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 (N_fix) 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 N_fix, 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 N_fix 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 non-legume 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 N_fix 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 N_fix under legume crops did not immediately increase soil N availability over the growing season.