The dynamic features and microbial mechanism of nitrogen transformation for hydrothermal aqueous phase as fertilizer in dryland soil

Hydrothermal aqueous phase (HAP) contains abundant organics and nutrients, which have potential to partially replace chemical fertilizers for enhancing plant growth and soil quality. However, the underlying reasons for low available nitrogen (N) and high N loss in dryland soil remain unclear. A cultivation experiment was conducted using HAP or urea to supply 160 mg N kg−1 in dryland soil. The dynamic changes of soil organic matters (SOMs), pH, N forms, and N cycling genes were investigated. Results showed that SOMs from HAP stimulated urease activity and ureC, which enhanced ammonification in turn. The high-molecular-weight SOMs relatively increased during 5–30 d and then biodegraded during 30–90 d, which SUV254 changed from 0.51 to 1.47 to 0.29 L−1 m−1. This affected ureC that changed from 5.58 to 5.34 to 5.75 lg copies g−1. Relative to urea, addition HAP enhanced ON mineralization by 8.40 times during 30–90 d due to higher ureC. It decreased NO3–N by 65.35%–77.32% but increased AOB and AOA by 0.25 and 0.90 lg copies g−1 at 5 d and 90 d, respectively. It little affected nirK and increased nosZ by 0.41 lg copies g−1 at 90 d. It increased N loss by 4.59 times. The soil pH for HAP was higher than that for urea after 11 d. The comprehensive effects of high SOMs and pH, including ammonification enhancement and nitrification activity inhibition, were the primary causes of high N loss. The core idea for developing high-efficiency HAP fertilizer is to moderately inhibit ammonification and promote nitrification.