Distribution of nitrification within surface soils under pasture

Abstract

Soil pH decreases rapidly with depth through the surface 10 cm of soils under pasture resulting in acidic subsurface soil layers. This laboratory study examined the influence of these pH changes on nitrification of added NH₄ ⁺. Five sites were sampled where the soils had been under pasture for at least 3 years and where there had been no lime or nitrogen (N) fertilizer application. Soil was sampled in 2-cm depth intervals to a total depth of 10 cm. Soil pH decreased between 0.2 and 1.4 units between the soil depth of 0–2 cm and 8–10 cm. Organic carbon (C) and total N concentration, and ECEC decreased with depth in all soils, while clay content was constant and exchangeable Al³⁺ concentration increased. Soils were moistened, amended with NH₄ ⁺ and incubated at 30°C for 21 days. Nitrification decreased with depth in all soils. Nitrification of all added NH₄ ⁺ only occurred in soil from two sites and then only in the surface 2 cm. In two other soils little nitrification occurred at depths below 6 cm. Even for samples collected in 2 cm intervals, nitrification across all soils and depths was highly correlated with soil pH (r=0.89, P<0.01) and organic C percentage (r=0.69, P<0.01). This indicates that the acidic subsurface layers in soil cause nitrification to be limited to the surface few centimeters of soil. The implications of this observation for N cycling and for further acidification are outlined.

Acknowledgments

S.Y. wishes to thank Charles Sturt University for a CSU Postgraduate Scholarship. The project was funded by The Farrer Centre for Conservation Farming at Charles Sturt University.