Symbiotic nitrogen fixation and soil N availability under legume crops in an arid environment

Purpose Legume crops often present an important option to maintain and improve soil nitrogen (N) quality and fertility in a dryland agroecosystem. However, the work on the integral assessment of the symbiotic N2 fixation (Nfix) and their effects on soil N availability under field conditions is scare. Materials and methods Five treatments consisted of legumes (capello woolly pod vetch and field pea), two non-legume crops (Saia oat and Indian mustard), and a nil-crop treatment as control (CK) in southeastern Australia to investigate the effects of legume crops on the amount of Nfix, which was estimated using a 15N natural abundance technique, and soil N pools, microbial biomass, microbial quotient, soil basal respiration, metabolic quotient (qCO2), net N mineralization rates, and substrate-induced respiration (SIR) using the MicroResp method. Results and discussion Crop 15N natural abundances under legume crops were lower, and the amounts of Nfix in the aboveground vetch and pea biomass were 42.1 and 37.3 kg ha'1, respectively, compared with the reference crops (oat and mustard). The crop treatments had higher soil pH, and lower moisture, NH4 +'N and NO3''N contents compared with the CK. The NO3 ''N was predominant form of soil inorganic N across the treatments. Although no significant differences were found in microbial biomass carbon (C) and N across the treatments, legume crops had lower soil basal respiration and metabolic quotient, indicating that soil organic carbon was less easily accessible to microorganisms in comparison with the nonlegume crops. In addition, no pronounced differences were found in soil available N pools (NH4 +'N, NO3 ''N, and soil soluble organic N) among the crop treatments. However, legume crops had lower soil net N mineralization rates and SIR, indicating lower soil potential N availability compared with the non-legume crops. These results showed that the amounts of Nfix by legume crops did not have immediate effects on soil N availability. Conclusions Compared with non-legume crops, legume crops exerted less negative effects on the soil microbial properties in this dry environment. However, the amount of Nfix under legume crops did not immediately increase soil N availability over the growing season.