SOIL-NITROGEN AVAILABILITY IN THE CEREAL ZONE OF SOUTH AUSTRALIA .2. BUFFER-EXTRACTABLE NITROGEN, MINERALIZABLE NITROGEN, AND MINERAL NITROGEN IN SOIL-PROFILE UNDER DIFFERENT LAND USES

Mineral nitrogen (nitrate-N+ammonium-N) and its distribution in soil p rofiles to 60 cm depth at sowing in 3 growing seasons, 1990-1992, were assessed for 123 field experimental sites in South Australia. The sit es were used to test N fertiliser responses with cereal crops followin g 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 i n 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 f rom 22 to 113 kg N/ha (median 69) following grain legumes, significant ly higher than 17 to 116 kg N/ha (median 47) following cereals. Only 2 6% of the variation in mineral N of soils (0-60 cm depth) could be pre dicted by soil total N, mineralisable N assessed by the aerobic incuba tion method, and previous land use. Ammonium-N extracted by phosphate- borate buffer from soils sampled at 0-10 and 10-20 cm depths was direc tly related to soil total N and N mineralised after soil incubation, b ut not to mineral N accumulating at sowing in the soil profiles to 60 cm depth. Utilisation of a soil containing N-15-labelled organic resid ues, and sampled to 100 cm depth at sowing in 5 successive growing sea sons, revealed a positive relationship between the N-15 atom% enrichme nts of soil profile mineral N, mineralisable N from soil incubations, and plant N. Enrichments of soil profile mineral N and plant N were al most 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-extr actable N was unrepresentative of plant-available N.