Comparison Of Contribution Of Previous Crop-Derived Carbon To Soil Organic Carbon Between Maize-Rice And Double Rice System

The contribution of crop photosynthetically fixed carbon to soil organic carbon components affects the carbon balance of farmland ecosystems and the stability of soil organic carbon.In order to clarify transfer of crop source carbon among different compartments of crop-soil system,and to provide scientific basis for understanding the carbon sequestration mechanism under maize-rice and double rice cropping in the middle reaches of the Yangtze River,the distribution of previous crop sourced carbon in crop-soil system and their contributions to soil organic carbon components（TOC,DOC,MBC,fPOC,mPOC）by root deposition and residues return were examined under maize-rice（MR）and double rice（RR）cropping systems in this study by using ¹³C labeling technique.The main results are shown as the following:（1）Annually carbon sequestration under MR treatment was significantly higher than that under the RR treatment,with 282 g C/m² and 319 g C/m² increase in 2016 and 2017,respectively.The proportion of recovered ¹³C_labeled were allocated in early rice plants by99.75%and 97.49%at 1 day after the labeling events and at the harvest of ealy rice,respectively.At the harvest of early rice,the soil received recovered ¹³C_labeled from the rice plants by 2.51%.While,the proportion of recovered ¹³C_labeled in maize plants were 99.42%and 98.85%at 1 day after labeling and the harvest,respectively.By the harvest of maize,the soil received recovered ¹³C_labeled from the maize plants by 1.15%.（2）Under the RR treatment,the contribution of root carbon to soil TOC was 0.71%and 1.03%higher than that under the MR treatment,and the amount of contribution were296.00 mg C/kg soil and 408.09 mg C/kg soil,respectively.In the RR treatment plots,the contribution of labeled straw carbon to soil TOC was higher than that in the MR treatmentby16.33%and 2.14 g C/kg soil at the heading stage of late rice.However,the contribution percentage and amount of early rice labeled straw carbon to soil TOC were not significantly different from that of corn straw at late rice harvest.Overall,the plants sourced carbon of the previous crop was more easily trapped in the soil under the RR treatment in comparison with MR treatemnt.（3）Under the MR treatment,the contribution of pre-crop root-derived carbon to soil DOC was greater than that under the RR treatment,and the contribution percentage was0.26%and 1.19%higher,respectively,at harvest time of the two crops,and the annual contribution was 5.28 mg C/kg soil higher than that under the RR treatment.At harvest time of late rice,the contribution of pre-season crop root-derived C to soil MBC under the MR treatment was significantly higher than that under the RR treatment,by increase of0.73%in contribution percentage and 1.66 mg C/kg soil in amount,respectively.Under MR treatment,the contribution percent in MBC and DOC of labeled straw-derived carbon were 4.21%and 3.37%higher than that of RR mode,respectively,an the contribution amount were 134.91 mg C/kg soil and 46.96 mg C/kg soil higer,respectively.It can be seen thatthe effect of maize source carbon on soil active organic carbon components is greater than that of early rice.（4）In the RR treatment,the contribution of root-derived C of the previous crop to fPOC and mPOC was greater than that of the MR treatment.Annually,RR treatment had significant higher contribution percent of root-derived C to soil fPOC and mPOC by 3.76%and 2.07%than that of MR treatment,and the contribution amount of root-derived C to soil fPOC and mPOC was 5.76 and 4.23 times higher than that of MR treatment.The contribution percent to fPOC and mPOC of maize straw-derived carbon were 6.21%and0.87%higher than that of early rice,respectively.The contribution amount in fPOC and mPOC were 0.19g C/kg soil and 0.25g C/kg soil higher than that of early rice,respectively.However,the difference was not significant.（5）After 2-year of field experiment,there was no significant difference in the carbon content of soil TOC,MBC,DOC,fPOC and mPOC under MR and RR treatments.However,at the late rice harvest in 2017,the natural abundance of ¹³C（δ13C）in fPOC,mPOC and TOC at 0-10 cm soil depth under MR treatment were significantly different from that under the RR treatment.According to the change in naturalδ13C,estimated contribution percentage and amount of C_{maize-derived} in soil fPOC were 4.13%and 172 mg/kg soil,3.76%and 606 mg/kg soil in mPOC,and 4.96%and 810 mg/kg soil in TOC,respectively.