Film mulching, residue retention and N fertilization affect ammonia volatilization through soil labile N and C pools

Ammonia (NH3) volatilization was a vital process of the nitrogen cycle in agricultural ecosystems and closely related to the atmospheric environment quality. To investigate the tillage practice and N fertilization impact on the soil labile C, N pools and their effects on ammonia volatilization from wheat fields. A two-year consecutive field experiment was conducted at 2017 and 2018. The split-plot design of three tillage systems (conventional tillage (CT), film mulching (FM), straw residue incorporated into soil (SM)) and three nitrogen rates (the control rate (N0), 144 kg N ha(-1) (N144), 180 kg N ha(-1) (N180)) was applied. The results showed that the NH3 volatilization occurred during 40 days and peaked in 2 weeks post N fertilization. FM could significantly cut down ammonia volatilization peaks, cumulative NH3 loss and ammonia loss rate during both seasons (p < 0.05). The average fertilizer ammonia loss rates (AFR) in two growth seasons were 3.70% - 3.95%, 1.90% - 2.10% and 3.47% - 3.87% for CT, FM and SM separately. The topsoil water content (SWC) and soil temperature (ST), mineral nitrogen (NH4+-N and NO3--N) were enhanced by FM during the wheat cultivation. The microbial biomass nitrogen (MBN) and dissolved organic carbon (DOC) contents of SM were higher than others. Water extraction organic nitrogen (WEON) and NO3--N could be augmented by straw addition during heading stage growth. The ammonia volatilization was closely related with soil NH4+-N content, which was originated from the organic N mineralization in no fertilization condition and significantly affected by ST and SWC. N fertilization increased soil NH4+-N content and ammonia volatilization, and the higher N rate was, the more NH3 volatilization. When film mulching, WEON, MBN and soil mineral nitrogen have been augmented by higher SWC and ST, which increased NH4+-N content and NH3 volatilization potential, nevertheless film barriers mitigated the ammonia volatilization ultimately. Under straw incorporation, DOC, MBN and ST, which significantly affected the immobilization, mineralization and nitrification through microorganism proliferation, were the key factors affecting soil NH4+-N contents and NH3 emission. Overall, N fertilization promoted the ammonia volatilization; FM could mitigate ammonia volatilization through physical barrier or blocking, while the crop residue incorporated into soil had no pronounced impact on the soil ammonia volatilization.