Effects of film mulching and straw return on soil labile nitrogen pool, enzymes and ammonia volatilization in the root zone of winter wheat in the drylands on the Loess Plateau

Film mulching and straw return are common conservation tillage measures in arid regions. A field experiment was conducted to investigate the changes in soil labile nitrogen pools and soil enzyme activities, as well as ammonia volatilization, in the root zone soil of winter wheat under film mulching and straw return. The experiment was conducted at the Caoxinzhuang Experimental Farm of Northwest A&F University starting from 2017. A split-zone experimental design was used. The main zones were conventional cultivation, straw return, and film mulching, while the secondary zones were no nitrogen application, conventional nitrogen application, and 20% reduction in nitrogen application rate. Wheat root zone soil (0–20 cm) was collected 42, 166, 194, and 217 days after winter wheat sowing in the fifth season, to determine contents of soil nitrate nitrogen, ammonium nitrogen, and microbial biomass nitrogen, and activities of soil nitrate reductase, hydroxylamine reductase, and urease. Ammonia volatilization from agricultural soils was also measured during the wheat season to analyze the response of ammonia volatilization to cultivation patterns, soil labile N pools, and soil enzymes. The results showed that nitrogen application could increase the liable soil nitrogen pool, soil enzyme activities, and ammonia volatilization. For the nitrogen application treatments, ammonia volatilization accumulation in wheat season was significantly lower (P<0.05) under the film mulching treatment (6.41−7.38 kg·hm⁻²) than under the straw return (8.37−9.45 kg·hm⁻²) and conventional cultivation treatments (8.81−11.06 kg·hm⁻²) with identical fertilization levels. During the pre-winter tillering, and the reviving and jointing periods, the microbial biomass nitrogen content was significantly higher (P<0.05) under the straw return treatment than under the conventional cultivation and film mulching treatments. Soil accumulative ammonia emissions was found to have a significant positive correlation with soil ammonium nitrogen content, microbial biomass nitrogen content, and urease activity under the straw return treatment (P<0.05 or P<0.01 or P<0.001). It also had a significant positive correlation with soil ammonium nitrogen content, microbial biomass nitrogen content, hydroxylamine reductase activity, and urease activity under the film mulching treatment (P<0.01 or P<0.001). Under the 20% nitrogen reduction treatment, compared to conventaional cultivation, film mulching significantly reduced accumulative ammonia emissions througout the wohle growth period, and significantly increased soil microbial biomass nitrogen content at pre-winter tillering period, which is feasible for local winter wheat cultivation. This study provides theoretical support for reducing ammonia volatilization losses and improving the utilization rate of nitrogen fertilizers in the drylands on the Loess Plateau.