Change Of Organic Carbon In The Purple Paddy Soil And Its Impact On Carbon Sequestration Under Tillage Management Practices

Terrestrial ecosystems have been considered as a possible missing carbon sink. Farm ecosystems are those under heavy interference by human management activities. Carbon sequestration in cropland soils has been concerned globally to be crucial in mitigation of greenhouse gases emmission in commitment to kyoto protocol, which was taken into effect in February of 2005. Paddys, a special kind of human managed wetland ecosystem, may plays important role in mitigation of climatic change and world food securiety especially for China. While researches has shown that paddy soil have great potential for C stock and sequestration in terretrial soils, its dynamics under changing environment deserves urgent study of priority. Up to now, the study of role of these soils in global change has been focused in greenhouse gases emission especially of CH₄ and N₂O. Carbon sequestration and the mechanism under different agricultural managements practices has not been very well described yet. Data has been reported of the increasing trend of carbon stock in China's paddy soils in literature. However, nature of SOC pools and the binding characteristic associated with this trend and the effect in soil quality and soil functioning has not been well understood.A holistic study of minerals-organic matter-microbes interaction at scale of microaggregates by combination of soil physical, chmeical and biological methodologies is necessary for characterizing the carbon stablizing mechanisms and its response to global change in order to elucidate the effect of rice farming management in carbon stock and possible sink of atmopsheric CO₂ and the option and strategy for enhancing the mitigation potential of these paddy soils. Such study may provide either scientific bases for carbon sequestration policy in sustainable agriculture or new insight into carbon cycling in paddys under intense tillage and croping systems in agricultural systems of high yield demand like in China.The paddys belong to a unique kind of fertile soils in Southwest China andareextensive in the basin of Sichuan, which has been traditionally considered as an area of fertile land for a great proportion of grain production in China. Many studies have been conducted on soil fertility of these soils in relation to fertilization, farming systems with much under short term field trials. However, data have been insufficient in carbon dynamics under different farming systems including tillage and cropping systems. Started with a c storage estimation of these paddys of Chongqing Municipality, this study focuses on carbon pool distrbution and characterization of carbon in soil aggregates affected by different farming systems by using a long term field experiment initiated in 1989 managed by the Southwest China Agricultural University. Special regards are paid to carbon accumulaton and the mechanism of carbon sequestration under particular hydroagric processes at microscale in order to give evidence to possible sink effect of paddy management and to supply dta for modeling carbon dynamics varied with tillage and cropping practices. The main findings are as follows:(1) Esimate of baseline C stock of soils of Chonqing MunicipalityBased on the data of SOC of 1411 soil profiles and the soil map completed in the 2nd national Soil survey, estimation of SOC stock and its variation in different eco-zones, under different land uses and with domint soisl types are accomplished by G1S. The total SOC stock of topsoil of 20cm depthand of whole soils of 100 cm depth amounts to 0.27 Tg and 1.0 Tg respectively with a mean carbon density of topsoil and whole in range of 0.33-30.36 kg/ m2 and of 1.27-72.69 kg /m7 resectively; The relative SOC accumulation index of dormant soil groupe, in major geopraphical ecozoners and in different landscape ranges from 0.46 to 1.95, showing the impacts of climate, vegetation and human land uses.(2) Changes of total organci carbon (TOC) in the field experimentProminent changes of TOC has taken place in different plots treated with different tillage and cropping systems of the paddy of long term field experiment with an initial TOC being 13 g/kg. With single rice cropping, TOC has been increased and stablized at 20 g/kg regardless of tillage treatments; However, different features of TOC increase has been observed in those treatment with rape-rice rotation farming and different tillages. With conventioanl tillage, TOC has been increased by 50% as compared to the original and TOC has been increased to 28.57g/kg under no tillage with ridge, being double as that in 1991 and higher than in other treatments by 8-10g/kg. Thus, no tillage with ridge and with rice rape croopping can be regarded as a acceptable farminmg system for carbon sequestartion as well as for enhancing soil fertility shown in early works.The aggregate size fraction composition is characterized by the prevalence of those of size of 0.25-0.02mm in diameter (being around 50%), followed by the size fraction of 0.02-0.002mm (approximately 30% with insignificant proportion of the size fration of >2mm and <0.002mm. Theinfluence of farming system in soil aggrgation is only detected in surface soil of 10 cm depth with a higher proportion of coarse fractions under no tillage than under tillage. Rotation has caused increase of coarse fractions pf soil aggregates as compared to that under winter-fallow. This implicates that the frequency and intensity of tillage may inpact on soil aggregation and, in turn, on physical protection of soil organci carbon.The TOC in aggregates varies with the size fractions. Carbon partitioning study shows that the dominant size fraction of aggregates for SOC storage is that of 0.25-0.02mm, which is different from that of paddy soils from the Tai Lake region of China.While defining a C enrichment index (the SOC content in a size fraction of aggrgates divided by the SOC content of the bulk sample), a general trend of SOC partitioning could be found. Generally, the size fractions of >2mm and of 2-0.25mm are highly enriched in SOC while those of 0.25-0.02mm and of <0.002mm are more or less depleted of SOC. The SOC enrichemnt becomes weaker in coarse size fractions and stronger in smaller size frafctions down to the profiles. This may be due to the incorpation of less decomposable organic residue in upper soil. The potential of carbon sequestration by improvement of farming system in this paddy are foundin this order NRR(no tillage and ridge, rice and rape)> TRR (Conventional tillage and ridge, rice and rape)> CRWF(conventional tillage, rice and winter fallow)> NRWF( No tillage and ridge, rice and winter fallow)> RRR( Rotation of waterloging rice and dry rape ) . Taking the paddy under conventional farming system as 100%, the C sequestration potential of topsoil of 30 cm could be raised by 14% under NRRfPTRR. Nevertheless, the potential of upper soil of 60cm is found to be raised only by NRR by 10% with the other teatments of nagetive potential. Therefore, the farming system of NRR could be only acceptable for C sequesrtration in such purple soil-derived paddy. (3) Change of labile carbon formsThe content of easily oxidizable SOC in soil aggregates ranges from 0.5g/kg to 20 g/kg, being concentrated in the size fraction of 2-0.25mm. The KOS ranges from 0.4 to 0.9, increasing with the smaller size fractions of the aggregates. This may evidence that the SOC in coarse size fractions are rich in labile and bioactive carbon. The range of POC is 2.94-10.13 g/kg at 0-10cm depth, being decreasing in following order: NRR>TRR>CRWF>NRWF. The microbial carbon pool falls in range of 40-500mg/kg, with the ratio of Cmic to TOC ranging from 0.24 to 1.85. The changing order is similar to that of POC. Dissolve organic carbon (DOC) is found in range of 0.03-0.34 mg/kg, in a proportion to TOC of 0.16%-1.25%; very well correlation is found between DOC, POC , MBC with TOC respectively. The order of all these labile carbon pools is found as same as for POC in surface 0-10cm soil. These findings suported again that nollage rice rape cropping with the ridged tiles could be the best option for SOC enrichment in soils for both enahncing carbon sequestrationn and biological activity in this paddy soil.