A Study On Greenhouse Gases (CO₂,CH₄ And N₂O) Emissions From Rice-Winter Wheat Rotations In Southeast China

The increasing concentration of greenhouse gases in the atmosphere, such as CO₂, CH₄ and N₂O has been paid much attention due to their substantial contribution to global warming and ozone depletion, the two severest impacts on the global climate. Agroecosystems play an important role in the budget of atmospheric greenhouse gases. Rice and wheat productions in China constitute a large portion of the world total cereal cultivation, and the rice-wheat rotation is one of major cropping types in China. Consequently, this study was carried out in rice-winter wheat rotations with static opaque chamber-gas chromatograph method to simultaneously measure CO₂, CH₄ and N₂O fluxes. The primary objective was to examine the effect of local conventional agricultural management on greenhouse gases emissions, and to explore their processes and the associated mechanisms in this typical cropping system in southeast China.In this study, field experiments were accomplished over three rice-winter wheat rotation cycles from the 2000 rice season to the 2003 winter wheat season. In 2000-01, a split-plot experiment was performed to study the effects of water regime and wheat straw application in the rice-growing season on CH4 and N₂O emissions over an annual cycle. Water regime in paddies consisted of the flooding-drainage-flooding (F-D-F) and the continuous flooding (F). Wheat straw was incorporated with the rate of 0, 225 or 450 g.m^-2 at the depth of 10 cm in the soil for each water regime just before rice transplanting. In 2001-02, a field experiment was conducted to investigate the effect of organic material application on CH4 and N₂O emissions from rice and winter wheat seasons. Five fertilizer treatments in rice paddies including chemical fertilizer (CK), chemical fertilizer + rapeseed cake (RC), chemical fertilizer + wheat straw (WS), chemical fertilizer + cow manure (CM), and chemical fertilizer + pig manure (PM) were dedicated to this experiment. The identical Chemical fertilizer was used as urea and NH₄HCO₃ for all treatments, and organic material (225 g.m^-2) was incorporated for organic treatments at the depth of 10 cm in the soil as the basal fertilizer just before rice transplanting. In 2002-03, fertilizer application treatmentswere adopted in a rice-winter wheat rotation agroecosystem to quantify the direct emission of N₂O for synthetic N fertilizer and crop residue application. To investigate the impact of plant residues decomposition on N₂O emission, additionally, laboratory incubations were carried out for a period of 21 days using urea and five plant residues with a wide range of C/N ratios from 8 to 118. Finally, outdoor pot and field experiments were oriented to assessing the role of growing plants in agricultural ecosystem N₂O emissions. The results of this study are presented as follows:1. In contrast with the water regime of Flooding-Drainage-Flooding (F-D-F), continuous flooding greatly decreased N₂O emissions in rice-growing season, but markedly enhanced CH4 emissions in rice season and N₂O emissions in the following wheat-growing season. The effect of straw incorporation on N₂O emissions differed with water regime. Under the water regime of F-D-F, incorporation of wheat straw reduced N₂O emissions from the following winter-wheat season, while there was no significant difference in N₂O emissions between the treatments of straw applied at the rates of 225 and 450 gm^-2. On the other hand, N₂O emissions were not significantly different among the treatments with three rates of straw application under the continuous flooding regime.2. Under the water regime of Flooding-Drainage-Flooding-Moisture (F-D-F-M), the effect of organic material incorporation on CH4 and N₂O emissions depended on the type of organic material. Compared with the CK treatment, rapeseed cake incorporation before rice transplanting increased CH4 and N₂O emissions by 252% and 22% in the rice season, respectively, while it had no obvious effect on N₂O emissions from the following wheat season. In contrast with the CK treatmen...