Changes in soil N status and N supply rates in agricultural land afforested with eucalypts in south-western Australia

Abstract

In south-western Australia, plantations of Eucalyptus globulus are being established on land that has previously been used for conventional agriculture. Sustaining the productivity of these tree plantations in second and subsequent rotations will depend partly on maintenance of soil fertility, especially soil nitrogen (N) supply rates. We compared soil N status and supply rates between adjacent pasture and 6–11 year old first-rotation eucalypt plantations at 31 paired sites in south-western Australia. Total soil N varied widely among sites (0.07–0.68% in the fraction <2 mm of the 0–10 cm soil layer), but concentrations averaged over all sites did not differ between land-use types. However, measurements of the indices of mineralization (mineral N produced during incubation of intact cores), potentially available N (from short-term anaerobic incubation) and model-predicted mineralization rates during 28-day aerobic incubations were generally lower in afforested soils than in pasture soils. This finding was supported by in situ field estimates of N mineralization over a full year at two contrasting paired pasture-plantation sites. At each site there was a marked reduction (2–3-fold) in net annual mineral N flux rates in soils under eucalypt plantations. Reduced N mineralization associated with tree plantations was due to both changes in soil organic matter quality and the generally lower soil moisture content under trees in comparison with pasture. These results suggest that N supply rates of pasture soils are likely to decline when the land is planted to successive crops of eucalypts. Eucalypt plantation managers will need to take account of this and implement management strategies to maintain adequate N nutrition to sustain tree growth in future rotations.