TREE STOCKING EFFECTS ON SOIL CHEMICAL AND MICROBIAL PROPERTIES AT THE TIKITERE AGROFORESTRY RESEARCH AREA

Soil fertility at the Tikitere Agroforestry Research Area near Rotorua has been monitored since the site was planted with Pinus radiata D. Don in 1973. Measurements made in 1991 and 1992 showed that after 18 years soil pH had declined and that Olsen phosphorus levels increased with increased tree stocking. Soil magnesium also declined at the higher tree stocking rates. We made further measurements on samples collected in 1991 and 1992 to determine reasons for the change in nutrient status. Soil carbon and soil organic phosphorus in the surface soil (0-75 mm depth) decreased with tree stocking, indicating increased net mineralisation of soil organic matter under P. radiata compared with pasture. Exchangeable cations and cation exchange capacity also declined with tree stocking, which is consistent with the loss of exchange sites in soil organic matter. Total soil inorganic phosphorus declined with increased tree stocking. This was expected because of reduced fertiliser phosphorus input at the higher stocking rates. However, phosphorus fractionation showed that bicarbonateextractable inorganic phosphorus increased and acid-extractable inorganic phosphorus declined with tree stocking. The latter indicated possible decreases in soil apatite phosphorus. These results were consistent with the mobilisation of soil phosphorus under P. radiata by dissolution of fluorapatite (in parent material deposited during the 1886 Tarawera eruption) due to lower soil pH values and mineralisation of organic phosphorus previously accumulated under pasture. Consequently, Olsen phosphorus has increased despite reduced phosphorus fertiliser applications. Microbiological activity (as indicated by microbial biomass and microbial respiration) and phosphatase activities of soil samples collected in 1995 also decreased with tree stocking, and so mineralisation of organic matter was not a result of microbial activity alone. However, the proportion of soil aggregates > 0.5 mm decreased under P. radiata, indicating there was less physical protection of the soil organic matter than under pasture where soil aggregates were maintained by fine roots.