Effects of continuous turnover of Astragalus sinicus on rice yield and N absorption, distribution and residue  in single-cropping rice regions of Fujian Province

【Objectives】Overturning Astragalus sinicus into paddy fields can improve soil physical and chemical properties and raise the output of after crop rice to some extent. However, it is still unclear that whether it is from the extra N input after overturning Astragalus sinicus or the improvement of nitrogen use efficiency. Data were collected from a 4-years continuous field experiment of overturning Astragalus sinicus into paddy fields and supplementary undistributed soil column simulation by 15N isotopic tracer, effects of the overturning on rice yield and N absorption, distribution and residue of 15N isotope of chemical fertilizer were investigated in single-crop rice regions of Fujian province.【Methods】 Field experiment with five treatments was implemented： 1)control, no overturning Astragalus sinicus and no chemical fertilizer(CK); 2) only applying chemical fertilizer(100%H); 3) Astragalus sinicus combined with 100% of chemical fertilizer (Z+100%H); 4) Astragalus sinicus combined with 60% of chemical fertilizer (Z+60%H); 5) only overturning Astragalus sinicus(Z). The amount of chemical nitrogen fertilizer was N 135 kg/ha, N∶P2O5∶K2O=1∶0.4∶0.7，and the amount of annually overturning Astragalus sinicus was 18000 kg/ha. Each treatment was set up in 3 replicates. The plot area was 15 m2 and cultivated with 2015 clumps plant. In the fourth year of the experiment, supplementary undistributed soil column simulation by 15N isotopic tracer(10% abundance) was investigated to reveal the N absorption, distribution of chemical fertilizer. The diameter of PVC tube was 25 cm and 35 cm in length,and 20 cm length was impressed into field. 2 soil columns were inserted into each plot and each column was cultivated 2 clumps of plants.【Results】Compared with 100% chemical fertilizer treatment, rice grain yield is significantly increased by 6.5% in the Z+100% H treatment. Overturning the same level of green mamure and reducing 40% of chemical fertilizer (Z+60%H), the grain yield is equivalent to that of 100%H treatment. Increasing N content of rice plant significantly is found in the Z+100%H treatment at the tillering stage. And N content of stem and leaf significantly is improved by 7.0% in comparison with 100%H treatment, in particular. Although no significant difference is observed for nitrogen use efficiency of chemical fertilizer after the overturning Astragalus sinicus into paddy field, total N absorption of rice plant in the Z+100%H treatment is increased by 23.0% and 18.0% in comparison with those in the 100%H treatment at the tillering and mature periods, respectively. The above result shows green manure combined with chemical fertilizer could promote N absorption from the exogenous N sources, and differences of absorption mainly come from Astragalus sinicus mineralization of organic nitrogen. Total N absorption of rice plant in the Z+60%H treatment is equivalent to that in the 100%H treatment. There is a trend of increasing soil nitrogen content for different fertilization treatments. The plough soil nitrogen residual ratio is the highest in the Z+60%H treatment, with significant difference compared with the Z+100%H treatment. 【Conclusions】The 4\|year continuous overturning Astragalus sinicus significantly improve the productivity of yellow paddy fields of single\|crop rice in Fujian. To maintain the production, 40% chemical fertilizer could be replaced during the period of rice growth. One of main mechanisms of reducing fertilizer and improving efficiency is that mineralized nutrients of Astragalus sinicus replace the chemical fertilizer.