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Gains and losses in C and N stocks of New Zealand pasture soils depend on land use

Schipper, L.A., Parfitt, R.L., Ross, C., Baisden, W.T., Claydon, J.J., Fraser, S.
Agriculture, ecosystems & environment 2010 v.139 no.4 pp. 611-617
carbon sinks, dairies, data collection, fertilizers, grasslands, grazing lands, hills, inventories, land use change, pastures, risk reduction, sheep, soil profiles, stocking rate, New Zealand
Previous re-sampling of 31 New Zealand pasture soil profiles to 1m depth found large and significant losses of C and N over 2–3 decades. These profiles were predominantly on intensively grazed flat land. We have extended re-sampling to 83 profiles, to investigate whether changes in soil C and N stocks were related to land use. Over an average of 27 years, soils (0–30cm) in flat dairy pastures lost 0.73±0.16MgCha⁻¹y⁻¹ and 57±16kgNha⁻¹y⁻¹ but we observed no significant change in soil C or N in flat pasture grazed by “dry stock” (e.g., sheep, beef), or in grazed tussock grasslands. Grazed hill country soils (0–30cm) gained 0.52±0.18MgCha⁻¹y⁻¹ and 66±18kgNha⁻¹y⁻¹. The losses of C and N were strongly correlated, and C:N declined significantly. Further, results reported to 60 and 90cm show that the pattern of losses and gains extend beyond the IPCC accounting depth of 30cm. Specific causes for the soil C and N changes are unknown, but appear to be related to land use. In general, the losses under dairying correspond to systems with greater stocking rates, fertiliser inputs and removal of C and N in exported products. Gains in hill country pastures may be due to long-term recovery from erosion and disturbance following land clearance. The unexpected and contrary changes of C and N in different pasture systems (initially thought to be at steady state) demonstrates the need for global and national-scale collection of robust data investigating soil biogeochemical changes, not only for grasslands but also for other land uses. Re-sampling of soils can constrain the directions and magnitude of soil C and N change associated with land use and management to underpin C and N inventories and correctly identify mitigation options.