Mineral nitrogen regimes in soils of natural and modified snow tussock grasslands of Canterbury and Otago, New Zealand

Abstract

Mineral nitrogen (N) levels in natural grasslands of the world fluctuate in response to a range of environmental influences but are generally low with ammonium (NH₄-N) levels usually below 10 µg g⁻¹ soil and nitrate (NO₃-N) levels even lower. This is considered to reflect active competition for mineral N between grass roots and soil micro organisms although it has been suggested that low nitrate levels may result from nitrification inhibition by plant exudates in grassland soils. Snow tussock (Chionochloa) grasslands cover approximately two million hectares of South Island, New Zealand. Such natural grasslands have been extensively modified since European settlement by grazing, burning and cultivation, factors which are postulated to have triggered changes in soi1 N losing ecosystems mineral N regimes leading to these becoming N losing ecosystems.

Seasonal mineral N levels were measured at intervals for up to three years in a range of intact tall tussock grasslands in Canterbury and Otago. First a sampling and extraction procedure was devised to avoid the changes in mineral N after sampling which are considered to have marred earlier studies of seasonal soil mineral N. Nitrogen levels in intact grassland soils were compared to levels in similar grasslands subjected to modification ranging from simulated grazing to burning and cultivation.

Intact grasslands showed sometimes high mineral N levels (up to 90 µg g⁻¹ soil), particularly in winter months and also after a spring dry-wet alternation. Most of this mineral N was located in the upper 100 mm of soil. Only in autumn did mineral N levels approach the levels considered characteristic of grassland ecosystems. Nitrifying bacteria numbers were generally low although this finding possibly reflected analytical limitations.

It was postulated that the surge in mineral N levels detected in winter resulted from alternate freezing and thawing of surface soil layers resulting in the mineralisation of nitrogen in the soil. A laboratory experiment simulating field freeze-thaw conditions in a range of soils induced a major increase in NH₄-N levels within these soils.

Repeated defoliation caused a general surge in soil NH₄-N levels, resulted in a change in the distribution of mineral N in the soil profile, altered soil moisture regimes and generally increased the number of nitrifying bacteria in soils particularly at higher altitude Chionochloa macra sites.

Urea application to defoliated sites amplified the changes in mineral N levels and nitrifying bacteria numbers caused by defoliation. However, urea application to intact grasslands did not cause a general increase in soil mineral N levels although it caused a marked increase in foliar N levels. It was therefore postulated that snow tussocks are capable of absorbing the quantities of mineral N available from periodic surges in mineralization.

Burning and cultivation of Chionochloa grasslands caused substantial increases in soil mineral N levels and nitrifying bacteria numbers. These studies reveal the magnitude of mineral N fluctuations in intact snow tussock grasslands and show the increased potential for N loss which may result from cultural modification of these natural grasslands by grazing, burning and cultivation.

The importance of maintaining snow tussock grassland to prevent the development of N losing ecosystems and to enable similar comparative studies to be done in the future is emphasised.