbon (NLC) as compared to microaggregates. Aggregate-associated carbon is less susceptible to physical, microbiological, and enzymatic degradation, and is a potential carbon reservoir that is protected from mineralization. Addition of FYM along with 100% NPK recorded highest SOC stocks, SOC sequestration rate and highest carbon buildup. Different aggregate fractions of TOC, OC, LC and NLC were found to be positively and significantly correlated with available N, P and K and negatively and significantly correlated with bulk density whereas, positively and non-significantly correlated with soil pH and electrical conductivity. Different aggregate fractions of TOC, OC, LC and NLC were positively and significantly correlated with grain and straw yield of wheat.

Effects of Fractal Dimension and Soil Erodibility on Soil Quality in an Erodible Region: A Case Study from Karst Mountainous Areas

Soil aggregate stability and soil erodibility (k) are crucial indicators of soil quality that exhibit high sensitivity to changes in soil function. Therefore, it is of great significance to explore the quantitative relationship between these indicators and soil quality for effective ecosystem monitoring and assessment. In this study, soil samples were collected from eight altitude gradients in a karst mountainous area; we analyzed 11 soil physical, chemical, and biological properties, and assessed soil quality using the minimum data set (MDS) method. The results revealed that soil aggregate stability, bulk density (BD), pH, and fungal community diversity exhibited a unimodal altitudinal pattern, whereas the soil organic carbon (SOC), total nitrogen (TN), and C:N ratio showed an increasing trend. Among the factors considered, SOC, BD, soil pH, mechanical composition, and fungal community diversity were found to explain the most variation in soil aggregate stability and soil erodibility (k). Principal component analysis (PCA) identified soil fungal community diversity, C:N ratio, coarse sand, and macro-aggregate (MA) content as highly weighted indicators for MDS. The integrated soil quality index (SQI) values, ranging from 0.30 to 0.62 across the eight altitude gradients, also exhibited a unimodal altitudinal pattern. The analysis indicated a significant linear relationship between the fractal dimension (D) and soil erodibility of the EPIC model (Kepic) with SQI, suggesting that D and Kepic can serve as alternative indicators for soil quality. These findings further enhance our understanding of the response of soil properties to altitude changes, and provide a novel method for assessing and monitoring soil quality in karst mountainous areas.

Grazing Decreases Soil Aggregation and Has Different Effects on Soil Organic Carbon Storage across Different Grassland Types in Northern Xinjiang, China

Soil aggregates, as the basic component of soil, make great contributions to the stability of soil structure and soil carbon (C) sequestration. Recently, grasslands have been experiencing continuous grazing, which has had a significant impact on soil aggregation and soil C storage. However, how soil aggregates and soil C in different grasslands respond to grazing remains unclear. Therefore, three national fenced grassland-monitoring sites that represented mountain meadow (MM), temperate steppe (TS), and temperate steppe desert (TSD) were selected to investigate the differences in the responses of soil aggregates and soil C among grazing of different types of grasslands. Soil samples of 0–10 cm was collected from both inside and outside the fence of each site to analyze soil properties and soil aggregate characteristics. The results showed that soil nutrients varied greatly among the three grassland types, with the highest values in MM. At each site, grazing increased the content of sand and decreased the contents of silt and clay compared to fenced plots. Soil aggregate composition showed significant responses to both grassland type and grazing, especially the proportions of soil aggregates &gt;2 mm, which significantly decreased by 51.7% on average in grazing plots compared with fenced plots. A significant decrease (on average, 25.1%) in the mean weight diameter (MWD) of soil aggregates under grazing was detected across all grassland types. The effect of grazing on nutrients in macroaggregates (&gt;0.25 mm) was greater than that in microaggregates (&lt;0.25 mm). Aggregate-associated SOC concentration decreased under grazing in MM and TS. However, grazing had no significant influence on the SOC density of MM, while it led to a significant decrease in TS and an increase in TSD. The magnitude of grazing effect size on aggregate-associated SOC varied with different soil particle sizes, with greater responses in aggregates &gt;2 mm and the biggest value in TDS. In addition, the results of the correlation analysis and redundancy analysis (PDA) indicated that soil bulk density and nutrients made the main contribution to soil composition and stability of soil aggregates. Overall, grazing had a significant influence on soil aggregation, stability, and SOC, playing a crucial role in grassland soil stability and the accumulation of SOC.

Effect of using Agrogeotextiles on soil carbon sequestration in the Indian Himalayas

Agrogeotextiles (AGT) have potential for soil conservation, but limited information is available on effect of AGT on C sequestration and soil aggregation in Indian Himalayan region (IHR). Hence, the study was conducted on a 4% slope at Selakui, Dehradun in a maize-based cropping system where Arundo donax mats were used as AGT mats for soil conservation. Soil sampling was done in December 2021 after vegetable pea harvest and results indicated that in 0–15 cm soil depth, Maize + Arundo donax mat (10 cm thick) on 0.5 m vertical interval vegetable pea - wheat (M+A10D0.5-V-W) had ~23% higher total soil organic C (TSOC) in bulk soils than the control (M-W) plots. Plots with Arundo donax mats exhibited ~12% higher TSOC than plots without the mats. Plots under M+A10D0.5-V-W and maize-vegetable pea-wheat under bench terracing (M-V-W)B showed similar impacts on C sequestration. M+A10D0.5-V-W plots had ~36% greater macroaggregate and ~35% higher mean weight diameter (MWD) than M-W plots in the 0–15 cm soil depth. Microbial biomass C (MBC) enhanced by ~86% under M+A10D0.5-V-W over M-W in the 0–15 cm soil depth due to higher root biomass, root exudates and metabolizable C. Thus, microbial quotient (MQ) was also increased. Mean annual soil loss value of 2020–2021 and 2021–2022 ranged from 0.4 t/ha/yr to 2.9 t/ha/yr which was correlated with carbon management index (CMI) value. The CMI was ~38% higher in M+A10D0.5-V-W plots than M-W due to emplacement of Arundo donax mats, resulting in better soil aggregate stability. Thus, AGT application can be a potential practice for soil aggregation stability and C sequestration in the IHR. Conservation soil practices along with AGT are more profitable than conservation soil practices alone which had ~10% less benefit : cost ratio (B:C) on 4% land slopes of IHR.

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.

Role of Bacterial Inoculant and Organic Matter in Improving some Soil Physical Properties and Water use Efficiency of Maize under Drought Conditions in Iraq

Field experiment was carried out at the Agricultural Research Directorate Research Station, Ministry of Science and Technology in Autumn 2018 to study the role of liquid bacterial inoculant and addition of organic matter in improving physical properties, water consumption and water use efficiency of the maize crop (Zea mays L.) under drought conditions prevailing in Iraq. Bacterial inoculated was added with two levels with, without adding, adding organic fertilizer at three levels (0, 1.5 and 3%) of the soil weight. Irrigation was scheduled when depleting (50, 60 and 70%) of the available water. Water consumption were 584, 437 and 317 mm under (50, 60 and 70%) of available water depletion respectively. The addition of bacterial inoculant and 3% organic matter was significantly superior to treatments at 50 and 60% of available water depletion. Saturated hydraulic conductivity and soil aggregate stability under these two treatments were 1.81 and 1.75 cm h−1 and 0.530 and 0.524% respectively. The grain yield was 12.321 and 12.162 t ha−1 under the above-mentioned treatments respectively. The highest rate of water use efficiency was 2.98 kg m−3 under 60 % moisture depletion, with bacterial inoculant and under addition of organic matter at the level of 3%.

Effect of Super Absorbent Polymer and Ceratophyllum Powder Application on Some Soil Physical Properties

A two factorial laboratory experiment was conducted according to randomized complete block design with three replications, to investigate the effect of super absorbent polymer (SAP) and ceratophyllum powder application on some physical characteristics of loamy sand soil. The first factor included three levels of super absorbent polymer, namely 0,0.2, and 0.4 %. While the second factor consists of three levels of ceratophyllum powder i.e., 0,2, and 4 %. The treated soil (treated with super absorbent polymer and ceratophyllum powder) was exposed to 14 cycles of wetting and drying. Effect of experiments factors on coefficient of linear extensibility, soil bulk density and total porosity were measured. The obtained results show that the super absorbent polymer and ceratophyllum powder application led to increase the dry soil bulk density, total porosity, and the coefficient of linear extensibility values for all application levels. Regarding the soil aggregate stability, a significant increase was observed under effect of super absorbent polymer and ceratophyllum powder application. Where the highest value of soil aggregate stability was recorded at 0.4% level of SAP reached 54.2%. Likewise, the highest value of soil aggregate stability under effect of ceratophyllum powder application reached 30.7% at 4% level of application, compare to the control treatment that gave 11.1 % with increasing rate reached 76.5%. In same direction, all moisture content had increased under effect of study factors. However, the super absorbent polymer was superior to ceratophyllum powder application.

Phosphatase activities and available nutrients in soil aggregates affected by straw returning to a calcareous soil under the maize–wheat cropping system

The objective of this study was to investigate the effects of different rates of straw returning on soil aggregate stability, phosphatase activities, and the available nitrogen (N) and phosphorus (P) within different soil aggregate sizes. The experiment included five treatments: 1) no straw returning and no chemical fertilizer, 2) chemical fertilizer only (150 kg N ha-1, 75 kg P ha-1, and 75 kg K ha-1), 3) 20% straw returning with chemical fertilizer, 4) 60% straw returning with chemical fertilizer, and 5) 100% straw returning with chemical fertilizer. Soil samples were collected 3.5 years after the start of the experiment and separated into four aggregate sizes (&amp;lt;0.25 mm, 0.25–1 mm, 1–2 mm, and 2–7 mm) using the dry sieving method. Soil acid phosphomonoesterase (AcP) and alkaline phosphomonoesterase (AlP); phosphodiesterase (PD); pyrophosphatase (PrA) activities; and soil NO3−−N, NH4+−N, and resin-P were determined within soil aggregates. The results showed that straw returning rates did not significantly impact soil aggregate distribution. However, straw returning increased soil AcP, AlP, and PD in &amp;lt;2 mm aggregates, and high rates of straw returning led to high enzyme activities. Soil phosphatase activities were also higher in 1–2 mm aggregates. All straw returning and chemical fertilization treatments increased soil NO3−−N and resin-P concentrations but had much less effect on soil NH4+−N concentrations. Additionally, the study revealed that soil pH, the concentrations of NH4+−N, NO3−−N, resin-P, and CaCO3 significantly influenced soil phosphatase activities, but their impact varied across different sizes of aggregates.

The effect of silvicultural systems on soil function depends on bedrock geology and altitude

Soil quality and function in forest environments are influenced by the interaction of soil-forming parameters and silvicultural systems. Hyrcanian forests were recently accepted as a UNESCO World Heritage Site, which extends across an area of approximately 1.8 million hectares and ascend to an elevation of 2800 m above sea level (m.a.s.l). In these woodlands, Oriental Beech (Fagus orientalis Lipsky) is the predominant tree species and could be observed at 700–1500 m.a.s.l., and occur on different parent rocks. Shelterwood and single-tree selection techniques have been the primary management methods for beech forests for the past forty years. Studies investigating the impacts of silvicultural systems have not yet been done on soil and forest floor features on different bedrock geology and altitudes. Therefore, in this study, we examined the influence of single-tree selection and shelterwood methods, 25 years after employing those methods, on soil quality and function compared to control areas (intact forests) in Hyrcanian beech stands. For this purpose, 15 forest floor (30 × 30 cm) and topsoil (0–10 cm depth) samples in each silvicultural systems (i.e., single-tree selection and shelterwood methods and control zones) × 4 regions (including Rasht, Nowshahr, Sari and Gorgan) × 4 altitude levels (with averages of 800, 1000, 1200 and 1400 m.a.s.l.) were considered. According to our findings, the investigated forest regions, forest floor and soil characteristics across various locations spots could be separated by principal component analysis output, and more than 85% of the variance was explained by the first and second axes. The structural equation model showed that the region, altitude and silvicultural systems had an effective role in the changes in soil biological activities by influencing the forest floor, and the soil physicochemical features. Based upon the network model, the C/N ratio, the sand content, the soil aggregate stability, the available K, the fulvic acid, and the Acarina density were found to be prominent factors with regard to soil function. In the control sites, increased soil organic material fractions, microbial/enzyme and biota activities were detected, particularly at the lower altitudes of the Nowshahr site, which had geological formations of dolomite and calcic layers. Taken together, it seems that the single-tree method, commonly referred to as the close-to-nature technique produces more suitable conditions for soil functioning compared to the shelterwood management approach. Silvicultural systems, bedrock geology and altitude can have major detrimental effects on soil function and fertility, over the long-term, impacts may increase with harvest intensity.

Estimating the indices of soil erodibility to wind erosion using pedo- and spectro-transfer functions in calcareous soils

Soil aggregate size and stability are two important factors affecting soil erodibility to wind erosion. In this study, we developed new models to quantify soil erodibility to wind erosion by looking at the mean weighted aggregate diameter (MWD) and the wind erodible fraction (EF) of the soil. These two erodibility indices together with the spectral reflection of soil in the range of Vis– NIR (400–2500 nm) were measured in 511 soil samples. Pedo-transfer functions (PTF) and Spectro-transfer functions (STF) were built using the Multiple Linear Regression (MLR), Partial Least Squares Regression (PLSR), and Support Vector Regression (SVR), considering train(70%) and test (30%) datasets. Result showed that shear strength (SS), organic matter (OM), penetration resistance (PR), and clay content had a significant coefficient (p < 0.05) for predicting MWD and EF indices. Vis– NIR spectroscopy method performed better than PTF method and wavelengths of 513, 773, 872, 1256, 1414, 1488, 1908, 2042, 2210, and 2311 nm were introduced as the key wavelengths for the estimation of soil erodibility indices based on the regression coefficient. Results of the predictive models revealed that SVR outperformed (RMSE = 0.12 mm, RPIQ = 2.56 for MWD; and RMSE = 9.32%, RPIQ = 2.24 for EF) PLSR (RMSE = 0.12 mm, RPIQ = 1.98 for MWD; and RMSE = 9.40%, RPIQ = 1.84 for EF). This study proved that Vis– NIR spectroscopy is a promising method for the prediction of MWD and EF indices as soil erodibility indices.

Warming-dominated climate change impacts on soil organic carbon fractions and aggregate stability in Mollisols

Mollisols contain high amounts of soil organic carbon (SOC), which is highly susceptible to climate change; thus, climate change could indirectly influence soil aggregate stability, but the dominant factor affecting aggregate stability remains controversial. Here, a soil transplanting test from high-latitude to low-latitude locations was initiated in 2004 to investigate the influences of warming-dominated climate change (approximately 3–4.7 ℃) on the quantity and molecular composition of OC fractions in surface (0–20 cm) soils, aggregate stability changes and underlying mechanisms. Different initial soil organic matter (SOM) contents of 50.6 g kg−1 (SOM5), 58.8 g kg−1 (SOM6), and 108.9 g kg−1 (SOM11) were established in situ soils and in transplanted soils to simulate warming. The 15-year warming-dominated climate change presented no noticeable change in the SOC content in the lower SOM Mollisols (SOM5 and SOM6) but increased the SOC content by 13.3% in the higher SOM Mollisol (SOM11). In terms of labile OC fractions, warming-dominated climate change significantly increased the dissolved organic carbon (DOC) content by 20.1%–47.7% but reduced the easily oxidizable organic carbon (EOC) and microbial biomass carbon (MBC) contents by 22.1%–33.6%. Irrespective of any treatment, warming-dominated climate change decreased soil aggregate stability, as evidenced by the reduction in mean weight diameter (MWD) and geometric mean diameter (GMD) of 41.7%–79.3% and an increase in fractal dimension (D) of 28.6%–58.5%. For hierarchically organized soil aggregates, warming-dominated climate change increased the proportion and OC content of particulate organic matter inside free microaggregates (Fm-POM) as well as nonaggregated silt + clay-sized organic matter (nA-MOM). However, climate change decreased the proportion and OC content of silt + clay-sized fractions inside microaggregates within macroaggregates (mM-MOM). Of importance, warming-dominated climate change increased the amount of carbohydrates and decreased the amount of lignin in the mM-MOM, Fm-POM, and nA-MOM fractions. Therefore, we speculated that chemical protection by mineral association within macroaggregates and nA-MOM fractions and physical protection by the occlusion of POM within free microaggregates might be the primary mechanisms for SOC stabilization under long-term climate change in Mollisols. The long-term warming-dominated climate change results demonstrated a positive effect on SOC content in higher SOM Mollisols, DOC, carbohydrate C, free microaggregate-associated C and nonaggregated silt + clay-sized associated C but exhibited a negative effect on EOC, MBC, lignin C, silt + clay-sized inside microaggregates within macroaggregates C and aggregate stability. These variables all contributed to the reduction in soil aggregate stability and might act as sensitive indicators of warming-dominated climate change in Mollisols, which in turn affect farmland ecosystem C fluxes in response to further climate change.

Effects of reduced tillage with stubble remaining and nitrogen application on soil aggregation, soil organic carbon and grain yield in maize-wheat rotation system

Global climate change has threatened stable food production and soil carbon storage. Whether reduced tillage with stubble remaining (RTS) and nitrogen (N) application improve soil carbon storage and grain yield simultaneously and its impact on soil aggregation and soil organic carbon (SOC) fractions are still controversial. A 4-year experiment, including four treatments (CT, rotary tillage with stubble removing; RTS; CT+N; RTS+N), was conducted to explore the effects of RTS and N application on soil aggregates and soil organic carbon fractions, and answer whether the combination of RTS and N application can simultaneously improve soil carbon storage, crop yield and water productivity in summer maize-winter wheat rotation (MWR) system. The results showed that soil aggregates stability and SOC content of RTS+N increased by 34.9% and 9.2% due to increased proportion of large macroaggregate (LM, increased by 6.6%) and small macroaggregate (SM, increased by 6.9%), respectively, compared to CT+N at the end of the 4-year experiment. RTS significantly increased particular organic carbon (POC) content, and N application significantly increased the mineral-associated organic carbon (MAOC) content. Compared with CT, the POC, MAOC and POC/MAOC increased by 42.7%, 18.7% and 20.4% in bulk soil, increased by 19.6%, 8.5% and 10.5% in LM, increased by 33.2%, 14.2% and 17.1% in SM, increased by 25.9%, 16.8% and 8.3% in microaggregate, respectively, under RTS+N. Meanwhile, the mean grain yield, yield stability and water productivity of RTS+N increased by 33.7%, 206.4% and 43.4% for summer maize, and increased by 7.0%, 29.3% and 3.0% for winter wheat compared with CT+N. Therefore, reduced tillage with stubble remaining combined with nitrogen application in semi humid and drought-prone areas can simultaneously increase soil carbon storage, grain yield and water productivity in the MWR system.

Physically protected organic matter drives soil carbon sequestration potential of a managed grassland ecosystem in Italian Alps

Mountain managed grasslands are characterized by a high biodiversity and are capable to store a considerable amount of carbon (C) in the soil. In this study, we evaluated the chemical characteristics of physically protected organic matter and its contribution to the soil C stock potential in a subalpine managed grassland ecosystem, combining low-temperature ashing (LTA), stable isotope ratio mass spectrometry (IRMS) and infrared photoacoustic spectroscopy (PAS). The physically protected C fraction, evaluated through LTA by oxygen plasma, amounted ∼74% of the whole soil organic C (SOC) in the first 15 cm depth. This C fraction was closely linked to mineral components in stable aggregates of ∼587 μm mean weight diameter. The increase of both C and nitrogen (N) stable isotope composition (δ13C; δ15N) in the physically protected organic matter, concomitant with a decrease of its C/N ratio, suggests higher microbial transformation degree, age and stability against decomposition of protected organic fraction compared to unprotected one. However, also the younger and less protected C fraction acted as cementing organic labile agent, creating larger but less stable soil aggregates. The infrared photoacoustic spectroscopy showed that the physically protected C fraction was characterized by a more marked presence of phenolic compounds and a reduced amount of proteinaceous and polysaccharides-like materials compared to the more labile and younger organic fraction. These results highlight that managed grasslands can effectively sequester large amount of long-residence C into the soil, promoting aggregate stability and soil fertility, and playing a key role for mitigation strategies against climate change.

Phosphorus-mediated succession of microbial nitrogen, carbon, and sulfur functions in rice-driven saline-alkali soil remediation

Although rice cultivation holds potential for restoring unproductive saline-alkali soils and increasing food production, the mechanisms underlying the relationship between microbial functions and soil element turnover remain unclear. To clarify this relationship, this study investigated the soil physicochemical properties and microbial functions during remediation in saline-alkali soil by rice cultivation over 2, 4, 6, 8, 11, 12, 20, and 23 years. The results indicated rice cultivation markedly improved soil nutrients, soil nutrient stoichiometry, and soil aggregate stability. Additionally, rice cultivation significantly increased the microbial functions involved in nutrient cycling, such as nitrogen fixation, carbon fixation, methanogenesis, dissimilatory sulfate reduction, and thiosulfate oxidation. However, these nitrogen (N), carbon (C), and sulfur (S) cycle-related functions exhibited a similar “increase-peak-decrease” successional pattern with the years of remediation, reaching optimal levels when rice was continuously grown for 11–16 years. Furthermore, correlation analysis demonstrated that the succession of soil microbial N, C, and S functions during saline-alkali soil restoration closely related to changes in soil properties, particularly the availability of phosphorus (P). Therefore, we propose to prioritize the management of P during saline-alkali soil remediation. In conclusion, this study provides a comprehensive understanding of the microbial N, C, and S functions and soil P in the remediation of saline-alkali soils mediated by rice crop.

Responses of Soil Labile Organic Carbon on Aggregate Stability across Different Collapsing-Gully Erosion Positions from Acric Ferralsols of South China

Soil labile organic carbon (LOC) is a valuable and sensitive parameter of the changes in soil carbon (C) pools and further affects soil structural stability. However, the influences of soil-aggregate stability on LOC fractions under erosion conditions are still unclear, especially under the collapsing gully area of south China. Soils of five positions of collapsing gully erosion, including the upper catchment (UC), collapsing wall (CW), colluvial deposit (CD), scour channel (SC) and alluvial fan (AF) from Acric Ferralsols were investigated and sampled. Soil aggregate stability and LOC fractions were measured and analyzed. Soil water-stable aggregate and passive C (passive-C) contents significantly increased by 67–76% and 8.7–13.0% at the UC, CW, CD and SC positions, respectively, while soil labile C (labile-C) content was lower at these positions as compared to the AF position (p &lt; 0.05). Moreover, the UC position’s soil C pool management index (CPMI) significantly increased by 37–40% compared to CW, CD, SC and AF soils, indicating that the soil of the UC position had a more stable C pool due to its stronger structural stability. SOC, silt, and amorphous iron oxide (Fea) contents significantly contributed to aggregate stability. We demonstrated that the depletion of soil aggregate stability could result in the decreases in soil LOC fractions, while soil properties of the OC but not the LOC pool regulated aggregate stability and thus affected soil structure across different collapsing gully erosion positions in the subtropical Acric Ferralsols region of south China. This study contributes to developing strategies to prevent soil erosion and improve global C cycle and soil quality, which could be beneficial to strengthen soil and water conservation, and improve soil fertility (e.g., SOC) and vegetation recovery, such as tea and tobacco.

The potential for soil C sequestration and N fixation under different planting patterns depends on the carbon and nitrogen content and stability of soil aggregates

The winter wheat-summer maize rotation system is common in the Huang-Huai-Hai Plain due to its consistent yield, however, it may cause soil quality degradation and increased risk of greenhouse gas emissions. To evaluate the effects of different planting patterns on soil organic carbon (SOC) and total nitrogen (TN) sequestration, as well as aggregate and C-N distribution, a three-year field experiment that included three annual double-cropping rotation patterns: winter wheat-maize (W-M), winter wheat-soybean (W-S), and winter wheat-sweet potato (W-SP) was conducted from 2020 to 2022, with W-M as the control. Our research revealed significant differences in soil carbon sequestration rates among the various planting systems. Specifically, the SOC stock in the W-S system was 12.21 % to 24.51 % higher than that of the W-M system and 10.28 % to 35.73 % higher than that of the W-SP system. While TN stock demonstrated an increase of 9.85 % to 37.39 % compared to the W-M system and 8.14 % to 67.43 % compared to the W-SP system. Moreover, SOC and TN sequestration were largely related to soil aggregates, with macroaggregates being the primary component in both W-S and W-M planting patterns, while microaggregates were more common in W-SP patterns. The accumulation of SOC and TN occurred mainly in macroaggregates, leading to a significant increase in C and N content in soil macroaggregates under the W-S planting pattern. The structural equation model suggested that the TN stock had both direct and indirect effects on SOC sequestration, with a total impact coefficient of 0.872. Our three-year field results indicate that the W-S model is advantageous in enhancing soil C and N sequestration capacity and had great potential in reducing greenhouse gas emissions in farmland.

Effects of biochar additions on the mechanical stability of soil aggregates and their role in the dynamic renewal of aggregates in slope ecological restoration

Mechanical stability of soil aggregates is important for resisting external disturbances in slope soils. Biochar (BC) is widely used in slope remediation. However, biochar application may not be conducive to the formation of mechanical-stable soil aggregates, and the effects of biochar additions on the mechanical stability of soil aggregates in slope restoration remain largely unclear. In this context, an incubation experiment was conducted in this study with four biochar levels added to artificial soil, namely 0 % (BC0), 1.5 % (BC1), 3 % (BC2), and 4.5 % (BC3), corresponding approximately to 0, 0.77, 1.53 and 2.30 M ha−1, respectively. The contributions of different soil aggregate fractions to maintaining the mechanical stability of aggregates, as well as the main influencing factors and pathways of biochar additions on soil aggregate stability in a dynamic renewal process of aggregates, were investigated in this study. The results showed a decreasing trend in the mean weight diameter (MWD) with increasing biochar levels and BC1 has no significant difference with BC0, showing MWD values of 2.74 and 2.75, respectively. In contrast, BC3 is significantly lower MWD value of 2.18. The BC3 exhibited negative impact on the mechanical stability of the aggregates. Redundancy analysis (RDA) showed that large macroaggregates (>5 mm) exhibited a stronger contribution on the aggregate mechanical stability between all soil aggregate fractions. The random forest (RF) algorithm and structural equation modeling (SEM) indicated that microaggregate-associated soil organic carbon (SOC) contents and soil pH values were the main factors driving the changes in the aggregate mechanical stability caused by biochar applications. Indeed, the biochar level of 1.5 % maintained the stability of macroaggregates and increased the microaggregate-associated SOC content by 35.7 %, which was conducive to the formation of microaggregates within macroaggregates. Our study suggests that the application of biochar at a level of 1.5 % is more beneficial for maintaining the mechanical stability of artificial soil aggregates.

Oriental beech roots improve soil aggregate stability and reduce soil detachment rate in forest lands

In many forestlands, such as Saravan Forest Park, the high capacity of soil detachment from deforestation and subsequent ecological restoration are important issues that need detailed information about plant and tree species for the objectives of soil conservation. To determine if some species are more helpful for the stability of soil aggregates and thus reducing soil erosion, we examined Oriental beech species, compared to three other species (Crataegus ambigue, Primula heterochroma and Rubus persicus) in the Saravan forest park, and measured how root characteristics (root biomass, root length and root weight density) influenced mean weight diameter of aggregates (MWD) and soil detachment rate (Dc). For measuring Dc, undisturbed samples collected from soils under the four studied species and subjected to five slopes (from 4.8 to 31.4%) and five water discharges (from 0.26 to 0.65 l m−1 s−1) by a hydraulic flume. Results showed that Oriental beech roots can significantly influence soil aggregate stability. MWD was greater in soil under Oriental beech species characterized by high values of root biomass, root length and root weight density. Moreover, Dc was significantly lower in soils with Oriental beech species compared to three other species. Pearson's correlation matrix showed a strong positive correlation between MWD and the root characteristics (P < 0.01) (R2 = 0.8). Also, the clustering of soils among the studied species by the principal component analysis (PCA) revealed a clear difference based on the root characteristics and MWD. Generally, this study showed that, compared with other species, the root system of Oriental beech is more effective in the stabilization of slope by the binding effect on the soil, thus improving aggregate stability and reducing soil detachment rates.