Soil nitrogen availability in the cereal zone of South Australia .2. Buffer-extractable nitrogen, mineralisable nitrogen, and mineral nitrogen in soil profile under different land uses

Abstract

Mineral nitrogen (nitrate-N+ammonium-N) and its distribution in soil profiles to 60 cm depth at sowing in 3 growing seasons, 1990-1992, were assessed for 123 field experimental sites in South Australia. The sites were used to test N fertiliser responses with cereal crops following different land uses. More than 90% of the variation in mineral N at cereal sowing was attributable to nitrate-N in the 60-cm soil profiles. Coefficients of variation (CV) for nitrate-N ranged from 37 to 45%, less than half of the CV values (88-113%) for ammonium-N. More than 70% of mineral N in soil to 60 cm depth was accounted for by mineral N in the top 20 cm of soil, and 49% by mineral N in the top 10 cm of soil. The amounts of mineral N in the 60-cm soil profiles at sowing ranged from 24 to 180 kg N/ha (median 75) at sites following pastures, and from 22 to 113 kg N/ha (median 69) following grain legumes, significantly higher than 17 to 116 kg N/ha (median 47) following cereals. Only 26% of the variation in mineral N of soils (0-60 cm depth) could be predicted by soil total N, mineralisable N assessed by the aerobic incubation method, and previous land use. Ammonium-N extracted by phosphate-borate buffer from soils sampled at 0-10 and 10-20 cm depths was directly related to soil total N and N mineralised after soil incubation, but not to mineral N accumulating at sowing in the soil profiles to 60 cm depth. Utilisation of a soil containing 15N-labelled organic residues, and sampled to 100 cm depth at sowing in 5 successive growing seasons, revealed a positive relationship between the 15N atom% enrichments of soil profile mineral N, mineralisable N from soil incubations, and plant N. Enrichments of soil profile mineral N and plant N were almost identical. However, the enrichment of buffer-extractable ammonium-N was comparatively low and unresponsive to the time of soil sampling, and unrelated to the other soil and plant N pools. Thus, buffer-extractable N was unrepresentative of plant-available N.