Effects Of Tillage On Soil Carbon And Nitrogen In Double Paddy Cropping System Of Southern China

Tillage practices play an important role in the turnover of soil organic carbon (SOC) and total nitrogen (TN), greenhouse gas (GHG) emissions, and reactive nitrogen (Nr) in the process of crop production. The objectives of this study were to identify tillage practices effects on SOC sequestration and dynamics of TN; to assess the tillage-induced environmental impacts based on the GHG and Nr emissions; and to provide suggestions or strategies for double rice (Oryza sativa L.) cropping system in Southern China. We analyzed the changes in SOC and TN pools and their relationship with crop productivity, and calculated carbon footprint (CF) and nitrogen footprint (NF) of double rice under different tillage practices. A long-term experiment, including no-till with residue retention (NT), rotary tillage with residue incorporation (RT), plow tillage with residue incorporation (PT) and plow tillage with residue removed (PTO) was established in2005. The data of2013-2014was used to analyze in current research, and the main results were list as follows:(1) There were slight variations in the concentrations of mineral-associated organic carbon (MOC) and mineral-associated total nitrogen (MTN) at0-20cm profile for PT, RT and PTO, except for NT. The concentrations of SOC, TN, heavy fraction organic carbon (HOC), heavy fraction total nitrogen (HTN), light fraction organic carbon (LOC), light fraction total nitrogen (LTN), particulate organic carbon (POC), and particulate total nitrogen (PTN) gradually decreased with increasing soil depth. With the increase of tillage intensity, the stratification ratio of SOC, TN, and related fractions decreased under residue retention treatments (e.g. PT, RT, and NT), except for MOC and MTN.(2) With regard to residue retention treatments, the SOC concentrations significantly decreased with increasing tillage frequency and intensity at0-5cm depth, following the order of NT> RT> PT at both the early and late rice harvest. Similar trends were observed for the TN concentration at the early rice harvest, and the TN concentration under NT was significantly higher than that under RT and PT at the late rice harvest. At5-10and10-20cm depths, the SOC and TN concentrations under NT were lower than those under RT and PT. Annual and inter-annual fluctuations of SOC and TN concentrations at20-30cm depth were also observed among the treatments. There was no significant difference in SOC and TN concentrations at30-50cm depth among the treatments.(3) The concentrations of HOC, HTN, MOC and MTN gradually decreased with increasing tillage intensity at0-5cm depth, NT was significantly higher than PT. However, NT reduced the concentrations of HOC, HTN, LOC, LTN, POC, PTN, MOC and MTN in contrast with those under PT at5-10cm depth. Tillage practices had a slight influence in the fractions of SOC and TN in the subsoil (>20cm).(4) Tillage and residue retention did not significantly impact on C:N, HOC:HTN, POC:PTN, and MOC:MTN. Under residue retention conditions, tillage practices had no influence on stocks of SOC and TN in the0-50cm layers at the early rice harvest, while inter-annual fluctuations were observed among the tillage treatments at the late rice harvest. Compared to PTO, PT significantly increased SOC stock in0-30cm layers at the early rice harvest and in0-20cm layer at the late rice harvest, and TN stocks in20- 30cm layers at both harvest seasons. Results showed that rice yield could increase by24.1kg ha-1with an increase of1Mg SOC stocks in the0-50cm profile.(5) Under residue retention conditions, the hidden GHG and Nr emissions due to agricultural inputs in double rice production system decreased gradually with a decrease in tillage intensity, following in the order of PT>RT> NT, ranging from3978.3to4271.9kg CO2-eq ha-1year-1and1149.2to1435.7kg N-eq ha-1year-1for the hidden GHG emissions and Nr emissions, respectively. Compared with PT, NT and RT reduced by7.4%and3.4%of the hidden GHG emissions, and24.9%and10.0%of the hidden Nr emissions from agricultural inputs application, respectively. Fertilizers application were the most primary contributor to the total hidden GHG emissions from agricultural inputs. However, machinery energy consumptions induced by tillage, irrigation, and harvest, were the largest contributor to the total Nr emissions from agricultural inputs.(6) Without considering SOC changes, the CFs of double rice under NT, RT, PT, and PT0were1.53,1.95,1.59, and1.37kg CO2-eq kg-1of grain, respectively, and the CFs for the early rice were lower than that for the late rice under residue retention treatments. Principal contribution to CFs were due to CH4emissions from paddy fields, followed by fertilizers application. In consideration with SOC changes, the CFs of double rice decreased by35.9-50.3%compared to that excluding SOC stock. In addition, the NFs of double rice under NT, RT, PT, and PT0were5.70,4.67,4.94, and5.17kg N-eq kg-1of grain, respectively, and the NH3volatilizations from paddy fields were the largest component of NFs of the double rice.