PLANNING FOR CHANGES IN TOPSOIL C AND N STOCKS–SIGNIFICANCE IN C AND N BUDGETS
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Abstract
New Zealand has a history of rapid land use change as trends in global commodity markets influence primary sector financial sustainability. Traditionally, low sheep and beef returns accelerate extensive pastoral land use change to forest, particularly if supported by afforestation schemes (e.g.AGS and ETS). High dairy payout accelerate forest change to intensive pasture. Current debate around the agricultural sector participating in a carbon(C) economy is spreading in New Zealand, coincident with debate on de-intensification to reduce impacts on water quality. Farms including planted forest lands may be rewarded if they are able to show a decrease in nitrogen (N) loss to water and an increase in the terrestrial sink of C. While soil carbon change is not accounted for in the ETS a change from forest to pasture penalises the landowner for the reduction in biomass C with no reward or penalty for change in soil organic matter C and N. To account for soil carbon change, protocols to measure and monitor topsoil organic C and N storage at the farm level are needed. Evidence for consistent quantifiable change is required to support inclusion of soil organic matter change in both C and N accounting. Previous research in the Taupo (Central North Island) area has shown that conversion of forest land back to productive permanent pasture caused a fast accumulation of soil organic C (6.1 t C/ha/year)and of N (450kgN/ha/year) as a response to fertiliser addition and plant productivity. In this paper we provide a case study of topsoil organic matter change in a forest to pasture conversion in the Taupo region. 42 paddocks from three sites (Tainui, Tauhara and Waimana; Wairakei Estate, Taupo)were monitored in 2017. The paddocks are currently under pasture management after recent (2-11yearsago) conversion from former planted forest. Marked differences in the storage of C (38to51tC/ha15cm) and N (1.8to 3.4 t N /ha15cm; Waimana site)were detected. The relevance of these changes to C and nutrient budgeting are discussed in relation to how such large and important changes can be accounted for.
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soil carbon