MICROBIAL BIOMASS C AND N, AND MINERALIZABLE-N, IN LITTER AND MINERAL SOIL UNDER PINUS-RADIATA ON A COASTAL SAND - INFLUENCE OF STAND AGE AND HARVEST MANAGEMENT

Microbial biomass C and N, and anaerobically mineralizable-N, were measured in the litter and mineral soil (0-10 cm and 10-20 cm depth) of Pinus radiata plantations in two trials on a nitrogen-deficient coastal sand. The trials comprised (a) stands of different age (1 to 33 years), with five of the seven stands studied being second rotation, and (b) a harvest-management trial, with stands established after different harvesting treatments of the first-rotation trees and understorey development controlled by manual weeding and chemical sprays. The harvest-management stands were sampled in the fifth year after the second-rotation establishment. In the stands of different age, the levels of microbial biomass C and N, and also mineralizable-N, in the litter and mineral soil showed no relationship with tree age and were similar to those in the oldest (33 years) stands of P. radiata. In the harvesting trial, five years after establishment of the second rotation, levels of microbial N and mineralizable-N in the litter and mineral soil were generally lowest where whole trees and the original forest floor had been removed; they were higher in associated plots in which the original forest floor had been removed but fertilizer N was regularly applied. No marked differences were then found between the other harvest treatments, viz. whole-tree harvest, stem-only harvest with slash remaining on site, and stem-only harvest plus extra added slash materials. In each trial, levels of microbial C and N and mineralizable-N were closely related to total C. and especially total N, in 0-10 cm depth mineral soil, but not generally in litter. Respiratory measurements strongly suggest that the microbial populations in mineral soil had a high metabolic activity. On an area basis in the harvest-management trial, total tree N and microbial N in the litter and mineral soil were lowest in stands where the original forest floor had been removed. In this particular treatment, microbial N in the litter plus mineral soil (0-20 cm depth) after five years of second-rotation growth comprised 7.3% of the total ecosystem N; values in the other treatments ranged between 5.6 and 6.0%. Our results emphasise the importance of slash and litter, and probably volunteer shrubs and herbaceous understorey species, in conserving pools of potentially available N during the early stages of tree development.