Responses of soil microbial community composition and enzyme activities to long-term organic amendments in a continuous tobacco cropping system

Organic amendment may be an effective management practice for alleviating soil degradation in continuous cropping systems. However, few studies focus on the effects of long-term organic amendments on soil microbial community and microbial nutrient status in continuous tobacco fields. Here, an 11-year field experiment was conducted to investigate the effects of organic amendments on plant and soil physiochemical properties, microbial communities, ecoenzymatic stoichiometry, and glomalin-related soil protein (GRSP) accumulation in a continuous tobacco field. Five treatments were applied: CK (no fertilizer); CF (chemical fertilizer only); CFS (chemical fertilizer plus corn straw); CFO (chemical fertilizer plus oilseed residues); CFM (chemical fertilizer plus composted pig manure). Organic amendments increased soil microbial abundances. Application of organic fertilizers significantly reduced gram-positive to gram-negative bacterial ratios, but significantly increased fungi/ bacteria ratios, probably because gram-negative bacteria and fungi are the dominant decomposers of increased available nutrients and recalcitrant materials, respectively. Organic amendments significantly enhanced soil enzyme activities, except for the difference in beta-1,4-N-acetylglucosaminidase, Leucine aminopeptidase, and polyphenol oxidase (PPO) activities between the CF and CFO. The lowest soil organic carbon (SOC) content in CFS among organic amendments treatments was probably because the positive priming effects of straw and the highest peroxidase and PPO activities increased stable SOC mineralization rates due to the high C:N:P ratio of nutrients input. Microbial activities were limited by carbon, nitrogen, and phosphorus in CFS, while carbon and phosphorus were the limiting factors in CK, CF, CFO, and CFM. Relative to CF, CFO significantly enhanced microbial carbon and phosphorus limitation, whereas CFM significantly decreased microbial carbon limitation. CFM had the highest GRSP content among treatments. Our results demonstrated that combined application of composted pig manure and chemical fertilizers is a consistently effective measure for alleviating soil degradation in continuous tobacco fields.