Influence of erosion and deposition on carbon and nitrogen accumulation in resampled steepland soils under pasture in New Zealand

Although erosion occurs under pastures in New Zealand North Island hill country, previous work shows that the soils on mid-slopes (>19 degrees) have large and significant average gains of soil carbon (C) and nitrogen (N) over a period of about 30 years. Both sheetwash erosion (from patches of bare soil) and soil deposition (from upslope) could have occurred in the intervening 30 years, and this could have led to a loss or gain of soil C. The fission-derived radionuclide ¹³⁷Cs is a unique tracer for studying erosion and sedimentation because it is strongly adsorbed onto soil particles, and its movement in the environment is predominantly due to physical processes. We hypothesized that accumulation and loss of ¹³⁷Cs and C would be correlated if erosion and deposition processes were responsible for the observed changes in soil C. We therefore measured ¹³⁷Cs in archived samples, previously used to assess changes in C and N, together with ¹³⁷Cs in the re-sampled soil horizons, to an average depth of 37 cm. The data show that, on average, there has been a small gain in total profile ¹³⁷Cs over this time, and complex patterns within individual soil profiles over 30 years. There was only a weak correlation between ¹³⁷Cs and C accumulation above 15 cm soil depth, but, in contrast, a much stronger relationship was found below 15 cm. A possible explanation for the observed pattern is soil thickening due to deposition, but the deposition may occur with variable ¹³⁷Cs contents. Generally deposition does not account for all of the increase in C, and regression analysis between Cs and C for all layers suggested that erosion/deposition accounted for 20-30% of the total change in C. In a detailed case study, the sampling of erosion scars caused by landslides showed that soil C stocks (0-20 cm) can increase from 10 to 80 tonne (Mg) C ha⁻¹ within 70 years. Use of the CENTURY model indicated that input of N from legumes in the pasture enabled N and C to accumulate in the surface soil. This suggests that gains in soil C and N under hill country pasture that occur in these facets of the landscape are probably largely due to the ongoing input from C and N in the grass-legume pastures rather than erosion or deposition of soil