Significance of soil N on dry matter production and N balance in soybean/sorghum mixed cropping system

Abstract

Abstract The importance of the soil nitrogen (N) level in dry matter production and N economy of soybean/sorghum mixed cropping system was studied using the 15N-dilution method. The N levels of the granite regosol used in the experiment varied due to previous management. The soil with a high nitrogen content (HSN) had been treated with animal waste materials and urea and cultivated with maize in the previous year, while the soil with a low N content (LSN) was not treated. Values of total nitrogen extractable with 2 mol L1 KC1 were 4.2 × 10-4 kg kg-1 for the HSN condition and 1.3 × 10-4 kg kg-1 for the LSN condition. Soybean and sorghum were planted as monocrops or in mixed cropping. The spacing was 18 cm × 18 cm for both cropping systems. Total dry weight was significantly larger under HSN than LSN conditions for both cropping systems. The dry weight in the soybean/sorghum mixed cropping system under HSN conditions was larger than that of the respective monocrops. However the total dry weight in the soybean/sorghum mixed cropping system under LSN conditions was smaller than that of the soybean monocrop. Biological nitrogen fixation (BNF) accounted for more than 90% of the N accumulated by soybean. The BNF of component soybean was higher than that of the monocrop. Furthermore, the BNF of soybean under LSN conditions was higher than that under HSN conditions, for both cropping systems. Soil N was the major source of N for both mono- and mixed cropped sorghum. Although the soybean BNF activity under LSN conditions was higher than that under HSN conditions, N transfer to component sorghum under LSN conditions was lower compared to that under HSN conditions. Nitrogen transferred accounted for about 7% of the total amount of N accumulated by component sorghum under HSN conditions. It is suggested that the N transfer may be due to the greater extent of root intermingling of component soybean and sorghum. Consequently, it is suggested that a certain amount of N is essential for a high dry matter production and N economy in the mixed cropping system.