Effect Of Rice Straw/Root Additions On The Composition Of Soil Organic Carbon In Red Paddy Soils

Profile red paddy soil samples were collected from the parent horizon?C?,hydragric horizon?Brs,Br,Br1,Br2?,plow pan?Ap2?and anthrostagnic epipedon?Ap1?of new paddy field?NP,20?30 yrs?,medium-old paddy field?MP,50?60yrs?and old paddy field?OP,400?500yrs?,which located at the Sunjia small watershed,Yingtan,Jiangxi Province.Soil organic carbon was fractionated into the free particulate organic carbon?fPOC?,occluded OC?oPOC?and mineral-associated OC?MOC?according the Six's method.The changes of TOC,fPOC,oPOC and MOC in the new paddy soils during the equal-carbon additions of rice straw?S?,rice root?R?and rice straw-root?S+R?from 0 day to 210 days were discussed;and the average daily mineralization?ADM?,cumulative mineralization?CM?,potential mineralization?C₀?,mineralization rate constant?k?and half-turnover time(T_1/2)also were calculated by the first-order kinetic equation.The main results are as follows:?1?TOC contents of red paddy soils changed in the order of OP>MP>NP,and decreased significantly with the increasing of soil depth.Compared with NP,the TOC content in Ap1 of MP and OP increased significantly by 110.2%and 92.0%,respectively,but there was no significant difference between MP and OP.The proportions of fPOC?fPOC_p?and MOC?MOC_p?were high to 72.2%and 72.9%,respectively,while the proportion of oPOC?oPOC_p?was not greater than 7.2%.Equal additions of rice straw,rice root and rice straw-root can significantly increased the TOC contents in profile soils,and presented significantly decreased with the increasing of incubation time.Compared with the results of30 days incubation,only the TOC content in AP1 of Treatment S was decreased by 19.6%after 120 days incubation,but the Treatment R and S+R no significantly differences.?2?The fPOC contents of paddy soils changed in order of OP>MP>NP,and decreased significantly with the increasing of soil depth.Compared with NP,the fPOC in Ap1,Ap2 of MP and OP increased significantly by 176.7%and 138.2%,76.4%and 51.9%respectively,but there was no significant difference between MP and OP.The fPOC_pin NP and MP were also decreased significantly with the increasing of soil depth,but OP no significantly change.Compare with NP,fPOC_pin Ap2 of MP increased significantly by31.9%,while OP had no significant change.Compared with MP,fPOC_pin Ap2 of OP decreased significantly by 15.2%.The fPOC contents in NP significantly increased with the increased of incubation time after equal additions of rice straw,rice root and rice straw-root.Compare with the incubation results of 30 days,only the fPOC of AP1 in Treatment S+R significantly increased by 44.8%at 210 days,but other treatments no significant difference.?3?The oPOC contents of paddy soils changed in order of OP>MP>NP,and decreased significantly with the increasing of soil depth.Compared with NP,the oPOC in Ap1 of OP increased significantly by 560.0%,but MP had no significant change.The oPOC_pin paddy soils were also decreased significantly with the increasing of soil depth.Compared with NP,oPOC_pin Ap1 of OP increased significantly by 157.5%,but MP had no significant change.The oPOC contents in NP was significantly increased with the increased of incubation time after equal additions of rice straw,rice root and rice straw-root.Compare with the incubation results of 30 days,the oPOC of Ap1 and Br1 in Treatment S significantly decreased by 40.3%at 210 days and 54.1%at 60 days respectively,the oPOC of Ap2 and Br1 in Treatment R significantly decreased by 46.6%at 60 days and 41.7%at 120 days respectively,and the oPOC of Ap1 and Ap2 in Treatment S+R decreased by 25.8%and 27.0%at 60 days.?4?The MOC contents of paddy soils changed in order of OP>MP>NP,and decreased significantly with the increasing of soil depth.Compared with NP,the MOC in Ap1 of MP and OP increased significantly by 79.5%and 140.5%respectively,but there was no significant difference between MP and OP.The MOC_pin paddy soils was increased significantly with the increasing of soil depth.Compared with NP,MOC_pin Ap2 of MP and OP decreased significantly by 27.4%and 13.7%respectively.Compared with MP,MOC_pin Ap2 of OP increased significantly by 18.8%.The MOC contents in NP were significantly increased with the increased of incubation time after equal additions of rice straw,rice root and rice straw-root.Compare with the incubation results of 30 days,the MOC of Ap1 and Ap2 in Treatment S,R and S+R significantly decreased high to 31.1%?21.2%after 60,120and 210 days incubation.?5?The average daily mineralization of soil organic carbon?ADM?in NP was significantly increased with the increased of incubation time after equal additions of rice straw,rice root and rice straw-root,and reached its maximum within first 3 days of incubation,while the ADM tended to decrease significantly with increasing incubation time and then change slowly.Compared with CK,the maximum ADM in Treatment S and S+R increased significantly by 6.1 to 54.8 times and 4.3 to 33.2 times,but Treatment S had no significant effect.Compared to day 1 of culture,the CM increased by 357.2%on first 7 days,while the increase was no more than 47.3%on days 7 to 35 days,with the lowest increase in CM after35 days incubation.The proportion of CM before ACM reach its stabilization in each layer was higher than 51.8%,which was the main contribution to the total mineralization amount.Compared with CK,C₀in Treatment S,R and S+R was significantly increased by 407.8%,115.0%and 254.5%respectively,k was increased significantly in Treatment S and decreased in Treatment R and S+R,T_1/2was decreased significantly in Treatment S and S+R and increased in Treatment R.With the increase of TOC,fPOC and oPOC contents,C₀and k were also significantly increased,while C₀significantly increased with the increase of fPOC_pbut significantly decreased with the increase of MOC_p.?6?The TOC contents in Treatment R or S+R were higher than that under Treatment S after equal additions of rice straw,rice root and rice straw-root in NP and the CM in Treatment S+R was the lowest during the incubation.Moreover,the TOC contents in Br1 of NP were significantly higher than in Ap1 and Ap2.Therefore,returning the rice straw-root to the Br1 layer?20?40 cm?can effectively increase the sequestration of organic carbon and reduce the mineralization of organic carbon in the soil of the new paddy.