Low-pyrolysis-temperature biochar promoted free-living N-2-fixation in calcareous purple soil by affecting diazotrophic composition

摘要

Free-living nitrogen fixation (FNF) is an important input of nitrogen to minimise the use of inorganic nitrogen fertiliser in agricultural ecosystems, but knowledge regarding response of FNF to biochar amendment is still limited. A microcosm study was performed to investigate changes in soil nitrogen content and FNF efficiency for calcareous purple soil applied with biochars prepared at 300 degrees C (CB300) and 600 degrees C (CB600). After 56 d of incubation, both CB300 and CB600 applied at any rates significantly (P < 0.01) increased soil total nitrogen (TN) concentrations relative to the controls. However, the soil TN is higher in the CB300 treatment group than in the CB600 group, especially for biochar applied at a 2% rate. Moreover, the concentrations of different N forms (NH4+, NO3-, microbial biomass nitrogen, and dissolved organic nitrogen) increased with biochar addition during the incubation, among which the peak value of NO3- was lagging behind NH4+. The acetylene reduction activity (ARA) as well as Mtn abundance significantly increased in soil applied with 2% of biochars and peaked at day 56. Notably, the maximum values of ARA and nifH abundance were similar to 50% greater in soil applied with CB300 than CB600. Further analysis on the soil diazotrophic community composition showed that Azotobacter, Bradyrhizobium, and Skermanella were the most dominant genera. The proportion of Azotobacter, reported as free-living diazotroph, reached the maximum in soil amended with 2% of CB300 at day 56. The redundancy analysis indicated that electrical conductivity was the main factor leading to the difference in diazotrophic community among treatments. Besides, the rapid consumption of dissolved organic carbon (DOC) in CB300 treatment group facilitated soil diazotrophic proliferation, and finally led to the increase of ARA. Increased pH and abundant sulfur in soil applied with biochar promoted the concentrations of available molybdenum, which was positively related with the abundance of Azotobacter. Overall, our study provided clear evidence that application of low-pyrolysis-temperature biochars in calcareous soils greatly contributes to FNF and increased soil N accumulation.

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