Land-use change: effects on soil carbon, nitrogen and phosphorus pools andfluxes in three adjacent ecosystems

Changes in land use can affect soil organic matter contents and fertility a nd also atmospheric CO2 concentrations and global warming through soil resp iration. We compared total and microbial C, N and P pools and C and N metab olism in sandy loam soils (Typic Udivitrands) under indigenous broadleaf-po docarp forest, grazed introduced pasture and 19-yr old Pinus radiata D. Don forest (planted on previous pasture) in New Zealand. Total and microbial C and N declined consistently with profile depth (except for total N in L an d FH samples), and in comparable depths of mineral soil (to 20 cm) tended t o be lower in the pine than in the other systems. Total P, organic P and ex tractable inorganic P concentrations at comparable depths were, in contrast , lowest in the indigenous forest. Microbial P concentrations did not diffe r significantly between the different systems. Microbial C-to-microbial N r atios differed little among soil profile depths and ecosystems. In 0-10 cm depth mineral soil, CO2-C production, metabolic quotients (qCO(2) values) a nd net N mineralization were all highest in the pasture samples. Net nitrif ication was high in the pine and pasture samples, but much lower in the ind igenous forest samples; nitrate-N was, however, consistently present in str eamwater from all three ecosystems. Changes in total C and microbial C and N pools on an area basis to 20 cm depth mineral soil were greatest after co nversion of the indigenous forest to pasture; total N contents were, howeve r, as high in the pasture as in the forest and net N mineralization was hig hest in the pasture. On this area basis, changes in total C contents were s mall after conversion of pasture to pines, although the distribution within the soil profile did differ considerably between the pine and pasture syst ems. (C) 1999 Elsevier Science Ltd. All rights reserved.