Monitoring the effects of pastoral use on upland and high country soils in South Island, New Zealand

Abstract

This paper reports on chemical changes that have occurred since 1978 in topsoils (O-7.5 cm depth) of upland and high country farms at 3 sites: Longslip and Glencaim Stations near Omarama, north Otago, and the east Otago uplands between Middlemarch and Alexandra. Between 1978 and 1992 on Longslip, on fertilised and oversown hilly and steep slopes, mean soil organic C increased by 67%, total N value changed little, and pH declined by 0.41 units. Associated with the soil changes were declines in the amount of bare ground and snow tussock cover, and increases in legume and hieracium cover. The organic C change was equivalent to an increase of about 11 t C/ha between 1978 and 1992. Over the same time period no topsoil changes on lower landscape positions were apparent. On Glencairn, under grazing but no topdressing, mean organic C declined by lo%, total N declined by 25% and pH declined by 0.43 units between 1978 and 1993). The organic C and N changes were equivalent to a decline of about 1.7 t C/ha and 0.4 t N/ha between 1978 and 1993. In contrast, within unfertilised and fertilised plots of exclosures that had not been mown or grazed for 9 years, organic C rose by 40-55%, total N rose by 21-36%, and there was no significant change of pH. Vegetation changes outside the exclosures included an increase of hieracium and briar cover. On the east Otago uplands under light grazing and little or no fertiliser application no significant changes of pH, organic C or total N were~recorded between 1978 and 1994. The results show that soil monitoring can give useful information about the changes associated with pastoral use and the options available to maintain or enhance soil organic matter and pH levels. Keywords: high country, monitoring, organic C, pH, sustainability, total N