| Soil organic matter(SOM)is an essential component of soil,which as the dirver of soil bilological process could promote the formation of soil aggregates and favorable soil structure,supply energy for microorganisms to hold soil biota,thus dirvering and regulating the cyclying of nutrient,water and energy and exerting soil ecosystem function service.Therefore,the interactions of soil organic matter-aggreates-soil microbial community-soil ecosystem function with soil development are the intrinsic properties of soil science.Paddy soils have been known to have high organic carbon storage and sequestration potential Paddy soil is a unique soil type(Anthrosols)which developed under long term rice cultivation with dynamic redox potential.In China,rice has been cultivated for more than7000-10,000 years.Long term rice cultivation and the management practices have contributed to build-up of paddy soils with particular morphology and organo-mineral intercations though derive from various kinds of parent soils.While paddy soils are known to have high organic carbon storage and sequestration potential,and showed stable crop productivity.Some researches have showed that soil organic carbon accumulated during rice cultivation and be with high chemical and biological stability.But the evolution of SOC accumulation and stabilization with duration rice cultivation during paddy soil development,as well as the interaction of organic carbon and soil mcirobes are still not fully addresed.In the costal area of Cixi City,Zhejiang Province,salt marsh lands have been reclaimed and cultivated with rice in different historical periods since 2000 years ago,creating a potential soil chronosequence for tracing the trends of organic carbon accumulation and stabilization with prolonging rice cultiavtion.In this study,topsoil samples of a series rice paddies cultivated for different lengths up to 700 years(P0,P50,P100,P300,P700)were collected and the changes in physic-chemical properties,in SOC pools and in soil microbial biomass and enzyme actitivities both of bulk soil and aggregate particle size fractions were analysed.Using a lab incubation with exostic carbon input,SOC sequestration potential of the rice paddies were also explored.The aim of the research was thus to assess the SOC accumulation and stabilization with respect to ecosystem functioning along a chronosequence of rice cultiavtion.The main findings are as follows:1.Organic carbon accumulation as rice cultivation prolonged Organic carbon has been found increasingly accumulated in the rice paddies with prolonged rice cultivation.Topsoil OC content was 6.32 g kg-1 in P0,increased rapidly to15.97 g kg-1 in P50 and up to 17.97 g kg-1 in P300.Conmpared to P0,the OC conetnt was several folds in soils cultivated for hundreds years with the extent of increase decraesing with rice cultivation length.Following the general trend of bulk OC,dissolved organic carbon(DOC),labile organic carbon(LOC),particulate organic carbon(POC),Fe bound organic carbon(Fe-OC),humic acid carbon(HA-OC)and humin carbon(HM-OC)contents in paddy soils were all much higher than in tidal marsh(PO)while the Ca bound OC(Ca-OC)content reduced with rice cultivation length.Meanwhile,the ratio DOC/SOC was higheer in P50 and P100,with a decreasing trend with prolonged rice cultivation.Similarly,LOC/SOC was highest in PO and decreased with prolonged rice cultivation in the paddy soils.On the contrary,POC/SOC,Fe-OC/SOC and HA-OC/SOC,indicators of OC stability,increased generally with the duration of rice cultivation.Whereas,the ratio of HM-OC/SOC showed hardly change with cultivation time.Thus,with the general increasing trend of bulk SOC,chemical stability of SOC enhanced with prolonged rice cultivation though labile carbon fractions showed relatively slight change over rice cultivation for 50 years.Overall,compared to the uncultivated tidal marsh,LOC/SOC and Ca-OC/SOC was decreased in the rice paddies by 28.73%-41.98%and by 67.02%-83.95%,while POC/SOC and HA-OC/SOC increased by 21.61%-50.42%and by 49.49%-127.67%,respectively.Fe-OC/SOC decreased by 29.21%in P50 and increased by 11.43%in P100 and 51.15%in P300.Therefore,the incraese in bulk OC in the rice paddies over rice cultivation could be largely attributed to the increase of particulate organic carbon and chemical bound OC with Fe oxides.The NMR spectra showed that HA-OC in the studied soils was mainly composed of Alkyl-C(30%-37%)and O-alkyl-C(17%-22%),followed by Aromatic-C(14%-17%).The relative portion of Alkyl-C and Carboxyl-C decreased after rice cultivation for 50 years while that of O-alkyl-C and Aromatic-C increased along the chronosequence of rice cultivation history.Over hundreds years of rice cultivation,the relative portion of Alkyl-C increased to 37.10%and O-alkyl-C decreased to 18.81%,showing a general increase in hydrophobicity of OC in the old rice paddies.2.Changes in microbial biomass carbon and enzyme activity with prolonged rice cultivation Microbial biomass carbon(MBC)increased from 63.41 mg kg-1 in P0 to 532.44 mg kg-1 in P100,and then relatively unchanged with cultivation length.A similar trend was followed by the microbial quotient,which was only 1.01%in P0 but 3.12%in P100.Except for acid phosphatase,activities of invertase,urease,?-glucosidase,?-cellobiosidase and peroxidase all showed increasing with prolonging rice cultivation,with an extent in range of 77.2%to 750%in P700 compared to P0.When normalized against OC content,enzyme activity increased from 0.11 in P0 to 0.30 in P700,with highest SOC enzyme density and microbial biomass carbon density in P0.For paddy soils,SOC enzyme density and microbial biomass carbon density enhanced with rice cultivation time.Soil basic respriation was lowest in P0 and increased with rice cultivation over 100 years.However,soil basic respiration against organic carbon content showed no significant changes with rice cultivation time.However,metabolic quotient was much lower in all the rice paddy soils than in the uncultivated tidal marsh,showing a slight variation among the old rice soils from P50 to P700.The results demonstrated that microbial biomass and enzyme activity were enhanced with rice cultivation time,which indicated SOC accumulation promoted microorganism abundance and their functional activity.3.Changes in SOC and mciorbial properties in particle size fractions along the rice cultivation chronosequence The soil micro-aggregates was overall dominated by the size fraction of 200-20?m(39.91%-46.53%)and of 20-2?m(36.97%-41.00%).The coarse macro-aggregates of2000-200?m fraction increased from 2.78%in P0 to 7.63%in P700,with a mean weight diameter(MWD)increased from 86.46?m in P0 to 132.24?m in P700,showing an icnraesing microaggregate stability along rice cultivation sequence.Generally,SOC and TN were enriched both in 2000-200?m and in<2?m fractions,increasing with prolonged rice cultiavtion.C/N in the farctions deceased with decreasing size of the farctions,but C/N in2000-200?m increased with rice cultivation length.As indicated by the ratio of stable organic carbon to SOC,organic carbon showed lower stability in the 2000-200?m fraction and higher in<2?m fraction,with the stability in a single fraction enhanced with rice cultivation.Compared to the uncultivated tidal marsh,stability of OC in 2000-200?m increased with increasing of phenol-C,aromatic-C and aliphatic-C.MBC was generally highest in 2000-200?m fraction,decreasing with the smaller size of the aggregate classes.Moreover,MBC in a single particle farction increased with rice cultivation length,especiall y in 2000-200?m fraction.Similarly,enzyme activity of invertase,?-glucosidase and?-cellobiosidase were mainly allocated in the 2000-200?m fraction,but of urease was the lowest.Acid phosphatase acitivity was much higher in 2000-200?m and<2?m particles than in the other farctions,with peroxidase being relatively unchanged among the fractions.For a single fraction,enzyme activities in the paddy soils were higher than in the uncultivated tidal marsh.Moreover,organic carbon fraction showed more significant relationship with microbial activities indicated by the enzyme activities were found more closely correlated to OC in the 2000-200?m farction than in the other particle size fractions.The results here indicated that long-term rice cultivation promoted soil micro-aggregation and their stability and microbial fucntioning.Furthermore,prolonged rice cultivation increased SOC content and the stability while improved the enhancement of microbial biomass and enzyme activity in the macroaggregtes of the rice paddies.4.Changes in carbon sequestration by the rice paddies along the chronosequence Using maize straw as exotic carbon input,topsoil samples were incubated for 180 days for analyzing the carbon sequestration potential of the rice paddies.After incubation,SOC increased from 1.38 g kg-1 in P0 to 2.86 g kg-1 in P700,with more organic carbon sequestrated in the paddy soils cultivated for over hundraeds.In companion with the SOC sequestration,mass content of the particle size fraction of 2000-200?m increased but of<20?m decraesed.In the incubated soils,maize straw carbon in 2000-200?m was higher in P0,P50 and P100 compared to older soils.However,maize straw carbon decreased in particle size fraction of 2000-200 and increased in of<2?m in P300 and P500.These finding could indicate that maize carbon was rather chemically stabilized in fine aggregates of<2?m farction of old soils.All these suggest that rice soils developed under long term rice cultivation promoted a high carbon sequestration potential with rather chemical stabilization in fine aggregates and less carbon decomposition primed by the exotic carbon input.The study here reports a dynamic figure of SOC accumulation and stabilizationin the rice soils derived from salt marsh soils.SOC showed a fast accumulation in the soil shortly shifted from the salt marsh and a steady accumulation over hundreds of years of prolonged rice cultivation.This could be supported by the concurrent changes in physical protection by aggregates and chemical binding with Fe oxyhydrates in the soils along the sequence with prolonged rice cultivation.The SOC accumulation in old rice soils was further evidenced by the increasing molecular recalcitrance of higher ratio of humic acids to SOC.Meanwhile,soil microbial biomass and enzyme activities were found generally increased with the prolonged rice cultivation.This indicated a potential enhancement of SOC functioning and service,characterized by the aggregate stability,carbon stability and sequenstartion potential as well as microbial biochemical activity,in line with the SOC trend in the rice soils along the sequence.This study addressed a dynamic interaction of soil organic carbon,soil aggregation and microbial activity with the SOC accumulation and stabilization over the long term rice cultivation.And this could offer new insights for soil development with regard to soil organic matter dynamics in Anthrosols over long term rice cultivation. |
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