Tuesday, November 14, 2006

Large Losses of Soil C and N from New Zealand Pastures during the Past 20 Years.

Louis Schipper1, Troy Baisden2, Roger Parfitt2, Craig Ross2, John Claydon2, and Greg Arnold2. (1) NEW ZEALAND, Landcare Research, University of Waikato, Private Bag 3105, Hamilton, NEW ZEALAND, (2) Landcare Research, Private Bag 11 052, Palmerston North, New Zealand

Following trends throughout developed nations, New Zealand indigenous forests and tussock grasslands have been converted to agriculture, primarily grazed pastures. The initial conversion of land to pasture decreased soil acidity and elevated N and P stocks, but caused little change in soil organic C stocks.  However, little is known about changes in basic soil characteristics during the last two decades, including soil organic matter (SOM) under long-term pastoral management. We resampled 32 soil profiles in pastures spanning seven soil orders with a latitudinal range of 36–46°S, which had originally been sampled 17 to 43 years ago. We measured total C, total N and bulk density for each horizon (generally to 1 m) and also reanalysed archived soil samples of the same horizons for C and N. On average, profiles had lost significant amounts of C (-2.1 kg C m-2) and N (-0.19 kg N m-2) since initial sampling. Assuming a continuous linear decline in organic matter between sampling dates, losses averaged 0.106 kg C m-2 y-1 and 0.009 kg N m-2 y-1. Removal of carbon through leaching and erosion appears too small to explain these losses, suggesting losses from respiration may have increased and/or inputs of photosynthate may have decreased. Whether losses are continuing is not known, but if so, would have implications for both New Zealand’s national CO2 emissions and sustainability of current land uses. Our results add credence to the suggestion that soil C losses may be occurring in developed temperate zone pastures at a global scale.