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).