Soil C turnover, microbial biomass and respiration, and enzymatic activities following rangeland conversion to wheat–alfalfa cropping in a semi-arid climate

Conversion from rangeland to cropland potentially influences soil organic matter (SOM) dynamics and biochemical properties. The purpose of the current study was to assess changes in soil properties and investigate the interaction between SOM and enzyme activities following cultivation of native rangelands for more than 40 years. The quantities of soil organic carbon (C) and microbial biomass C (MBC), aggregate stability, microbial respiration rate (MRR), potential C mineralization (PCM) and the activities of soil enzymes involved in C, N, P and S cycling at the 0–20 and 20–40 cm depths were evaluated in never-cultivated natural rangelands and their corresponding farmlands with similar soil type and climatic conditions at Dehnow located in Central west, Iran. Cultivation in native rangelands resulted in a significant decline of most soil properties in farmlands, depending on soil sampling depths. Conversion of rangelands to their farmland counterparts decreased soil C (50–66 %), MBC (48–61 %), MRR (20–44 %), PCM (45–59 %), the activities of urease (5–17 %), invertase (7–28 %) and arylsulfatase (17–20 %) per soil mass, while increased C turnover rate (61–63 %), microbial metabolic quotient (qCO₂) values (41–54 %) and enzyme activities per unit C and MBC (61–190 %) without affecting microbial quotient (MBC/C ratio) values and the activities of soil phosphatases per soil mass. The decreases in soil microbial and biochemical properties were largely the result of a substantial decline in soil C contents and aggregate stability by tillage activities in farmlands. Although the absolute activities of soil enzymes (i.e., activity per dry soil mass) showed inconsistent responses to land use changes and were even less responsive than soil organic C and MBC contents to changing land uses, the specific enzyme activities expressed either per unit of C or MBC tended to increase considerably and consistently with this conversion and were more sensitive than soil C and MBC contents. Soil C turnover rates and metabolic quotients were found to correlate positively with the specific enzyme activities, indicating an interaction between SOM and enzyme activities. The specific enzymatic activity could be an appropriate and integrative simple index to (1) assess soil quality changes, (2) detect changes in soil microbial community; and even (3) associate the losses of SOM with the presence of metabolically more active decomposer biota in rangelands cultivated for the long term.