Effects of long-term cycling of organic nutrient on soil nitrogen supplying capacity in a red soil paddy ecosystem

Nitrogen supplying capacity played an important role in growth and yield of rice.In order to obtain efficient nitrogen and organic nutrient utilization and reduce the possible environmental effects,it is essential to determine the fertilization model which is based on the soil nitrogen supplying capacity.A pot experiment based on long-term field experiment was conducted to determine the effects of long-term cycling of organic nutrient or application of chemical fertilizer on soil in total nitrogen,soil organic carbon,mineralizable nitrogen,microbial biomass nitrogen and carbon content,nitrogen uptake by rice.The field experiment was carried out on a reddish paddy soil in taoyuan,Hunan,during 1990-2004,8 treatments(CK,N,NP,NPK,C,N+C,NP+C,NPK+C) and three replicates were set up.Levels of fertilization were N 182.3 kg/ha,P 39.3 kg/ha,K 197.0 kg/ha after 1997(N 262.5 kg/ha,P 39.3 kg/ha,K 137.0 kg/ha before 1997).For some treatments with recycling of crop nutrients,rice straw was fully returned to the field after harvesting,80% of full grains(50% after 1995) and all of the empty or blighted grains were fed to pigs,and the pig manure subsequently was spread in the field,and the last step was that the Chinese milk vetch was cultivated in winter and then ploughed into the field before spring plowing.The pot experiment was carried out in taoyuan(2004),level of fertilization were NaH2PO4 0.26 g/pot,KCl 0.27 g/pot.The results indicated that there was a significant positive relationship among soil organic carbon,total N,amount of mineralizable N,the microbial biomass N(MB-N) and N uptake of rice,and they were good indicators of soil nitrogen supplied capacity.The recycling application of organic nutrient or application combined with chemical fertilizer significantly increased soil organic carbon content,total nitrogen content and amount of net N mineralization(6 weeks) by 20.1%～40.9%,0.46～0.60 g/kg and 55.0%,respectively;And it obviously increased soil MB-N content by 70.3%.However,there was less effect of chemical fertilizer application on the content of soil nitrogen,soil carbon and mineralizable N.The pot experiment indicated that compared with zero application of fertilizer,there were significant effects of long-term recycling application of organic nutrient on the soil nitrogen supply capacity,and the amount of nitrogen uptake by rice increased by 33.7%～89.0%, while there was less effects of application of chemical fertilizer N and NPK,which only increased by 2.1% and 6.2%.Additionally,soil nitrogen supplying capacity increased with the combination degree between the recycling application of organic nutrient and fertilizer NPK enhancement.