Mean Weight Diameter Of Aggregates Research Articles

Forest conversion effects on soil organic carbon are regulated by soil aggregate stability and not by recalcitrance: Evidence from a reforestation experiment

Preservation of biochemically recalcitrant organic matter and protection within soil aggregates are two important mechanisms of soil organic carbon (SOC) preservation, but their relative importance remains unclear. In this study, we converted part of a mature secondary forest to two young plantation forests (Castanopsis carlesii and Chinese fir) and an assisted naturally regenerated (ANR) forest in 2012. The three young forests (five years old at the time of study) and the remaining mature secondary forest constitute a soil disturbance intensity gradient ideal for exploring effects of forest conversion on soil aggregate size classes and stability as well as SOC concentration and chemical composition. Taking advantage of the disturbance gradient, we examined the relative importance of physical protection versus recalcitrance in SOC preservations. We found that the young ANR forest conserved a similar amount of SOC to that of the mature secondary forest, which was 18 %–21 % higher than those retained in the two young plantations. Compared with the mature secondary forest, the young ANR forest retained 87 % of >2 mm aggregates, whereas the two young forest plantations retained only 45 %–53 %. Compared to the mature secondary forest, aggregate mean weight diameter (MWD) decreased by 10 % in the young ANR forest and 37 %–43 % in the two young plantations. Moreover, the young Castanopsis carlesii plantation had proportionally more recalcitrant aromatic carbon (C) and less labile O-alkyl C compared to the mature secondary forest. Total SOC was positively related to >2 mm aggregates mass percent, soil aggregate MWD, and labile O-alkyl C, but not with recalcitrant alkyl and aromatic C. Our results indicate that SOC preservation is more related to soil aggregate stability than recalcitrant C abundance, and advocate that reducing soil disturbance is key to future forest management aiming to maximize C sequestration.

Effect of Rainfall on Soil Aggregate Breakdown and Transportation on Cultivated Land in the Black Soil Region of Northeast China

To clarify the characteristics of soil aggregate breakdown and transportation by rainfall, the cultivated land (0–10 cm) of the black soil region of Northeast China was taken as the research object, with rainfall intensities of 78 and 127 mm·h−1 (RI78, RI127), to analyze the differences in the mass transported, fraction size distribution, mean weight diameter (MWD) and enrichment rate of aggregates before and after runoff generation. Before runoff generation, the total mass of transported aggregates, the total mass percentage of the aggregate fraction size < 1 mm and the MWD of the transported aggregates were not significantly different at rainfall intensities of 78 and 127 mm·h−1. After runoff generation, the mass of transported aggregates was greater than that before runoff generation, and the mass of transported aggregates under RI78 was significantly higher than that under RI127, by 15.6%. The mass percentage of the aggregate fraction size < 0.053 mm after runoff generation was significantly reduced by 8.4% and 19.4% compared to that before runoff generation. After runoff generation, compared with before runoff generation, the MWD of the transported aggregates was significantly reduced by 50.9% and 50.3% under RI78 and RI127, respectively. Compared with before runoff generation, the mass percentage of small macroaggregates increased gradually with the increase in the transport distance. The aggregate fraction size > 0.25 mm was lost, and the fraction size < 0.25 mm was enriched, before and after runoff generation. A comparative study on the characteristics of black soil aggregate breakdown and transportation before and after runoff generation can provide a theoretical basis for the mechanism of soil erosion and the transportation of cultivated soil in the black soil region of Northeast China.

Impact of Long-Term Nutrient Supply Options on Soil Aggregate Stability after Nineteen Years of Rice–Wheat Cropping System

Continuing soil degradation remains a serious threat to future food security. Soil aggregation can help protect soil organic matter from biodegradation; it affects soil physical (aeration), chemical (water infiltration), and biological (microbial) activities. The integrated plant nutrition system (IPNS) and organic farming (OF) options have been contemplated as a sustainable strategy to sustain soil aggregate stability under adverse climatic conditions and a possible tool to restore degraded soil systems. Results suggested that the application of plant nutrients based on IPNS and soil test crop response (STCR) including mineral fertilizers and organic manure (farmyard manure: FYM) improved soil aggregate stability and mean weight diameter (MWD) under rice–wheat cropping systems. A long-term (19 year) cropping system (rice–wheat) experiment was examined to identify best nutrient management practices. Seven nutrient supply options were applied: organic, mineral fertilizer in combination with IPNS, IPNS + B/IPNS + C to improve soil aggregate stability and MWD after completing 19 cropping cycles of rice–wheat cropping systems. Results showed that significantly higher (+31%) macroaggregates were dominant in the surface soil layer than in the subsurface soil. The significantly highest macroaggregates were observed under OF (60.12 g 100 g−1 dry soil) management practices followed by IPNS options. The MWD was significantly increased (+17%) between surface and subsurface soil. Maximum MWD was reported with OF (0.93 mm) management practices followed by the IPNS + C (0.78 mm), IPNS + B (0.77 mm), IPNS (0.70 mm), STCR (0.69 mm), NPK (0.67 mm), and unfertilized control (0.66 mm) plots. Overall, results suggest that the adoption of IPNS options, such as organic farming (OF), RDF, STCR, and inclusion of pulses (berseem and cowpea), significantly improved all soil aggregation fractions in the soil system and also offered an additional benefit in terms of soil sustainability.

Soil aggregate structure, stability, and stoichiometric characteristics in a smelter-impacted soil under phytoremediation

Smelter-impacted soils often result in soil degradation and the destruction of the soil structure. Although soil aggregate typically plays a crucial role in soil structure, the influence of phytoremediation on soil aggregate structure stability and stoichiometric characteristics remains unclear. To study the influence of phytoremediation on soil aggregate structure, stability and stoichiometric characteristics, a 3-year in situ experiment was conducted. After hydroxyapatite was applied, Elsholtzia splendens, Sedum plumbizincicola, and Pennisetum sp. were planted in a smelter-impacted soil. After 3 years, the soil aggregate structure, stability, and stoichiometric of chemical elements were analyzed. The results showed that the three phytoremediation treatments increased the content of >0.25 mm mechanically-stable (DR0.25) and water-stable (WR0.25) aggregates by 6.6%–10.4% and 13.3%–17.5%, respectively. Aggregate mean weight diameter (MWD), geometric mean diameter, and aggregate stability rate (AR, %) were significantly increased, and the soil mechanically stable aggregate fractal dimension (D) was significantly reduced after the 3-year remediation. Soil total nitrogen and phosphorus in aggregates with different particle sizes were significantly increased by 11.4%–46.4% and 107%–236% after different plant treatments. For the stoichiometric characteristics of the aggregates, the combined remediation only significantly reduced the value of N:P and C:P in different particle size aggregates and had no significant effect on the C:N in all particle size aggregates. Meanwhile, the combined remediation of hydroxyapatite and Elsholtzia splendens, Sedum plumbizincicola, and Pennisetum sp. in heavy metal heavily contaminated soil could reduce the availability of Cu and Cd by 54.1%–72.3% and 20.3%–47.2% during the 3 years, respectively. In summary, this combined remediation method can be used for the remediation of farmland that is contaminated by heavy metals.

Reduced basal and increased topdressing fertilizer rate combined with straw incorporation improves rice yield stability and soil organic carbon sequestration in a rice-wheat system.

Fertilizer management is vital for sustainable agriculture under climate change. Reduced basal and increased topdressing fertilizer rate (RBIT) has been reported to improve the yield of in–season rice or wheat. However, the effect of RBIT on rice and wheat yield stability and soil organic carbon (SOC) sequestration potential is unknown, especially when combined with straw incorporation. Here, we report the effect of RBIT with/without straw incorporation on crop yields, yield stability, SOC stock, and SOC fractions in the lower Yangtze River rice–wheat system region over nine years. RBIT with/without straw incorporation significantly increased nine–year average and annual rice yields but not wheat yields. Compared with conventional fertilization (CF), RBIT did not significantly affect wheat or rice yield stability, but combined with straw incorporation, it increased the sustainable yield index (SYI) of wheat and rice by 7.6 and 12.8%, respectively. RBIT produced a higher C sequestration rate (0.20 Mg C ha−1 year−1) than CF (0.06 Mg ha−1 year−1) in the 0–20 cm layer due to higher root C input and lower C mineralization rate, and RBIT in combination with straw incorporation produced the highest C sequestration rate (0.47 Mg ha−1 year−1). Long–term RBIT had a greater positive effect on silt+clay (0.053 mm)–associated C, microbial biomass C (MBC), dissolved organic C, and hot water organic C in the surface layer (0–10 cm) than in the subsurface layer (10–20 cm). In particular, the increases in SOC pools and mean weight diameter (MWD) of soil aggregates were greater when RBIT was combined with straw incorporation. Correlation analysis indicated that topsoil SOC fractions and MWD were positively correlated with the SYI of wheat and rice. Our findings suggest that the long–term application of RBIT combined with straw incorporation contributed to improving the sustainability of rice production and SOC sequestration in a rice–wheat system.

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.

Paraplough and Mouldboard Plow Performance Evaluation for Seedbed Preparation and Supporting Conservation Tillage

Abstract The final yield of agricultural products depends on the effective factors during the growing season of plants, especially the size of soil aggregates and proper size distribution of aggregates. Therefore, it is very important to select appropriate tillage implement and to provide a suitable seedbed in terms of aggregate size with the least energy consumption. It is a new idea to use paraplough and winged-paraplough as tools for seedbed preparation. To measure and determine the factors affecting the mean weight diameter (MWD) of aggregates and percentage of crop residues on the soil surface, a series of field trials were performed in a randomized complete block design (RCBD) with five replications. The trials were conducted at three forward speeds of 2, 5 and 7 km·h−1 and three operating depths of 0–10; 0–20; 0–30 cm using the mouldboard plough and the paraplough with different wing configurations (without wings, with forward wings, with backward wings). The main impacts of implemented type, forward speed and operating depth and their mutual binary effects were significant at P <1% for both the MWD of aggregates and percentage of remaining residue. The highest MWD of aggregates was related to mouldboard plough (5.59 mm) and the lowest value was related to paraplough with backward wings (2.9 mm). It was found that the winged paraplough created a good pulverization in the soil and sustained a high percentage of crop residues on the soil surface. This issue can be very useful and important in the conservation tillage and the conservation of soil resources, especially in the water deficient areas where the soil is prone to erosion.

The persistently breaking trade-offs of three-decade plastic film mulching: Microplastic pollution, soil degradation and reduced cotton yield

Plastic mulching has been extensively used for near 40 years in China because of the commonly recognized trade-offs between economic profit and adverse impacts. Whether the trade-offs is well kept after decades of mulching usage is poorly understood. In this study, microplastic (MP) pollution, soil physical properties, as well as the root traits and yields of cotton under plastic mulching for different years (up to 32 years) in northwest China were investigated. The results showed that average abundance of MPs in soil ranged from 28.00 to 1426.67 items kg−1 and increased with time whereas size of MPs decreased with time. With respect to soil physical properties, bulk density and penetration resistance significantly increased with mulching time while saturated hydraulic conductivity and mean weight diameter of water-stable aggregates in cotton fields decreased over time, indicating that long-term plastic mulching has irreversible adverse impact on soil function. The significantly increased soil penetration resistance and decreased hydraulic conductivity substantially restricted root growth and this finally led to remarkable reduction of seed cotton yield. It is urgent for the agricultural departments to take appropriate measures to mitigate the constantly increased negative trade-offs of plastic mulching in the arid areas.

Organic amendments and gypsum reduce dispersion and increase aggregation of two sodic Vertisols

Sodic soils are widespread in semi-arid and arid regions of the world, with agricultural production on these soils constrained due to structural instability resulting from high concentration of exchangeable sodium (Na). The structural instability results in clay dispersion, reduced infiltration, crust formation and poor seed germination. One way to decrease undesirable effects of Na on soil structure is to apply gypsum (G) and organic amendments. We evaluated the effect of G and four organic amendments [polyacrylamide (PAM), feedlot manure (FLM), chicken manure (CM), and lucerne pellets (LP)] on two sodic soils. These amendments differed in their chemical nature as assessed by 13C cross-polarisation magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy after amended soils were incubated for 60 d at field water capacity. Gypsum reduced dispersion but had no effect on macroaggregates or aggregate mean weight diameter (MWD). Using principal component analysis, we found that aryl C and O-aryl C functional groups were positively correlated with microaggregates and the silt + clay fraction, and O-alkyl C and di-O alkyl C were positively correlated with soil dispersion after 1 d of soil incubation. The CM reduced dispersion more than FLM, LP, or PAM, although LP increased the proportion of large macroaggregates and MWD. We also found that large macroaggregates and MWD were positively correlated with microbial respiration and dissolved organic carbon. Except for the G + LP amendment, G had no synergistic effect with other organic amendments on macroaggregates and MWD of the two sodic soils. This study demonstrates that the functional groups of organic amendments and microbial activity play important roles in aggregation and dispersion of sodic soils. This study also demonstrates that a combination of gypsum and organic amendments will be a better option to ameliorate sodic soils by decreasing dispersion and increasing aggregate formation.

Determination of the threshold velocity of soil wind erosion using a wind tunnel and its prediction for calcareous soils of Iran

AbstractDetermination of the threshold velocity (TV) is a crucial step for wind erosion evaluation. Due to the difficulties of direct field measurements, pedotransfer functions (PTFs) and easily measurable soil properties could be used to save time and cost in predicting TV. Therefore, the present study was conducted to predict the TV using PTFs and to assess its influential parameters for calcareous soils of Fars Province, southern Iran. To this end, the TV was measured by a portable wind tunnel at 72 locations in different land uses and soil types across the study site. Various physicochemical and mechanical soil properties were used to develop six PTFs using multiple linear regression. Results showed that the TV varied from 3.0 m s−1 in poor rangelands to 12.83 m s−1 in saline lands. Soil surface shear strength (SS) with a correlation coefficient of 0.85 was the most influential parameter affecting the TV, followed by aggregate mean weight diameter (MWD). Results of the predictive models revealed that PTF 5, which was developed using SS and penetration resistance (PR;R2 = 0.86, RMSE = 0.85 m s−1), and PTF 6, which was developed using MWD and PR (R2 = 0.81, RMSE = 1.07 m s−1), had the highest performance for predicting the TV. PTF 5 was selected as the final model for predicting the TV since it only needed easily measurable soil properties without soil sample collection. We concluded that the use of PTFs could be an applicable alternative way to predict the TV, particularly at large scales.

How does no-till affect soil-profile compactibility in the long term?

How no-till management affects soil-profile compactibility in the long-term is unknown. The few previous studies measured soil compactibility for the upper 20 cm soil depth, which limits a complete understanding of the potential changes in compactibility for deeper depths. The objective of this study was to determine Proctor bulk density at different soil water contents, Proctor maximum bulk density (MaxBD), water content at MaxBD termed as critical water content (CWC), and relationships of MaxBD and CWC with soil organic C (SOC) and mean weight diameter of water-stable aggregates (MWD) for the 0–15, 15–30, and 30–60 cm depths in two long-term (32 and 36 yr) tillage experiments in the western US Corn Belt on silty clay loam soils. Treatments were: moldboard plow, disk, and no-till under continuous corn (Zea mays L.) and corn-soybean (Glycine max L.) rotation at the 32-yr site, and moldboard plow, chisel plow, double disk, and no-till under corn-soybean rotation at the 36-yr site. No-till reduced Proctor bulk density and MaxBD only at the 32-yr site and shallow depth (0–15 cm) by 5% compared with moldboard plow. However, disking reduced Proctor bulk density compared with plowed systems to 60 cm depth at the 32-yr site and to 30 cm depth at the 36-yr site. Disking at both sites reduced MaxBD by 7% (0.10 Mg m−3) to 30 cm depth compared with plowed treatments. Compared with moldboard plow, disk till increased CWC by 3% for the 30–60 cm depth at the 32-yr site, while both disk and no-till increased CWC by 5% at the 0–15 cm depth at the 36-yr site. Maximum compactibility decreased as SOC concentration and MWD increased, while CWC increased as SOC concentration increased at most soil depths. No-till reduces soil compactibility only near the surface compared with plowed systems but reduced till (disk) reduces compactibility to deeper depths in the profile, suggesting it can be more effective than no-till at reducing compactibility.

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.

Effects of soil erosion and soil amendment on soil aggregate stability in the cultivated-layer of sloping farmland in the Three Gorges Reservoir area

Soil aggregate stability is an important parameter that indicates soil resistance to erosion and the potential productivity of sloping farmland. The cultivated-layer (0–20 cm) of typical sloping purple soil (entisol) farmland around the Three Gorges Reservoir was studied. In-situ controls test and shovel test were used to determine the response of water stable aggregates to two factors, degree of erosion and soil amendment (CK, no fertilizer; F, chemical fertilizer; and BF, biochar with chemical fertilizer) and to analyze regulating pathways of the rational cultivation layer for sloping farmland. Water stable aggregate content for each soil particle size fraction in the cultivated-layer was significantly different for all degrees of erosion. The fraction of particle size > 5 mm in subsurface soil (10–20 cm) was greater than in topsoil (0–10 cm). Erosion significantly affected soil aggregate stability (WR0.25, >0.25 mm particle size water-stable aggregates content; MWD, mean weight diameter of aggregates; GMD, geometric mean diameter of aggregates) in the cultivated-layer. Both MWD and GMD reached maximums for S-0 (erosion of 0 cm). Comparing all treatments, it is BF that showed greater weight increase for MWD and GMD in topsoil (0–10 cm). Percentage of aggregate disruption consistently increased along downward direction of erosion degrees. Shear strength and penetration resistance, which are soil macrostructure parameters, were used to quantify soil stability in a short time. Treatment BF resulted in the greatest increase in shear strength in topsoil for S-10 (erosion of 10 cm) and significantly decreased penetration resistance. This treatment could significantly improve soil tillage performance and facilitate the formation of a loose upper and compact lower cultivated-layer. Shear strength responded very significantly to the interactive effects of erosion and soil treatment and was more sensitive to soil treatment. These results demonstrated that erosion makes soil less sustainable by disintegrating macroaggregates, and BF is the better soil management practice to effectively improve aggregate stability of erodible cultivated purple soil.

Biological conservation measures are better than engineering conservation measures in improving soil quality of eroded orchards in southern China

AbstractBoth biological and engineering measures have proven to be effective soil and water conservation practices for eroded environments, whereas how the soil quality varies with them remains unclear. In this study, we measured soil properties in the 0‐to‐100‐cm soil layer of eroded orchards in southern China after 17‐yr of application of different conservation measures. The purpose of the experiment was to examine the effect of biological (grass planting, intercropping) and engineering measures (terrace) on the soil quality of eroded orchards. We selected an orchard plot with traditional clean‐tillage as control. In comparison, soil pH, organic carbon, total phosphorus, microbial carbon, available potassium, and available micro‐elements contents increased significantly in orchards with biological measures, and the improvements were more dramatic in orchards planting grass in strips. However, biological measures significantly reduced the mean weight diameter of aggregates. Engineering measures significantly increased the soil pH of orchards but decreased the contents of available iron and available manganese. The soil quality index (SQI) was 4–55% higher in the biological treatment than in the control. Moreover, higher SQIs were found in the orchards with grass planting than that with intercropping, and this effect was greater in the topsoil than in deeper soils. On the contrary, engineering measures had similar SQI with clean tillage. These results suggest that biological measures are more effective than engineering measures in maintaining soil quality and nutrient content in eroded orchards in southern China.

Interactive effects of conservation tillage on the aggregate stability and soil organic carbon

AbstractBackgroundSoil carbon sequestration is of great significance to achieve carbon neutralization at an early date.AimsThe present study was conducted in order to explore the interactive effects of conservation tillage on soil organic carbon (SOC) and aggregate stability, and to understand the response of soil organic carbon to aggregate structure and distribution characteristics.MethodsUndisturbed soil from varying depths was collected from the conservation tillage demonstration base under differential tillage practices (NT, no‐tillage mulch; DT, deep‐tillage mulch; CT, conventional‐tillage mulch). SOC and stable‐aggregates of different particle sizes, their number, and structures were analyzed and the relationship between the stability of soil aggregates and SOC was presented.ResultsThe SOC content was the highest under CT treatment (1.67 g kg–1) and the lowest under NT treatment (1.31 g kg–1). In surface soil, the number of macroaggregates (>0.25 mm, M) under NT treatment was the largest (89.1%), the mean weight diameter (MWD) and geometric mean diameter (GMD) of water stability and mechanical aggregates were the highest, and the fractal dimension (D) was the lowest. This indicated that the NT treatment could maintain the aggregate structure of surface soil well. In the short term, DT and CT could increase SOC content, while NT could decrease SOC content. There was a significant correlation between SOC content and MWD (R2 = 0.40), GMD (R2 = 0.38), and D (R2 = 0.44). The more stable the aggregate structure, the higher the soil fixation and maintenance of SOC.ConclusionsSOC content and aggregate stability have a synergistic effect, and NT can promote the formation of soil aggregates, improve their stability, and balance the distribution of carbon in soil aggregates with different particle sizes.

Rejuvenation of iron oxides enhances carbon sequestration by the ‘iron gate’ and ‘enzyme latch’ mechanisms in a rice-wheat cropping system

The ‘enzyme latch’ theory believes that oxygen constraints on phenol oxidase can restrain the activity of hydrolytic enzymes responsible for decomposition, while the ‘iron (Fe) gate’ theory suggests that Fe oxidation can decrease phenol oxidase activity and enhance Fe-lignin complexation under oxygen exposure. The objective of this study was to explore the roles of the ‘enzyme latch’ and ‘Fe gate’ mechanisms in regulating soil organic carbon (SOC) sequestration in a rice-wheat cropping system subjected to six fertilization treatments: control (CT), chemical fertilizer (CF), CF plus manure (CFM), CF plus straw (CFS), CF plus manure and straw (CFMS), and CF plus organic-inorganic compound fertilizer (OICF). Soil samples were collected after the rice and wheat harvests and wet sieved into large macroaggregates, small macroaggregates, microaggregates, and silt and clay particles. Variations in amorphous and free Fe oxides, Fe-bound organic carbon and phenol oxidase activity were examined. After nine years, compared with the initial soil, the activation degree of free Fe oxides increased by 1.3- to 1.6-fold and the topsoil SOC stock increased by 13–61% across all treatments. Amorphous Fe oxide content, phenol oxidase activity and aggregate mean-weight diameter were higher after the wheat harvest than after the rice harvest. Amorphous Fe oxide content was positively correlated with Fe-bound organic carbon content (P < 0.001) but negatively correlated with phenol oxidase activity (P < 0.001). Therefore, seasonal alternation of wetting and drying can progressively drive the rejuvenation of Fe oxides and simultaneously affect the activity of phenol oxidase. Oxidative precipitation of amorphous Fe oxides promoted the formation of organo-Fe complexes and macroaggregates, while flooding of the paddies decreased the activity of phenol oxidase, thereby resulting in year-round hindered decomposition. Organic fertilization strengthened the roles of the ‘Fe gate’ and ‘enzyme latch’ mechanisms, and thus accelerated SOC sequestration in the rice-wheat cropping system.

Temporal variations in soil aggregation following olive pomace and vineyard pruning waste compost applications on clay, loam, and sandy loam soils.

This study was aimed to determine the temporal effects of olive pomace (OPC) and vineyard pruning waste (VPC) composts on soil aggregation of three different textured soils. Both OPC and VPC are the most common agricultural wastes in the Mediterranean Region. The application of these composts to soils with different textures (clay, loam, and sandy loam) at different rates and knowing how long after the application the aggregate stability (AS) reaches the maximum (for different compost, texture, and rate) increases the novelty value of this study. Composts were prepared in automatic temperature and aeration-controlled reactors. The experiment consisted of three different soil textures (clay, loam, and sandy loam), five different treatments (control, 3% and 6% OPC, 3% and 6% VPC), 10 different incubation times, and four replications. The highest AS values were obtained with 6% VPC application on the 210th day of incubation for clay and loam soils while it was the highest for 6%OPC on the 120th day of incubation for sandy loam soil. Soil carbon (TC) was the strongest and positively correlated with AS 90days after VPC and OPC applications in all soils. Final TC stocks increased by 131% and 417% in clay, 72% and 251% in loam, and 21 to 257% in sandy loam soil, compared to 15days of incubation. OPC and VPC amendments to clay, loam, and sandy loam soils increased AS while reducing the mean weight diameter (MWD) of aggregates. It took a shorter time (90days) for the VPC application to increase the amount of TC in soils.

Aggregate stability and size distribution regulate rainsplash erosion: Evidence from a humid tropical soil under different land-use regimes

Soil erosion by water is a serious global ecological issue that leads to land degradation and threatens ecosystem sustainability. Splash erosion resulting from raindrop impact is the first stage of the erosion process and is mainly responsible for the detachment and migration of soil surface aggregates. In humid tropical regions, however, there has been little quantitative analysis of the relations between splash erosion and variable aggregate characteristics such as stability, particle size distribution and organic matter content. The objective of the current study was to determine how rain splash erosion is related to soil aggregate characteristics under land-use change, i.e., the conversion of tropical rainforest (TR) into rubber plantation (RP). Splash cups containing dry-sieved aggregate samples with eight size classes were exposed to natural rainfall to measure the splash erosion rate. The results showed that the initial aggregate organic carbon, water-stable aggregate index (WSA) and mean weight diameter (MWD) decreased by 31%, 9% and 48%, respectively, after 32 years of rubber cultivation. These degenerations in aggregate properties increased the susceptibility of soil aggregates to erosion and mutually contributed to a higher average splash erosion rate in RP (1.20 kg m−2) than in TR (1.09 kg m−2), regardless of aggregate size fractions. Splash erosion rates for all aggregate sizes were significantly positively correlated with rainfall kinetic energy, rainfall amount and intensity during the study period. The average splash erosion rate of aggregates first increased and then decreased with a decline in aggregate size in both TR and RP. The minimum and maximum splash rates were observed within 10–8 mm and 0.5–0.25 mm size fractions, respectively. In particular, the proportion of small-sized aggregates (i.e., 1–0.15 mm) with a relatively high splash erosion rate increased remarkably after land-use change. This exerted a negative impact on splash erosion control as well as on the sustainable development of rubber cultivation. Increasing the additional input of organic materials (e.g., intercropping cash crops with rubber trees) may help to enhance large aggregate formation and stabilization, which could minimize the risk of splash erosion at the aggregate scale for eroded rubber plantation ecosystems.

Effects of long-term manure substitution regimes on soil organic carbon composition in a red paddy soil of southern China

Long-term fertilization regimes can substantially change soil organic carbon (SOC) dynamics within different soil fractions. However, the underlying mechanisms of these effects, particularly under manure substitution regimes, remain elusive. Soils from different fertilization regimes [i.e., control, mineral NPK fertilization, and NPK with three manure substitution rates (30%, 50%, and 70%)] in a 33-year red paddy field experiment in southern China were sampled. The mean weight diameter (MWD) of soil aggregates and soil microbial properties (PLFAs), SOC content, and chemical composition of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) under different treatments were determined to investigate the effects of manure substitution regimes on SOC quantity and quality. The results showed mineral NPK fertilization alone significantly (p < 0.05) decreased the MWD of soil aggregates by 14.4%, whereas 30%, 50%, and 70% manure replacement significantly (p < 0.05) increased the MWD by 12.0%, 19.9% and 27.2%, respectively. Compared to the mineral NPK fertilization regime, manure substitution practices continuously and significantly (p < 0.05) increased SOC along with its content in microbial biomass C and POC when the substitution rate of manure increased from 30% to 70%. Regardless of the substitution rates, manure substitution regimes decreased the O-alkyl C and aryl C proportions of POC, but increased the recalcitrant alkyl C resulting in higher alkyl C to O-alkyl C ratios compared with mineral NPK fertilization. Pearson analysis further showed that the O-alkyl C of POC were more associated with the soil microbial biomass C and N and total PLFAs. Partial least squares path modeling analysis indicated that microbial biomass C, POC, and MAOC were positive factors impacting SOC, whereas the promotion of manure substitution on SOC stabilization progressed mainly through increasing POC content. Our results suggested that substitution of 70% mineral N with manure N is the most effective fertilizer practice for improving POC-related SOC sequestration in rice paddy soil of subtropical regions.

Effects of saline-water furrow irrigation on the stability of soil water-stable aggregates in cotton field

It is of great importance to explore the effects of saline-water furrow irrigation on soil water-stable aggregates for safe and efficient utilization of saline water resources. We conducted a long-term cotton experiment with six levels of saline-water furrow irrigation (1, 2, 4, 6, 8, 10 g·L-1) since 2006 and analyzed the variations of soil salinity and water-stable aggregates in the 10th and 15th years under saline irrigation. The results showed that soil salinity in the 0-30 cm layer at the ditch increased with increasing salinity level of irrigation water. There were significant differences between the 6, 8, 10 g·L-1 and 1 g·L-1 treatments. Soil salinity in each treatment increased gradually with increasing soil depth. Saline-water furrow irrigation tended to reduce the stability of soil water-stable aggregates. When the salinity level of the irrigation water was ≥6 g·L-1, the mass fraction of macroaggregates (&gt;0.25 mm), the mean weight diameter and geometric mean diameter of water-stable aggregates significantly decreased. In contrast, the fractal dimension and mean weight specific surface area increased significantly. The stability of soil water-stable aggregates decreased with soil depth in all treatments. Under the condition of saline-water furrow irrigation for several years, there was no accumulation of soil salinity and instability of water-stable aggregates in the 0-30 cm soil layer at the ditch with each passing year. With the irrigation scheduling of this study, saline-water furrow irrigation with salinity ≤4 g·L-1 did not affect soil salinity and water-stable aggregate stability of cotton field in this area.