Changes in soil carbon and nitrogen storage after conversion from tussock grassland to pine plantation, Glendhu catchment, Otago, New Zealand

Abstract

Globally, soil and surface litter store 2 to 3 times more organic carbon than vegetation and 3 times more carbon than the atmosphere. The soil organic stock at a soil depth of 1 m is globally about 1500 to 1600 Pg (Pg = 1015 g). The amount of carbon and nitrogen stored in the soil is influenced by many factors. An important factor affecting soil properties, carbon and nitrogen storage is land use and land cover change (LULCC). Conversion of grasslands into tree plantations is common with the aim of increasing aboveground carbon stocks to mitigate climate change. This study investigated the changes that happened in the soil in the Glendhu catchment (Otago, New Zealand) after conversion of tussock grassland to pine plantation 36 years ago. The objectives of this study were to  Quantify the soil carbon and nitrogen stocks to 1 m soil depth  Determine the physical parameters of soil  Investigate the relationships between soil parameters, soil depths and land use types  Investigate the relationships between tussock density and soil carbon density A total of 72 mineral soil cores (to 1 m depth, five depth increments, in total 360 mineral soil samples) and 72 organic layer samples from tussock grassland, and 244 mineral soil samples from pine plantation were collected and analysed for pH, total carbon and nitrogen concentration, dry bulk density (BD), and soil texture. The median soil pH values (0-10 cm depths) was 4.00 in tussock grassland and 3.91 in pine plantation, respectively. The pH values and bulk densities under both land use types increased with soil depth while carbon and nitrogen concentration decreased with depth. The texture of tussock grassland and pine plantation investigated in this study were dominated by sand (>50%). Carbon stocks in the organic layer were significantly different between land use types. 1.14 kg C/m2 in tussock grassland and 6.78 kg C/m2 in pine plantation. The median of soil organic carbon stock (SOC, 0-100 cm) in tussock and pine plantation was 21.36 kg C/m2 and 20.93 kg C/m2, respectively. The differences in SOC stock between these two land types were not significantly different. However, the SOC stock at 0-10 cm was significantly higher under tussock grassland (5.4 kg C/m2) than that under pine plantation (4.8 kg C/m2). Nitrogen stock was significantly higher in the pine plantation than that in tussock grassland. A significant negative correlation was found between SOC stock and soil pH in tussock grassland. This study suggests that pine plantation can store similar level of SOC as tussock grassland in mineral soils in the depth of 0-100 cm in Glendhu catchments, South Island, New Zealand.