Partial substitution of chemical fertilizer with organic amendments affects soil organic carbon composition and stability in a greenhouse vegetable production system

Understanding soil organic carbon (SOC) composition and stability is necessary for long-term carbon (C) sequestration. Fertilization is known to modify SOC stability and composition; however, inconsistent results have been reported. Here, we evaluated the changes in SOC composition and stability through multiple methods in an 8-year fertilization experiment under greenhouse vegetable production (GVP) systems in Tianjin, China. This experiment was designed to supply an equivalent amount of nutrients (N, P2O5 and K2O) but variable amount of C through different exogenous organic resources (ORs) input. Five treatments were included based on substitution of chemical fertilizer nitrogen (N) with different exogenous ORs: no N (no nitrogen inputs), 4/4CN (CN, nitrogen in chemical fertilizer), 2/4CN + 2/4 MN (MN, nitrogen in pig manure, high-quality ORs), 2/4CN + 2/4SN (SN, nitrogen in corn straw, low-quality ORs) and 2/4CN + 1/4 MN+1/4SN. Organic fertilization strategies promoted SOC accumulation and reduced SOC stability by increasing labile C fractions, whereas chemical fertilizer alone had a negative effect on SOC accumulation by consuming labile C fractions but enhanced SOC stability. Notably, the characteristics of high temperature, nutrient availability and C inputs in organically managed soils in GVP systems weaken the influence of ORs quality on SOC stability. Consequently, high doses of C inputs (11.5 and 18.5 Mg ha(-1) year(-1) in 2/4CN + 1/4 MN+1/4SN and 2/4CN + 2/4SN treatments) induced lower SOC stability than low doses of C inputs (4.5 Mg ha(-1) year(-1) in the 2/4CN + 2/4 MN treatment), irrespective of ORs quality. In addition, strong correlations were found between SOC thermal characteristics and SOC chemical and C-13 NMR spectral properties, indicating that thermal analysis techniques can characterize SOC stability effectively.