Effects Of Elevated Atmospheric CO₂ Concentration On Soil Organic Carbon Stability In Paddy Fields

With the continuous development of industry,the global atmospheric CO₂ concentration shows an obvious increase trend.Elevated atmospheric CO₂ concentration can improve the photosynthetic rate and net primary production of vegetation,and increase the carbon input to soil from plant root exudates,root biomass and other rhizosphere sediments.Therefore,elevated atmospheric CO₂ concentration may affect the transformation and regeneration of soil organic carbon（SOC）,and thus affect SOC stability.Current studies mainly focus on the effects of elevated atmospheric CO₂concentration on SOC concentration,chemical structure composition and decomposition rate,and rarely involve the concept of SOC stability.Studying the effects of elevated atmospheric CO₂ on SOC stability of paddy fields is not only can evaluate the feedback effect of farmland soil carbon pool on climate change,but also has important significance for the sustainability of farmland soil quality under future climate change.In this study,the only two open rice field climate change experiment platforms（FACE platforms）in China--Jiangdu Rice FACE Platform（Jiangdu）and Nanjing Agricultural University Changshu FACE Platporm（Changshu）.From the perspective of SOC stability,the effects of long-term elevated atmospheric CO₂ concentration on SOC stability in paddy fields were analyzed by using biochemical indices and isotope techniques.The changes of SOC stability in aggregates at different levels were analyzed by soil physical classification technique with increasing atmospheric CO₂ concentration.By adding ¹³C-labeled glucose and different levels of nitrogen fertilizer to the soil to explore the priming effect of paddy SOC under elevated CO₂ concentration.The main conclusions are as follows:（1）Paddy soils may be a potential carbon sink in the future under elevated atmospheric CO2 concentration.In the paddy soil of Jiangdu,elevated CO₂ concentration did increase SOC content at medium and long-term temporal scale（14 years）,and the SOC content increased more and more with the increase of time,but the average annual increase was small,ranging from 0.16～0.26 g kg^-1 soil;In the short-term（1 year）temporal scale,elevated atmospheric CO₂ concentration led to a slight downward trend of SOC content,but the decline was not significant.For paddy soil in Changshu,elevated atmospheric CO₂ concentration in 10years significantly increased SOC content in paddy field（increased by 31.36%）,the average annual increase was 0.347 g kg^-1 soil.（2）Elevated atmospheric CO2 concentration decreased stability of SOC in paddy fields,and the degree of stability decline slowed down with the increase of treatment years.The relative stability of SOC was negatively correlated with POC/SOC,ROC/SOC and DOC/SOC（P<0.01）,was negatively correlated with MBC/SOC（P<0.05）.The reliability of SOC relative stability and biochemical stability indexes（POC/SOC,ROC/SOC,DOC/SOC and MBC/SOC）were verified.The relative stability of SOC in paddy soil decreased by 10.56～12.45%and the sensitive stability index of SOC increased by 7.89～48.29%with the continuous elevated atmospheric CO₂ concentration for 10～14 years.Both indicated that the decrease of SOC stability slowed down with the increase of treatment years.（3）Elevated atmospheric CO2 concentration increased the structure stability of soil aggregates,but decreased the SOC stability of paddy fields,and SOC stability decline mainly occurred in soil>0.5 mm particle size aggregates.Elevated atmospheric CO₂concentration increased the MWD,GMD,R_0.25 of paddy soil,and improved the structural stability of soil aggregates.However,small aggregates were conducive to long-term preservation of SOC.Therefore,high CO₂ concentration actually reduced the physical stability of SOC.The comprehensive analysis of SOC stability index shows that elevated atmospheric CO₂ concentration decreased the SOC stability of>0.5 mm particle size aggregates,and the SOC stability of 1-2 mm particle size aggregates decreased the most.The SOC stability of aggregates with particle size<0.5 mm increased.（4）Isotope labeling experiments confirmed that the SOC stability of paddy soil was decreased with the elevated atmospheric CO2 concentration,but the addition of nitrogen fertilizer effectively alleviated the SOC stability decline,and the mitigating effect increased with the increase of nitrogen fertilizer dosage.Compared with natural CO₂concentration,total soil respiration under high CO₂ concentration increased by 2.62～20.32%,indicating that the transformation ability of SOC was enhanced.From the cumulative total priming effect of the whole incubation cycle,the soil priming effect of high CO₂ concentration was significantly increased by 55.02～89.75%,and the relative priming effect was significantly increased by 41.07～72.67%,the increase of total priming effect and relative priming effect decreased with the increase of nitrogen application rate.In conclusion,elevated atmospheric CO₂ concentration increased the SOC content of paddy fields,but decreased the SOC stability,and the SOC stability decreased mainly in soil>0.5 mm particle size.At the same time,the addition of nitrogen fertilizer could effectively alleviate the decrease of SOC stability caused by the elevated CO₂ concentration,and the mitigating effect increased with the increase of nitrogen fertilizer dosage.