Changes in soil fauna and soil conditions under Pinus radiata agroforestryregimes during a 25-year tree rotation

Pinus radiata was planted in a grazed pasture at Tikitere, New Zealand in 1 973; final tree stocking rates were 0, 50, 100, 200 and 400 stems ha(-1); t rees were harvested in 1999. We sampled the loamy sand for chemistry, earth worms, enchytraeids and nematodes during crop growth. There were no visible changes in the soil profile between years 2 and 25 of the P. radiata rotat ion, but marked changes in soil chemistry with pH declining at all depths. Topsoil pH declined but plant-available P and the soil C:N ratio increased. That similar trends occurred in pasture may have reflected reduced inputs. Lumbricid earthworms declined with increasing time and tree stocking rate; significant populations were found only in pasture at 25 years; no related changes in soil bulk density were detected. Enchytraeids were abundant but declined in the plots with 200 and 400 stems ha(-1). The composition of th e nematode fauna at a soil depth of 0-10 cm varied, and there were changes in the "control" pasture. Pratylenchus sp. and Pungentus sp. were character istic of pasture; Diphtherophora sp. appeared at higher tree stocking rates . In addition to changes related to the change in landuse there were tempor al changes that affected faunal composition across tree stocking rates: Het erodera sp. and Paratylenchus sp. were rare after year 6; Prismatolaimus sp . was most abundant in years 2, 13 and 25; Axonchium sp. occurred in year 6 and Alaimus sp. mainly in year 25. Overall, the changes in soil chemistry reflected the change from pasture to forest. Parallel changes in soil micro fauna and macrofauna also reflected the change in landuse, from fertilised pasture to coniferous forest, and are considered reversible.