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

Abstract

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.