| Climate warming is a global environmental problem generally cared by people at present. As a major emission source of significant atmospheric greenhouse gases CH4 and N2 O, rice field gains increasing attention of scholars all over the world, and how to find effective greenhouse gas emission reduction measures for the rice field, at the same time ensure yield stability and increase of rice, and relatively accurately estimate the emission amount of the greenhouse gas of the rice field are important subjects for agricultural sustainable development.The study taken black soil rice field in cold area as the object, and with the combination of field experiment and pot experiment, CH4 and N2 O in black soil rice field in cold area were studied and estimated by using methods such as model simulation and theoretic analysis. Two water treatment methods of controlled irrigation and ordinary irrigation were set in field experiment, four fertilization methods, i.e. nitrogen phosphorus and potassium reduction II, nitrogen phosphorus and potassium reduction I, balanced fertilization and nitrogen phosphorus and potassium increase, were adopted to compare and analyze seasonal emission regulation of CH4 and N2 O and related factors of the rice field, thereby evaluating greenhouse effect and greenhouse gas emission intensity in different irrigation and fertilization modes, and studying the optimal irrigation and fertilization mode for greenhouse gas emission reduction of black soil rice field in cold area. On the basis of field experiment, saturated optimal test design was adopted in pot experiment, a relationship model of seasonal emission regulation of CH4 and N2 O and nitrogen phosphorus and potassium fertilizer and irrigation amount（lower limit of relative water content in soil at the controlled late tillering stage） under the controlled irrigation condition was established, the influence of water and fertilizer factors on emission of CH4 and N2 O under the controlled irrigation condition was analyzed, and the optimal irrigation and fertilization scheme for comprehensive CH4 and N2 O emission reduction of the rice field with stable yield was optimized. Simulation effectiveness of Denitrification-Decomposition model, DNDC was verified on the basis of field experiment data, by taking Qingan County in Heilongjiang province for example, the total emission amount of CH4 and N2 O of rice field in the County region was simulated by using the DNDC model, contribution of multiple influence factors to rice field CH4 and N2 O emission was studied through sensitivity analysis, and reasonable and feasible emission reduction measures were put forward. The main study conclusion is as follows:（1） In the all-bearing period of rice, the seasonal emission amount of rice field CH4 with different controlled irrigation fertilization treatment is 1.446-3.217g·m-2, and is reduced by 49.9%-61.2% when being compared with that of different basin irrigation fertilization treatment. Under two irrigation modes, nitrogen phosphorus and potassium increase treatment has the largest seasonal emission amount of CH4, seasonal emission amount of CH4 in nitrogen phosphorus and potassium reduction II is smaller than that in nitrogen phosphorus and potassium reduction I, and balanced fertilization has the smallest seasonal emission amount of CH4. CH4 emission in controlled irrigation rice field shows single peak seasonal variation, the emission peaks mainly appear at the front and middle tillering stages, and the two emission peaks at the tillering stage are somehow brought forward when being compared with those in waterflooding irrigation. CH4 emission in waterflooding irrigation rice field also shows single peak seasonal variation, and the emission peaks mainly appear at the middle and late tillering stages.（2） In the all bearing period of rice, the seasonal emission amount of rice field N2 O with different controlled irrigation fertilization treatment is 43.04-72.37mg·m-2, and is increased by 4.70%-9.02% when being compared with that of different basin irrigation fertilization treatment. Under controlled irrigation and basin irrigation modes, the seasonal emission amount of N2 O is increased along with increase of fertilization. The N2 O emission in controlled irrigation rice field shows single peak seasonal variation, the emission peaks mainly appear at the front and late tillering stages, small peaks also appear at the sprouting and blossoming stages and milk-ripe stage, and the emission amounts in other bearing periods are relatively small. N2 O emission in waterflooding irrigation rice field also shows single peak seasonal variation, the emission peak mainly appears at the late tillering stages, another small peak appears at the yellow ripening stage, and the emission amounts in other bearing periods are very small.（3） The seasonal emission regulation of CH4 and N2 O is mainly controlled by irrigation mode, and fertilization decides variation and amplitude of emission peaks of CH4 and N2 O in the rice field. CH4 under different irrigation and fertilization modes is the main greenhouse gas of the rice field, and the greenhouse gas emission reduction in the area shall take CH4 as the main target. In comprehensive consideration of rice yield, comprehensive greenhouse effect and greenhouse gas emission intensity, controlled irrigation is an ideal irrigation mode for greenhouse gas emission reduction, and balanced fertilization under the controlled irrigation mode has the highest yield and the lowest comprehensive greenhouse effect and greenhouse gas emission intensity, so that controlled irrigation balanced fertilization can be a relatively ideal irrigation and fertilization mode for greenhouse gas emission reduction of black soil rice field in cold area.（4） Under the controlled irrigation conditions, nitrogen, phosphate and potash fertilizers and irrigation amount（lower limit of relative water content in soil at the controlled late tillering stage） can all influence the seasonal emission amount of CH4 and N2 O of the rice field to different extents. The nitrogen fertilizer has a remarkable inhibition function on seasonal emission amount of CH4, but there is a lowest point, excessive nitrogen fertilizer can increase seasonal emission amount of CH4, on the contrary, the nitrogen fertilizer can greatly increase the seasonal emission amount of N2 O as a whole, and the effect is remarkable with a great amount of nitrogen fertilizer; the seasonal emission amounts of CH4 and N2 O are not remarkably influenced by potash and phosphate fertilizers, the potash fertilizer of low amount can inhibit seasonal emission amounts of CH4 and N2 O, and that of high amount can promote seasonal emission amounts of CH4 and N2O; the phosphate fertilizer of low amount can inhibit seasonal emission amount of CH4, that of high amount can promote seasonal emission amount of CH4, and the phosphate fertilizer can promote emission of N2O; medium irrigation amount has a very good inhibition function on seasonal emission of CH4, but also can remarkably increase emission of N2 O.（5） With the combination of emission characteristic that CH4 and N2 O are increased and decreased mutually, by optimizing the relationship model of the seasonal emission amount of CH4 and N2 O of the rice field and irrigation and fertilization, the yield can be ensured, at the same time the seasonal emission amount of CH4 of the rice field can be reduced by 40-60%, the seasonal emission amount of N2 O of the rice field can be increased by less than 10%, and the optimal comprehensive irrigation and fertilization scheme is: N 97.21-105.75 kg/hm2, K2 O 36.93-63.57 kg/hm2, P2O5 33.11-39.83 kg/hm2, W（lower limit of relative water content in soil at the controlled late tillering stage） 69.07%-70.93%.（6） Comparison validation on observation result and simulation result of CH4 and N2 O of the rice field shows that the DNDC model is feasible for simulation and estimation on CH4 and N2 O of the black soil rice field in cold area. Furthermore, the total emission amounts of CH4 and N2 O of the rice field in Qingan County are estimated as 2.74×106kg C and 3.16×104kg N respectively. The model further simulates the seasonal emission amounts of CH4 and N2 O of the rice field in Qingan County in the conventional and optimal modes, in the optimal mode, the seasonal emission amounts of CH4 of the rice field all over the county is reduced by 55.93% when being compared with that in the conventional mode, that of N2 O is increased by 24.81%, and with the combination of the DNDC simulation and sensibility factor analysis result of CH4 and N2 O of the rice field, the following operable and feasible greenhouse gas emission reduction measures for the black soil rice field in cold area are put forward: water-saving irrigation is adopted for the rice field, CH4 emission is particularly controlled, and fertilize reasonably to control emission of N2 O, so that comprehensive greenhouse effect caused by CH4 and N2 O of the rice field can be alleviated, and environment pollution can be relieved; in emission reduction operation, straw turnover ratio shall be well controlled or reasonable straw turnover modes shall be selected.To sum up, the optimized irrigation and fertilization scheme in the controlled irrigation mode can ensure the yield, save the water, and remarkably relieve comprehensive greenhouse effect of CH4 and N2 O of the rice field, and can be a relatively ideal emission reduction method for greenhouse gases of rice fields. With the combination of ecological model and conventional observation, simulation on CH4 and N2 O under different irrigation and fertilization modes can be achieved, and it can be an effective method for regional estimation on greenhouse gases of black soil rice field which is widened rapidly in cold area. |
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