Soil phosphorus pools and cycling as affected by changing land-uses in a semi-steppe ecosystem

Land-use changes in native rangelands can greatly affect nutrient cycling processes. While phosphorus (P) is the second most important macronutrient that limits plant growth and productivity in semi-arid rangelands, the effect of land-use conversions on P pools and turnover has been rarely studied. The main objective of this study was to determine soil P pools and dynamics after conversion of rangelands to rainfed wheat croplands and re-conversion of these rainfed farming systems back to rangelands (i.e., cultivation abandonment) in West Central Iran. Soil samples (0–150 and 150–300 mm) were collected from wheat-cultivated rangelands, wheat-abandoned rangelands and uncultivated rangelands on north-facing and south-facing slopes, and analyzed for P pools, P mineralization and phosphatase activities. Results showed a strong impact of land-use changes on soil P pools and turnover, which varied greatly with slope aspect. Converting rangelands to wheat fields declined soil inorganic P (46%), organic P (26%), microbial biomass P (52%), potentially mineralizable P (78%) and P-acquiring enzyme activities (15–20%). Conversely, abandonment of wheat cultivation resulted in a major recovery of P pools and cycling with greater inorganic P (36%) and organic P (21%) pools, microbial biomass P content (79%), P mineralization (191%) and P-cycling enzyme activities (4–7%) after 15 years. In summary, short-term cessation of wheat cultivation to allow secondary rangeland succession without disturbance would rebuild soil P storage, restore P cycling and supply, and hence would improve soil functions and quality. Therefore, cropland abandonment should be adopted to maintain soil P fertility and the sustainability of native rangelands in semi-arid regions.