| Rice is an important food crop of the world, and plays an important role in ensuring global food security. Nitrogen (N), phosphorus (P), and potassium (K) are essential nutrients that require careful management in intensive rice systems, since insufficient amounts might result in yield losses, and excessive application might harm the environment. However, farmers often tend to apply a large excess of N fertilizer but insufficient P and K fertilizer, primarily because of the absence of reliable methods to estimate optimal fertilizer application rates.Through the project of soil testing and fertilizer recommendation in2005-2009, a lot of data about soil nutrients in paddy fields were acquired. Based on the data obtained in2008, status quo of the soil fertilities in seven rice producing regions of Hubei province were systematic analyzed and compared with the data obtained during the second national soil survey to explore laws and causes of the variation of soil nutrients in paddy field. Besides, we conducted a large-scale study comprising626fields in seven rice regions of Hubei province. The overall goals were to assess yield and profit responses of rice to N, P and K fertilizer, investigate N, P and K fertilizer use efficiency, establish indigenous soil N, P and K supply classification systems, and provide recommendations for N, P and K fertilizer application rates in this large-scale region, thereby helping farmers perform reasonable fertilizer management. The main results are summarized below.1. At present, the paddy soils in Hubei Province were10-40g/kg or26.1g/kg on average in organic matter,>90mg/kg or124.2mg/kg on average in alkalystic N,5-40mg/kg or13.1mg/kg on average in available P, and50-150mg/kg or89.1mg/kg on average in available K, and5.0-7.5in pH or6.3on average. The soil nutrients varied to a varying extent from region to region. In view of the province as a whole, the distribution of organic matter was characterized by being high in the east and south and low in the west and north, that of alkalystic N being high in the west and south and low in the east and north, and that of available K and pH both being high in the west and north and low in the east and south, while that of available P did not show any apparent rule. Compared with the findings of the second soil survey, the contents of organic matter, alkalystic N and available P increased while pH decreased as a result of long-term application of N and P fertilizers at an increasing rate and increased area and rate of straw incorporation. However, inadequate application rate of K fertilizer and the adoption of high K-demanding rice cultivar led to decrease in available K in the soil. 2. The application of N, P and K fertilizer increased the yield and profit of rice significantly. Compared with PK (without N) treatment of early, mid and late rice, yields of NPK treatment increased1631,2021and1631kg/hm2, and profits of NPK treatment increased2205,2965and2262Yuan/hm2, respectively. Compared withNK (without P) treatment of early, mid and late rice, yields of NPK treatment increased850,937and646kg/hm2, and profits of NPK treatment increased1201,1423and921Yuan/hm2, respectively. Compared with NP (without K) treatment of early, mid and late rice, yields of NPK treatment increased648,876and750kg/hm2, and profits of NPK treatment increased582,966and661Yuan/hm2, respectively. The average yield of NPK treatment of early, mid and late rice was2128,2594and2068kg/hm2higher than that of CK (without N, P and K) treatment, respectively. Compared with CK treatment, profits of NPK treatment of early, mid and late rice increased2187,3071and2080Yuan/hm2, respectively. These results showed that the combined application of N, P and K fertilizer significantly increased both rice yield and profit. The responses of different rices to fertilizer were different. The response to N and P fertilizer application for early rice was strongest, whereas the response to K fertilizer application for late rice was strongest.3. At present production conditions, nitrogen, phosphorus and potassium use efficiency of rice were partial factor productivity of applied fertilizer (PFP) of46.6kg/kg N,131.6kg/kg P2O5and92.1kg/kg K2O, agronomic efficiency (AE) of11.3kg/kg N,14.7kg/kg P2O5and9.8kg/kg K2O, recovery efficiency (RE) of29.2%,15.0%and43.0%, and physiological efficiency (PE) of36.5kg/kg N,78.6kg/kg P2O5and18.7kg/kg K2O in Hubei province, respectively. The average dependent rate of rice to soil N, P and K was65.3%,86.0%and80.1%, respectively. As a whole, the fertilizer use efficiency of early, mid and late rice was different, however N, P and K absorbed by three kinds of rice primarily came from soil but not fertilizer. As a result, sustaining and increasing soil fertility is an effective measure to increase rice yield and save fertilizer resources.4. The grain yield of no-N, rather than soil alkaline hydrolyzable-N and total N, is a better predictor of indigenous N supply (INS) in paddy fields. The classification of INS for early, mid and late rice was determined based on the relationship between relative yield and grain yield of no-N. Compared to the complete treatment, the relative yields of50%,60%,70%,80%and90%obtained from the no-N treatment were selected to establish the classification indices of the grain yield without N for rice. Both soil available P and grain yield of no-P are feasible predictor of indigenous P supply (IPS) in paddy fields. The classification of IPS for early, mid and late rice was determined based on the relationship between relative yield and soil available P or grain yield of no-P. Compared to the complete treatment, the relative yields of75%,80%,85%,90%and95%obtained from the no-P treatment were selected to establish the classification indices of soil available P and grain yield without P for rice. Both soil available K and grain yield of no-K are feasible predictor of indigenous K supply (IKS) in paddy fields as well. Compared to the complete treatment, the relative yields of80%,85%,90%and95%(or75%,80%,85%,90%and95%) obtained from the no-K treatment were selected to establish the classification indices of soil available K (or the grain yield without K) for5. The average economic optimum fertilizer rate of early, mid and late rice, which was determined according to fertilizer response models, was135,149and158kg/hm2for N,53,55and45kg/hm2for P2O5,71,74and78kg/hm2for K2O in Hubei province. Compared with the N, P2O5and K2O rate of the medium application level, average economic optimum N rate (EONR), economic optimum P rate (EOPR) and economic optimum K rate (EOKR) decreased by6.5%-19.2%,14.1%-23.2%and15.5%-24.3%, respectively. The50%confidence interval of optimum fertilizer rate of early, mid and late rice was110-180,120-180and130-200kg/hm2for N,45-65,40-75and35-60kg/hm2for P2O5,45-90,50-95and60-95kg/hm2for K2O in Hubei province. The obvious increasing trend, which was shown by the average EONR with decreasing INS, the average EOPR with decreasing IPS, and the average EOKR with decreasing IKS in Hubei province, proved the importance of indigenous nutrient supply for recommending fertilizer application rates of rice. With decreasing INS, IPS and IKS, an increasing trend of EONR, EOPR and EOKR was also shown for most rice regions. The economic optimum fertilizer rate differed among the seven rice regions in Hubei province. In conclusion, the recommended fertilizer application, based on regional mean optimal fertilizer rates in combination with indigenous nutrient supply, is feasible for regional rice production in China and other countries that have large numbers of small farmland areas and where agricultural testing equipment is absent or less modern. |
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