Changes In Organic Carbon And Extracellular Enzyme Activity Of Paddy Soils With Different Field Managements

Soil organic carbon is not only the critical carbon pool for mitigation of climate change, but also the crucial driver and regulator for soil ecosystem. While the impacts of different envionment variables and field managements on soil carbon accumulation have been widely studied, the linkage of soil biological properties and ecosystem function to them have not yet been fully understood. Paddy soil is Hydragric Anthrosols with typical redoxi-morphic features. It is commonly accepted that rice cropping enhances accumulation and stabilization of soil organic carbon(SOC), thus possessing higher nitrogen productivity and climate stability. For this reason, the storage of SOC associating with soil biological properties and function changed with agricultural management is the scientific foundation for sustainable carbon sequestration and productivity in rice paddies. The effects of irrigation management and fertilization on the variation of SOC content and soil enzyme activity in plough layer and soil aggregates were studied under different scales, to discuss ecosystem function change of SOC and provide scientific basis for sustainable carbon sequestration.Changes in plow layer thickness and topsoil organic carbon content of rice paddies were analyzed using data obtained in a county-level survey of soil fertility conducted in 2005 and 2006 in Guichi County, Anhui Province, China. Both soil thickness and organic carbon content of plow layer showed skewed normal distributions,with their averages of 14.58 ± 3.92 cm, and 16.45 ± 6.02 g/kg,respectively. The irrigation method was found to have significant influences on both plow layer thickness and soil organic carbon content, as the plow layer thickness and soil organic carbon content had an inverse response to different irrigation methods.The land-level performance of irrigation/drainage infrastructure and the irrigation water sources were detected to have significant effect on plow layer thickness, but little influence on soil organic carbon content. While the capacity of irrigation/drainage infrastructure had a remarkable effect on soil organic carbon content but little impact on plow layer thickness. However, the irrigation condition for surveyed fields was detected to have little effect on both plow layer thickness and soil organic carbon content. The change range of topsoil organic carbon is greater than plow layer thickness both affected by different irrigation managements.Further analysis for topsoil organic carbon content changed with fertilization treatments under oilseed rape-rice cropping system in the Tai Lake region was conducted. The experiment include four fertilization treatments: (1) none fertilizer(NF); (2) chemical fertilizer only (CF); (3) chemical fertilizer (without nitrogen)combined pig-manure (MPK) and; (4) chemical fertilizer combined straw return(CFS). The results showed that the increases of SOC in aggregates were consistent with the increases in bulk soil while comparing to the control, as SOC content was increased significantly by applying CFS. The increased organic carbon incrasement was found mainly in macroaggregates (250-2000 μm) and microaggregates (53-250μm),while a small mount was observed in large macroaggregates (> 2000 μm),litte increasement was observed in silt-sized (2-53 μm) and clay-sized (< 2 μm) fractions.The increasements of SOC sequential extracted from soil aggregates were coincident with the increases of total SOC content in bulk soil and aggregate-size fractions as well. Comparing to the control, the SOC component bound to minerals by weak linkages was increased by average of 51.3%; SOC components bound to cations was increased by average of 22.7%; and the massive SOC extracted by NaOH was increased by average of 15.9. These results indicated that soil aggregate hierarchy has well and protective effect on SOC accumulation. Labile mineral-associated organic carbon was increased more easily than steable mineral-associated organic carbon by application of fertilizer.Accoding to the experimental result of that, enhanced topsoil OC content in paddy soil was observed by application of fertilizer, further study on the relationship between soil micarobial properties on SOC accumulation was conducted. The results showed that soil biological properties and soil enzyme activities under different fertilization treatemts exhibited a diverse variation. Comparing to the control, soil microbial biomass was decreased by application of MPK and CFS at oilseed rape harvest time, while the activities of P-glucosidase and cellobiohydrolase were increased significantly by application of CFS. Whereas there was no difference observed at rice harvest time. Soil phosphomonoesterase activity was significantly decreased by applying MPK and CFS at both oilseed rape and rice harvest time. In addition,comparing to the control,phenol oxidase acivity was declined by application of chemical fertilizer combining straw return (CFS), whereas peroxidase activity increased significantly by application of fertilization. Positive correlation between microbial biomass and enzyme activity in soil matrix was detected by using correlation test. Associating with higher soil moisture in soils sampled at rice harvestthan in soils sampled at oilseed rape harvest, these results indicating that the effects of water management and redox process on soil microbial activity and enzyme activity are moe critical driver to soil organic carbon dynamics than fertilization treatments.In accordance with the mechanism of carbon sequestration in soil aggregates affected by different fertilization managements, further study on the distribution of SOC content and enzyme activity in soil aggregates was conducted. The results showed that the activities of carbon-degrading enzymes, chitinase and phosphomonoesterase were mainly distributed in the fractions the aggregate-size more than 53 μm, while lower activities in silt-sized and clay-sized fractions. In comparison, phenol oxidase activity were primarily distributed in clay-sized,silt-sized fractions and macroaggregates, as lower activities in microaggregates and large macroaggregates were observed. However, peroxidase activity was observed the lowest in clay-sized fractions and higher in the other aggregate-size fractions.These results indicated that most soil enzymes are sensitive to clay-sized and silt-sized fractions, while stable in the fractions aggregate-size more than 53 μm.While the distribution characters of pheol oxidase and peroxidase activities in aggregate-size fractions were related to the turnover of stable carbon components.According to the distribution of soil enzymes and their substrates in soil matrix where possible, a conceptual framework for the activity of soil enzyme distributed in soil matrix was established. The meaning of measured enzyme activities, and the relationship between measured enzyme activities and the real enzyme activities could be understood by the means of this conceptual framework.In conclusion, our study indicated that irrigation management has significant impacts on topsoil organic carbon dynamics, as water-saving irrigation like furrow irrigation could balance the effects of irrigation activites on plow layer thickness and SOC content in paddy soil. While long-term fertilization had positive effect on soil carbon sequestration. The effect of soil moisture on soil microbial activity and soil enzyme activity are prior to the effect of fertilization. Further analysis on the distribution of SOC content and soil enzyme activity indicated that soil aggregate hierarchy can harmonize the relationship between SOC stabilization and soil biological properties. In addition, labile SOC was increased mostly and stable SOC increased little by application of fertilizer. These results indicated that irrigation management should keep the balance between surface erosion on plow layer thickness and soil organic carbon accumulation. Hence, developing new technique for good irrigation infrastructure and water management in future will help soil organic carbon accumulation as well as improve soil fertility for enhancing crop growth in rice agriculture.