Straw return increases crop production by improving soil organic carbon sequestration and soil aggregation in a long-term wheat-cotton cropping system

Straw return is a promising strategy for managing soil organic carbon(SOC)and improving yield stability.However,the optimal straw return strategy for sustainable crop production in the wheat(Triticum aestivum L.)-cotton(Gossypium hirsutum L.)cropping system remains uncertain.The objective of this study was to quantify the long-term(10 years)impact of carbon(C)input on SOC sequestration,soil aggregation and crop yields in a wheat-cotton cropping system in the Yangtze River Valley,China.Five treatments were arranged with a single-factor randomized design as follows:no straw return(Control),return of wheat straw only(Wt),return of cotton straw only(Ct),return of 50%wheat and 50%cotton straw(Wh-Ch)and return of 100%wheat and 100%cotton straw(Wt-Ct).In comparison to the Control,the SOC content increased by 8.4 to 20.2%under straw return.A significant linear positive correlation between SOC sequestration and C input(1.42-7.19 Mg ha^(−1)yr^(−1))(P<0.05)was detected.The percentages of aggregates of sizes>2 and 1-2 mm at the 0-20 cm soil depth were also significantly elevated under straw return,with the greatest increase of the aggregate stability in the Wt-Ct treatment(28.1%).The average wheat yields increased by 12.4-36.0%and cotton yields increased by 29.4-73.7%,and significantly linear positive correlations were also detected between C input and the yields of wheat and cotton.The average sustainable yield index(SYI)reached a maximum value of 0.69 when the C input was 7.08 Mg ha^(−1)yr^(−1),which was close to the maximum value(SYI of 0.69,C input of 7.19 Mg ha^(−1)yr^(-1))in the Wt-Ct treatment.Overall,the return of both wheat and cotton straw was the best strategy for improving SOC sequestration,soil aggregation,yields and their sustainability in the wheat-cotton rotation system.

Phosphorus Fertilizer Effects on Near-Surface Soil Aggregation in Furrow-Irrigated Rice on a Silt-Loam Soil

Well-aggregated soil has been shown to improve soil infiltration and reduce runoff and soil erosion, making well-aggregated soil important for productive, sustainable agriculture. One factor that may influence near-surface soil aggregate stability is fertilizer application. Rapid dissolution of fertilizers, which are mostly salts, can potentially disperse clays and destabilize aggregates. The objective of this study was to evaluate the potential effect of various fertilizer-phosphorus (P) and -nitrogen (N) sources [i.e., triple superphosphate (TSP), monoammonium phosphate (MAP), chemically precipitated struvite (CPST), electrochemically precipitated struvite (ECST), environmentally smart nitrogen (ESN)] and soil depth on water-stable aggregates (WSA) in furrow-irrigated rice on a silt-loam soil (Typic Albaqualf). Total WSA (TWSA) concentration was unaffected (P > 0.05) by fertilizer treatment or soil depth, while WSA concentration was numerically largest (P ∙g-1), which did not differ from CPST, ECST, and ESN in the 0 - 5 cm depth or the unamended control in the 0 - 5 and 5 - 10 cm depths, and was at least 1.7 times larger than ESN in the 5 - 10 cm depth (0.03 g∙g-1). Results indicated that WSA concentration among non-struvite fertilizer-P sources was generally similar to that from the struvite fertilizer materials. Principal component analysis determined that 32% of the variation of TWSA was mainly explained by changes in soil bulk density, pH, and electrical conductivity. Long-term, continual annual application of fertilizer-P and N could negatively impact soil aggregate stability, soil structure, and potentially erosion.

The features of soil aggregation and its eco-environmental effects under different subalpine forests on the east slope of Gongga Mountain, China

Structural properties of forest soils have important hydro-ecological function and can influence the soil water-physical characters and soil erosion. The experimental soil samples were obtained in surface horizon (0-10 cm) from different subalpine forest types on east slope of Gongga Mountain in the upriver area of Yangtze River China in May 2002. The soil bulk density, porosity, stable infiltration rate, aggregate distribution and particle-size distribution were analyzed by the routine methods in room, and the features and effects on eco-environment of soil aggregation were studied. The results showed that the structure of soil under mixed mature forest is in the best condition and can clearly enhance the eco-environmental function of soil, and the soil structure under the clear-cutting forest is the worst, the others are ranked between them. The study results can offer a basic guidance for the eco-environmental construction in the upper reaches of Yangtze River.

Long-Term Manure Amendments Enhance Soil Aggregation and Carbon Saturation of Stable Pools in North China Plain

Organic amendment is considered as an effective way to increase soil organic carbon （SOC） stock in croplands. To better understand its potential for SOC sequestration, whether SOC saturation could be observed in an intensive agricultural ecosystem receiving long-term composted manure were examined. Different SOC pools were isolated by physical fractionation techniques ofa Cambisol soil under a long-term manure experiment with wheat-maize cropping in North China Plain. A field experiment was initiated in 1993, with 6 treatments including control （i.e., without fertilization）, chemical fertilizer only, low rate of traditional composted manure （7.5 t ha-h）, high rate of traditional composted manure （15 t ha-~）, low rate ofbio-composted manure （7.5 t ha-h） and high rate of bio-composted manure （15 t ha-h）. The results showed that consecutive （for up to 20 years） composted manure amendments significantly improved soil macro-aggregation, aggregate associated SOC concentration, and soil structure stability. In detail, SOC concentration in the sand-sized fraction （〉53 ~tm） continued to increase with manure application rate, while the silt （2-53 I.tm） and clay （〈2 ~tm） particles showed no further increase with greater C inputs, exhibiting the C saturation. Further physical separation of small macro-aggregates （250-2 000 tam） into subpools showed that the non-protected coarse particulate organic matter （cPOM, 〉250 pro） was the fraction in which SOC continued to increase with increasing manure application rate. In contrast, the chemical and physical protected C pools （i.e., micro-aggregates and silt-clay occluded in the small macro- aggregates） exhibited no additional C sequestration when the manure application rate was increased. It can be concluded that repeated manure amendments can increase soil macro-aggregation and lead to the increase in relatively stable C pools, showing hierarchical saturation behavior in the intensive cropping system of North China Plain.

Soil Aggregation and Its Relationship with Organic Carbon of Purple Soils in the Sichuan Basin,China

The interaction of soil aggregate dynamics with soil organic carbon is complex with varied spatio-temporal processes in macro-and micro-aggregates. This paper is to determine the aggregation of soil aggregates in purple soils （Regosols in FAO Taxonomy or Entisols in USDA Taxonomy） for four types of land use, cropland [corn （Zea mays L.）], orchard （citrus）, forestland （bamboo or cypress）, and barren land （wild grass）, and to explore their relationship with soil organic carbon in the Sichuan basin of southwestern China. Procedures and methods, including manual dry sieving procedure, Yoder＇s wet sieving procedure, pyrophosphates solution method, and Kachisky method, are used to acquire dry, wet, and chemically stable aggregates, and microaggregates. Light and heavy fractions of soil organic carbon were separated using 2.0 g·mL^-1 HgI2-KI mixed solution. The loosely, stably, and tightly combined organic carbon in heavy fraction were separated by extraction with 0.1 M NaOH and 0.1 M NaOH-0.1M Na4P2O7 mixed solution （pH 13）. The results show that the contents of dry and wet macroaggregates 〉0.25 mm in diameter were 974.1 and 900.0 g·kg^-1 highest in red brown purple soils under forestland, while 889.6 and 350.6 g·kg^-1 lowest in dark purple soil and lowest in grey brown purple soils under cropland, respectively. The chemical stability of macroaggregates was lowest in grey brown purple soil with 8.47% under cropland, and highest in red brown purple soil with 69.34% under barren land. The content of microaggregates in dark purple soils was 587g·kg^-1 higher than brown purple soils, while 655g·kg^-1 in red brown purple soils was similar to grey brown purple soils （651g·kg^-1）. Cropland conditions, only 38.4% of organic carbon was of the combined form, and 61.6% of that existed in light fraction. Forestland conditions, 90.7% of organic carbon in red brown purple soil was complexed with minerals as a form of humic substances. The contents and stability of wet aggregates 〉 0.25 mm, contents and stability of chemically stable aggregates 〉0.25 mm, contents of microaggregates 〉 0.01 mm, contents of aggregated primary particle （d〈0.01 mm） and degree of primary particles （d 〈0.01 mm） aggregation were closely related to the concentrations of total soil organic carbon, and loosely and tightly combined organic carbon in heavy fraction. Soil microaggregation could be associated with organic carbon concentration and its combined forms in heavy fraction. There was a direct relationship between microaggregation and macroaggregation of soil primary particles, because the contents of wet aggregates 〉 0.25 mm and its water stability of aggregates were highly correlated with the contents of aggregated primary particle （d 〈 0.01 mm） and the degree of primary particles （d 〈 0.01 mm） aggregation.

Subsoil tillage enhances wheat productivity,soil organic carbon and available nutrient status in dryland fields

Tillage practices during the fallow period benefit water storage and yield in dryland wheat crops.However,there is currently no clarity on the responses of soil organic carbon(SOC),total nitrogen(TN),and available nutrients to tillage practices within the growing season.This study evaluated the effects of three tillage practices(NT,no tillage;SS,subsoil tillage;DT,deep tillage)over five years on soil physicochemical properties.Soil samples at harvest stage from the fifth year were analyzed to determine the soil aggregate and aggregate-associated C and N fractions.The results indicated that SS and DT improved grain yield,straw biomass and straw carbon return of wheat compared with NT.In contrast to DT and NT,SS favored SOC and TN concentrations and stocks by increasing the soil organic carbon sequestration rate(SOCSR)and soil nitrogen sequestration rate(TNSR)in the 0-40 cm layer.Higher SOC levels under SS and NT were associated with greater aggregate-associated C fractions,while TN was positively associated with soluble organic nitrogen(SON).Compared with DT,the NT and SS treatments improved soil available nutrients in the 0-20 cm layer.These findings suggest that SS is an excellent practice for increasing soil carbon,nitrogen and nutrient availability in dryland wheat fields in North China.

Impact of wetting-drying cycles and acidic conditions on the soil aggregate stability of yellow‒brown soil

Soil aggregate is the basic structural unit of soil,which is the foundation for supporting ecosystem functions,while its composition and stability is significantly affected by the external environment.This study was conducted to explore the effect of external environment(wetting-drying cycles and acidic conditions)on the soil aggregate distribution and stability and identify the key soil physicochemical factors that affect the soil aggregate stability.The yellow‒brown soil from the Three Gorges Reservoir area(TGRA)was used,and 8 wetting-drying conditions(0,1,2,3,4,5,10 and 15 cycles)were simulated under 4 acidic conditions(pH=3,4,5 and 7).The particle size distribution and soil aggregate stability were determined by wet sieving method,the contribution of environmental factors(acid condition,wetting-drying cycle and their combined action)to the soil aggregate stability was clarified and the key soil physicochemical factors that affect the soil aggregate stability under wetting-drying cycles and acidic conditions were determined by using the Pearson’s correlation analysis,Partial least squares path modeling(PLS‒PM)and multiple linear regression analysis.The results indicate that wetting-drying cycles and acidic conditions have significant effects on the stability of soil aggregates,the soil aggregate stability gradually decreases with increasing number of wetting-drying cycles and it obviously decreases with the increase of acidity.Moreover,the combination of wetting-drying cycles and acidic conditions aggravate the reduction in the soil aggregate stability.The wetting-drying cycles,acidic conditions and their combined effect imposes significant impact on the soil aggregate stability,and the wetting-drying cycles exert the greatest influence.The soil aggregate stability is significantly correlated with the pH,Ca^(2+),Mg^(2+),maximum disintegration index(MDI)and soil bulk density(SBD).The PLS‒PM and multiple linear regression analysis further reveal that the soil aggregate stability is primarily influenced by SBD,Ca^(2+),and MDI.These results offer a scientific basis for understanding the soil aggregate breakdown mechanism and are helpful for clarifying the coupled effect of wetting-drying cycles and acid rain on terrestrial ecosystems in the TGRA.

Calcium carbonate promotes the formation and stability of soil macroaggregates in mining areas of China

We studied changes in the concentrations of aggregate-cementing agents after different reclamation times and with different fertilization regimes,as well as the formation mechanism of aggregates in reclaimed soil,to provide a theoretical basis for rapid reclamation of soil fertility in the subsidence area of coal mines in Shanxi Province,China.In this study,soil samples of 0–20 cm depth were collected from four fertilization treatments of a longterm experiment started in 2008:no fertilizer (CK),inorganic fertilizer (NPK),chicken manure compost (M),and50%inorganic fertilizer plus 50%chicken manure compost (MNPK).The concentrations of cementing agents and changes in soil aggregate size distribution and stability were analysed.The results showed that the formation of>2 mm aggregates,the aggregate mean weight diameter (MWD),and the proportion of>0.25 mm water-stable aggregates (WR_(0.25)) increased significantly after 6 and 11 years of reclamation.The concentration of organic cementing agents tended to increase with reclamation time,whereas free iron oxide (Fed) and free aluminium oxide(Ald) concentrations initially increased but then decreased.In general,the MNPK treatment signi?cantly increased the concentrations of organic cementing agents and CaCO_(3),and CaCO_(3) increased by 60.4%at 11 years after reclamation.Additionally,CaCO_(3) had the greatest effect on the stability of aggregates,promoting the formation of>0.25 mm aggregates and accounting for 54.4%of the variance in the proportion and stability of the aggregates.It was concluded that long-term reclamation is bene?cial for improving soil structure.The MNPK treatment was the most effective measure for increasing maize grain yield and concentration of organic cementing agents and CaCO_(3).

Impact of biochar amendment on soil aggregation varied with incubation duration and biochar pyrolysis temperature

Soil aggregation is one of the crucial processes that facilitate carbon sequestration and maintain soil fertility.So far,the effect of biochar amendment on soil aggregation remains inconclusive.Here,we tested the hypothesis that the response of soil aggregation to biochar addition varied with incubation duration and biochar chemistry.A one year microcosm experiment of soil with biochar was conducted that included biochar produced at three different temperatures(300,450,and 600°C),and three biochar application rates,i.e.,0,1,and 3 wt%.It was observed that after one and three months,biochar mainly(>90%)distributed in the micro-aggregates,and slightly reduced aggregate stability and increased proportion of micro-aggregates,which was demonstrated to result from the mechanical mixture of amended biochar with soil.Contrastingly,when the duration was prolonged to six months and one year,a significant increase in macro-aggregates(6.6-38.5%)and aggregate stability(7.3-29.4%)was detected,with the increasing extent being apparently higher for low-temperature biochar.This was related to the comparatively strong interaction of biochar particles with soil minerals or microbes after long-time incubation.The strong interaction was directly supported by the significant increase in H/C,O/C ratios of isolated biochar from treated soils,the detection of typical soil mineral elements on the surface of isolated biochar,and the increase in microbial biomass carbon of treated soils.The findings of this study highlighted the role of biochar type and amendment duration in mediating the effect of biochar application on soil aggregation.

Enhancing soil aggregation and acetamiprid adsorption by ecofriendly polysaccharides hydrogel based on Ca^(2+)-amphiphilic sodium alginate

Soil aggregation plays an important role in agricultural production activities.However,the structure of soil aggregation is destroyed by the natural environment and unreasonable farming management,resulting in the loss of water,fertilizers and pesticides in soil.At present,hydrogels have been widely reported to promote the formation of soil aggregation.In this paper,amphiphilic calcium alginate(ASA/Ca^(2+))was applied to promote the formation of soil aggregation and enhance pesticide retention.Initially,an ASA was obtained through the one-pot Ugi condensation(a four-component green chemical reaction).Then,ASA/Ca^(2+) hydrogel is prepared by Ca^(2+) cross-linking.The formation of soil aggregation was determined through the Turbiscan Lab Expert stability analyzer,Confocal Laser Scanning Microscope(CLSM),and Transmission Electron Microscope(TEM).And the effect of soil aggregation on acetamiprid environmental behavior was investigated by adsorption kinetics,adsorption isotherms,and leaching.The results shown that the three-dimensional network structure of ASA/Ca^(2+) hydrogel can promote the formation of soil aggregation.Aggregate durability index(ADI)was 0.55 in the presence of ASA/Ca^(2+) hydrogel,indicating that amphiphilic hydrogel can enhance the stability of soil aggregation.The adsorbing capacity of acetamiprid was 1.58 times higher than pure soil,and the release of acetamiprid only about 20%in the presence of ASA/Ca^(2+) hydrogel.These results would be helpful for the formation of soil aggregation and pesticides adsorption on soil aggregation.Thus,ASA/Ca^(2+) hydrogel is likely to improve soil quality,simultaneously it can minimize the mobility of pesticides in the agricultural system.

Changes in the soil microbial communities of different soil aggregations after vegetation restoration in a semiarid grassland,China

Soil aggregate fractions can regulate microbial community composition and structure after vegetation restoration.However,there has been less focus on the effects of soil aggregate fractions on the distributions of microbial communities.Here,we used phospholipid fatty acid(PLFA)analysis to explore the effects of different years of vegetation restoration(a 35-year-old Thymus mongolicus community(Re-35yrs)and a 2-year-old nongrazing grassland(Ug-2yrs))on microbial communities within different soil aggregate sizes(<0.25 mm,0.25–1 mm,1–2 mm,2–3 mm,3–5 mm and>5 mm).The results indicated that the amount of total PLFA in Re-35yrs was 10 times greater than that in Ug-2yrs.The soil aggregate stability increased with increasing duration of vegetation restoration.In Re-35yrs,the total PLFA shown an increase as the soil aggregate size increased,and the highest values were observed in 3–5 mm.Ug-2yrs differed from Re-35yrs,the soil microbial diversity was higher in medium particle sizes(1–2 mm and 2–3 mm)and lower in microaggregates(<0.25 mm and 0.25–1 mm)and macroaggregates(3–5 mm and>5 mm).Soil microbial diversity was highest in large particle size aggregates,which resulted in low environmental stress and strong stability.The same tendency was observed in the high values of cyc/prec,S/M and soil organic matter,which indicated a lower turnover speed(F/B)of fungal energy utilization and a higher fixation rate.After years of natural restoration,the soil microbial community generally transformed from nutrient-rich to heterotrophdominant,especially in microaggregates(reflected in the G^(+)/G^(–)ratio).

Effects of the combined application of organic and chemical nitrogen fertilizer on soil aggregate carbon and nitrogen:A 30-year study

To understand the long-term effects of combined organic and chemical nitrogen fertilization on soil organic C(SOC) and total N(TN), we conducted a 30-year field experiment with a wheat–maize rotation system on the Huang-HuaiHai Plain during 1990–2019. The experimental treatments consisted of five fertilizer regimes: no fertilizer(control), chemical fertilizer only(NPK), chemical fertilizer with straw(NPKS), chemical fertilizer with manure(NPKM), and 1.5 times the rate of NPKM(1.5NPKM). The NPK, NPKS, and NPKM treatments had equal N inputs. The crop yields were measured over the whole experimental duration. Soil samples were collected from the topsoil(0–10 and 10–20 cm) and subsoil(20–40 cm) layers for assessing soil aggregates and taking SOC and TN measurements. Compared with the NPK treatment, the SOC and TN contents increased significantly in both the topsoil(24.1–44.4% for SOC and 22.8–47.7% for TN) and subsoil layers(22.0–47.9% for SOC and 19.8–41.8% for TN) for the organically amended treatments(NPKS, NPKM and 1.5NPKM) after 30 years, while no significant differences were found for the average annual crop yields over the 30 years of the experiment. The 0–10 cm layer of the NPKS treatment and the 20–40 cm layer of the NPKM treatment had significantly higher macroaggregate fraction mass proportions(19.8 and 27.0%) than the NPK treatment. However, the 0–10 and 20–40 cm layers of the 1.5NPKM treatment had significantly lower macroaggregate fraction mass proportions(–19.2 and –29.1%) than the control. The analysis showed that the higher SOC and TN in the soil of organically amended treatments compared to the NPK treatment were related to the increases in SOC and TN protected in the stable fractions(i.e., free microaggregates and microaggregates within macroaggregates), in which the contributions of the stable fractions were 81.1–91.7% of the increase in SOC and 83.3–94.0% of the increase in TN, respectively. The relationships between average C inputs and both stable SOC and TN stocks were significantly positive with R2 values of 0.74 and 0.72(P<0.01) for the whole 40 cm soil profile, which indicates the importance of N for soil C storage. The results of our study provide key evidence that long-term combined organic and chemical nitrogen fertilization, while maintaining reasonable total N inputs, benefited soil C and N storage in both the topsoil and subsoil layers.

Fragmentation process of soil aggregates under concentrated water flow in red soil hilly region with different land use patterns

The hilly area of red soil in the central subtropical region of China has a long history of severe soil erosion due to its abundance of water,heat,and intense agricultural and forestry activities.The Sandshale red soil area is hot and rainy,the local land utilization rate and replanting index are high,and the soil easily weathers and erodes,resulting in infertile and sandy soils,extensive soil erosion and large erosion,with far-reaching impacts.In this study,the stability of soil aggregates was studied by the wet sieving method and Le Bissonais(LB)method in six land use patterns in the Sandshale red soil area,including natural forest(NF),Pinus massoniana(PM),Eucalyptus urophylla×E.grandis(EU),orchard(OR),wasteland(WL)and arable land(AL).The transport damage characteristics of the soil aggregates under concentrated water flow were analyzed by using the soil aggregates to simulate the soil surface roughness in the field using a steel scouring flume with a variable slope.The results showed that:(1)the total soil porosity of the natural forest was the highest,with 56.51%in A layer,which was 4.99%higher than the B layer,and the organic matter content ranged from 10.69 to 29.94 g.kg-1 and was highest in NF and lowest in AL;(2)the maximum mean weight diameter(MWD)obtained by the wet sieving method was 4.81 mm for natural forest,and the MWD was the lowest in OR and AL at 2.45-2.77mm.The MWD measured by the LB method was also highest in NF and lowest in AL.The contents of Fed and Ald have a strong correlation with the stability parameters of soil aggregates;(3)the Wr/Wi results for the six land use patterns were NF>PM>EU>WL>OR>AL;the NF had the strongest soil aggregate stability,followed by WL,PM and EU,and AL and OR had the weakest;the stability of soil aggregates gradually weakened as the soil depth increased.Comprehensive analysis shows that forest land has high soil stability and obvious advantages in soil erosion resistance.Strengthening the construction of artificial forests can be an important means to reduce soil erosion in red soil hilly region.

Preliminary Study on the Effect of Different Ecological Cultivation Modes on the Water Stability of Soil Aggregates in Rubber Based Agroforestry Systems

Rubber trees (Hevea brasiliensis Müll. Arg.) have been commercially cultivated for a century and a half in Asia, particularly in China, and they constitute a common element of plantation ecosystems in tropical regions. Soil health is fundamental to the sustainable development of rubber plantations. The objective of the study is to explore the influence of different complex ecological cultivation modes on the stability of soil aggregates in rubber based agroforestry systems. In this study, the ecological cultivation mode of rubber—Alpinia oxyphylla plantation, the ecological cultivation mode of rubber—Phrynium hainanense plantations, the ecological cultivation mode of rubber—Homalium ceylanicum plantations and monoculture rubber plantations were selected, and the particle size distribution of soil aggregates and their water stability characteristics were analyzed. The soil depth of 0 - 20 cm and 20 - 40 cm was collected for four cultivation modes. Soil was divided into 6 particle levels > 20 cm. soil was divided into 6 particle levels > 5 mm, 2 - 5 mm, 1 - 2 mm, 0.5 - 1 mm, 0.25 - 0.5 mm, and 0.053 - 0.25 mm according to the wet sieve method. The particle size proportion and water stability of soil aggregates were determined by the wet sieve method. The particle size proportion and water stability of soil aggregates under different ecological cultivation modes were analyzed. The results showed that under different ecological cultivation modes in the shallow soil layer (0 - 20 cm), the rubber—Alpinia oxyphylla plantation and the rubber—Phrynium hainanense plantation promoted the development of dominant soil aggregates towards larger size classes, whereas the situation is the opposite for rubber—Homalium ceylanicum plantation. In soil layer (20 - 40 cm), the ecological cultivation mode of rubber—Phrynium hainanense plantation developed the dominant radial level of soil aggregates to the diameter level of large aggregates. Rubber—Alpinia oxyphylla plantation and rubber—Homalium ceylanicum plantation, three indicators, including the water-stable aggregate content R0.25 (>0.25 mm water-stable aggregates), mean weight diameter (MWD), and geometric mean diameter (GMD), were all lower than those in the rubber monoculture mode. However, in the rubber—Phrynium hainanense plantation, the water-stable aggregate content R0.25, mean weight diameter, and geometric mean diameter were higher than in the rubber monoculture mode, although these differences did not reach statistical significance.

Nitrogen rhizodeposition from corn and soybean,and its contribution to the subsequent wheat crops

Nitrogen(N)is a key factor in the positive response of cereal crops that follow leguminous crops when compared to gramineous crops in rotations,with the nonrecyclable rhizosphere-derived N playing an important role.However,quantitative assessments of differences in the N derived from rhizodeposition(NdfR)between legumes and gramineous crops are lacking,and comparative studies on their contributions to the subsequent cereals are scarce.In this study,we conducted a meta-analysis of NdfR from leguminous and gramineous crops based on 34 observations published worldwide.In addition,pot experiments were conducted to study the differences in the NdfR amounts,distributions and subsequent effects of two major wheat(Triticum aestivum L.)-preceding crops,corn(Zea mays L.)and soybean(Glycine max L.),by the cotton wick-labelling method in the main wheat-producing areas of China.The meta-analysis results showed that the NdfR of legumes was significantly greater by 138.93%compared to gramineous crops.In our pot experiment,the NdfR values from corn and soybean were 502.32 and 944.12 mg/pot,respectively,and soybean was also significantly higher than corn,accounting for 76.91 and 84.15%of the total belowground nitrogen of the plants,respectively.Moreover,in different soil particle sizes,NdfR was mainly enriched in the large macro-aggregates(>2 mm),followed by the small macro-aggregates(2–0.25 mm).The amount and proportion of NdfR in the macro-aggregates(>0.25 mm)of soybean were 3.48 and 1.66 times higher than those of corn,respectively,indicating the high utilization potential of soybean NdfR.Regarding the N accumulation of subsequent wheat,the contribution of soybean NdfR to wheat was approximately 3 times that of corn,accounting for 8.37 and 4.04%of the total N uptake of wheat,respectively.In conclusion,soybean NdfR is superior to corn in terms of the quantity and distribution ratio of soil macro-aggregates.In future field production,legume NdfR should be included in the nitrogen pool that can be absorbed and utilized by subsequent crops,and the role and potential of leguminous plants as nitrogen source providers in crop rotation systems should be fully utilized.

Distribution of soil water-stable aggrega-tes and organic carbon content affected by tillage systems:a meta-analysis

A better understanding of soil carbon(C)distribution within aggregate fractions is essential to evaluating the potential of no-till for sustaining productivity and protecting the environment.A meta-analysis on 744 comparisons from 34 studies was conducted to determine the effects of three different tillage treatments(conventional mouldbould ploughing tillage(CT),reduced tillage(RT)and no tillage(NT))on water-stable aggregate size distribution,soil C concentration in aggregate fractions.The meta-analysis indicates that compared with CT treatment,NT/RT significantly(P<0.05)increases macro-aggregate above 20 cm by 20.9%-82.2%(>2.00 mm)and 5.9%-19.1%(0.25-2.00 mm),whereas NT/RT significantly reduces micro-aggregate and silt clay fractions above 20 cm.NT/RT significantly(P<0.05)increases the SOC in macro-aggregate(>0.25 mm)and micro-aggregate(<0.25 mm)size classes above 20 cm soil depth compared with CT.The results suggest that soil sampling depth should be considered to evaluate the influence of tillage systems on the distribution of soil aggregate,and the content of aggregate-associated C content.

Effects of Fertilization Patterns on Chemical Forms of Nitrogen in Dark Brown Soil and Its Distribution in Different Aggregates

[Objective] This study was conducted to investigate the effects of tradition-al fertilization and formula fertilization by soil testing on the chemical forms of nitro-gen in dark brown soil and its distribution in different aggregates. [Method] A physi-co-chemistry method was adopted in a comparative study on the chemical forms of nitrogen and their distribution in different-sized aggregates of dark brown soil under traditional fertilization and formula fertilization by soil testing respectively. [Result] Compared with traditional fertilization in spring and autumn, the formula fertilization by soil testing averagely decreased, the total nitrogen in soil by 23.2% in spring and by 20% in autumn in the soil layer of 0-20 cm, by 48.8% in the layer of 20-40 cm. Ammonium nitrogen was so sensitive to the methods of fertilization that the content of ammonium nitrogen was reduced much more under formula fertitization by soil testing in autumn than under traditional fertilization. Nitrogen in soil under traditional fertilization pattern was mainly distributed in the aggregates of 0-0.25 and 0.5-1 mm, while in formula fertilization by soil testing it was mainly distributed in the aggregates of 0.25-0.5 and 0-0.25 mm. [Conclusion] The study proved that for-mula fertilization by soil testing helped to reduce the risk of nitrogen pol ution and had huge effects on the chemical forms and distribution of nitrogen in different ag-gregates in dark brown soil.

Paddy Soil Stability and Mechanical Properties as Affected by Long-Term Application of Chemical Fertilizer and Animal Manure in Subtropical China

Wet stability, penetration resistance （PR）, and tensile strength （TS） of paddy soils under a fertilization experiment for 22 years were determined to elucidate the function of soil organic matter in paddy soil stabilization. The treatments included no fertilization （CK）, normal chemical fertilization （NPK）, double the NPK application rates （2NPK）, and NPK mixed with organic manure （NPK＋OM）. Compared with CK, Fertilization increased soil organic carbon （SOC） and soil porosity. The results of soil aggregate fragmentation degree （SAFD） showed that fast wetting by water was the key fragmentation mechanism. Among the treatments, the NPK＋OM treatment had the largest size of water-stable aggregates and greatest normal mean weight diameter （NMWD） （P ≤ 0.05）, but the lowest PR and TS in both cultivated horizon （Ap） and plow pan. The CK and 2NPK treatments were measured with PR 〉 2.0 MPa and friability index 〈 0.20, respectively, in the Ap horizon, suggesting that the soils was mechanically unfavourable to root growth and tillage. In the plow pan, the fertilization treatments had greater TS and PR than in CK. TS and PR of the tested soil aggregates were negatively correlated to SOC content and soil porosity. This study suggested that chemical fertilization could cause deterioration of mechanical properties while application of organic manure could improve soil stability and mechanical properties.

Long-Term Effect of No-Tillage on Soil Organic Carbon Fractions in a Continuous Maize Cropping System of Northeast China

Increasing evidence has shown that conservation tillage is an effective agricultural practice to increase carbon （C） sequestration in soils. In order to understand the mechanisms underlying the responses of soil organic carbon （SOC） to tillage regimes, physical fractionation techniques were employed to evaluate the effect of long-term no-tillage （NT） on soil aggregation and SOC fractions. Results showed that NT increased the concentration of total SOC by 18.1% compared with conventional tillage （CT） under a long-term maize （Zea mays L.） cropping system in Northeast China. The proportion of soil large macroaggregates （〉 2 000 μm） was higher in NT than that in CT, while small macroaggregates （250-2 000μm） showed an opposite trend. Therefore, the total proportion of macroaggregates （〉 2 000 and 250-2 000μm） was not affected by tillage management. However, C concentrations of macroaggregates on a whole soil basis were higher under NT relative to CT, indicating that both the amount of aggregation and aggregate turnover affected C stabilization. Carbon concentrations of intra-aggregate particulate organic matter associated with microaggregates （iPOM-m） and microaggregates occluded within macroaggregates （iPOM-mM） in NT were 1.6 and 1.8 times greater than those in CT, respectively. Carbon proportions of iPOM-n and iPOM-mM in the total SOC increased from 5.4% and 6.3% in CT to 7.2% and 9.7% in NT, respectively. Furthermore, the difference in the microaggregate protected C （i. e., iPOM-m and iPOM-mM） between NT and CT could explain 45.4% of the difference in the whole SOC. The above results indicate that NT stimulates C accumulation within microaggregates which then are further acted upon in the soil to form macroaggregates. The shift of SOC within microaggregates is beneficial for long-term C sequestration in soil. We also corroborate that the microaggregate protected C is useful as a pool for assessing the impact of tillage management on SOC storage.

Organic carbon stratification and size distribution of three typical paddy soils from Taihu Lake region,China

Developing realistic soil carbon （C） sequestration strategies for China＇s sustainable agriculture relies on accurate estimates of the amount, retention and turnover rates of C stored in paddy soils. Available C estimates to date are predominantly for the tilled and flood-irrigated surface topsoil （ca. 30 cm）. Such estimates cannot be used to extrapolate to soil depths of 100 cm since soil organic carbon （SOC） generally shows a sharp decrease with depth. In this research, composite soil samples were collected at several depths to 100 cm from three representative paddy soils in the Taihu Lake region, China. Soil organic carbon distribution in the profiles and in aggregate-size fractions was determined. Results showed that while SOC decreased exponentially with depth to 100 cm, a substantial proportion of the total SOC （30%-40%） is stored below the 30 cm depth. In the carbon-enriched paddy topsoils, SOC was found to accumulate preferentially in the 2-0.25 and 0.25-0.02 mm aggregate size fractions. δ^13C analysis of the coarse micro-aggregate fraction showed that the high degree of C stratification in the paddy topsoil was in agreement with the occurrence of lighter δ^1313C in the upper 30 cm depth. These results suggest that SOC stratification within profiles varies with different pedogenetical types of paddy soils with regards to clay and iron oxyhydrates distributions. Sand-sized fractions of aggregates in paddy soil systems may play a very important role in carbon sequestration and turnover, dissimilar to other studied agricultural systems.