Effect Of Elevated Atmospheric [CO₂] On The Stability Of Organic Carbon In Paddy Soil Aggregate Sizes And Crop Growth

With the continuous development of industry,the global atmospheric CO₂concentration（[CO₂]）is increasing significantly.Elevated atmospheric[CO₂]can increase the carbon input to the soil through plant root exudates,root biomass and other rhizosphere sediments.Meanwhile,elevated atmospheric[CO₂]indirectly affects soil microbial activity and microbial decomposition and mineralization of soil organic carbon（SOC）.In recent years,many studies have directly or indirectly confirmed that elevated atmospheric[CO₂]reduces the stability of surface SOC.As an important indicator of soil quality and agricultural sustainability,SOC stability is believed greatly controlled by soil aggregates components.However,In the background of elevated atmospheric[CO₂],little is known about the law of SOC stability reduction at the aggregate scale and whether the decrease of SOC stability may feedback to crop growth by changing the carbon and nitrogen cycle.In this study,soil samples were collected from the surface layer（0～20 cm）of different plots(elevated[CO₂]plot（FACE plot）and control[CO₂]plot（Ambient plot）under the long-term rice field climate change experimental platform（FACE platform）.The SOC transformation and stability in each size aggregate influenced by elevated atmospheric[CO₂]and different levels of nitrogen were studied.Through arranging soil（Soils from different plots were placed under the same[CO₂]environment）replacement pot experiments under elevated and non-elevated[CO₂]environment with different nitrogen fertilizer levels,the possible feedback effect of SOC stability on crop growth under elevated atmospheric[CO₂]were revealed.The main conclusions are as follows:（1）Elevated atmospheric[CO₂]reduced SOC stability from paddy fields mainly happened in the middle aggregates（1～5 mm）.The addition of nitrogen could alleviate the decrease of SOC stability in each size aggregate.The increase of atmospheric[CO₂]reduced the SOC stability in each size aggregate,so that the MBC/SOC and cumulative mineralization rate in each size aggregate increased due to the increase of[CO₂],especially in the medium aggregates（1～5 mm）.Therefore,the increase rate of SOC content in the medium aggregates（1～5 mm）was significantly lower than that in the large aggregates（>5 mm）and small aggregates（<1 mm）.The addition of appropriate nitrogen can alleviate the decrease of SOC stability in each size aggregate.（2）The change of SOC stability caused by long-term elevated atmospheric[CO₂]had a negative feedback effect on spinach growth.Compared with the Ambient plot soil,the net photosynthetic rate and stomatal conductance of spinach under FACE plot soil decreased by 4.74～19.77%and 7.01～26.23%,respectively,which resulted in the decrease of spinach stem thickness and dry weight by-2.90～18.51%and-3.80～50%,respectively.Spinach growth was mainly affected by soil ammonium nitrogen content,p H,and conductivity from different plots.While soil ammonium nitrogen content,p H,and conductivity were significantly correlated with SOC stability（ROC/SOC,DOC/SOC）.Compared with the Ambient plot soil,the ROC/SOC of FACE plot soil increased-8.91～97.67%,which indicated the decrease of SOC stability by elevated atmospheric[CO₂].ROC/SOC was found negatively correlated with plant height,leaf length,stem thickness,fresh weight,and dry weight,especially spinach stem thickness and dry weight.（3）Application of nitrogen fertilizer could weaken the negative feedback effect of spinach growth through the change of SOC stability under elevated atmospheric[CO₂]condition.Compared with the no nitrogen fertilizer treatment,nitrogen fertilizer application decreased the soil plot effect（FACE plot vs Ambient plot change rate）of spinach stem thickness and fresh weight from-1.04～18.51%and 12.21～45.95%to-0.48～13.11%and4.95～41.07%,respectively.It’s mainly attributed to the decrease of the soil plot effect of spinach net photosynthetic rate and intercellular CO₂concentration from 16.49～19.77%and12.31～16.76%to 4.74～9.62%and 8.67～16.60%,respectively by nitrogen application.Compared to the treatment without nitrogen fertilizer,nitrogen fertilizer application decreased the soil plot effect of ROC/SOC from-8.91～83.18%to-7.81～97.67%,which implied the increase of SOC stability and further affected the soil nitrate nitrogen content,p H,and electrical conductivity and altered spinach growth.（4）Elevated atmospheric[CO₂]would further amplify the negative feedback effect of spinach growth through the change of SOC stability.Compared to a CO₂,e CO₂increased the soil plot effects of spinach leaf length,fresh weight,and dry weight from-3.37～11.04%,-15.28～28.23%,and-3.80～50.00%to 9.73～16.49%,12.21～45.95%%,and11.89～40.63%,respectively.The strengthened negative feedback effect of spinach growth was mainly attributed to the increase of spinach net photosynthetic rate,intercellular CO₂concentration,and transpiration rate from 4.74～16.49%,8.67～12.32%,and 11.11～39.71%under a CO₂to 9.62～19.71%,16.60～16.75%,and 15.52～44.94%,respectively under e CO₂.e CO₂increased the soil plot effects of ROC/SOC from-7.91～83.18%to-8.91～97.67%,which implied the reduce of SOC stability,and in turn altering spinach growth by decreasing soil p H and conductivity.Obviously,under the scenario of elevated atmospheric[CO₂]in the future,the sustainability of crop yield increase by using CO₂fertilization effect is limited by the change of SOC stability,and the use of appropriate nitrogen fertilizer is helpful to weaken the above limitations,and these results can provide a scientific basis for future farmland fertilization management decisions.