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.

An evaluation on using soil aggregate stability as the indicator of interrill erodibility

Aggregate stability is a very important predictor of soil structure and strength, which influences soil erodibility. Several aggregate stability indices were selected erodibility of four soil properties from temperate for estimating interrill types with contrasting and subtropical regions of China. This study was conducted to investigate how closely the soil interrill erodibility factor in the Water Erosion Prediction Project （WEPP） model relates to soil aggregate stability. The mass fractal dimension （FD）, geometric mean diameter （GMD）, mean weight diameter （MWD）, and aggregate stability index （ASI） of soil aggregates were calculated. A rainfall simulator with a drainable flume （3.0 m long × 1.0 m wide × 0.5 m deep） was used at four slope gradients （5°,10 °,15° and 20°）, and four rainfall intensities （0.6, 1.1, 1.7 and 2.5 mm/min）. Results indicated that the interriU erodibility （Ki） values were significantly correlated to the indices of ASI, MWD, GMD, and FD computed from the aggregate wet-sieve data. The Kihad a strong positive correlation with FD, as well as a strong negative correlation with ASI, GMD, and MWD. Soils with a higher aggregate stability and lower fractal dimension have smaller Ki values. Stable soils were characterized by a high percentage of large aggregates and the erodible soils by a high percentage of smaller aggregates. The correlation coefficients of Ki with ASI and GMD were greater than those with FD and MWD, implying that both the ASI and GMD may be better alternative parameters for empirically predicting the soil Ki factor. ASI and GMD are more reasonable in interrill soil erodibility estimation, compared with Ki calculation in original WEPP model equation. Results demonstrate the validation of soil aggregation characterization as an appropriate indicator of soil susceptibility to erosion in contrasting soil types in China.

Soil Aggregates, Organic Matter, and Labile C and N Fractions after 37 Years of N, P and K Applications to an Irrigated Subtropical Soil under Maize-Wheat Rotation

Abstract： Physical, chemical and biological soil properties in surface （0-5 cm） and subsurface soil （5-15 cm） were determined in a field experiment conducted with seven treatments consisted of different combinations of fertilizer N （0, 100 and 200 kg N ha^-1）, P （0, 22 and 44 kg P2O5 ha^-1） and K （0, 41 and 82 kg K2O ha^-1） applied both to summer-grown maize （Zea mays L.） and winter-grown wheat （Triticum aestivum L.） crops continuously for 37 years under irrigated subtropical conditions. Application of N, P and K significantly increased water stable aggregates and had profound effects in increasing the mean weight diameter as well as the formation of macro-aggregates, which were highest in both surface （81%） and subsurface （74%） soil layers with application of 100 kg N ＋ 22 kg P2O5 ＋ 41 kg K2O ha^-1 （N100P22K41）. The N100P22K41 treatment also enhanced total organic C （TOC） from 4.4 g kg^-1 in no-NPK control to 4.8 g kg^-1in surface layer and from 3.3 to 4.1 g kg1 in subsurface layer leading to the 20% higher TOC stocks in 0-15 cm soil. The labile C and N fractions such as water soluble C, particulate and light fraction organic matter, potentially mineralizable N and microbial biomass were also highest under the optimized balanced application of N100P22K41. Relatively higher increase in all labile fractions of C and N as proportion of TOC and total N, respectively suggested that these are potential indicators to reflect changes in management practices long before changes in TOC and TN are detectable. These results demonstrated that optimized balanced application of N, P and K is crucial for improving soil health ensuring long-term sustainability of farming systems in semiarid subtropical soils.

Soil Aggregates and Fractal Features under Different Land Use Types in a Frequent Debris Flow Area

The stability of soil aggregates and the fractal characteristics of four typical land use types(farmland,grassland,woodland,and bare land) in the Jiangjiagou Ravine(Yunnan,China),a frequent debris flow occurring area,were studied according to the normal mean mass diameter and fractal theory.The present research showed that the stability of the soil aggregates was different for the different land use types.When the soil depth was 0-30 cm,farmland soil formed more aggregates with diameters greater than 0.25 mm,i.e.,the farmland soil was more stable than that of the other three land uses.When the soil depth was 30-45 cm,the order of stability of the soil aggregates was woodland > grassland > farmland > bare land.The fractal dimensions had a significant linear positive correlation with the amount of soil particles with diameters of <0.25 mm,and a significant negative linear correlation with the amount of soil particles with diameters of 0.25-0.5 mm,0.5-1 mm and 1-2 mm.Smaller fractal dimensions of the soil particles correlated with more stable soil aggregates.The fractal dimensions had a positive linear correlation with the soil bulk density and a negative correlation with the concentration of organic matter.These results showed that soil aggregates can be used as a parameter for characterizing the soil structures and properties.According to these results,the soil particle fractal dimensions could not only objectively characterize the stability of the soil structure but also could be used to indicate soil structure and properties.In addition,these results have great significance for the discussion of the comprehensive evaluation of soil.

Effects of Different Organic Residues on Rice Yield and Soil Quality

Calcaric regosols are a valuable land resource, distributed widely across western China. Soil quality has deteriorated considerably in recent years owing to the blind pursuit of economic benefits. A 2-year field experiment was carried out to evaluate the effects of using spent mushroom compost, leguminous plant (Vicia sepium L.) compost, and a combination of the two (at a 1:1 and 2:1 ratio), on rice yield and soil quality in a suburb of China. Vicia sepium L. composted with spent mushroom compost at a 1:1 ratio produced the highest grain and stover yield, grain and stover phosphorus concentration, and phosphorus uptake of rice; they were 56.5%, 93.2%, 89.3%, 198.6% and 22.2% greater than control soil, respectively. The 2:1 ratio (Vicia sepium L.: spent mushroom compost) produced the highest grain N concentration, stover N concentration, and N uptake; they were 31.6%, 31.4%, and 40.7% higher than control, respectively. Soil physical, chemical, and environmental properties were improved with the application of Vicia sepium L. composted with spent mushroom compost at a 2:1 ratio. In particular, soil water-stable aggregates, organic carbon, particulate organic carbon, total nitrogen, available potassium, and cation exchange capacity increased, whereas bulk density, pH, and phytoavailable heavy metals decreased. This organic treatment is beneficial to improve soil quality indicators, and contribute to soil restoration.

土壤试样单轴压缩试验与离散元法模拟对比研究

土壤是典型的松散介质,利用粘连的颗粒离散元模型模拟土壤聚团的行成及破裂一直是散体力学的热点之一。本研究详细描述了利用带粘连的颗粒离散元模型制备土壤聚团的方法,利用粘连颗粒离散元模型对土样单轴压缩试验进行数值模拟分析,将模拟结果与试验结果进行对比分析。结果表明:试验与模拟得到的压缩曲线走势吻合,均呈拟线性上升,达到峰值后光滑下降;试验和模拟得到的土壤压缩时的强度极限分别为21.5和24 kPa,模拟值高于试验值11.6%。

Carbon Mineralization Associated with Soil Aggregates as Affected by Short-term Tillage

Abstract： Tillage practice has received much attention due to its effects on greenhouse gas emissions from agricultural fields. The understanding of carbon mineralization associated with soil aggregates helps to explore the influence mechanisms of tillage practice on soil carbon dynamics. Total carbon and carbon mineralization rates associated with various sizes of soil aggregates under no-tillage and tillage treatments were studied with a volcanic ash soil. Total carbon content in microaggregates （〈0.25 mm） was higher than that in macroaggregates （〉0.25 mm） for both the no-tillage and tillage treatments, since microaggregates of the volcanic ash soil include more fine silts and clay particles absorbing more organic agents. The carbon mineralization rate and total carbon were highly correlated （R2 = 0.6552, P= 0.002） for both treatments, suggesting that soil aggregate size is an important factor to influence the carbon mineralization rate. The no-tillage system showed the advantage of improving soil structure for volcanic ash soil. A larger proportion of microaggregates with relatively high carbon mineralization might contribute to the greater carbon loss from tilled soils. Unlike aggregate size, short-term tillage showed no significant effects on carbon mineralization rates associated with aggregates in a specific size class.

Impact of Land Use and Soil Fertility on Distributions of Soil Aggregate Fractions and Some Nutrients

The size distribution of water-stable aggregates and the variability of organic C,N and P contents over aggregate size fractions were studied for orchard,upland,paddy,and grassland soils with high,medium,and low fertility levels.The results showed that > 5 mm aggregates in the cultivated upland and paddy soils were 44.0% and 32.0%,respectively,less than those in the un-tilled orchard soil.Organic C and soil N in different size aggregate fractions in orchard soil with high fertility were significantly higher than those of other land uses.However,the contents of soil P in different size aggregates were significantly greater in the paddy soil as compared to the other land uses.Soil organic C,N and P contents were higher in larger aggregates than those in smaller ones.The amount of water-stable aggregates was positively correlated to their contribution to soil organic C,N and P.For orchard and grassland soils,the > 5 mm aggregates made the greatest contribution to soil nutrients,while for upland soil,the 0.25-0.053 mm aggregates contributed the most to soil nutrients.Therefore,the land use with minimum disturbance was beneficial for the formation of a better soil structure.The dominant soil aggregates in different land use types determined the distribution of soil nutrients.Utilization efficiency of soil P could be improved by converting other land uses to the paddy soil.

Enzyme Activity in Water-Stable Soil Aggregates as Affected by Long-Term Application of Organic Manure and Chemical Fertiliser

The activities of invertase, protease, urease, acid phosphomonoesterase, dehydrogenase, and catalase in different fractions of waterstable aggregates (WSA) were examined in long-term (26 years) fertilised soils. The long-term application of organic manure (OM) with chemical fertiliser (CF) significantly increased macroaggregate and decreased microaggregate percentages, enhanced the mean weight diameter, and significantly increased soil total carbon (TC) and total nitrogen (TN) contents of WSA in different size fractions. Combined fertilisation with OM and CF also increased invertase, protease, urease, acid phosphomonoesterase, dehydrogenase, and catalase activities of WSA in different size fractions. Enzyme activities were higher in macroaggregates than in microaggregates. The distribution of enzyme activities generally followed the distribution of TC and TN in WSA. The geometric mean of the enzyme activities in different WSA of OM-treated soils was significantly higher than that in soils treated with 100% CF or no fertiliser. The results indicated that the long-term combined application of OM with CF increased the aggregate stability and enzyme activity of different WSA sizes, and consequently, improved soil physical structure and increased soil microbial activity.

Effects of Tillage Practices and Land Use Management on Soil Aggregates and Soil Organic Carbon in the North Appalachian Region,USA

Promoting soil carbon sequestration in agricultural land is one of the viable strategies to decelerate the observed climate changes. However, soil physical disturbances have aggravated the soil degradation process by accelerating erosion. Thus, reducing the magnitude and intensity of soil physical disturbance through appropriate farming/agricultural systems is essential to management of soil carbon sink capacity of agricultural lands. Four sites of different land use types/tillage practices, i） no-till （NT） corn （Zea mays L.） （NTC）, ii） conventional till （CT） corn （CTC）, iii） pastureland （PL）, and iv） native forest （NF）, were selected at the North Appalachian Experimental Watershed Station, Ohio, USA to assess the impact of NT farming on soil aggregate indices including water-stable aggregation, mean weight diameter （MWD） and geometric mean diameter （GMD）, and soil organic carbon and total nitrogen contents. The NTC plots received cow manure additions （about 15 t ha-1） every other year. The CTC plots involved disking and chisel ploughing and liquid fertilizer application （110 L ha-l）. The results showed that both water-stable aggregation and MWD were greater in soil for NTC than for CTC. In the 0-10 cm soil layer, the 〉 4.75-mm size fraction dominated NTC and was 46% more than that for CTC, whereas the 〈 0.25-mm size fraction was 380% more for CTC than for NTC. The values of both MWD and GMD in soil for NTC （2.17 mm and 1.19 mm, respectively） were higher than those for CTC （1.47 and 0.72 mm, respectively） in the 0-10 cm soil layer. Macroaggregates contained 6%-42% and 13%-43% higher organic carbon and total nitrogen contents, respectively, than microaggregates in soil for all sites. Macroaggregates in soil for NTC contained 40% more organic carbon and total nitrogen over microaggregates in soil for CTC. Therefore, a higher proportion of microaggregates with lower organic carbon contents created a carbon-depleted environment for CTC. In contrast, soil for NTC had more aggregation and contained higher organic carbon content within water-stable aggregates. The soil organic carbon and total nitrogen stocks （Mg ha-1） among the different sites followed the trend of NF 〉 PL 〉 NTC 〉 CTC, being 35%-46% more for NTC over CTC. The NT practice enhanced soil organic carbon content over the CT practice and thus was an important strategy of carbon sequestration in cropland soils.

Dynamic Relationship Between Biologically Active Soil Organic Carbon and Aggregate Stability in Long-Term Organically Fertilized Soils

Biologically active soil organic carbon （BASOC） is an important fraction of soil organic carbon （SOC）, but our understanding of the correlation between BASOC and soil aggregate stability is limited. At an ecological experimental station （28° 04＇-28° 37＇ N, 116° 41＇-117° 09＇ E） in Yujiang County, Jiangxi Province, China, we analyzed the dynamic relationship between soil aggregate stability and BASOC content over time in the red soil （Udic Ferrosols） fertilized with a nitrogen-phosphorus-potassium chemical fertilizer （NPK） without manure or with NPK plus livestock manure or green manure. The dynamics of BASOC was evaluated using CO2 efflux, and soil aggregates were separated according to size using a wet-sieving technique. The soils fertilized with NPK plus livestock manure had a significantly higher content of BASOC and an improved aggregate stability compared to the soils fertilized with NPK plus green manure or NPK alone. The BASOC contents in all fertilized soils decreased over time. The contents of large aggregates （800-2 000 μm） dramatically decreased over the first 7 d of incubation, but the contents of small aggregates （〈 800 μm） either remained the same or increased, depending on the incubation time and specific aggregate sizes. The aggregate stability did not differ significantly at the beginning and end of incubation, but the lowest stability in all fertilized soils occurred in the middle of the incubation, which implied that the soils had a strong resilience for aggregate stability. The change in BASOC content was only correlated with aggregate stability during the first 27 d of incubation.

Enzyme Activities and Microbial Communities in Subtropical Forest Soil Aggregates to Ammonium and Nitrate-Nitrogen Additions

A laboratory incubation experiment was established to examine the impacts of nitrate and ammonium nitrogen additions on soil microbial attributes of a subtropical Pinus elliottii forest ecosystem in southern China. Soils were subjected to three different treatments： the control with no nitrogen addition （CK）, the ammonium nitrogen addition （NH4^＋-N）, and the nitrate nitrogen addition （NO23^-N）. Samples from bulk and two different size fractions （macroaggregate （〉250 pm） and microaggregate （53-250 μm）） were analyzed for soil properties, enzyme activities and microbial communities on day 7 and 15 of the incubation. Our study demonstrated that NH4^＋-N had a 9rearer influence on soil microbial activities than NO3-N. NH4^＋-N additions resulted in significant increases in 13-1,4-glucosidase （βG） and β-1,4-N-acetyl glucosaminidase （NAG） enzyme activities in bulk, macroaggregate and microag- gregate soils after 7 and 15 days incubation. NO3^-N additions only significantly increased in βG and NAG enzyme activities in bulk, macroaggregate soils after 7 and 15 days incubation, but not in microaggregate. All NH4^＋-N and NO3-N additions resulted in significant increases in gram-positive bacterial PLFAs in microaggregates. Only a significant correlation between soil nutrient contents and enzyme activities in macroaggregates was founded, which suggests that the soil aggregation structure played an important role in the determining enzyme activities.

Meso-damage cracking characteristics analysis for rock and soil aggregate with CT test

The aim of this paper is to investigate the damage cracking characteristics of rock and soil aggregate(RSA)by X-ray computed tomography(CT)under uniaxial compressive loading.The mean CT value for the region of interest(ROI)is used to analyze the cracking characteristics.Also,the mathematical morphology method based on the image threshold segmentation is used to obtain characteristic parameters of cracks to describe the cracking evolution of RSA.Results show that the elastic mismatch between rock blocks and soil matrix is the primary reason for RSA cracking.The mean CT value for the RSA specimen,rock block inclusions,and their adjacent soil regions decreases with the increasing stress level.However,it is more sensitive for block inclusions than soil regions.Using the image segmentation method,length,area and mean width of cracks obey to power function distribution.Crack statistical characteristics are closely related to the rock block’s distribution and morphology.These results may be useful to reveal the mesoscopic cracking mechanism,establish meso-damage evolution equation,and constitutive relation for RSA.

Soil Aggregate Stability and Iron and Aluminium Oxide Contents Under Different Fertiliser Treatments in a Long-Term Solar Greenhouse Experiment

Soil in greenhouses is likely to suffer a gradual decline in aggregate stability. Determination of the effects of different fertiliser practices on soil aggregate stability is important for taking advantage of solar greenhouses. Soil aggregate stability and iron （Fe） and aluminium （A1） oxide contents were investigated in a 26-year long-term fertilisation experiment in greenhouse in Shenyang, China, under eight fertiliser treatments： manure （M）, fertiliser N （FN）, fertiliser N with manure （MN）, fertiliser P （FP）, fertiliser P with manure （MP）, fertiliser NP （FNP）, fertiliser NP with manure （MNP）, and control without any fertiliser （CK）. A wet sieving method was used to determine aggregate size distribution and water-stable aggregates （WSA）, mean weight diameter and geometric mean diameter as the indices of soil aggregate stability. Different fertiliser treatments had a statistically significant influence on aggregate stability and Fe and A1 oxide contents. Long-term application of inorganic fertilisers had no obvious effects on the mass proportion of aggregates. By contrast, manure application significantly increased the mass proportion of macroaggregates at the expense of microaggregates. All treatments, with the exception of FNP, significantly increased the stability of macroaggregates but decreased that of microaggregates when compared with CK. Aggregation under MP and MN was better than that under M and MNP; however, no significant differences were found among inorganic fertiliser treatments （i.e., FN, FP, and FNP）. A positive relation was found between pyrophosphate-extractable Fe and WSA （r=0.269）, but no significant relations were observed between other Fe and Al oxides and aggregate stability.

Process and Mechanism for the Development of Physical Crusts in Three Typical Chinese Soils

To compare the development of physical crusts in three typical cultivated soils of China, a black soil （Luvic Phaeozem）, a loess soil （Haplic Luvisol）, and a purple soil （Calcaric Regosol） were packed in splash plates with covered and uncovered treatments, and exposed to simulated rainfall. Meshes covered above the surfaces of half of soil samples to simulate the effects of crop residue on crusting. The results indicated a progressive breakdown of aggregates on the soil surface as rainfall continued. The bulk density and shear strength on the surface of the three soil types increased logarithmically as rainfall duration increased. During the first 30 min of simulated rainfall, the purple soil developed a 7-8 mm thick crust and the loess soil developed a 3-4 mm thick crust. The black soil developed a distinguishable, but still unstable, crust after 80 rain of simulated rainfall. Soil organic matter （SOM） content, the mean weight diameter （MWD） of soil aggregates, and soil clay content were negatively correlated with the rate of crust formation, whereas the percentage of aggregate dispersion （PAD）, the exchangeable sodium percentage （ESP）, and the silt and sand contents were positively correlated with crusting. Mechanical breakdown caused by raindrop impact was the primary mechanism of crust formation in the black soil with more stable aggregates （MWD 25.0 mm, PAD 3.1%） and higher SOM content （42.6 g kg-1）. Slaking and mechanical eluviation were the primary mechanisms of crust formation in the purple soil with low clay content （103 g kg-1）, cation exchange capacity （CEC, 228 mmol kg-1）, ESP （0.60%）, and SOM （17.2 g kg-1）. Mechanical breakdown and slaking were the most important in the loess soil with low CEC （80.6 mmol kg-1）, ESP （1.29%）, SOM （9.82 g kg-1）, and high PAD （71.7%） and MWD （4.6 mm）. Simulated residue cover reduced crust formation in black and loess soils, but increased crust formation in purple soil.

Aggregate Development and Organic Matter Storage in Mediterranean Mountain Soils

Soil aggregation and organic matter of soils from the pre-Pyrenean range in Catalonia (NE Spain) were studied,in order to assess their quality as carbon sinks and also to select the best soil management practices to preserve their quality.Aggregate stability,organic carbon and micromorphology were investigated.The highest amount of organic carbon was found in alluvial,deep soils (228 Mg C ha -1 ),and the lowest was in a shallow,stony soil with a low plant cover (78 Mg C ha -1 ).Subsurface horizons of degraded soils under pastures were the ones with smaller and less-stable aggregates.Fresh residues of organic matter (OM) were found mostly in interaggregate spaces.Within the aggregates there were some organic remains that were beginning to decompose,and also impregnative nodules of amorphous OM.Although OM was evenly distributed among the aggregate fractions,the larger blocky peds had more specific surface,contained less decomposed OM and had a lower organic/mineral interphase than smaller crumb aggregates,which were also more stable.Soil carbon storage was affected primarily by the OM inputs in the surface horizons.In order to store organic carbon over the mid- and long-term periods,the mechanisms favouring structuration through biological activity and creating small aggregates with intrapedal stable microporosities seemed to be the most effective.

Reconstruction of Soil Particle Composition During Freeze-Thaw Cycling： A Review

Studies conducted over several decades have shown that the freeze-thaw cycles are a process of energy input and output in soil, which help drive the formation of soil structure, through water expansion by crystallization and the movement of water and salts by thermal gradients. However, most of these studies are published in Russian or Chinese and are less accessible to international researchers. This review brought together a wide range of studies on the effects of freezing and thawing on soil structure. The following findings are summarized： i） soil structure after freeze-thaw cycles changes considerably and the changes are due to the mechanical fragmentation of soil coarse mineral particles and the aggregation of soil fine particles; ii） the particle size of soil becomes homogeneous and the variation in soil structure weakens as the number of freeze-thaw cycles increases; iii） in the freezing process of soil, an important principle in the variation of soil particle bonding is presented as： condensation →aggregation→ crystallization; iv） the freeze-thaw cycling process has a strong effect on soil structure by changing the granulometric composition of mineral particles and structures within the soil. The freeze-thaw cycling process strengthens particle bonding, which causes an overall increase in aggregate stability of soil, showing a process from destruction to reconstruction.

Responses of soil labile organic carbon and water-stable aggregates to reforestation in southern subtropical China

Aims Reforestation can enhance soil carbon(C)stability and promote soil C accumulation.Experimental results are,however,highly variable,and the efficacy of reforestation in enhancing soil C stability is still in debate.Consequently,it remains unclear how the different soil C pools respond to reforestation in forest ecosystems.Methods The response of different soil C fractions to reforestation was examined in five subtropical forests,including the plantations of Eucalyptus urophylla(EU),Acacia crassicarpa(AC),Castanopsis hystrix(CH)and 10 species mixed(MX),and a naturally recovered shrubland(NS).Soil labile C fractions(readily oxidized organic C by KMnO4:ROC;dissolved organic C:DOC),distribution of aggregate-size classes and aggregate-associated C from different soil layers(0–10,10–20,20–40 and 40–60 cm)were evaluated.Important Findings We found that reforestation and forest type did not affect ROC concentration,yet the highest DOC concentration was detected in NS at four soil layers.Aggregate C concentration was the highest in all aggregate-size classes of CH at 0–10 cm depth.In addition,forest type did not alter the proportion of soil water-stable aggregates at four soil layers.However,soil depths significantly affected the distribution of soil aggregates with>0.25 mm aggregates dominating in the topsoils(0–20 cm),but 0.053–2 mm aggregates being dominant in the deep soils(20–60 cm).These results indicate that reforestation and forest type affected soil DOC(0–60 cm)and aggregate C(0–10 cm).Furthermore,soil DOC and aggregate C were more susceptive to reforestation than ROC.The findings suggest that plantations reduce soil DOC concentration,highlighting that C leaching loss may decrease compared with natural recovery.Moreover,C.hystrix plantation may enhance soil C stability by physical protection in topsoil.This study provides valuable information on tree species selection for reforestation concerning soil C sequestration in southern subtropical China.

Effect of Rotational Tillage on Soil Aggregates, Organic Carbon and Nitrogen in the Loess Plateau Area of China

In rain-fed semi-arid agroecosystems, continuous conventional tillage can cause serious problems in soil quality and crop production, whereas rotational tillage （no-tillage and subsoiling） could decrease soil bulk density, and increase soil aggregates and organic carbon in the 0-40 cm soil layer. A 3-year field study was conducted to determine the effect of tillage practices on soil organic carbon （SOC）, total nitrogen （TN）, water-stable aggregate size distribution and aggregate C and N sequestration from 0 to 40 cm soil in semi-arid areas of southern Ningxia. Three tillage treatments were tested： no-tillage in year 1, subsoiling in year 2, and no-tillage in year 3 （NT-ST-NT）; subsoiling in year 1, no-tillage in year 2, and subsoiling in year 3 （ST-NT-ST）; and conventional tillage over years 1-3 （CT）. Mean values of soil bulk density in 0-40 cm under NT-ST-NT and ST-NT-ST were significantly decreased by 3.3% and 6.5%, respectively, compared with CT, while soil total porosity was greatly improved. Rotational tillage increased SOC, TN, and water-stable aggregates in the 0-40 cm soil, with the greatest effect under ST-NT-ST. In 0-20 and 2（}-40 cm soils, the tillage effect was confined to the 2-0.25 mm size fraction of soil aggregates, and rotational tillage treatments obtained significantly higher SOC and TN contents than conventional tillage. No significant differences were detected in SOC and TN contents in the 〉 2 mm and 〈 0.25 mm aggregates among all treatments. In conclusion, rotational tillage practices could significantly increase SOC and TN levels, due to a fundamental change in soil structure, and maintain agroecosystem sustainability in the Loess Plateau area of China.

Changes in Soil Organic Carbon After Five Years of Biowaste Compost Application in a Mediterranean Vegetable Cropping System

Biowaste compost can influence soil organic matter accumulation directly or indirectly. A 5-year experiment was conducted to assess the influence of biowaste compost on the process of soil aggregation and soil organic carbon （SOC） accumulation in a Mediterranean vegetable cropping system. The study involved four treatments： biowaste compost （COM）, mineral NPK fertilizers （MIN）, biowaste compost with half-dose N fertilizer （COMN）, and unfertilized control （CK）. The SOC stocks were increased in COM, COMN, and MIN by 20.2, 14.9, and 2.4 Mg ha-1 over CK, respectively. The SOC concentration was significantly related to mean weight diameter of aggregates （MWD） （P 〈 0.05, R^2 = 0.798 4） when CK was excluded from regression analysis. Compared to CK, COM and COMN increased the SOC amount in macroaggregates （〉 250 μm） by 2.7 and 0.6 g kg-1 soil, respectively, while MIN showed a loss of 0.4 g kg-1 soil. The SOC amount in free microaggregates （53-250 ttm） increased by 0.9, 1.6, and 1.0 g kg-1 soil for COM, COMN, and MIN, respectively, while those in the free silt plus clay aggregates （~ 53 ~m） did not vary significantly. However, when separating SOC in particle-size fractions, we found that more stable organic carbon associated with mineral fraction 〈 53 μm （MOM-C） increased significantly by 3.4, 2.2, and 0.7 g kg-1 soil for COM, COMN, and MIN, respectively, over CK, while SOC amount in fine particulate organic matter （POM） fraction （53-250 μm） increased only by 0.3 g kg-1 soil for both COM and COMN, with no difference in coarse POM 〉 250 μm. Therefore, we consider that biowaste compost could be effective in improving soil structure and long-term C sequestration as more stable MOM-C.