Changes in carbon and nitrogen stocks following conversion of plantation forest to dairy pasture on pumice soils, in the Central North Island

Since 1990 approximately 36,000 ha of land in the Waikato has been converted from pine plantation to dairy farms. By changing land use from plantation forest to pasture there is potential to change soil properties. The property of interest, due to international discussion (Kyoto Protocol), is the potential for soils to store carbon. My main objective was to determine the rate and magnitude of change in soil carbon, soil nitrogen, and C:N ratio following land conversion from pine plantation to pasture. My study examined three areas (Atiamuri, Tokoroa and Wairakei) in the Central North Island of New Zealand. At each study area sites ranging from pine plantation, through 2, 3, 4, 5 and 11 years since conversion, to long-term (>40 years) dairy or sheep/beef pasture, were identified. Three transects were established at each site and 7 soil core samples each to a depth of 60 cm, were taken at random intervals along each transect. Soil cores were split into horizons with samples from each horizon bulked together for each transect. At the midpoint of each transect a pit was dug and soil dry bulk density samples were taken from each horizon. At one pit from each site a sample of each soil horizon was taken. In the laboratory all soil samples were passed through a 2 mm sieve and air dried. Air dried samples were crushed and analysed for total carbon and total nitrogen with a TruSpec CN Carbon/Nitrogen Determinator. Soil dry bulk density was lower (P<0.05) in the pine sites Ap horizons (0.47 to 0.54 g cm-3) than the Ap horizon for all other sites (0.59 to 0.76 g cm-3) at the Atiamuri, Tokoroa and Wairakei areas. There was no significant difference in Ap horizon depths between pine forest, recent conversion and long-term pasture sites at any of the study areas. Carbon concentration in the Ap horizon (5.5 to 9 %) showed few significant differences between land-use sites. The total soil carbon at Atiamuri and Wairakei was higher (P<0.05) in the long-term pastures (88 to 100 t/ha) than in the pine (42 to 54 t/ha), however there was no significant difference between pine and long-term pasture at Tokoroa and data from recently converted sites were variable with no apparent significant differences. Total soil nitrogen at all three study areas was higher (P<0.05) in the long-term pasture sites (5 to 8 t/ha) than in the pine sites (9 to 12 t/ha). The C:N ratio in the Ap horizon was higher (P