Development of nitrogen mineralisation gradients through surface soil depth and their influence on surface soil pH

Following mixing of the surface soil to about 7.5 cm depth in the field, soil layers (0-2.5, 2.5-5, 5-10 and 10-15 cm) were separately incubated in the laboratory to determine the rate of development of net N mineralisation gradients through surface soil depth under fallow, wheat and subterranean clover plots. Gradients in net N mineralisation were compared with those observed in the field, and their contribution to the observed pH changes was investigated. Heterotrophic activity, and thus net N mineralisation, decreased only slightly with depth immediately after soil mixing. This pattern persisted over time in soil layers sampled from fallow plots. In contrast, within 1 growing season after soil mixing, heterotrophic activity and net N mineralisation decreased significantly with depth in soil sampled from wheat and clover plots. In 0-15 cm soil sampled from under senescing plants, 32-38% of CO2-C produced and net N mineralised originated from the surface 2.5 cm, while 52-56% originated from the surface 5 cm of soil. This resulted from an increase of pH and organic substrate concentration within the surface 2.5 cm of soil following plant residue return. Limitations of the in situ measurement of net N mineralisation in fallow soil was identified. Laboratory incubation studies showed that since most net N mineralisation occurred within the surface 2.5 cm of soil under senescing plants, nitrification and acidification were also concentrated at this depth. Despite this, compared to fallow soil, high potential acidification rates of 0-2.5 cm soil under senescing plants were not realised in the field due to the exposure to prolonged dry periods and moist-dry cycles. As a consequence, in the field the large magnitude of surface soil pH gradient which resulted from the return of alkaline plant residues was maintained over summer and autumn.