Effect of contrasting farm management on vegetation and biochemical, chemical, and biological condition of moist steepland soils of the South Island high country, New Zealand

A question of economic, social, and land-use importance in the predominantl y steep South Island high country tussock grasslands of New Zealand is whet her these lands can be sustainably farmed by oversowing introduced grasses and legumes and using fertilisers. To help answer this question, we compare d vegetation and soil chemical, biochemical, and biological properties on B rown soils (Dystrudepts) on adjacent land areas which have been differently managed since 1978. One area had never been fertilised or oversown. The ot her had been oversown with grasses and clovers and received about 1100 kg/h a of sulfur-superphosphate between 1979 and 1997. Oversowing and fertilising reduced the amount of bare ground and transforme d the vegetation to a species composition dominated by the introduced adven tives Anthoxanthum odoratum and Agrostis capillaris. Fertilising raised soi l carbon (C) and nitrogen (N) content, increased A-horizon thickness, and r aised exchangeable cation values. All of the phosphorus (P) applied to the fertilised area was accounted for in the top 15 cm of soil, but has accumul ated in the relatively unavailable organic form. Oversowing and fertilising significantly (P < 0.05) increased microbial biomass C, N, and P relative to values in unfertilised soils. The microbial biomass C: N and C: P ratios were significantly (P < 0.05) lower in fertilised soils. Fertilised soils had significantly more (P < 0.05) mineralised N than unfertilised soils. Populations of 5 groups of soil fauna (Scarabaeidae, nematodes, enchytraeid s, rotifers, tardigrades) were higher in fertilised soils. Nematode species parasitic or pathogenic for clovers were present in greater numbers on fer tilised soils. A combination of fertility decline (in particular, P immobil isation as organic P) and nematode damage may be the reason for the low clo ver cover on fertilised sites, and may explain the widely observed clover ' flush' and decline common to oversown high country. We conclude that there is no simple measure of 'soil quality' that can be u sed as a sustainability indicator in moist New Zealand high country. Nor ca n sustainability be judged purely on biological, biochemical, and chemical critieria. Although many of the erects associated with oversowing and ferti lising, such as increased organic matter content of A horizons and increase d biological activity, are positive, these erects must be balanced against the economic risk associated with being committed to a high-input farming s ystem, the loss of low-producing but resilient native species, and increase of plant-pathogenic nematodes.