Study on soil nitrogen supplying capacity and nitrogen requirements　for crop under long-term rice-barley cropping system

Long-term (over 18 years) rice-barley crop rotation experiment with seven fertilization treatments was conducted to investigate soil natural productivity, indigenous nitrogen (N) supplying capacity, internal N use efficiency and apparent recovery efficiency of N applied. The long-term experimental results indicated the contribution of the soil natural productivity was 68.9% for barley yields and 75%-81% for rice. The contribution of chemical fertilizers increases barley yield by 31% and by 19%-25% for rice. The natural productivity of paddy soil could annual maintain 2.3 t/ha barley and 6-7 t/ha rice grain yields. The soil indigenous N supplying capacity to barley was 75.9%, which was lower 3.3%-7.2% than rice. The amount of soil and environment indigenous N supplying was 118-299 kg/ha for barley-early rice-late rice rotation cropping system and 86-199 kg/ha for barley-rice rotation system. The internal N use efficiency in soils received N fertilizer was 31.0-56.3 kg/kg for barley and 23.6-50.2 kg/kg for rice. The recovery efficiency of N applied were 27.5%-41.2% and 14.6%-41.2% for barley and rice, respectively. To produce 12 t/hm2 grains, crops require N 226-337 kg/ha under the rice-barley rotation systems. Our study suggests that in order to achieve higher crop yields, yield targets, soil and environment indigenous N supplying capacity and crop N requirement should be considered when making N fertilizer recommendations and developing strategies for intensively managed rice-barley cropping systems.