Recovery of pasture production and soil properties on soil slip scars in erodible siltstone hill country, Wairarapa, New Zealand

Abstract

Landslide scars previously dated (1977, 1961 and 1941) and sampled in 1984 for pasture production and topsoil characteristics were re-sampled. Pasture dry matter (DM) production and selected soil properties were re-measured on the same scars and uneroded control sites for 2 years, beginning in 2007. The results show that after a further 25 years of recovery, no significant increase in pasture production had occurred on the 1941 and 1961 slip scars. Average DM pasture production on eroded sites increased from 63 to 78% of pasture production levels on uneroded sites, but improvement was restricted to the youngest 1977 slip scars, where DM production increased from 20 to 80% of uneroded levels. Regression analysis revealed that maximum pasture recovery occurred within about 20 years of landsliding and further recovery beyond 80% of uneroded level was unlikely. The recovery of pasture production on slip scars follows similar recovery to soil physical (e.g. soil depth, particle density, bulk density) and chemical properties (e.g. total C, total N). Topsoil depths on eroded sites were roughly a third of topsoil depths on uneroded sites, indicating reduced profile available water capacity on eroded soils. This research was unable to determine if total C would recover to uneroded levels because of the high variability in total C at eroded sites. However, given that uneroded soils were formed under native forest (over a long period of time) and that new soils are forming under pasture (over a short period of time), it is unlikely that total C will recover to uneroded soil levels. This research verifies the conclusion of the previous work carried out in 1984 that it is unlikely pasture production on slip scars will return to production levels on uneroded sites in human timescales. The sustainability of pastoral agriculture on steeper slopes in soft-rock hill country is increasingly under threat from the progressive reduction of pasture production from cumulative erosion.