Effects of November–April grazing pressure on hill country pastures 3. Interrelationship with soil and pasture variation

Abstract

Abstract Variations in soil moisture content, nutrient levels, and temperature were measured for a range of micro-sites located on a steep, north-facing hillside. Species distribution and plant responses to changing moisture conditions and different grazing treatments were related to these physical conditions. Soil nutrient levels (organic carbon; total, inorganic, and mineralised nitrogen; Olsen phosphorus; potassium) were highest in camp/old track areas reflecting nutrient transfer by grazing animals. Variation in nutrient level between other flat and sloping areas was low. Soil moisture holding capacities were similar for most areas, but sloping areas were consistently drier in the surface horizon (0–50 mm) because of less effective re-wetting compared with flatter areas. Maximum soil surface temperatures of 45–50°C were reached on clear days where low pasture cover existed on sloping areas. The effects of grazing treatments on ryegrass and browntop tiller number, white clover leaf number, and seedling establishment were similar across a wide range of micro-sites. However, pasture species content varied between micro-sites: ryegrass was dominant on camps and tracks; browntop increased with steepness of slope; white clover peaked on rolling and easy sloping areas; grass seedlings were most numerous on camps and tracks; and legume seedlings were most numerous on steeper slopes. Pasture type would appear to be determined more by nutrient fluxes (e.g., mineralisation rate, excreta return) and summer rewetting of the soil surface than by nutrient pool size and inherent water-holding capacity of the soil.