Abstract:【Objective】This study aimed to understand the role of glomalin in maintaining soil organic carbon (SOC) balance and soil aggregate stability, and construct management strategies for improving soil structure and soil quality. 【Method】 To fully understand glomalin, published data in recent years (332 sets of data from 19 literature) were collected, the distribution characteristics of glomalin in soil aggregates were quantitatively analyzed, and its influencing factors were systematically analyzed. Moreover, the distribution of glomalin in soil aggregates under different land uses was compared. 【Result】The results showed that the mass percentages of the > 2 000 μm and 2 000-250 μm aggregates (about 40%, respectively) were significantly higher than that of the 250-53 μm aggregates ( about 20%). The proportion of easily extractable glomalin in total glomalin was 20% in<53 μm aggregates, which was lower than other particle sizes (> 30%). There was no significant difference in the glomalin-C in SOC in different aggregates. The proportion of easily extractable glomalin-C in aggregates of different particle sizes was about 2%, while the proportion of total glomalin-C was about 8%.【Conclusion】In the >250 μm aggregates, GRSP (glomalin related soil protein) increased with the increase of temperature and precipitation, but decreased with the increase of pH. Although these correlations were not found in <250 μm aggregates, there was a significant positive correlation between glomalin and SOC. By comparing different land use patterns, it was found that the glomalin in the aggregates of forest soil was more than those in farmland and grassland, which indicated that forest soil was more conducive to the accumulation of glomalin than farmlands and grasslands.

Abstract:【Objective】 Mineral-associated organic carbon (MAOC) is the most important of soil organic carbon(SOC), and its mineralization characteristics have an important impact on soil carbon sequestration and global climate change. As an important topographic factor, slope position significantly affects the interaction and stability of organic carbon and soil minerals. However, the influence of slope positions on mineralization characteristics of MAOC in soils is not fully understood. 【Method】 In this study, typical citrus orchard soils at different slope positions were sampled, and the aggregates with sizes of >2, 2～0.25, 0.25～0.053, and <0.053 mm were obtained by physical fractionation. Moreover, the MAOC in aggregates were separated to investigate the mineralization characteristics of MAOC at varying slope positions (upper slope, middle slope, and lower slope) through indoor cultivation. The influence of soil physicochemical factors and hydrophobicity on MAOC mineralization was analyzed by Infrared spectroscopy (FTIR), Redundancy analysis (RDA), and Hierarchical partitioning analysis. 【Result】 The results showed that the cumulative mineralization (Ct), mineralization rate and potential mineralization (Co) of MAOC in citrus orchard soil at lower slopes were significantly higher than those at upper and middle slopes, but the ratio of Co/MAOC at lower slope was significantly lower compared with upper and middle slopes. With the decrease in aggregate size, the Ct, mineralization rate, and Co of MAOC in citrus orchard soil at each slope position showed an upward trend, while the mineralization intensity of MAOC gradually weakened. RDA results showed that the Co was significantly positively correlated with pH, SOC, MAOC, TN, and C/N (P<0.05), and significantly negatively correlated with iron and aluminum oxides (Fed/Ald, Feo/Alo, and Fep/Alp) and hydrophobicity (P<0.05). Co/MAOC was significantly positively correlated with iron and aluminum oxides and hydrophobicity, but significantly negatively correlated with Co, Ct, pH, SOC, MAOC, TN, and C/N. Hierarchical partitioning analysis revealed that Alo, Alp, and Fep emerged as significant factors influencing the mineralization of MAOC. Variation decomposition analysis showed that the combined effects of Alo, Alp, Fep, C/N, MAOC, and Feo significantly affected MAOC mineralization in aggregates with different particle sizes at different slope positions. 【Conclusion】 The slope positions have obvious effects on the mineralization characteristics of MAOC in aggregates in citrus orchard soils. The findings of this study are of great significance for understanding the formation mechanisms and stability of mineral-bound organic carbon in soil aggregates and in enhancing soil organic carbon sequestration in citrus orchards at different slope positions in hilly regions of southern China.

Abstract:Soil aggregates are the essential building blocks of soil, which impact the retention and distribution of water, air, heat, and nutrients. Binding materials play a pivotal role in the formation of soil aggregates. However, the specific functions of different types of binding materials as well as the mechanisms of the complex interactions in the formation of these aggregates still lack a systematic summary. This article reviews the key theories related to soil aggregates and sorts out types, forms, transformations, and interaction mechanisms of binding materials in soil aggregates under different geographical contexts and human activities. It also describes the effects of binding materials (e.g., organic matter, minerals, roots, organisms and their derivatives such as Extracellular Polymeric Substances) on the structure and stability of soil aggregates. Furthermore, it introduces models illustrating the self-organization process driven by binding materials within soil aggregates and elucidates the mechanisms of the formation and transformation of soil aggregates. Finally, the future development of soil aggregates is suggested. Specifically, future research could investigate the in-situ analysis of soil aggregates, quantitative description of the formation process of soil aggregates, spatial structure of soil aggregates at the landscape scale in relation to its stability, and development of product/technology in cultivating good soil aggregates for applications. This research holds significant scientific and practical value in promoting the development of healthy soil aggregates, unraveling the processes of soil elemental cycling and evolution, and enhancing overall soil quality and productivity.

Abstract:【Objective】Nitrifiers play an important role in the process of farmland soil nitrogen transformation. A study on the distribution of nitrifying microorganisms in aggregates will help to reveal the interaction mechanism between soil structure, microorganisms and soil nutrient cycling.【Method】The changes in soil aggregate properties and nitrification potential (NP) in maize monoculture (M-M) and maize peanut rotation (M-P) were compared, and the abundance and community composition of different nitrifying functional genes in soil aggregates were evaluated by quantitative PCR and high-throughput sequencing.【Result】Compared with M-M, M-P significantly increased pH, NH4+ and total carbon (TC) in soil aggregates. M-P also significantly enhanced NP, but the aggregate size had no significant effect on NP. The abundance of the AOB amoA gene was higher in M-P and also more abundant in smaller aggregates. The distribution pattern of AOA and comammox amoA genes was opposite to that of AOB, indicating that AOB can better adapt to the micro-environment of smaller aggregates, while AOA and comammox tended to dominate in larger aggregates. In addition, compared with M-M, the difference in AOA/AOB and comammox/AOB ratio between M-P aggregates decreased, indicating that rotation promoted an even distribution of soil nitrifiers among different aggregate sizes. By further analysis of the nitrifying community based on the 16S rRNA gene sequencing, the results showed M-P increased the proportion of Nitrolancea-like NOB and Candidatus Nitrosocosmicus-like AOA, decreased the proportion of Nitrospira-like NOB, and had no significant effect on AOB compositions. The aggregate size only had a significant effect on the proportion of Nitrosospira-like AOB. NH4+ content and pH were the main factors affecting soil NP and the nitrifying microbial community structure among soil aggregates. NP was positively correlated with AOB amoA gene abundance and negatively correlated with AOA amoA gene abundance. However, in terms of community composition, Nitrosospira-like AOB, Candidatus Nitrosocosmicus-like AOA and Nitrospira-like NOB all showed positive correlations with NP.【Conclusion】Soil aggregate size and cropping system can greatly affect the distribution of nitrifying microorganisms in soil aggregates. However, nitrifying microorganisms have different adaptation mechanisms among aggregates. This study provides a theoretical support for improving the ecological adaptation mechanism of soil nitrifying microorganisms in the micro-environments under Gramineae-Legume rotation.

Abstract:[Objective] Soil aggregate size distribution evolves due to soil aggregation and breakdown processes that occur under natural and anthropogenic impact. A change in aggregate size distribution can alter soil pore structure, which might also influence soil water transportation, gas exchange and microbial activities. Thus, this may potentially affect soil organic carbon (SOC) mineralization. Therefore, this study aimed to investigate the effect of aggregate size distribution on soil pore structure and the SOC mineralization.[Method] Soil samples were collected from a long-term field experimental site with treatments receiving different amount of pig manure (No manure, CK; Low manure, LM; High manure, HM). The soil samples were passed through sieves with 5.0(S5), 2.0(S2), or 0.5 mm(S0.5) openings to create different aggregate size distributions, with the maximum aggregate sizes corresponding to 5.0, 2.0 and 0.5 mm, respectively. The sieved aggregates were repacked into soil columns (diameter 2.9 cm, height 4 cm) with a bulk density of 1.3 g·cm-3. Soil columns were incubated for 57 days and SOC mineralization was measured during this period. Also, the soil pore structure was quantified using X-ray micro-computed tomography (μ-CT) imaging.[Result] Sieving altered soil aggregate size distribution, which resulted in significantly different soil pore structure in the repacked columns. Compared to S5 and S2 treatments, the S0.5 treatment significantly decreased the image-based porosity (>16 μm) by 83.0%~93.9% and pore connectivity by more than 95%. The differences in macroporosity and pore connectivity between S5 and S2 treatmentwere significant for the HM soil, but not for the CK and LM soil. Also, aggregate size distribution had a significant effect on SOC mineralization. The breakdown of aggregates increased SOC mineralization. The cumulative SOC mineralization amount of S0.5 treatment was 64.2%~79.1% and 14.1%~19.3% higher than that of S5 and S2 treatments for the CK and HM soils, respectively. However, there was no significant difference in the cumulative SOC mineralization between the S0.5 and S5 treatment in the LM soil. The correlation analysis indicated that the cumulative SOC mineralization amount was negatively correlated with the porosity of 16~30 μm pores.[Conclusion] Sieving broke down large aggregates and decreased soil macroporosity in the repacked soil columns. The breakdown of soil aggregates promoted SOC mineralization partially due to the release of the protected SOC. The change of aggregate size distribution and the resulting alteration of pore structure also correlated with SOC mineralization. This study can serve as a reference for future research related to SOC mineralization and the effect of soil aggregation on this process.

Abstract:[Objective] In order to improve soil structure of the subsoil layer of black soil and solve the problem of how to return maize straw in Northeast China, a three-year (2015—2018) field experiment was carried out.[Method] The field experiment was designed to have three treatments in returning pattern, i.e. CK (no straw return), QS (returning of chopped straw) and KL (returning of pelletized straw), and three treatments in straw returning dosage, i.e. low dosage (15 000 kg·hm-2), medium dosage (45 000 kg·hm-2, 3 times as much as the low dosage) and high dosage (75000 kg·hm-2, 5 times as much as the low dosage), and all had the straw buried in the soil 30-40 cm depth. Effects of the treatments on soil bulk density, soil compactness and soil water content were explored, and effects of soil aggregate distribution regulating soil physical properties was analyzed.[Result] The tests of the soil samples collected at the maize maturity stage of the three year experiment show:1) The practice of deep straw returning reduced soil bulk density and soil compactness and increased soil water content, and the effects were more significant in the 20-40 cm layer than in the 0-20cm soil layer. Moreover, the effects become more significant with increasing of straw dosage, and less with the time going on, and the effects of straw returning once at a high rate might last for a few years. In the third year of the experiment, only QS5 reduced soil bulk density significantly or by 10.9% (P < 0.05), and QS5 and KL5 lowered soil compactness by 24.1% and 22.0% (P < 0.05), but raised soil water content by 8.9% and 7.4% (P < 0.05), respectively; 2) The treatments of medium and high in dosage significantly increased the content of macroaggregates and reduced the content of microaggregates in the subsoil, and the fractions of > 0.25 mm water stable aggregates and of the > 2 mm large aggregates increased significantly with rising straw dosage. Whats more, the return of pelletized straw tended to promote the formation of macroaggregates than the return of chopped straw. The first and second years of straw returning were the main period for formation of macroaggregates, of which the highest content reached up to about 5 times and 1.5 times than that in CK, respectively; 3) Significant relationships were observed of 0.25-0.053 mm micro aggregate and > 0.25 mm water stable aggregate with soil bulk density, soil compactness and soil water content in the subsoil layer (P < 0.05), and the 0.25-0.053 mm, > 2 mm, and > 0.25 mm fractions of aggregates were the key factor driving changes in soil physical properties in 2016 and 2017, but the 1-0.5 mm fraction of aggregates was found to be the key factor driving the changes in soil physical properties in 2018.[Conclusion] Therefore, from the perspective of recycling maize straw in Northeast China, the return of chopped straw with a rate of 75 000 kg·hm-2 is a good management to regulate the distribution of soil aggregates and improve the physical properties of subsoil. The finding may provide a theoretical basis and technical guidance for improvement of soil structure of the black soil in China.

Abstract:Nitrous oxide (N2O), a potent greenhouse gas, is produced and reduced mainly under the mediation of functional microorganisms in soil. In terrestrial ecosystems, soil is an important source of N2O emission. Soil aggregates, a key structural component of the soil, consist of sand, silt, clay (primary particles), organic matter (binding agents) and pore spaces. According to the hierarchy theory, soil aggregates can be divided into four fractions by size, that is, large macroaggregates (>2 mm), small macroaggregates (2-0.25 mm), microaggregates (0.25-0.053 mm) and silt plus clay-sized particles (<0.053 mm). Large macroaggregates are high in pore connectivity and oxygen diffusion rate, fast in turnover, and rich in organic matter, and microaggregates high in water retention capacity and stable carbon content, and capable of protecting microorganisms from being predated. Hence, soil aggregates different in size may offer heterogeneous microhabitats for fungi and bacteria. And each independent microhabitat could be regarded as a biogeochemical reactor producing greenhouse gas. Nitrifiers and denitrifiers, which carry functional genes amoA, narG/napA, nirK/nirS, are identified as the major contributors to N2O production. However, N2O reduction is primarily a single process catalyzed by N2O reductase, encoded by nosZI and nosZII genes, which are present in bacteria and archaea capable of complete denitrification and acting as non-denitrifiers in N2O reduction to N2. These microorganisms are distributed separately in polymerized reactors different in size, driving N2O production and transportation as affected by soil moisture status, substrate availability, and porous connectivity. However, so far little is known about community structure of the nitrifiers and denitrifiers in aggregates relative to particle size and its influences on N2O emission. Nowadays, a numerous of studies have been reportedly devoted to soil N2O emission characteristics in different ecosystems, but limited knowledge was achieved on N2O emission and relative contribution of soil aggregates relative to size fraction. Therefore, with the clarification of functional microbial distribution at the aggregate scale, hot-spots of N2O production and reduction in soil microhabitats could be specified. In this review, advances in the recent research are summarized on divergence of N2O emission from soil aggregates. Large macroaggregates and small macroaggregates were found emitting more N2O than microaggregates did. However, studies were also found reporting conversely that microaggregates emitted N2O more vigerously. Papers in the literature also reported relationships between aggregate turnover(the formation, stabilization and disintegration of soil aggregates)and microbial structure dynamics. Bacteria contribute strongly to the formation of both macro- and microaggregates, while fungi play an important role in the formation of large macroaggregates. Hence, the mechanisms of soil microbes producing and reducing N2O in soil microhabitats could be summed up. A large number of studies have shown that ammonium oxiders are abundant in macroaggregates (>0.25 mm) and a dominant denitrifier community in microaggregates (<0.25 mm), and environmental factors affect N2O emission via redistributing these functional microorganisms. Based on the current results, discussions are done of some perspectives for future investigations: potential hot-spots for soil N2O production at the aggregate scale as heterogenetic living niches existing in soil aggregates different in size, critical values of key environmental parameters impacting soil N2O production and reduction, and holistic research on functional gene groups and enzymes instead of some individual gene due to the complex participation of soil microbes in N2O production and reduction. It is expected that this study will provide a reference for modeling and parameter optimization and a solid theoretical basis for mitigation of N2O emissions.

Abstract:【Objective】Farmlands on long gentle slopes of black soil in Northeast China is the main source of soil erosion. Soil erodibility is an important factor affecting soil erosion. The soil erosion process in the black soil region of Northeast China is characterized by overlapping and/or coupling of wind-hydraulic-gravity-freeze-thaw, multiple in force and in process. In spring, freezing and thawing accompanies melting water, and temperature fluctuation and freezing and thawing are the dominant factors affecting soil properties, while in summer and autumn, water erosion is the main form of erosion and rainfall runoff and dry-wet alteration are the two leading factors. However, so far little has been reported in the literature about field experiments on soil erosion resistance in the black soil region under the multi-forced compound erosion, thus making it hard to effectively implement the task of controlling the multi-forced compound erosion. 【Method】On a typical cultivated long gentle slope in the Keshan Farm in Heilongjiang Province, a 150 m long section of the slope was delineated along the same direction as the farmland ridges go from south to north. Soil samples were collected from the 0～30 cm soil layer of the slope at sampling sites 30 m apart along the slope. Soil shearing force of undisturbed soil was determined in April and September 2018. At the same time, the collected soil samples were analyzed for content of water-stable aggregates with the Le Bissonnais method via slow-setting and mean weight diameter (MWD), geometric mean diameter (GMD) and mass fractal dimension (D) of the soil aggregates were measured in an attempt to characterize soil erodibility parameters of the long gentle slope of black soil.【Result】Results show that in terms of shearing force, MWD, GMD, D and content of >0.2 mm soil aggregates, the sampling sites exhibited a decreasing order of 0 m > 30 m > 150 m > 60 m > 120 m > 90 m in spring, and generally an order of 30 m > 0 m > 150 m > 120 m > 60 m > 90 m in fall, which indicate that the soils in the middle of the slope at 60 m, 90 m and 120 m are quite low in soil erosion resistance, and that the slope varies with posiiton of the sampling site in strength of soil erosion, sediment transport and deposition. And soil anti-erodibility increases with soil depth. MWD and GMD is 1.27 times and 1.37 times as high in fall as in spring, respectively, and the content of >0.2 mm soil aggregates and shearing force is higher in autumn than in spring, but D is in a reverse trend, which suggest that the soil erosion resistance of the long gentle slope of black soil is higher in fall than in the spring. The correlations between the indices of water-stable aggregates were significant, but they have nothing to do with shearing force. Soil water-stable aggregates can be used as a stable index to assess soil erodibility. Shearing force is not so good when used as an index for the assessment because it is not stable, which is mainly attributed to its variability with soil properties, susceptible to and complex in change, especially under the influence of freeze-thaw erosion force. As the change of soil erodibility has certain uncertainties, it calls for further in-depth studies on impacts of soil shearing force on soil erodibility on long gentle slopes of black soil in Northeast China.【Conclusion】To control soil erosion in cultivated slopes of black soil in the region, focuses should be laid on adoption of erosion controlling practices in spring, like reducing length of the slope and adopting corresponding soil and water conservation measures in the middle of the slope. Soil water-stable aggregates can be used as a stable index in assessing soil erodibility in the black soil region of Northeast China. Shear force is not so good as soil water stable aggregates when used as index. The findings of this study may be used as a scientific basis for prevention and control of compound erosion in cultivated long gentle slopes of black soil in Northeast China.

Abstract:【Objective】This study was done to elucidate C, N and P distributions in soil aggregates and their stoichiometric characteristics in eroded red soil under vegetation restoration relative to history of the restoration. 【Method】Six sample plots were set up on slope lands of typical eroded red soil under vegetation restoration with different restoration history (0, 5, 10, 15, 30 and 80 years) located in Hetian Town, Changting County, Fujian Province of Subtropical China. Soil samples were collected from different soil layers (0~20 cm and 20~40 cm) of the six plots for analysis of contents of organic carbon, total nitrogen, total phosphorus in soil aggregates different in particle size (>5 mm, 2～5 mm, 0.5～1 mm, 1～2 mm, 0.25~5 mm and <0.25 mm). 【Result】 Results show that the contents of organic carbon, total nitrogen, total phosphorus in soil aggregates varied in the range of 2.06~27.71 g•kg-1, 0.54~2.12 g•kg-1 and 0.034~0.189 g•kg-1, respectively, and C:N, C:P and N:P did in the range of 3.06~13.05, 21.4~185.6 and 5.62~18.20, respectively. On the whole the contents of organic carbon, total nitrogen, total phosphorus and C:N increased in all fractions of soil aggregates in both soil layers with the restoration going on (P<0.05), and the trend was more significant in the 0~20 cm soil layer than in the 20~40 cm layer, while soil C:P and N:P displayed a rising-falling-rising trend, and . C:P deceased with soil depth, and N:P did not vary much. The contents of organic carbon, total nitrogen and total phosphorus, and C:N and C:P on the whole increased with the aggregates going down in particle size (P<0.05), except for the plot of 0 year, while N:P did not vary much with aggregate particle size (P>0.05). The content organic carbon, total nitrogen and total phosphorus in soil aggregates were remarkably positively related to their respective ones in the soil. In the soil aggregates the contents of organic carbon and total N positively related to C:N, the content of organic carbon was to C:P, and the content of total phosphorus was to N:P. 【Conclusion】All the findings in the study demonstrate that vegetation restoration mitigates soil erosion and significantly increases the contents of organic carbon, total nitrogen and total phosphorus and improves the function of soil aggregates as carbon and nitrogen pools, and what is more, P in the aggregates is the major factor restraining rehabilitation of the degraded ecosystem.

Abstract:【Objective】 The objective of this research was to investigate effects of long-term fertilization on formation of soil aggregates and the fraction of soil organic nitrogen therein in fluvo-aquic soil in the North China Plain, and consequently nitrogen supplying capacity of the soil and its mechanism.【Method】 Undisturbed soil samples were collected from the four treatments, i.e. CK (no fertilization), NPK (Application of chemical fertilizers only), 1/2OM (Application of half the rate of chemical fertilizer in Treatment NPK and half of the rate of organic manure in Treatment OM) and OM (Application of organic manure only) of the long-term (27 years) fertilization experiment at the State Agro-Ecological Experimental Station in Fengqiu, for analysis of contents of mechanically-stable soil aggregates and organic nitrogen therein, with the dry-sieving method and the Bremner method, separately. 【Result】 Results show that application of organic compost increased the proportion of > 2 mm aggregates in the topsoil, significantly or by 33% and 17% as compared with CK and NPK, whereas it decreased the content of <0.25 mm aggregates. Long-term fertilization significantly increased the content of organic nitrogen in aggregates in the fluvo-aquic soil, especially Treatment OM. The content of organic nitrogen in aggregates >2 mm, 2~0.25 mm and <0.25 mm was 776.4 mg·kg-1, 837.7 mg·kg-1 and 625.3 mg·kg-1, respectively. The organic nitrogen in soil aggregates was dominated with acidolyzable ammonium N, which was followed by amino acid nitrogen and unknown-acidolyzable nitrogen, aminosaccharide nitrogen, the least. Treatment NPK increased the proportion of acidolyzable nitrogen in > 2mm aggregates, while Treatment OM did the content and proportion of amino acid nitrogen and unknown acidolyzable nitrogen. 【Conclusion】 long-term application of organic manure can improve the soil structure of fluvo-aquic soil significantly, and stimulate accumulation of total nitrogen and all fractions of organic nitrogen, with amino acid nitrogen, amico sugar nitrogen and non-acidolyzable organic nitrogen accumulated mainly in 2~0.25 mm, and acidolyzable and unknown acidolyzable nitrogen in >2 mm aggregates, thus significantly increasing nitrogen supplying capacity of the soil.