Effects of Long-Term Winter Planted Green Manure on Distribution and Storage of Organic Carbon and Nitrogen in Water-Stable Aggregates of Reddish Paddy Soil Under a Double-Rice Cropping System

In agricultural systems, maintenance of soil organic matter has long been recognized as a strategy to reduce soil degradation. Manure amendments and green manures are management practices that can increase some nutrient contents and improve soil aggregation. We investigated the effects of 28 yr of winter planted green manure on soil aggregate-size distribution and aggregateassociated carbon（C） and nitrogen（N）. The study was a randomized completed block design with three replicates. The treatments included rice-rice-fallow, rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass. The experiment was established in 1982 on a silty light clayey paddy soil derived from Quaternary red clay（classified as Fe-Accumuli-Stagnic Anthrosols） with continuous early and late rice. In 2009, soil samples were collected（0-15 cm depth） from the field treatment plots and separated into water-stable aggregates of different sizes（i.e., 〉5, 2-5, 1-2, 0.5-1, 0.25-0.5 and 〈0.25 mm） by wet sieving. The long-term winter planted green manure significantly increased total C and N, and the formation of the 2-5-mm water-stable aggregate fraction. Compared with rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass, the rice-rice-fallow significantly reduced 2-5-mm water-stable aggregates, with a significant redistribution of aggregates into micro-aggregates. Long-term winter planted green manure obviously improved C/N ratio and macro-aggregate-associated C and N. The highest contribution to soil fertility was from macro-aggregates of 2-5 mm in most cases.

Effects of Long-Term Fertilization on the Distribution of Carbon, Nitrogen and Phosphorus in Water-Stable Aggregates in Paddy Soil

We investigated the size distribution of water-stable aggregates and the soil carbon, nitrogen and phosphorus concentration over aggregate size fractions based on a long-term （1990-2006） fertilization experiment in a reddish paddy soil. The results showed that the largest water-stable aggregate （WSA） （〉5 mm） and the smallest WSA （〈0.25 mm） took up the first largest proportion （38.3%） and the second largest proportion （23.3%）, respectively. Application of organic materials increased the proportion of the large WSA （〉2 mm） and decreased the proportion of the small WSA （〈1 ram）, resulting in an increase in the mean weight diameter of WSA, whereas application of chemical fertilizer had little effect. Application of organic materials, especially combined with chemical fertilizers, increased total carbon, nitrogen and phosphorus concentrations in all sizes of WSA, and total carbon, nitrogen and phosphorus were prone to concentrate in the large WSA. Further more, application of organic materials improved the supply effectiveness of available phosphorus, whereas had little influence on the labile carbon in WSA. Application of chemical fertilizers improved concentrations of total and available phosphorus in all sizes of WSA, whereas had little influence on total carbon and nitrogen contents. Economical fertilization model maintained the soil fertility when compared with full dose of chemical fertilizers, indicating that using organic materials could reduce chemical fertilizers by about one third.

Relationship Between Water-Stable Aggregates and Nutrients in Black Soils After Reclamation

Water-stable aggregates, which are an index for the evaluation of the structural properties of the soil, are affected by many factors. Zhaoguang Farm, Longzhen Farm, and Jiusan Farm were chosen as the representative study sites in the region of black soils, a typical soil resource in Northeast China. The variation in the content of 〉 0.25 mm water-stable aggregates and its relationship with the nutrients in black soil were investigated after different years of reclamation. The results showed that the 〉 0.25 mm water-stable aggregates were more in the surface than in the subsurface soil and they changed in the following order： Longzhen Farm 〉 Zhaoguang Farm 〉 Jiusan Farm. The water-stable aggregates decreased sharply at the initial stage of reclamation and then became stable gradually with time. They were significantly correlated with the contents of organic C, total N, total P, and CEC in black soil, with the correlation coefficients r being 0.76, 0.68, 0.61, and 0.81 （P 〈 0.01）, respectively; however, their relationships with available P, available K, and total K were unclear. These showed that organic matter was the cementation of soil water-stable aggregates. Increasing decompositions and decreasing inputs of organic matter after reclamation were responsible for the amount of reduction of the water-stable aggregates. Thus, to maintain good soil aggregate structure, attention should be paid to improvement of soil nutrient status, especially the supply of organic C and N.

Light Fraction Carbon and Water-Stable Aggregates in Black Soils

The distribution of light fraction carbon （LF-C） in the various size classes of aggregates and its relationship to water- stable aggregates as well as the influence of cultivation on the organic components in virgin and cultivated black soils were studied by wet sieving and density separation methods. The total organic carbon （TOC） and LF-C were significantly higher （P≤ 0.05） in the virgin soils than in the cultivated soils. The LF-C in aggregates of different size classes varied from 0.9 to 2.5 g kg^-1 in the cultivated soils and from 2.5 to 7.1 g kg^-1 in the virgin soils, whereas the ratio of LF-C to TOC varied from 1.9% to 7.3% and from 5.0% to 12.2%, respectively. After being incubated under constant temperature and controlled humidity for three months, the contribution of LF-C to TOC sharply decreased to an amount （1.7%4.5%） close to the level in soils that had been cultivated for 20 to 25 years （1.3%-8.8%）. As a result, the larger water-stable macro-aggregates （especially 〉 1 mm） decreased sharply, indicating that the LF-C pool in virgin soils declined quickly after cultivation, which reduced the water stability of soil aggregates.

Influences of Seasonal Freezing and Thawing on Soil Water-stable Aggregates in Orchard in High Cold Region,Northeast China

Soil aggregate stability,as an important indicator of soil functions,may be affected by seasonal freezing and thawing(SFT)and land use in high cold and wet regions.Therefore,comprehensive understanding the effects of SFT on aggregate stability in orchards during winter and spring is crucial to develop appropriate management strategies that can effectively alleviate the degradation of soil quality to ensure sustainable development of orchard ecosystems.To determine the mechanism of degradation in orchard soil quality,the effects of SFT on the stability of water-stable aggregates were examined in apple-pear orchards(Pyrus ussuriensis var.ovoidea)of four different ages(11,25,40,and 63 yr)on 0 to 5%slopes before freezing and after thawing from October 2015 to June 2016 in Longjing City,Yanbian Prefecture,Northeast China,involving a comparison of planted versus adjacent uncultivated lands(control).Soil samples were collected to investigate water-stable aggregate stability in three incremental soil layers(0–20,20–40 and 40–60 cm).In the same samples,iron oxide,organic matter,and clay contents of the soil were also determined.Results showed that the destructive influences of SFT on water-stable aggregates were more pronounced with the increased orchards ages,and SFT exerted severe effects on water-stable aggregates of older orchards(40 and 63 yr)than juvenile orchards.Undergoing SFT,the soil instability index and the percentage of aggregate destruction increased by mean 0.15 mm and 1.86%,the degree of aggregation decreased by mean 1.32%,and the erosion resistance weakened,which consequently led to aggregate stability decreased.In addition,soil free,amorphous,and crystalline iron oxide as well as soil organic matter and clay contents are all important factors affecting the stability of water-stable aggregates,and their changes in their contents were consistent with those in the stability of water-stable aggregates.The results of this study suggest that long-term planting fruit trees can exacerbate the damaging effects of SFT on aggregate stability and further soil erosion increases and nutrient losses in an orchard,which hider sustainable use of soil and the productivity orchards.

Contents of soil organic carbon and nitrogen in water-stable aggregates in abandoned agricultural lands in an arid ecosystem of Northwest China

Soil organic matter content in water-stable aggregates（WSA） in the arid ecosystems（abandoned agricultural lands especially） of China is poorly understood. In this study, we examined the WSA sizes and stability, and soil organic carbon（OC） and nitrogen（N） contents in agricultural lands with abandonment ages of 0, 3, 12, 20, 30 and 40 years, respectively, in the Minqin Oasis of Northwest China. The total soil OC and N contents at depths of 0–20, 20–40 and 40–60 cm in abandoned agricultural lands were compared to those in cultivated land（the control）. Agricultural land abandonment significantly（P0.25 mm） as the age of agricultural land abandonment increased. The effect of abandonment ages of agricultural lands on MWD was determined by the changes of OC and N accumulation in WSA sizes ＆gt;2 mm. The total OC and N contents presented a stratification phenomenon across soil depths in this arid ecosystem. That is, both of them decreased significantly at depths of 0–20 and 40–60 cm while increased at the depth of 20–40 cm. The WSA sizes ＆lt;0.053 mm had the highest soil OC and N contents（accounting for 51.41%–55.59% and 42.61%–48.94% of their total, respectively）. Soil OC and N contents in microaggregates（sizes 0.053–0.25 mm） were the dominant factors that influenced the variations of total OC and N contents in abandoned agricultural lands. The results of this study suggested that agricultural land abandonment may result in the recovery of WSA stability and the shifting of soil organic matter from the silt＋clay（＆lt;0.053 mm） and microaggregate fractions to the macroaggregate fractions. However, agricultural land abandonment did not increase total soil OC and N contents in the short-term.

The Effects of Different Ratios of Feldspathic Sandstone and Sand Compound Soil on Water-stable Aggregates in 4 Years Crop Planting

To study the formation process of feldspathic sandstone and sand compound soil in the Mu Us Desert,1∶ 1,1∶ 2 and 1∶ 5 ratios of feldspathic sandstone and sand were mixed to obtain compound soil to plant crops,and analyze the rules of changes in water-stable aggregates of the compound soil among the 4 years crops growing process. The results showed,before crop planting,the order of mass percent of> 0. 25 mm and 0. 25-2. 00 mm water-stable aggregates in three kinds of compound soil was 1∶ 1 > 1∶ 2 > 1∶ 5,showing that the overall content was low; the mass percent of > 0. 25 mm water-stable aggregates remained at 18. 38%-28. 22%; the mass percent of 0. 25-0. 50 mm,0. 50-2. 00 mm,2. 00-5. 00 mm,and > 5. 00 mm water-stable aggregates was close with each other in each kind of compound soil. After4 years of planting,the mass percent of > 0. 25 mm water-stable aggregates in 1∶ 2 compound soil increased significantly and exceeded other2 kinds of compound soil,reached 32. 34%; the main components of > 0. 25 mm water stable aggregates in 1∶ 1,1∶ 2,and 1∶ 5 compound soil were 0. 25-0. 50 mm( 53. 54%),0. 25-0. 50 mm( 59. 43%),0. 05-2. 00 mm( 52. 16%),aggregates; 0. 25-2. 00 mm aggregates increased significantly in all three kinds of compound soil,with the highest increase in 1∶ 2 compound soil; the organic matters of 1∶ 2 compound soil were significantly correlated with 0. 25-0. 50 mm and 0. 25-2. 00 mm water-stable aggregates. The results showed that the ratio of 0. 25-2. 00 mm aggregates in the three kinds of compound soil was increased after 4 years of crop planting and 1∶ 2 compound soil was most favorable for the formation of aggregates.

Synthesis and Characterization of a Water-stable Metal-organic Framework Based on Mixed Ligands

A new water-stable metal-organic framework,[Cu_2（bdc）_2（tpt）_3]·2H_2O（1,H_2 bdc = terephthalic acid,tpt = 2,4,6-tri（4-pyridinyl）-1,3,5-triazine）,has been synthesized and characterized by elemental analysis,infrared（IR） spectrum,thermogravimetric analysis（TGA）,powder X-ray diffraction（PXRD） and single-crystal X-ray diffraction. Compund 1 crystalizes in monoclinic,space group C2/c with a = 30.147（5）,b = 11.0548（17）,c = 20.867（4） ？,β = 115.035（2）o,C_（70）H_（48）Cu_2N_（18）O_（10）,Mr = 1428.34,V = 6301.0（18） ？~3,Z = 8,D_c = 1.506 g/cm^3,μ = 0.754 mm^（–1）,F（000） = 2928,GOOF = 1.058,λ（MoK α） = 0.71073 ？,the final R = 0.0387 and wR = 0.1091 for 6230 observed reflections with I 〉 2σ（I）. In compound 1,the half metallacycles [Cu_2（tpt）_3] are linked by the bdc2-ligands to form a 1D bent ladder-like chain featured with a nano-sized channel,and these chains link each other via intermolecular hydrogen bonds and π-π stacking interactions to form a 3D supramolecular architecture with the voids filled with free water molecules. In addition,the water stability of 1 was investigated by PXRD patterns,which indicate that compound 1 keeps its structural integrity in boiling water for at least three days.

Accumulative Effect of Soil Organic Carbon and Nitrogen in Water-Stable Aggregates and Soil Stability Characteristics of Robinia pseudoacacia Plantation in the Loess Hilly-Gully Region

Soil water-stable aggregates (WSAs) are the basic unit of soil constitution and can contribute to remaining the stable soil constitution. The objective of this study was to clarify the distribution and stability of WSAs and the soil organic carbon (SOC), the total nitrogen (TN), and the total phosphorus (TP) concentrations in 0 - 20 cm and 20 - 40 cm soil layers under the different ages of Robinia pseudoacacia plantations. The 20, 25, 40, and 50 years-old Robinia pseudoacacia plantations were selected. Stepwise regression analysis showed that >5 mm and 1 - 2 mm WSAs, SOC concentration in 2 - 5 mm WSAs, and TN and TP concentrations in < 0.25 mm WSAs were dominant independent variables affecting aggregate stability and that SOC in 0.25 - 0.5 mm WSAs, TN in <0.25 mm and 1 - 2 mm WSAs and TP in 2 - 5 mm WSAs were dominant independent variables affecting SOC, TN, and TP concentrations in bulk soils.

A water-stable high-voltage P3-type cathode for sodium-ion batteries

The Na-deficient P3-type layered oxide cathode material usually experience complex in-plane Na^(+)/vacancy ordering rearrangement and undesirable P3-O3 phase transitions in the high-voltage region,leading to inferior cycling performance.Additionally,they exhibit unsatisfactory stability when exposed to water for extended periods.To address these challenges,we propose a Cu/Ti co-doped P3-type cathode material(Na_(0.67)Ni_(0.3)Cu_(0.03)Mn_(0.6)Ti_(0.07)O_(2)),which effectively mitigates Na^(+)/vacancy ordering and suppresses P3-O3 phase transitions at high voltages.As a result,the as-prepared sample exhibited outstanding cyclic performance,with 81.9%retention after 500 cycles within 2.5–4.15 V,and 75.7%retention after300 cycles within 2.5–4.25 V.Meanwhile,it demonstrates enhanced Na^(+)transport kinetics during desodiation/sodiation and reduced growth of charge transfer impedance(R_(ct))after various cycles.Furthermore,the sample showed superb stability against water,exhibiting no discernible degradation in structure,morphology,or electrochemical performance.This co-doping strategy provides new insights for innovative and prospective cathode materials.

A water-stable organolead iodide material for overall photocatalytic CO_(2) reduction

The utilization of perovskites as photocatalysts to convert CO_(2) into fuels and chemicals has received wide attention recently.However,their instability in water hinders their long-term application for overall photocatalytic CO_(2) reduction.Herein,we integrate the water-stable perovskite-like organolead iodide crystalline material[Pb8I8(H2O)3]8+[-O2C(CH2)4CO_(2)-]4(TJU-16)with Au co-catalyst for photocatalytic CO_(2) reduction in aqueous solution without sacrificial reagent.Under the AM 1.5 G simulated illumination,the TJU-16 with 0.19 wt.‰ Au co-catalyst steadily generated electrons for CO_(2) reduction reaction,which was 2.2 times of pure TJU-16.The Au0.19/TJU-16 catalyzed CO_(2) reduction at a rate of 84.2μmol·g-1·h-1,and achieved a solar-to-fuel(STF)conversion efficiency of 0.034%.Our work will motivate the rational design of water-stable perovskite-like materials for photocatalytic applications.

The effect of tea plantation age on soil water-stable aggregates and aggregate-associated carbohydrate in southwestern China

Soil carbohydrates constitute an important component of soil organic matter(SOM),and substantially contribute to the stabilization of soil aggregates.Here,we aimed to investigate the distribution of water-stable aggregates and carbohydrates within water-stable aggregates of soil in tea plantations located in Zhongfeng Township of Mingshan County,Sichuan,which is in southwest China.Samples were collected from tea plantations of different ages(18,25,33,and 55 years old)and an area of abandoned land was used as a control(CK).We also examined correlations between soil carbohydrates fractions and aggregate stability.The results showed that the mean weight diameter(MWD)of soil aggregates in the tea plan-tations was significantly higher than that the control.Furthermore,the soil aggregate stability was significantly enhanced in tea plantations,with the 25-year-old plantation showing the most pronounced effect.Soils in the plantations were also characterized by higher concentrated acid-extracted carbohy-drate content,and carbohydrate content in both surface and sub-surface layers were higher in the 25-year-old plantation.We also detected a significant positive correlation between the carbohydrate con-tent of soil and MWD after tea plantation(P<0.01).Notably,the association between dilute-acid extracted carbohydrate and the aggregate stability showed the highest correlation,indicating this car-bohydrate fraction could be used as an index to reflect changes in soil quality during tea plantation development.We should develop a potential fertilisation programme to maintain SOM-Carbohydrates within aggregates and the appropriate pH for preventing soil structure degradation after 25 years of tea planting.

A Water-Stable 3D Eu(Ⅲ)-Organic Framework as a Bi-Functional Ratiometric Luminescent Sensor for Fast, Sensitive and Selective Detection of ODZ and Hg^(2+) in Aqueous Media

The fast,sensitive and selective detection of some antibiotics and heavy metal cations in water is highly desirable for environmental protection and human health,but it is still currently challenging.In this work,a new luminescent Eu(Ⅲ)-based metalorganic framework(MOF),{[(CH_(3))_(2)NH_(2)][Eu(L)2(H_(2)O)_(2)]·xDMF}n(1)[H_(2)L=4,4’-((naphthalene-1,4-dicarbonyl)bis(azanediyl))dibenzoic acid],was solvothermally synthesized.Complex 1 exhibits good water stability and luminescent property and could serve as a bifunctional ratiometric luminescent sensor for fast,sensitive and selective detection of ornidazole(ODZ)and Hg^(2+)in aqueous solution.The corresponding luminescent mechanism has also been discussed.This work indicates that 1 as a promising luminescent material exhibits luminescent quenching behavior for ODZ and luminescent enhancement behavior for Hg^(2+)in H_(2)O,which will promote the practical application of Ln-MOF-based ratiometric luminescent sensors in monitoring antibiotics and metal ions pollutants in the environmental water matrices.

Physical and Microbiological Properties of Alfalfa-Established Soil in the Semiarid Horqin Sandy Land in Northern China

In a previous article, we reported that a local variety of alfalfa (Medicago sativa L. cv. Aohan) had high potential to be a pioneer plant for ecological restoration in the Horqin Sandy Land, China. The plantation of Aohan significantly improved the organic matter, clay, total carbon and nitrogen contents of the soils. In this study, we investigated the physical properties such as dispersion ratio, water-stable aggregates content, and the soil microbiomes, five years after alfalfa establishment in the same study site. We found no significant difference in the dispersion ratios between the soils before and after alfalfa establishment, and all the soils at the study site were erosive. Water stable aggregates mainly distributed in 96%, suggesting that it would take longer time for improving soil structure. However, large-size aggregates (2 - 5 mm) content was slightly higher in the alfalfa planting plots. This slight increase is presumed to have long-term importance for soil and ecosystem recovery in semi-arid areas like Horqin Sandy Land. Moreover, we also found that Actinomycetes dominated the microbial community in both bulk and rhizosphere soils, and two kinds of rhizobia, Bradyrhizobium and Sinorhizobium fredii, were identified in the rhizosphere soil.

Role of Organic Matter in Formation and Stability of Aggregates in Mulberry Plantation Soils

The role of organic matter in the formation and stability of soil aggregates in mulberry plantation in the Hang-Jia-Hu Plain, northern Zhejiang Province, was evaluated in this study. A positive correlation was found between water-stable aggregate contents and organic matter contents in the mulberry plantation soils, which supported the hypothesis that organic matter was the main cementing agent in formation of aggregates. A close correlation was also found between stability of aggregate and organic matter contents. Regression analysis showed that total nitrogen content was also an indicator of water-stable aggregate content and stability. The aggregate size distribution indicated that the water-stable aggregates 1--0.25 mm in diameter were the major component of the aggregates in the mulberry plantation soils. The organic matter contents of aggregates ranging from 5 to 0.25 mm in diameter increased with the decrease of aggregate sizes, and the aggregates 1-0.25 mm in diameter had the maximum organic matter content.

Short-Term Impacts of No Tillage on Aggregate-Associated C in Black Soil of Northeast China

In order to get a good indicator to evaluate the impacts of no tillage （NT） on soil structure and soil quality, we studied the dynamics of total soil organic carbon （SOC） and aggregate-associated SOC, and their relationships in the plow layer （30 cm） in black soil of Northeast China under NT practice. The tillage experiment was established in Dehui County, Jilin Province, China, in 2001. The total SOC and aggregate-associated SOC under 5-yr tillage treatments were measured. NT practices did not lead to the increase of average SOC content at 0-30 cm depth, but it did significantly increase SOC at the top soil （0-5 cm）. In NT plots, the change of SOC in 〉 1 000 μm aggregate was the same with that of total SOC, but the effect of NT on SOC in 〉 1 000 lain aggregate was greater than the effect on total SOC, suggesting that 〉 1 000 μm aggregate had more sensitive response to the impact of tillage practices. Also, significant positive correlation occurred between total SOC and SOC in 〉 1 000 μm aggregate in black soil. Consequently, in the short term soil macroaggregate 〉 1 000 μm could be used as an indicator to evaluate the impacts of tillage practices on soil structure in black soil of Northeast China.

Soil aggregation and aggregate associated organic carbon and total nitrogen under long-term contrasting soil management regimes in loess soil

This study investigated the effects of three contrasting soil management regimes and different nutrient treatments on the distribution of water-stable aggregates（〉2, 1–2, 0.5–1, 0.25–0.5, and 〈0.25 mm） and associated soil organic carbon（SOC） and total nitrogen（TN） content in loess soil. A 21-yr long-term experiment was performed, in which soil management regimes include cropland abandonment（Abandonment）, bare fallow（Fallow） and wheat-fallow cropping（Cropping）. Under Cropping, the following nutrient treatments were employed： control（CK, no nutrient input）, nitrogen only（N）, nitrogen and potassium（NK）, phosphorus and potassium（PK）, NP, NPK, and manure（M） plus NPK（MNPK）. Results demonstrated that Abandonment significantly increased the content of soil macro-aggregates（〉0.25 mm） and mean weight diameter（MWD） at 0–10 and 10–20 cm soil horizons compared with Cropping, whereas Fallow yielded lower values of above two parameters. Abandonment increased SOC and TN contents in all aggregate sizes by 17–62% and 6–60%, respectively, at 0–10 cm soil layer compared with Cropping. Conversely, Fallow decreased SOC and TN contents in all aggregates by 7–27% and 7–25%, respectively. Nevertheless, the three soil management regimes presented similar SOC contents in all aggregates at 10–20 cm soil horizon. Only Cropping showed higher TN content in 〉0.5 mm aggregates than the two other regimes. Consequently, Abandonment enhanced the partitioning proportions of SOC and TN in 〉1 mm macro-aggregates, and Fallow promoted these proportions in micro-aggregates compared with Cropping. Under Cropping, long-term fertilization did not affect the distribution of aggregates and MWD values compared with those under CK, except for NPK treatment. Fertilizer treatments enhanced SOC and TN contents in aggregates at all tested soil depths. However, fertilization did not affect the partitioning proportions of SOC and TN contents in all aggregates compared with CK. Comprehensive results showed that different soil management regimes generated varied patterns of SOC and TN sequestration in loess soil. Abandonment enhanced soil aggregation and sequestered high amounts of SOC and TN in macro-aggregates. Long-term amendment of organic manure integrated with NPK maintained soil aggregate stability and improved SOC and TN sequestration in all aggregates in loess soil subjected to dryland farming.

Effects of Long-Term Fertilization on Organic Carbon and Nitrogen Dynamics in a Vertisol in Eastern China

Mineral fertilizers and organic amendment can affect the various soil organic matter (SOM) pools and the distribution of organic carbon (OC) and nitrogen (N) in these pools. It is unknown how OC and N are distributed in different SOM pools under different long-term fertilization regimes. Therefore, this study aimed to examine the effects on OC and N concentrations in various SOM pools after 33 years of application of chemical fertilizer and organic amendment in Anhui Province in the Huang-Huai-Hai Plain, eastern China. This long-term experiment consisted of five fertilization treatments measuring changes in the OC and N concentrations in the soils and different SOM fractions of each experiment plot. Organic amendment increased the OC and N concentrations in the mineral-associated fraction, the coarse mineral-associated fraction and the aggregates compared with the values obtained without fertilizer application. Mineral fertilizer application alone increased the abovementioned indexes, but this increase was small. There was a small but significant increase in the OC and N concentrations in the free particulate fraction, and the change in magnitude had no obvious effect on the total OC (TOC) and total N (TN) concentrations in soils. More than 80% of the water-stable aggregate-associated C was stored in macroaggregates >2 mm in size. More than 60% of the TOC and TN accumulated within mineral associations in the soil, and organic amendment increased this proportion to 80%. These results suggest that the OC in Vertisols is dominated by mineral-associated OC and that the effect of organic amendment on mineral-associated OC is obvious.

How freezing and thawing processes affect black-soil aggregate stability in northeastern China

Laboratory experiments were carried out to investigate the effect of freezing and thawing processes on wet aggregate stability （WAS） of black soil. Wet aggregate stability was determined by different aggregate size groups, different water contents, various freeze-thaw cycles, and various freezing temperatures. The results showed that, when at suitable water content, aggregate stability was enhanced, aggregate sta-bility will be disrupted when moisture content is too high or too low, especially higher water content. Temperature also had a significant ef-fect, but moisture content determined the suitable freezing temperatures for a given soil. Water-stable aggregate （WSA〉0.5）, the total aggre-gate content, and mean weight diameter decreasing with the freeze-thaw cycles increase, reached to 5 percent significance level. The reason for crumbing aggregates is the water and air conflict, thus raising the hypothesis that water content affects the aggregate stability in the process of freezing and thawing.

Evaluation of Sawah Rice Management System in an Inland Valley in Southeastern Nigeria. II: Changes in Soil Physical Properties

Establishment of effective sawah management system in parts of southeastern Nigeria may involve the manipulation of certain soil physical properties in form of ecological engineering works. This practice may affect the soil physical properties adversely. The objective of the study were basically to compare the influence of sawah and non sawah water management practices on the soil physical properties following rice cultivation with various inorganic and organic amendments. Parameters determined were soil bulk density, total porosity, moisture contents at field capacity (FC) and wilting point (WP), water-stable aggregates, dispersion ratio (DR), and hydraulic conductivity (Ks). Sawah managed soils reduced significantly the soil bulk density in the first and second year of planting thus increasing the soil total porosity during the same period Moisture content also improved in sawah management while WP increased significantly in the second year of planting. In spite of the destruction of soil structure as a result of cultural practices during rice cultivation the DR is improved on the long run by sawah water management. Moisture contents at FC and WP relates significantly with soil bulk density which also relates negatively with total porosity during the 2 years of cultivation. However, FC and WP may be very good tools in the estimation of bulk density. Again, the amendments were identified as promoting the development of soil aggregates and Ks on a long term.