Characteristic And Mechanism Of Organic Carbon Sequestration Of Purple Paddy Soil Under Long-term Fertilization

The soil carbon?C?cycle is the vital process of terrestrial ecosystem,and the soil C is the important source or sink of greenhouse gases in the atmosphere.Meanwhile,soil organic C?SOC?can directly or indirectly affect the soil physical,chemical and biological processes,therefore the accumulation of SOC is beneficial to maintain soil fertility,increase productivity and ensure food security.Purple paddy soil is the one of the important agricultural soils in China,which is concentrated in Sichuan Basin,an important region for food production in China.Fertilization is one of important management practices to affect SOC.It is necessary to reveal the characteristic and mechanism of SOC sequestration based on the long-term experiment because the change of SOC is a slow process.Based on a 22-year long-term experiment of "National Monitoring Station of Soil Fertility and Fertilizer Efficiency on Purple Soil" located in Beibei district,Chongqing city,China,this article is to study the effect of long-term fertilization on SOC and its mechanism of purple paddy soil under a rice-wheat cropping system.In total,this study includes 9 treatments: unfertilized control?CK?,chemical fertilizer N?N?,chemical fertilizer PK?PK?,chemical fertilizer NK?NK?,chemical fertilizer NP?NP?,chemical fertilizer NPK?NPK?,NPK plus straw return?NPKS?,high amount of NPK plus straw return?1.5NPKS?,NPK plus manure?NPKM?.This study aims at promoting the soil C sequestration,improving the soil productivity and achieving the agricultural sustainable development by including the following primary coverages: firstly,using the saved soil sample and the data of crop yield over years,the characteristic of SOC sequestration was study to reveal the C sequestration dynamic,trend and efficiency under different long-term fertilizations;Secondly,the response of SOC fractions with different labilities to long-term fertilization was studied to investigated the SOC quality change and the mechanism of chemical protect by soil minerals;Thirdly,the chemical structure and turnover of SOC under different long-term fertilization was studied to reveal the mechanism of biochemical protection based on the technique of 13 C stable isotope and nuclear magnetic resonance?NMR?;Finally,the quantity and quality of SOC associated aggregate with different sizes was analyzed to clarify the relationship between SOC and aggregates turnover,and the mechanism of physical protection by aggregate.The main results as follows: 1.Characteristic of carbon sequestration of purple paddy soil under long-term fertilizationLong-term fertilization significantly increased the SOC content and stock than CK and the initial level.Soils with combined application of organic and chemical fertilizer had higher SOC than that without fertilizer and with chemical fertilizer alone;After 22 years,there were 18%-25% increase than the initial level.Among treatments with chemical fertilizer alone,NP and NPK treatments showed greater increase while CK,N,PK kept steady in a relatively low SOC level.After 22-year fertilization,the C sequestration was 5.4-8.3 t/ha with annual rate of 244-378 t/ha/yr for treatments under combined application of organic and inorganic,and was 6.6-7.1 t/ha with the annual rate of 302-322 kg/ha/yr.In compared with CK and unbalanced fertilization,NPK and NPK plus organic fertilizers enhanced the C inputs,and therefore the C stock,sequestration and rate.There existed saturation level and steady state of SOC.With increased C inputs,the marginal efficiency was declined.There was significant and positive relationship between SOC content and crop yield,illustrating that the rice and wheat grain yield would increase 627 and 313 kg/ha with each 1 g/kg C increase.Consequently,the combined application organic and inorganic fertilizer was the effective practice to promote both C sequestration and yield increase in purple paddy soil.2.Effects of long-term fertilization on SOC fractions of purple paddy soilCompared with CK,long-term fertilization increased the labile organic carbon content,and therefore improved the quality of SOC with the greatest increase for NPK plus straw return.The dissolved organic C?DOC?,permanganate oxidizable C?POXC?,the C of light fraction?LF-C?and particulate organic matter?POM-C?were increased by 30.5%,43.8%,59.5% and 43.8%,respectively.The labile organic carbons were significantly related to total SOC,indicating that they could sensitively indicate the change of total SOC.Compared with CK,long-term fertilization improved the stable organic C,including C in mineral associated organic matter?MOM-C?and heavy fraction?HF-C?,with the greatest increase for NPK plus straw return fertilizer.Change of Fe/Al-OC and Humin-OC accounted for 43.1% and 48.0% change of total SOC by long-term fertilization,while the percentage for Ca-OC was very small.Therefore,the role of chemical binding of Fe and Al in organic-metal complexes and the further formation of recalcitrant humin might be the potential mechanism for SOM accumulation in purple paddy soil.The micro-aggregate?250-53?m?was mainly the composition of soil matrix,followed by macro-aggregate?>250?m?and silt plus clay?<53?m?.Compared with CK,long-term fertilization promoted the formation of macro-aggregate,thus the proportion of macro-aggregate increased,the micro-aggregate decreased,and the mean weight diameter increased.Long-term fertilization enhanced the physical protect for SOC by aggregate.In all aggregates with different size,the SOC and labile organic C were enhanced by long-term fertilization,with a higher improvement for NPK plus straw return than other treatments.The accumulation of total SOC by long-term fertilization was determined by aggregates with different sizes because there were significantly positive relationships between SOC content in bulk soil and macro-aggregate,micro-aggregate and silt plus clay.However,the contribution of SOC change in aggregates to total SOC ranged as micro-aggregate > micro-aggregate > slit and clay fraction.The physical protection from macro-aggregate and micro-aggregate play an important role in the soil C sequestration,and the SOC associated with macro-aggregate was sensitive to fertilization,while the micro-aggregate was the main place of C sequestration due to greatest proportion of weight.4.Effects of long-term fertilization on SOC structure of purple paddy soilUsing the technique of 13 C isotope and solid-state NMR analysis,the turnover and structure of SOC in bulk soil and aggregate for CK,NPK and NPKS treatment were studied.The results of 13 C stable isotope showed that after 22 year fertilization the ?¹³C values were lower than the commencement,and tended to the values of rice and wheat plant,indicating that the return of crop residues promote the SOC turnover.Long-term fertilization especially NPKS accelerated the C turnover in all aggregates with different sizes than CK because their ?¹³C values were significantly lower than that in CK.Results from solid-state ¹³C-NMR showed that the structure of SOC of purple paddy soil was aliphatic,rather than the aromatic nature,because the relative proportion of O-Alkyl-C was highest,followed by Alkyl-C,while the Aromatic-C and Carbonyl-C only accounted for a small part.Compared with CK,long-term fertilization improved the structural stability of SOC in plough layer,but reduced the structural stability in deep soil layer.In the plough layer,compared with CK,the relative proportion of O-Alkyl-C in NPK treatment decreased,the Aromatic-C and Carbonyl-C increased,therefore the Alkyl-C/O-Alkyl-C ratio and the aromaticity increased;For NPKS treatment,the Alkyl-C increased while the O-Alkyl-C decreased,thus the Alkyl-C/O-Alkyl-C decreased.In the deep soil,compared with CK,NPK and NPKS increased the O-Alkyl-C,but decreased the Alkyl-C,Aromatic-C and Carbonyl-C;therefore,the Alkyl-C/O-Alkyl-C ratio and aromaticity were decreased by long-term fertilization,with a larger change for NPKS than NPK treatment.The SOC structure for the aggregates showed that in plough layer the structural change of SOC associated with micro-aggregate and silt plus clay were consistent with that in bulk soil,indicating that the improvement of the C structural stability of in bulk soil by long-term fertilization was mainly due to the micro-aggregate and silt plus clay fraction.However,long-term fertilization decreased the proportion of Alkyl-C,increased the O-Alkyl-C,thus the Alkyl-C/O-Alkyl-C ratio was decreased while the aromaticity was increased in the macro-aggregate.In the deep soil layer,effects of long-term fertilization on SOC structure of aggregates with different sizes were similar with total SOC,showing an increased O-Alkyl-C,declined Alkyl-C,Alkyl-C/O-Alkyl-C ratio and aromaticity.In conclusion,long-term fertilization,especially combined application chemical organic fertilizers,increased the C inputs and turnover;with the physical protection from macro-aggregate and micro-aggregate,the chemical protection from minerals,and the enhanced structural stability,the quantity and quality of SOC in purple paddy soil were improved.