Organic amendment increases soil respiration in a greenhouse vegetable production system through decreasing soil organic carbon recalcitrance and increasing carbon-degrading microbial activity

Purpose: Recent works have shown that fertilization has an important influence on soil respiration (Rs); however, the underlying mechanisms involved in regulating Rs in greenhouse vegetable production (GVP) systems remain unclear.Materials and methods: Samples from six kinds of soils that were amended with different fertilization patterns (8 years) were incubated for 36 days to determine soil microbial community (PLFA), enzyme activities, soil organic C (SOC) quality (13C NMR), and Rs in a GVP system in Tianjin, China. Treatments included 100% chemical N (CN) and different substitution rates of CN with manure-N and/or straw-N.Results and discussion: Compared with 100%CN treatment, organic amendment strongly promoted microbial (e.g., fungi, bacteria, and actinomycetes) growth, enhanced the majority of C-degrading enzyme activities, affected SOC chemical composition with increasing O-alkyl (labile) C and reducing aromatic (stable) C, decreased SOC recalcitrance, and enhanced Rs. Redundancy analysis indicated that variations in microbial community and SOC chemical composition were closely linked to light fraction organic C (LFC) and readily oxidizable C (ROC), respectively. Further, structural equation modeling and linear regression analysis revealed that SOC recalcitrance (negative effects) and C-degrading enzyme activities (positive effects) together mediate Rs rates; meanwhile, microbial community can indirect affect Rs rates through altering C-degrading enzyme activities. Conclusions: Agricultural soil abiotic properties (mainly labile C fractions, i.e., LFC and ROC) are altered by adding organic resources (i.e., manure and straw), the changes of which can promote soil microbial growth, enhance C-degrading microbial activity, and reduce SOC recalcitrance, and in turn accelerate Rs in GVP systems