Responses of extracellular enzyme activities and microbial community in both the rhizosphere and bulk soil to long-term fertilization practices in a fluvo-aquic soil

The influence of inorganic or organic fertilization on soil microbial ecology has been emphasized recently, but less is known about rhizosphere effects on extracellular enzyme activities and microbial community structure. Eleven extracellular enzymes involved in C, N, P. and S cycling and microbial community structure in both the rhizosphere and bulk soil samples from a long-term (31-year) fertilizer experimental field at the wheat reproductive stage were investigated by microplate fluorometric assay and phospholipid fatty acid analysis (PLFA), respectively. The samples were taken from six treatments: control (CK, without fertilization), fertilizer N (N), fertilizer N and P (NP), fertilizer N, P and K (NPK), organic manure (M), and organic manure plus fertilizer N, P and K (MNPK). Responses to inorganic or organic fertilizers in the rhizosphere were significantly different from those in the bulk soil. Except for NO3--N, thus, nutrient concentrations were generally higher in the rhizosphere than in the bulk soil. M and MNPK treatments greatly increased organic C, total N, NH4+-N and total S. Inorganic fertilizers (N, NP, and NPK) generally maintained or reduced most enzyme activities in the rhizosphere, but markedly increased these enzyme activities in the bulk soil. However, organic treatments (M and MNPK) enhanced most enzyme activities in both the rhizosphere and bulk soil. Higher total PLFA and lower ratios of bacteria to fungi and of actinomycetes to fungi were observed in the rhizosphere compared with the bulk soil. In the bulk soil, the ratios of bacteria to fungi and of actinomycetes to fungi were highest in the N treatment and lowest in the M treatment. However, in the rhizosphere there were no statistically significant differences in the abundance of bacteria, fungi and actinomycetes between the inorganic and organic treatments. Organic fertilization increased total PLEA and Gram+ to Gram bacteria ratio in both the rhizosphere and bulk soil. Our results indicated that changes in fertilization regime had a greater impact on the bulk soil microbial community than in the rhizosphere. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.