Contribution of decomposing harvest residues to nutrient cycling in a second rotation Eucalyptus globulus plantation in south-western Australia

Amounts of nutrients in harvest residues and their contribution to nutrient cycling were quantified following logging of a Eucalyptus globulus plantation in south-western Australia. An estimated 64 t ha^-1 of leaf, bark and branch material less than 3 cm in diameter was deposited on the forest floor during harvesting. Leaves contributed about one-third of the residue dry weight but accounted for almost three-quarters of residue-N stores (299 of 428 kg N ha^-1) and 36% to 52% of P, K, Ca, and Mg stores. Stores of nutrients in slash were significant in comparison to amounts in surface soil (0-20 cm). Residue-N amounted to 11% of total surface soil N and cations stored in residues were equivalent to 23-114% of surface soil exchangeable cations. Decomposition of the leaf fraction of slash was rapid with more than 90% of dry weight released during the 105-week study. Bark and branch fractions of diameters 0.5, 1 and 2 cm lost 39%, 37%, 32% and 29% of dry weight, respectively, during the same period. Single and double exponential decay models fitted to the data indicated half lives ranging from 20 weeks for leaves and from 3 to 4 years for bark and the branch fractions. During decomposition, K was leached rapidly from all residue fractions, Mg and P were released at similar rates to dry weight, and Ca and N were released more slowly than dry weight. In the 105-week study period, 250 kg ha^-1 of N, 20 kg ha^-1 of P, 213 kg ha^-1 of Ca, 298 kg ha^-1 of K, and 63 kg ha^-1 of Mg were returned to the soil from decomposing harvest slash. The leaf fraction was the major contributor to nutrient cycling, accounting for almost all of the N and Ca release and from half to three-quarters of the K, Mg and P released. Amounts of nutrients released from residues in the year following logging greatly exceeded quantities likely to be taken up by the newly established tree crop.