Soil microbial biomass, metabolic quotient, and carbon and nitrogen mineralisation in 25-year-old Pinus radiata agroforestry regimes

To understand the effects of agroforestry on soil biological processes we assessed the conditions in Pinus radiata plantations of 50, 100, 200, and 400 stems/ha after 25 years of growth, and in a grassland. Agroforestry resulted in a 15–25% decline in soil organic C and N compared with grassland, and had a significant negative influence on soil microbial biomass. There was less microbial C and N in soils under 50–400 stems/ha of P. radiata than in soils under grassland (0 stems/ha). Soil carbon decomposition and microbial activity were measured by trapping the carbon dioxide produced by incubating soils over a 60-week period. The results showed that soil C decomposition rates were ~1.5 times as much (c. 15 mg CO2-C/kg soil) in soil from grassland as in that from plots with 50 or100 stems/ha (c. 10 mg CO2-C/kg soil), and were further reduced to one half (c. 5.5 mg CO2-C/kg soil) in the plots with 200 or 400 stems/ha. The soils under P. radiata gave off less carbon dioxide per unit of biomass (the metabolic quotient) than soils under grassland. These shifts in microbial biomass and its metabolic quotients appear to be associated with differences in the quantity and ‘quality’ of inputs and soil organic matter decomposition rates, and to reflect the land use change from grassland to forest. Given the general ability of soil microbial biomass to recolonise depopulated areas after tree harvest, we see no problem in restoring populations of these soil organisms vital in controlling nutrient cycling after tree felling, provided adequate adjustments to soil pH are made.