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

Film mulching and straw return are common conservation tillage measures in arid regions. A long-term experiment was conducted to investigate the changes in soil labile nitrogen pools and soil enzyme activity, as well as ammonia volatilization, in the root zone of winter wheat under film mulching and straw retention. The experiment was conducted at the Cao Xinzhuang Experimental Farm of Northwest A&F University in 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 conventional nitrogen application. Wheat root zone soil (0–20 cm) was collected 42, 166, 194, and 217 days after winter wheat sowing in the fifth season, and the inter-root soil of wheat was collected 42 days after sowing to determine soil nitrate-ammonium nitrogen, microbial biomass nitrogen, soil nitrate reductase, hydroxylamine reductase, and urease activities. 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 significantly increased the unstable soil nitrogen pool, soil enzyme activity, and ammonia volatilization. Total season ammonia volatilization accumulation in wheat to be significantly lower (P<0.05) in the film mulching treatments (6.41−7.38 kg·hm⁻²) than in the straw return (8.37−9.45 kg·hm⁻²) and conventional tillage treatments (8.81−11.06 kg·hm⁻²) with identical fertilization levels. The microbial biomass N content was significantly higher (P<0.05) in the straw return treatment than in the conventional tillage and film mulching treatments. Soil ammonia volatilization was found to have a significant positive correlation with soil ammonium nitrogen, microbial biomass nitrogen, and urease activity under the straw return treatment (P<0.05). It also had a significant positive correlation with soil ammonium nitrogen, microbial biomass nitrogen, hydroxylamine reductase, and urease activity under the mulch treatment (P<0.01). Under the 20% nitrogen reduction treatment, film mulching and straw return can reduce ammonia volatilization and increase soil unstable nitrogen pool content and soil enzyme activity. This study provides theoretical support for reducing ammonia volatilization losses and improving the utilization of N fertilizers.