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The formation, stabilization and breakdown processes of soil aggregates determine soil organic carbon (SOC) sequestration, in turn, soil aggregate dynamics are mediated by SOC changes. However, the interactions between them remain elusive. Herein, three types of ¹³C-labelled residues were added to two textured soils. Rare earth oxides (REOs) and ¹³C isotope were used as dual tracers to simultaneously track aggregate transfer pathways and SOC sequestration during a 56-day incubation period. Residue-derived CO₂ followed the sequence of Vetch > Maize > Decomposed maize during the first two weeks. Residue-derived CO₂ was significantly negatively correlated with the aggregate turnover time in both investigated soils (P < 0.01), indicating that aggregate turnover was a controlling factor of residue decomposition in addition to its inherent features. Generally, residue addition decreased the aggregate turnover time in the sequence of Vetch < Maize < Decomposed maize. In Red clay soil, macroaggregates attained a higher turnover rate than that of microaggregates, while a similar change pattern was not observed in Sandstone soil with residue application. Aggregates turnover occurred faster in Sandstone soil than in Red clay soil under a given residue application. The aggregate turnover time was significantly reciprocally correlated with the residue-derived C sequestration rate (P < 0.01), suggesting that aggregate turnover was the key factor in C sequestration. A C flow conceptual model was proposed, residue-derived C firstly accumulated in macroaggregates in the formation process, and then relocated from macroaggregates to microaggregates with the breakdown processes at the mid-to-late stage. This study highlights the importance of aggregate turnover in SOC sequestration and demonstrates that these interactions are further affected by residue features and soil texture., Geoderma, 430, ISSN:0016-7061, ISSN:1872-6259

To mitigate the serious problem of Cd-contaminated paddy soil, we investigated the remediation potential of combining in-situ immobilization with a low-Cd-accumulation rice cultivar. A three-season field experiment compared the soil pH, available Cd and absorption of Cd by three rice cultivars with different Cd accumulation abilities grown in Cd-contaminated paddy soil amended with lime (L), slag (S), and bagasse (B) alone or in combination. The three amendments applied alone and in combination significantly increased soil pH, reduced available Cd and absorption of Cd by rice with no effect on grain yield. Among these, the LS and LSB treatments reduced the brown rice Cd content by 38.3-69.1% and 58.3-70.9%, respectively, during the three seasons. Combined with planting of a low-Cd-accumulation rice cultivar (Xiang Zaoxian 32) resulted in a Cd content in brown rice that met the contaminant limit (≤0.2mgkg-1). However, the grain yield of the low-Cd-accumulation rice cultivar was approximately 30% lower than the other two rice cultivars. Applying LS or LSB as amendments combined with planting a low-Cd-accumulation rice cultivar is recommended for the remediation of Cd-contaminated paddy soil. The selection and breeding of low-Cd-accumulation rice cultivars with high grain production requires further research.

CO2 fixation is the central mechanism of primary production in almost all ecosystems,and plays a major role in regulating the concentration of atmospheric CO2.Carbon dioxide accounts for about 50% of the current global warming potential.Soil microorganisms that assimilate CO2 are widely distributed,and have a great ability to adapt to environmental extremes,such as within volcanic sediments,lake wetlands and the submarine realm.Microbial CO2 assimilation is of great significance to climate change mitigation and sustainable development for human beings. It has now been well established that atmospheric concentration of CO2 can be reduced significantly by adopting management practices to enhance CO2 sequestration(storage) in cropland soils.Among the approaches for increasing CO2 sequestration of croplands are soil fertilization management practices,such as returning crop residues to the soil,planting temporarily retired land with grass for stabilization,and integrating nutrient management strategies to diversified cropping systems. Paddy soils are distributed widely throughout the world,and generally are known for their production of greenhouse gasses(N2O and CH4).Numerous studies have used molecular techniques to investigate these microbial driving mechanisms.However,few studies have addressed the importance of microbial CO2 fixation processes in paddy soils.Investigation of the impacts of long-term fertilization on the structure and abundance of CO2 assimilating bacteria in paddy soils can provide a theoretical basis for the application of information on fertilization of paddy-rice fields.This type of research also may benefit the reduction of greenhouse gas emissions and increase carbon sequestration. Although carbon fixating microorganisms exhibit a wide range of physiological and ecological traits,most photo-and chemoautotrophic bacteria use ribulose-1,5-biphosphate carboxylase/oxygenase(RubisCO),which catalyzes the first rate-limiting step in the Calvin cycle to incorporate atmospheric CO2.The cbbL gene,which encodes the large subunit of the form I RubisCO is especially useful as a functional marker for significant phylogenetic analyses of CO2 fixating microorganisms in different ecological systems. In this study,soil samples were collected from a long-term fertilization experiment,which included a station that received no fertilization(CK),one treated with chemical fertilization(NPK),and one with NPK plus crop residue return(NPKS).The abundance,diversity,and composition of soil-CO2 fixating bacteria were determined using Polymerase Chain Reaction(PCR),cloning and sequencing,and real-time quantitative PCR of the cbbL gene to explore the effects of long-term fertilization on the structure and abundance of the CO2 fixation bacterial community.Based on sequence libraries of cbbL genes,our results clearly demonstrate that there was a significant response difference in community composition with respect to long-term fertilization regimes.A total of 165 cbbL genotypes from three different treatment soils were divided into 14 bacterial cbbL gene phylotypes,mainly including Bradyrhizobium,Ralstonia,and Thiobacillus.Facultative autotrophic bacteria,such as Bradyrhizobium and Ralstonia,dominated both in NPK and NPKS treatment soils,while growth of the obligate autotrophic bacteria,such as Thiobacillus and Nitrosospira,were suppressed.LUBSHUFF statistical analyses also demonstrated that cbbL gene libraries of CK,NPK and NPKS treatments were significantly different from one another.The Rarefaction curve indicated that fertilizer addition increases cbbL gene diversity.Thus,the NPK treatment had the highest curve.The abundance of the bacterial cbbL gene ranged from 3.35×108—5.61×108 copies g-1 soil.Crop residue return NPKS had the highest abundance of bacterial cbbL gene,which was about one half the value of CK.Finally,application of chemicals and organic fertilizers in paddy ecosystems significantly influenced the CO2 fixating bacterial community structure,and increased bacterial abundance and diversity.These results provide a strong basis for further investigation of fertilization on soil carbon sequestration potential and its related microbial mechanisms.

The contents of soil organic C (SOC), total N (TN), total P (TP), dissolved N (DN), Olsen-P, and microbial biomass C, N, P (BC, BN, BP) of 254 paddy soils (0–18 cm in depth) in a hilly red soil region of subtropical zone of China were studied. The results showed that the contents of SOC, TN, BC, BN and DN of paddy soils at the bottom of hills were 14.6%, 13.6%, 24.6%, 20.4% and 95.8% higher than those at the foothill, respectively. The Olsen-P content of paddy soils at the foothill was 33.3% higher than that at the bottom of hills. However, the differences in TP, BP and available P (the sum of BP and Olsen-P) contents were not significant between the two positions. In addition, the ratios of soil C/P, BC/BP and BC/SOC of paddy soils at the bottom of hills were 12.7%, 28.5% and 8.2% higher than those at the foothill, respectively, but the differences in ratios of soil C/N, BC/BN, BN/TN and BP/TP were not statistically significant between various positions.

Aims: To provide a soil quality assessment frame work and threshold limits for assessing soil quality in Ultisol of subtropical China region. Study Design: Selected minimum data set for soil quality assessment and threshold limits for the study were total carbon, nitrogen, soil pH and phosphorus, biomass carbon, nitrogen and phosphorus, maize grain and fresh potato tuber yields. Soil data (2000-2010), maize grain and fresh potato yield data (2000-2009) from a long term experiment under the Institute of subtropical Agriculture, China were analyzed using the SAS statistical package and means were graphically compared to determine threshold limits for selected data set and fitted into a soil quality model. Place and Duration of Study: The key Laboratory for Agro-ecological Processes in Subtropical Regions, Chinese Academy of Sciences; Institute of Subtropical Agriculture, Changsha, Hunan China long-term experimental site in Taoyuan county, conducted from the year 2000 to 2010. Methodology: Soils samples at the experimental fields were obtained from depths 0-20 cm using an auger at each replicate in triplicates and homogenized to obtain a composite sub sample, air-dried, sieved through 2.0 mm to obtain samples for analysis in the Laboratory. Parameters analyzed for were organic carbon concentration, measured by the combustion method using an automated C/N analyzer (Vario MAX CN, Elemental Co., Germany) while total nitrogen was by the Kjeldahl method of ISSCAS (1978). Microbial carbon, nitrogen and phosphorus levels were determined using the chloroform-fumigationextraction method (Jenkinson and Powlson, 1976; Vance et al., 1987; Brookes et al., 1982) and adopting the conversion factors 0.45 (Wu et al., 1990), 0.45 (Brookes et al., 1985), and 0.29 (Wu et al., 2000) respectively for the C, N and P. Extractable N and Olson P were taken from values obtained from the non fumigated soil samples. Data obtained were statistically analyzed using the SAS package for ANOVA and significant means were separated using the Duncan’s New Multiple Range Test (DNMRT). Treatment means were also matched graphically to delineate critical threshold limits between classes for each parameter. Soil quality was assessed by using the Parr et al. (1992) equation; SQ =ƒ(SP,P,E,H, ER,BD,FQ, MI); where SQ= soil quality, SP= soil properties, P = potential productivity, E=environmental factor, H= health (human/animal), ER= erodibility, BD= biodiversity, FQ= food quality and MI= management input. A score scale of 1 to 5 was used in the assessment of parameters in the model; where 1 is best and 5 is the worst condition. However, E, H, ER, FQ and MI were each scored 1.0 because the long-term experiment has an environmental component, health factor, biodiversity, food quality and management input components that are being optimally managed. Therefore SQ= f(SP, P) was used to assess quality of the Ultisol at the uplands and slope land locations. Results: At the uplands, the practice of maize-rape/marsh residue+NK (8.54gkg-1 C, 1.0 gkg-1 N and 5.67 mgkg-1 P) treatments could be rotated with Maize-rape/nil fertilizer (7.51 gkg-1 C, 0.87 gkg-1 and 0.39 mgkg-1 P) to encourage improved soil quality by allowing for more years with soil carbon sequestration, nitrogen and phosphorus credit than years of depletion and discourage soil degradation. At the slope lands, treatments that combined application of organic and inorganic fertilizer materials [Sweet potato-rape/NP+straw (7.18 gkg-1 C, 0.88 gkg-1 N and 0.38 mgkg-1 P) and Peanut-broadbean/NP+straw (6.81 gkg-1 C, 0.86 gkg-1 N and 0.38 mgkg-1 P)] improved soil quality significantly over time by sequestering significantly higher total carbon, nitrogen and phosphorus better than sole inorganic fertilizer [Sweet potato-rape/NPK (6.52 gkg-1 C, 0.81 gkg-1 N and 0.38 mgkg-1 P)]. Conclusion: Ultisol at the upland positions had better quality (SQ1) than those at the slope (SQ2) positions. Threshold limits for nutrients, pH and yield of maize and Fresh Potato tubers in the subtropical China region Ultisol was developed.