Effects of Optimizing Fertilization on N2O and CH4 Emissions in a Paddy-Cowpea Rotation System in the Tropical Region of China

Abstract

In-situ measurement of nitrous oxide (N2O) and methane (CH4) emissions in a typical paddy-cowpea rotation system in Southern Hainan was conducted to determine the characteristics of greenhouse gas emissions under different optimum fertilization treatments. The experiment consisted of 5 treatments:conventional farming fertilization (CON), optimized fertilization (OPT), organic-inorganic fertilization (ORG), slow-controlled optimization fertilization (SCOPT), and no nitrogen as the control (CK). The N2O and CH4 emissions were measured using static chamber-gas chromatography during the all the paddy-cowpea rotation seasons. Global warming potential (GWP) and greenhouse gas intensity (GHGI) were also estimated in this study. The cumulative N2O emission during the rice growth season was 0.19-1.37 kg·hm-2. Compared with the CON treatment, other treatments reduced N2O emission by 50% to 86%. The cumulative N2O emission during the cowpea growth season was 1.29-3.55 kg·hm-2. In addition, N2O emission increased by 14% as a result of the ORG treatment, whereas that of the remaining treatments decreased by 16% to 59%. The cumulative CH4 emissions during the paddy growth season were 4.67-14.23 kg·hm-2. The CH4 emissions following the CK, OPT, and ORG treatments were higher by 116%, 22%, and 102%, respectively, whereas that of SCOPT was lower by 29%, than that following the CON treatment. Moreover, the cumulative CH4 emission during the cowpea growth season was 0.03-0.26 kg·hm-2, and CH4 absorption occurred during the same period. With regard to the contribution rate of different periods to GWP, the cowpea growth season still had a proportion of 44.7%-54.5%, despite extremely low CH4 emission. Regarding the two greenhouse gases, N2O contributed 66.7%-77.2%. During the entire rotation system, both GWP and GHGI processed by SCOPT were significantly lower than those of the CON treatments. To sum up, the SCOPT treatment was determined to be the optimal fertilization scheme in this study and had the most significant effects on increasing production and reducing greenhouse gas emissions.