4.2 Alteration of abundance of gene cbbL and nifHand their mediated factors during 20 year of IM
The significant difference in abundance of cbbL gene in both two soil layers indicted that bamboo IM practice with MCM affects CO2 fixation bacteria. The change in cbbL gene copy numbers was positively related with AK in both topsoils (r = 0.63,P < 0.05) and subsoils (r = 0.69, P < 0.05), and also with AN (r = 0.69, P < 0.05) in subsoils. It has been reported that the abundance of cbbL gene could affected by several factors including pH, SOC, AN, and C:N (Guo et al., 2015; Yuan et al., 2013; Yuan et al., 2015), and the major factor responsible may have varied over time. In this study, soil AK was in short supply and may have become the limiting nutrient for CO2 fixation bacteria, as so too may have AN in subsoils. The lower abundance during the middle years may reflect lower AK. The IM management induced a great decline in copy numbers of genenifH in both soil layers (P < 0.05) for IM6, although they recovered gradually afterward to some extent. Diazotrophic bacteria were logically more active when N was limiting (Pereira e Silva, Semenov, van Elsas, & Salles, 2011; Reardon, Gollany, & Wuest,2014). The increased soil AN and NO3--N may be responsible for sharp reductions in gene nifH copy numbers. However, no significantly negative relationship was observed between nifH gene number and AN, nor with NO3--N. The opposite relationship between nifH gene copy number and NH4+-N was observed in the topsoil and in subsoil. The biological N2 fixation is a high energy consuming process and need an ample of OC supply (Pfister, Meyer, & Antonopoulos,2018). The positive relationship between nifH gene number and C : N in topsoil was in accordance with result from long-term experienced 25 years of inorganic fertilization (Wang et al., 2017).