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

Pinus radiata was planted in a grazed pasture at Tikitere, New Zealand in 1973; final tree stocking rates were 0, 50, 100, 200 and 400 stems ha(-1); trees were harvested in 1999. We sampled the loamy sand for chemistry, earthworms, enchytraeids and nematodes during crop growth. There were no visible changes in the soil profile between years 2 and 25 of the P. radiata rotation, 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 the 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 characteristic of pasture; Diphtherophora sp. appeared at higher tree stocking rates. In addition to changes related to the change in landuse there were temporal changes that affected faunal composition across tree stocking rates: Heterodera 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 microfauna and macrofauna also reflected the change in landuse, from fertilised pasture to coniferous forest, and are considered reversible.