Effects of Exclosure and Management on Biomass and Soil Nutrient Pools in Seasonally Dry High Country, New Zealand

We examined the effects of grazing and three exclosure treatments (no grazing, no-grazing plus oversowing, and no grazing plus oversowing plus fertilizer) on soils (Ustochrepts) and biomass of tussock grasslands on hill country under a mean annual rainfall of about 500–600 mm in the Benmore Range, South Island, New Zealand. Excluding grazing by sheep and rabbits for 15 years, with no oversowing or fertilizer, resulted in a two- to three-fold increase in the total biomass (roots+litter+herbage) relative to the grazed treatment. On all three exclosure treatments root biomass was approximately doubled in relation to root biomass on the grazed treatment. With the exception of Mg, total nutrient pools in biomass plus soil were lowest in the grazed treatment. Oversowing alone, without added fertilizer, maintained soil pH at pre-exclosure values and prevented exchangeable Ca, Mg and K decline, but had no significant effect on soil C, N or P. In contrast, oversowing and fertilizing had the effect of increasing soil C and N by 5·7 t/ha and 0·6 t/ha respectively, relative to the ungrazed treatment, and by 8·7 t/ha and 0·87 t/ha respectively, relative to the grazed treatment. Under ungrazed, grazed, and the oversown+fertilizer treatments pH declined relative to pre-exclosure values and values on the oversown treatment. The lower pH under the oversown+fertilizer treatment was attributed to the direct and indirect acidifying effects of elemental S fertilizer, while the lower pH of the ungrazed treatment was attributed, in part, to the acidifying effect ofHieracium pilosella, with both cation removal by animals andHieraciumeffects likely to have lowered pH in the grazed treatment.A positive balance of 425 kg/ha of Ca, 680 kg/ha of K and 1900 kg/ha of N remained in total biomass plus soil on the oversown+fertilizer treatment, after fertilizer additions and previous herbage removals were taken into account. The positive Ca and K balance was attributed to release of nutrients by soil weathering, and the N balance to N fixation by legumes. Grazing of seasonally dry South Island tussock grasslands on hilly land, even at the present low levels, without fertilizing, appears incompatible with the maintenance of soil pH and carbon and total soil and biomass nutrients.