Gross nitrogen transformations and ammonia oxidizers affected by nitrification inhibitors and/or organic amendments in a calcareous soil: A 15N tracing study

Nitrification inhibitors (NIs) combined with organic amendments are considered effective strategies for improving soil nitrogen availability and reducing nitrogen loss. However, these effects might depend on the types of organic amendments and nitrification inhibitors, and whether their combined application could generate a synergistic effect on nitrogen transformation rate has not been well investigated. In this study, a 15N aerobic incubation experiment was conducted to investigate the effects of nitrification inhibitors (3,4-dimethylpyrazole phosphate [DMPP, 1 % of applied NH4+-N] or methyl 3-(4-hydroxyphenyl) propionate [MHPP, 5 % of applied NH4+-N]) and/or organic amendments (straw [1 %, w/w] or straw-derived biochar [1 %, w/w]) on the gross nitrification rates of mineralization, nitrification, and immobilization in calcareous soil. Our findings revealed that compared with the control treatment, biochar, straw, and MHPP_straw boosted gross mineralization rates by 1.19-2.69 times. The addition of nitrification inhibitors and/or organic amendments inhibited the gross nitri-fication rates by 15.8 %-89.7 %, which ranged from 0.85 mg N kg-1 day-1 to 8.28 mg N kg-1 day-1 across treatments. DMPP_straw showed the greatest inhibiting effect, whereas 5 % MHPP treatment had the least effect. Gross NH4+ immobilization rates increased by 4.12-18.18 times, where NO3- immobilization rates were only stimulated with the treatments of straw, DMPP_straw, and MHPP_straw. DMPP was more efficient in inhibiting nitrification than MHPP, but it also significantly enhanced ammonia (NH3) volatilization, which rose by 2.77 times over that in MHPP. The high C/N ratio and inorganic-N adsorption of straw and biochar might play prominent roles in affecting NH4+ content by stimulating mineralization and inorganic immobilization. The decrease in the abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) can be partially explained by the inhibitory effects of DMPP and MHPP. Changes in the community of AOB were more sensitive to management measures than those of AOA. In conclusion, our study demonstrates that treatments with DMPP_straw and MHPP_straw efficiently promote inorganic nitrogen immobilization, mitigate NH3 vola-tilization, inhibit nitrification to the greatest extent, and thus yield the most positive synergistic effects on soil nitrogen retention.