EFFECTS OF SLASH AND LITTER MANAGEMENT ON FLUXES OF NITROGEN AND TREE GROWTH IN A YOUNG PINUS-RADIATA PLANTATION

In a young P. radiata plantation on a sandy Podzol in SE Australia, treatments were slash and litter retained (SL), litter only retained (L), litter ploughed (LP), and slash and litter removed (SLR). Soils without slash or litter cover (LP and SLR) were up to 4°C warmer on average than soils overlaid by slash or litter and were subjected to greater extremes of temperature. Treatments had relatively little effect on soil water content and needle water potential in trees. Carbon in surface soil increased from 1.14 to 1.83% after incorporation of litter by ploughing, but decreased to 1.37% during the next 40 months. Smaller but significant decreases in C also occurred in other treatments. LP and SLR led to the highest rates of N mineralization in the 1st year. During the first 3 yr after clear-felling, rates of N mineralization increased in SL, L, and LP but decreased in SLR. During the 4th year, rates of N mineralization were low (20-30 kg N.ha-1.yr-1) in all treatments. Over 4 years, 211, 170, 210, and 147 kg N.ha-1 were mineralized in treatments SL, L, LP, and SLR, respectively. Rates of mineralization and leaching were strongly correlated and leaching <30 cm accounted for 75-85% of N mineralized irrespective of treatment. Incorporation of litter by ploughing doubled concentrations of mineral N during the 1st summer after planting and increased early tree growth, but rates of N mineralization in the slash and litter treatments, which were high compared with potential rates of uptake, were weakly correlated with tree growth. Factors controlling N supply were of little consequence for tree growth during this early phase of plantation establishment. -from Authors