Soil warming decreases carbon availability and reduces metabolic functions of bacteria

Plastic film mulch (PFM) is effective to save soil water and increase temperature and consequently, to increase crop yield. Therefore, PFM has become one of the most widely used on-farm management practices for maize cultivation in semi-arid regions. The effects of PFM-induced warming on the labile carbon (C) pools and microorganisms remain unclear. We used high-throughput genomic sequencing to assess bacterial community structure and metabolic functions in soils after short- (2 years) and long-term (10 years) cover with PFM in a dryland agriculture system. Strong decrease of dissolved organic C (DOC) pool (14–18 % less than in Control soil) by warming (2.4 °C) raised bacterial ⍺-diversity. The short-term mulch reduced the absolute abundance of bacteria by 36–43 % due to the temperature rise and labile C reduction. Bacteria developed with lower abundance (e.g., smaller colonies) but with higher diversity in soils with less available resources. During the long-term mulching and microbial acclimation to increased temperature and the reduced labile C, the bacterial community strongly changed towards an oligotrophic life history. The PFM increased the abundance of bacterial species with high nutrient uptake (e.g. Patescibacteria increased by 83 %), while chemotrophs that prefer eutrophic conditions were reduced in the PFM soil (e.g. Actinobacteriota by 11 %). The PFM increased the complexity of co-occurring networks of bacterial communities and decreased their stability. Almost all bacterial marker taxa screened by random forest algorithm differed between Control and Mulch soils. Long-term mulching reduced the rate of bacterial metabolism associated with organic matter degradation, such as metabolism of carbohydrates, esters (propanoate and butanoate decreased strongly), lipids (the greatest reduction was for fatty acids), and amino acids (lysine, valine, leucine, and isoleucine degradation pathways dropped the most). Structural equation modeling indicated that bacterial metabolism was mainly influenced by bacterial community structure. Consequently, the metabolic functions of bacteria were reduced after the decrease of C availability induced by warming under PFM, and this reduction was dependent on the duration of PFM application.