Mechanisms behind the inhibition of autotrophic nitrification following rice-straw incorporation in a subtropical acid soil

The effectiveness of rice-straw incorporation to alleviate environmental deterioration and increase soil fertility is widely accepted, whereas, the effect of this management on stimulating soil nitrogen (N) transformation is not fully understood. This study was conducted to investigate the effect of rice-straw incorporation on soil N transformation. An incubation experiment was conducted with rice-straw incorporated at rates of 0 (RS0), 1.67 (RS1), 3.33 (RS2) and 6.67 g kg−1 soil (RS3). Tracing experiments with 15NH4NO3 and NH415NO3 was conducted in the first (Week 1) and tenth week (Week 10) after straw incorporation, and a numerical model was used to calculate gross rates of N transformations. Incorporation of rice-straw increased gross rates of soil organic N mineralization, ammonium (NH4+) and nitrate (NO3−) immobilization and oxidized organic-N to NO3−, by 0.2–1.7 times, 4.6–11.6 times, 20.4–74.9 times and 6.2–20.3 times, respectively. However, the stimulation of soil N transformation via rice-straw incorporation was insignificant by week 10. Over the incubation period, the stimulation of soil inorganic N production pathways (organic N mineralization and oxidized organic-N to NO3−) via rice-straw incorporation was less than on consumption pathways (NH4+ and NO3− immobilization), leading to soil inorganic N supply capacity decreasing with straw incorporation rates. Dissimilatory NO3− reduction to NH4+ was stimulated by rice-straw incorporation, as observed in both the first and tenth week. Compared with RS0, autotrophic nitrification decreased by 14%, 25% and 46% in RS1, RS2 and RS3, respectively, but this effect disappeared by week 10. However, nitrification capacity (NC, the ratio of autotrophic nitrification rate to total mineralization rate) was constrained following rice-straw incorporation both in the first and tenth weeks. Decreasing autotrophic nitrification was the most important factor contributing to decreased NO3− content with straw incorporation, followed by increasing NO3− immobilization. The gross rate of autotrophic nitrification was negatively correlated with NH4+immobilization, indicating that autotrophic nitrification inhibition may be attributed to increased NH4+ immobilization. Therefore, based on the observations of this study, rice-straw incorporation is recommended for reducing nitrification capacity and reducing risks of N losses in subtropical acid soil.