| Crop residues were used as an important renewable resource of agricultural production. Straw incorporation to field not only can improve soil fertility, at the same time can reduce the pollution of agricultural ecological environment, achieving high and stable crop yield to maintain soil productivity. With the promotion and application of the technology of returning residues to field, how to regulate chemical fertilization in order to ensure efficient utilization of resourses has become a research hotspot. In this study, multiple-sites field experiments were carried out in Hubei province to study the regulation of nitrogen(N) fertilizer and potassium(K) substitution effects under different paddy-upland rotation systems. Our objectives were to study the effects of different amounts and dressing ratio of N fertilization and chemical K substitution under straw incorporation, aiming to provide a scientific basis for the reasonable utilization of N and K fertilization with returning residues to field. The main results were as follows:1. The results of the regulation of N-fertilizer showed the crop yield, aboveground biomass and total N accumulation with residues incorporation were better than that without residues incorporation under equal nitrogen rates for either single reason, annual and/or 2 years continue returning. Compared with N fertilization in 3 times, the crop yield, aboveground biomass and total N accumulation were not improved by high N application rate(i.e., 195 kg N/hm2 for rice, 210 kg N/hm2 for oilseed rape, and 180 kg N/hm2 for wheat)under residues incorporation for the single reason. And the results for annual and 2 years continue returning residues to field showed that high N application rate would significantly increase crop yield for the rice-wheat rotation but impact slightly for the rice-oilseed rape rotation. Compared with annual N fertilization in 3 times for the rice-wheat cropping rotation, high N fertilization rate in 3 times with residues incorporation increase 632 and 564 kg/hm2 for rice and wheat on average, with the increasing rates of 6.85% and 10.67%, respectively. Total N accumulation of rice and wheat increments were 11.54 and 23.57 kg/hm2 on average, with the increasing rates of 7.88% and 21.88%, respectively. For 2 years continue returning residues to field, yield increments were 429 and 377 kg/hm2 for rice and wheat on average, with the increasing rates of 4.71% and 7.63%, respectively. Total N accumulation of rice and wheat increments were 13.98 and 15.27 kg/hm2 on average, with the increasing rates of 8.81% and 13.10%, respectively.2. The results showed that N application at 2 times under residues incorporation would satisfy crop yield, aboveground biomass and total N accumulation compared with conventional N fertilization at 3 times for either single reason, annual and/or 2 years continue returning. For N efficiency, regular application rate of N obtained high partial factor productivity of applied N(PFPN)for rice, oilseed rape and wheat. Partial factor productivity of applied N(PFPN)with residues incorporation was better than that without residues incorporation under equal nitrogen rates, especially for the treatment with N application at 2 times under residues incorporation(i.e., moving the 3rd N fertilization forward with residues returning). With residues returning, moving N-fertilizer forward would achieve high and stable crop yield and enhance N efficiency under different paddy-upland rotation systems.3. The results of the potassium substitution for chemical K fertilizer showed the application of K fertilizer and returning residues to field could increase the rice, oilseed rape and wheat yield, aboveground biomass and total K accumulation in the single reason, annual and 2 years continue returning residues to field under paddy-upland rotation systems. Yield increase rates showed as low K levels(12.51%-126.33%)>middle K levels(5.57%-24.54%)>high K levels(2.55%-12.77%) for the three kinds of soil K supply levels(deviding by relative yield). The residues incorporation with the K fertilization treatment(S+K) had the highest yied and K accumulation. Compared with the treatment of chemical K fertilization(+K), the residues incorporation only would satisfy crop yield for soil with high K supply level. For soil with middle K supply level, applying half rate of chemical K fertilizer with residues incorporation could reach crop yield level of +K treatment. In case of soil with low K supply level, with residues incorporation, chemical K application rate could reduce 25% for rice and 50% for oilseed rape and wheat. For K efficiency, both K fertilizer agronomic efficiency(KAE) and K fertilizer recovery efficiency(KRE) were higher in the treatments without residues incorporation when we taken K input from straw returing into account. The KAE and KRE were higher in the rice season than that in oilseed rape and wheat seasons. The values were also higher in the soil with low(KAE 1.6-37.1 kg/kg, KRE 19.7%-119.3%) and the middle K supply levels(KAE 0.8-12.2 kg/kg, KRE 11.9%-119.3%) than the soil with high K supply levels(KAE 0.7-4.5 kg/kg, KRE 9.0%-94.9%).4. After calculating the apparent K balance, we found that K deficit occurred in all the treatments across the sites in the rice reason especially in the treatments without residues incorporation. For the oilseed rape and wheat reasons, K deficit could be offset with returning residues(21.83-103.15 kg K2O/hm2 surplus for oilseed rape, 23.50-95.93 kg K2O/hm2 surplus for wheat). Apparently, returning residues to field can significantly relieve the situation of soil potassium deficiency. Using quadratic and linear models to simulate the relationship between K fertilization rate and crop yield with residues incorporation, comparison of crop yield under +K treatment would obtain the suitable K application rate. With straw incorporation, suitable K application rates were 29 kg K2O/hm2, 32 kg K2O/hm2 and 48 kg K2O/hm2 on rice, oilseed rape, and wheat for high K level, middle K level and low K level on average, respectively. Compared with current K fertilization rate(60 kg K2O/hm2), chemical K rate could reduce 51.7% for soil with high K level, 46.7% for soil with middle K level, 20.0% for soil with low K level. For annual straw incorporation, annual suitable K rate would be 31 kg K2O/hm2 for rice-oilseed rape rotation and 33 kg K2O/hm2 for rice-wheat rotation, saving K fertilizer 48.3% and 45.0%, respectively. |
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