| Two seasons of field experiments were conducted in South China Agriculture University, Guangzhou, Guangdong Province from March 2015 to June 2015, and August 2015 to November 2015. Experiments were designed as randomized block design with two rates of N fertilizer(N1, 300 kg·hm-2 and N2, 360 kg·hm-2) and four cropping patterns [sweet corn monoculture(SS), sweet corn //soybean(2:3 rows) intercropping(S2B3), sweet corn //soybean(2:4 rows) intercropping(S2B4) and soybean monoculture(SB)]. Three replicates were used for each treatment. We explored the patterns of carbon and nitrogen cycling, concluding the plant's Nitrogen assimilation and utilization, changing of Carbon pool and Nitrogen pool in agricultural soil, soybean nitrogen-fixation and greenhouse gas emission, under the conditions of reduced nitrogen application. The main results were as follows:1. Both three year of field experiments showed that there was no significant difference in the yield of sweet corn and fresh pod soybean in different intercropping treats, while the mono-cropping soybean was significant higher than other intercropping patterns. It's showed that it could meet the needs of sweet corn growth of nitrogen at the dose of 300 kg·hm-2 nitrogen, which excessive nitrogen did not increase the sweet corn yield and quality.2. Given the same cropping patterns, the plant organic carbon and nitrogen accumulation of sweet corn were not significantly different under the different nitrogen application level. Besides, the plant organic carbon and the nitrogen accumulation of stem, leaves and pod in monoculture soybean were significant higher than that of other intercropping patterns. In 2015 different planting patterns had a significant effect on the plant organic carbon and nitrogen accumulation of sweet corn leaves and spikes.3. Compared with those before planting, the soil organic carbon(SOC) had been improved after sweet corn harvest in the spring of 2015 under all treatments, except for the soybean mono-cropping. The SOC was found in the treatments S2B3-N2, S2B4-N1, S2B4-N2 and SB in autumn of 2015. Soil nitrogen loss was also detected after sweet corn harvest in the spring of 2015. While in autumn of 2015, soil nitrogen surplus was only found under the treatment of S2B3 after sweet corn harvest, and soil nitrogen loss was revealed under all other treatments.4. Cropping patterns and nitrogen level both did not affect the amounts of nitrate leached in the system of sweet corn farmland significantly. Besides, there was no significant effect on the loss of ammonia nitrogen between all treatments in the spring of 2015. The loss of ammonia nitrogen in S2B3-N2 was significantly higher than that in S2B3-N1. There was no significant effect on other treatments.5. The study of autumn in 2015 found that soybean nitrogen fixation efficiency of monoculture was significantly higher than other patterns, and the soybean nitrogen fixation efficiency under reduced nitrogen were significantly higher than that under the conventional fertilization. There was no significant effect among all of treatments of spring in 2015. The study of two seasons showed that the quantity of soybean nitrogen fixation was no significant difference among treatments, except that a significant higher quantity of soybean nitrogen fixation was found in the monoculture soybean system.6. Straw returning plays a key role in the carbon cycling of agroecosystems. Without corn straw, N2-S2B3 mode performance as a carbon source in 2015 spring, and net carbon sequestration value reduce in other modes; all modes performance as a carbon source in 2015 autumn except S2B3 and S2B4.7. The result showed the nitrogen were surplus in both sweet corn//soybean intercropping ecosystems and monoculture ecosystems. The amount of nitrogen surplus under the treatment of conventional nitrogen application was higher than that of reducing nitrogen application. Surplus nitrogen application had increased the risk of the nitrogen loss of ecological systems. |
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