[Effects of Straw Combined with Lime Application on Bacterial Communities and Carbon Cycle Functional Genes in Acidic...

Acidic paddy soil is an important cultivated land resource in southern China. The strongly acidic soil environment leads to reduced availability of soil nutrients， rapid mineralization of organic matter， and enhanced activity of heavy metals， which severely restricts rice production and the stability of soil ecological functions. Straw combined with lime application is an effective measure for improving acidic paddy soils in southern China， to explore the regulatory effects of straw return combined with lime application on bacterial community structure and carbon cycle functional genes in acidic paddy soil， a field experiment was conducted with three treatments： conventional fertilization （CK）， straw return + conventional fertilization （RS）， and straw return combined with lime + conventional fertilization （RS+L）. Illumina MiSeq high-throughput sequencing technology was used to analyze the effects of different treatments on bacterial community diversity， community structure， and carbon cycle functional genes in acidic paddy soil. The results showed that compared with those in CK， the RS+L treatment significantly increased the activities of soil β-1，4-glucosidase （βG）， β-cellobiosidase （CBH）， and β-N-acetylglucosaminidase （NAG） by 14.59%-29.38%， 41.19%-98.86%， and 28.82%-63.36%， respectively （P<0.05）. Compared with that in RS， the RS+L treatment significantly reduced the activity of soil acid phosphatase （ACP） by 16.94%-20.47% （P<0.05）. Both the RS and RS+L treatments significantly increased the Chao 1 index and PD index compared with those in CK （P<0.05）. Compared with those in CK， the RS and RS+L treatments increased the relative abundances of Proteobacteria and Actinobacteria， while decreasing the relative abundances of Acidobacteria and Nitrospirae. PICRUSt2 functional prediction analysis showed that the RS+L treatment significantly increased the abundances of functional genes related to starch， hemicellulose， and pectin degradation compared with those in CK （P<0.05）. Specifically， genes involved in starch degradation （amyA， glucoamylase， pulA， malQ， malZ）， pectin degradation （yfiH）， hemicellulose degradation （xylA， rfbB， xylB）， cellulose degradation （bglB， endoglucanase）， chitin degradation （chi， nagA）， and aromatic compound degradation （leuA， prpB， AceA， AceB） were significantly enriched （P<0.05）. Compared with those in CK， the RS+L treatment increased the abundances of carbon fixation functional genes involved in the Calvin cycle and reductive tricarboxylic acid （TCA） cycle pathways （P<0.05）， including genes in the Calvin cycle （cbbS）， reductive TCA cycle （por/nifJ， pps， korD， korC， porA， porB）， and reductive acetyl-CoA pathway （cooS， acsB， metF） （P<0.05）. The Mantel test and redundancy analysis （RDA） indicated that the key environmental factors affecting carbon degradation functional genes were MBC， NAG， CBH， SOC， AP， and POC； those affecting carbon fixation functional genes were MBC， POC， NAG， pH， βG， AP， and CBH； and those affecting methane metabolism functional genes were UREA and NAG. Structural equation modeling showed that straw combined with lime application indirectly drove the expression of carbon cycle genes by increasing soil pH， organic carbon， and nutrient contents； altering bacterial communities； and activating enzyme activities. In conclusion， straw return combined with lime application can enhance enzyme activities， optimize bacterial community structure， and strengthen the expression of carbon cycle functional genes in acidic paddy soil， achieving a synergistic effect of improving soil quality and enhancing carbon sequestration potential.