| Nitrogen(N) transformation and circulation is an important part of elements biogeochemical cycles. Furthermore, N is the vital constraint of crop growth and development and yield. As one of the three major crops cultivated in C hina, rice( Oryza sativa L.) is commonly grown in southern regions of Yangtze River. Since 1978, chemical fertilizers especially N fertilizer was largely used in agriculture. Total nitrogen fer tilizer quantity and application rate in China was high and China became one of the countries whose agricultural production heavily relied on chemical fertilizers. To the contrary, organic fertilizer applied to agriculture sharply decreased. However, the using efficiency of N fertilizer by crops was not high owing to over-fertilization and inappropriate fertilization. It caused a large amount of nitrate nitrogen cumulating in soil, which led to various path losses of N such as infiltration leaching losses, NH3 volatilization and nitrification-denitrification(NOx) etc. In addition, the benefit in yield from chemical nitrogen declined year by year, and the nitrogen cumulated in soil, water and atmosphere caused lots of environmental problems. Planting and using winter catch crops is the essence and an important part of traditional agriculture in C hina. The nitrogen from decomposition of catch crops returned back to fields can be absorbed by crops like rice. It also has impacts on the distribution of various fo rms of nitrogen in soil nitrogen tank and their transformation and fates. Previous literatures on catch crops mainly focused on dry land or its impact on enriching soil and on supplying nutrition to succeeding crops. Few literatures have studied the influe ncing mechanism of catch crops on nitrogen supplying capacity, nitrogen transformation and rice productivity changes in the flooding paddy soil of rice based cropping systems. Hence, the research aimed to study the impact and the mechanism of catch crops on nitrogen transformation and nitrification-denitrification, on nitrogen supplying capacity of paddy fields and on the productivity of paddy fields. Chinese milk vetch(CMV, Astragalus sinicus L.) as the model winter catch crop was used. Long-term fixed location experiments, pot culture experiments as well as 15 N isotope tracer micro-experiments were conducted. It is meaningful for the rice cultivation system in Southern China to utilize catch crops properly, to improve nitrogen storage and nitrogen circulation as well as utilization efficiency, to reduce nitrogen losses and environmental risks caused by nitrogen losses and to maintain productivity and sustainability of rice production system. The results were as follows:1. A pot culture experiment was carried out to evaluate the gaseous loss of nitrogen in the mono-rice cropping system, the results showed that there were more than 20% and less than 1% of the applied nitrogen which lost in the form of NH3 and N2 O respectively from a mono-rice based cropping system. Compared with solely urea application, integrating urea with C hinese milk vetch(CMV) remarkably decreased the ammonium nitrogen content in surface soil water and the activities of hydroxylamine reductase at the tillering stage of rice plants. It also decreased the populations of nitrification and denitrification bacteria as well as the activities of nitric and nitrite reductase. Thus, it significantly decreased the NH3 volatilization and N2 O emission from soil by 14.6% and 45.2% respectively, and then decreased the greenhouse warming potential(GWP) and greenhouse gas intensity(GHGI) by 45.2%, 55.8%, 45.2% and 46.8% respectively.2. Compared with solely applying urea, integrating urea with CMV increased the contents of fixed nitrogen(FA), soil microbial carbon and nitrogen as well as the NH4+ and non-acid hydrolysable nitrogen while decreased the NO3- content in paddy soil at each growth stage of rice plant. In addition, corporation of urea with CMV increased the contents of amino acid nitrogen(AAN), amino sugar nitrogen(ASN) and acid hydrolysable ammonia nitrogen(AHAN) when decreased the content of acid hydrolysable unidentified nitrogen(AHUN). As a result, the contents of total nitrogen, total inorganic nitrogen and acid hydrolysable nitrogen(AHN) increased by 5.13%-24.2%, 3.44%-8.36% and 9.17%-10.9% respectively. Furthermore, the AAN and AHAN were the main components of the new synthesized AHN in paddy soil.3. comparing with the solely application of urea, integrating urea with CMV increased the amount of bacteria, fungi, actinomycetes and nitrogen- fixing bacteria by 14.5%-36.6%, and the activities of urease, protease and sucrase by 15.3%-31.1%, while reduced the populations of ammonia oxidizing bacteria(AOB) and abundance of ammonia-oxidizing archaea gene(AOA amo A) by 24.6% and 66.4% respectively. In addition, cooperating urea with CMV decreased the contributions of 15 NU to ASN, AHUN, AHAN and NAHN at tillering stage as well as to the AAN and ASN at maturing stage, but increasing the contributions of 15 NU to the AHUN, NAHN and the FA at maturing stage. What was more, the path analysis results showed that the AHAN was a temporary pool and the NAHN was a stable reserving pool of 15 NU in paddy soil of mono-rice field.4. Compared to solely app lying urea, cooperating urea with CMV increased the cumulative absorption of 15 NU and its distribution in the full grain. However, it decreased absorption and accumulation of SoilN in the aboveground parts of rice plants as well as the distribution of 15 NU in abortive grain. Thus, it significantly increased the utilization efficiency of 15 NU by 46.1%-83.0%, but reducing the dependence rate of rice plant to SoilN by 14.0%. Moreover, the number of filled grain per panicle, se tting rate and 1000-grain weight were improved, and as a result, the grain yield increased by 3.02%.5. Planting CMV in winter after harvesting rice plants increased the contents of total nitrogen, FA, SMBN and NH4+ when decreased the NO3- content in paddy soil at the flowering stage of CMV plants. Furthermore, it also significantly increased the contents of AHN, ASN and NAHN by 10.3%-37.2%, but decreasing the AHUN content 14.6%, thus the contents of AHN and NAHN were increased by 13.0% and 15.9% respective ly at the flowering stage of CMV plants. Moreover, planting CMV in winter significantly improved the contributions of the residue 15 NU to NH4+, AAN, ASN, AHAN by 6.31%-22.3%, however, it reduced the contributions of the residue 15 NU to the total nitrogen, NO3-, FA and the AHUN, AHN, NAHN by 8.28%-85.7% and 6.37%-33.2% respectively in soil at flowering stage of CMV plants.6. A field experiment was conducted to evaluated the effects of 100% FN(N100) and different substitution rates of FN by GM(80%, 60%, 40% and 20% FN plus 20%, 40%, 60% and 80% N through GM, and represented respectively by N80M20, N60M40, N40M60 and N20M80) on the rice productivity and N-supplying capacity of paddy soil in double-rice system from 2008 to 2013. The results showed that soil organic matter and total N content in the 0-15 cm layer and rice grain yield of early and late rice annually increased in N80M20 and N60M40 plots, but decreased in N100, N40M60 and N20M80 plots. Compared with N100 plots, the NH4+-N content and agronomic efficiency of applied N significantly increased in N80M20 and N60M40 plots. The grain yield and sustainable yield index of rice crops were improved in N80M20 and N60M40 plots, while declined in N40M60 and N20M80. Soil NO3--N content decreased significantly under partial substitutions of CMV for FN. |
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