Base cation availability and leaching after nitrogen fertilisation of a eucalypt plantation

Increasing use of nitrogen fertiliser in eucalypt plantations is affecting base cation availability via changes in concentrations in soil solution and leaching. While low base cation availability will probably limit future productivity of some eucalypt plantations, the extent and temporal patterns of availability as affected by N fertilisation were unknown. After applying urea in a fertiliser experiment in a 9-year-old Eucalyptus nitens plantation, soil solution chemistry was monitored for 13 months and maximum potential leaching estimated. The cumulative effects of earlier ammonium sulfate, urea, and superphosphate fertiliser applications had enhanced tree growth, base cation uptake, and probably leaching, but led to decreased pH and concentrations of exchangeable pools of Mg and K in surface soil (0–0.1 m). In soil solution before re-fertilisation, in the high N treatment, concentrations of K in soil solution were significantly lower, and of Ca significantly higher than the treatment that received no fertiliser. On many occasions during the 13-month period after this re-fertilisation event, increased concentrations of NH4, NO3, Ca, Mg, and K in soil solution were significant (0–0.6 m depth), and were consistently highest in the high fertiliser treatment at 0.3–0.6 m depth. We conclude that N fertilisation increased base cation availability during the study and probably for several subsequent years, but there was a risk that significant pools of N and base cations were leached off-site. Future research and modelling of base cation availability in plantations should consider these changes induced by N-fertiliser that increase availability for several years but lead to longer term decreases in availability.