Losses of nitrogen from an effluent-irrigated plantation

Abstract

Effluent-irrigated tree plantations are becoming a popular alternative to the direct discharge of sewerage effluent into inland rivers. However, when irrigated at the water-use rate of the plantation, nitrogen (N) applied in the effluent often exceeds the demand of the trees both in the early stages of growth and after canopy closure when much of the N is supplied from the decomposition of litter. With the ample supply of water and large loadings of N, conditions are conducive to significant N losses. The principal potential N loss mechanisms are leaching of nitrate-N to groundwater and biological denitrification. In 4 separate experiments between winter 1992 and summer 1994, ¹⁵N-labelled N (total application of 2 g N/m²; 30 atom%) was applied to 1-m² subplots or small plastic (PVC) cylinders. In the first 2 experiments, using 1-m² subplots, the conservative tracer Br was applied as KBr with the ¹⁵N at a total application of 12·8 g Br^-/m². In Expt 1, the high recovery of the ¹⁵N-labelled N, and the comparable distribution of ¹⁵N and Br^- in the soil profile, implied that there was little loss of N by denitrification, even though the soil was sufficiently wet for leaching of both tracers. Although less conclusive, the other experiments also showed a high potential for leaching and minimal loss of applied ¹⁵N-labelled N by denitrification. The small contribution to N loss of denitrification, despite conditions being otherwise suitable for it, is attributed to the free-draining nature of the soil, which meant that aeration was rarely sufficiently limited to encourage denitrification. Similar results are expected at other effluent irrigation sites on freely draining soils.