Impact of harvest residue management on soil nitrogen dynamics in Eucalyptus globulus plantations in south western Australia

More than 200,000 ha of short rotation Eucalyptus globulus plantations have been established in south-western Australia to supply wood for the pulp and paper industries. Sustaining the productivity of these tree crops over successive rotations will depend in part on maintenance of soil fertility, especially soil nitrogen (N) supply. We investigated the impact of four alternative strategies for management of harvest residues on soil N dynamics in recently logged first rotation plantations. The experiments were conducted over 5 years following harvesting at two sites with contrasting soils—a coarse textured grey sand over laterite (Podzol) with low natural fertility and a relatively fertile red earth soil (Ferralsol). At the grey sand site, 31 t ha⁻¹ of residues containing 219 kg N ha⁻¹ were deposited following harvest while at the red earth site the equivalent figures were 51 t ha⁻¹ of residues and 347 kg N ha⁻¹. Experimental treatments applied included residues burned, removed, retained and retained with double the amount of residues. The impact of treatments on soil nitrogen supply was investigated by incubating intact soil cores in the field to determine rates of net N mineralization. Additionally, the effect of treatments on soil moisture and temperature, the resident pool of soil mineral N and the amount of N potentially available for mineralization was assessed. The mulching effect of retained residues resulted in higher soil moisture where residues had been retained and a trend for soil on these treatments to dry out more slowly with the onset of the dry summer season, especially in the first year following harvest. Diurnal variations in soil temperature were moderated and average soil temperatures were reduced during summer where residues were retained. Concentrations of mineral N in soil were high in the 2 years following harvest at both sites and declined as newly established seedlings developed. At the more fertile site, where mineral N occurred predominantly as nitrate, retention of residues resulted in lower pools of soil mineral N following harvest. The effect of residue treatments on soil mineral N pools was less marked at the grey sand site. Concentrations of potentially mineralizable soil N and the amounts of N mineralized annually were greater where residues were retained at both sites. The results indicate that retention of harvest residues will favour the conservation of N following logging. However, accumulation of soil mineral N following harvesting due to reduced plant uptake will result in leaching of N early in the rotation that is largely independent of residue management. Retaining harvest residues will contribute to enhanced N supply for the next tree crop through mineralization in the long term. However, on some sites, additions of nitrogenous fertilizers will still be required to maximise the rate of tree growth.