Impacts of conversion from forestry to pasture on soil physical properties of Vitrands (Pumice Soils) in the Central North Island, New Zealand

Abstract At least 30 000 hectares of land has been converted from plantation forest to pasture in the central North Island of New Zealand between 2000 and 2010. When forests are cleared for pasture the soil may undergo changes in soil structure affecting physical properties. The overall objective of this study was to investigate changes in soil physical properties in Pumice Soils following land use change from forest to pasture. Two study areas were identified; one near Tokoroa (Maxwell Farms), and one near Taupo (Wairakei Estate). At each study area a series of sites including: plantation pine forest, pasture converted from pine forest 2,3,4 and 5 years ago, and long term (>50 years) dairy, and sheep and beef pasture, were identified, all on Taupo Pumice Soil. Field and laboratory methods included measurement of; penetration resistance, degree of packing, soil dry bulk density, soil hydrophobicity, unsaturated and saturated hydraulic conductivity, soil moisture retention, aggregate stability, particle size distribution, and soil pH. There was increased soil compaction in the A horizon on recently converted sites compared to pine forest sites as evidenced by higher soil dry bulk density, increased penetration resistance, and degree of packing. At the Tokoroa study area the pine forest soil A horizon had a significantly lower dry bulk density ( mean 0.58 g/cm3) ( P < 0.05 ) than any of the pasture sites (mean 0.65 - 0.72 g/cm3). At the Taupo study area only long term dairy (mean 0.78 g/cm3) and the youngest pasture site (3 years since conversion from pine forest) (mean 0.74 g/cm3) had higher (P < 0.05) bulk density than the plantation pine forest soil. At the Tokoroa study area, the penetration resistance and degree of packing in the A horizon of the pine forest site was lower ( P< 0.01 ) than any other site. The degree of packing and penetration resistance at the Taupo study area in the A horizon of the pine forest site was lower ( P< 0.05 ) compared to 3 years old conversion and long term dairy pasture but was not significantly different from pastures converted 4 and 5 years ago. The water repellency of recently converted pastures was higher (P < 0.05) than pine forest at both study areas. Two of the three long-term pasture sites investigated had very low water repellency. The long term pasture sites had markedly higher total available water holding capacity than either the recently converted pastures or the pine forest sites. The pine forest soil water content at 10 kPa (field capacity) and 100 kPa (readily available water) was generally lower than pasture sites. At the Tokoroa study area there was no significant difference in unsaturated hydraulic conductivity between the pine forest soil and any other site. At Taupo the pine forest soil unsaturated hydraulic conductivity was higher (P < 0.05) than the most recent conversion site (3 years old) but not significantly different from other sites. At the Taupo study area none of the land uses had significantly different saturated infiltration rate compared to the pine forest. At the Tokoroa study area the pine forest saturated hydraulic conductivity was higher (P < 0.05) than the 5 years old conversion or the long term sheep and beef farm. The A horizon, at both study areas, was observed to be deepest under long term dairy farm followed by forest and shallowest on sites recently converted from pine plantation. There was no clear pattern of changes in soil colour, pedality or boundary distinctiveness and shape, between different sites. Aggregate stability of the Ap horizon was noticeably lower in recently converted pastures than in the pine forest or long term pasture sites. Soil pH values were generally in 4.5 to 5.9 ranges across all land uses in all horizons. The exception was the most recent conversions to pasture (2 and 3 years ago converted) at the Tokoroa study area which exceeded pH 6, presumably due to high rates (3.5 T/ha) of lime application during the conversion process