CARBON AND PHOSPHORUS TRANSFORMATIONS DURING DECOMPOSITION OF PINE FOREST FLOOR WITH DIFFERENT PHOSPHORUS STATUS

Information on the mineralization of inorganic phosphate (P-i) from or ganically bound P (Po) during decomposition of forest floor and soil o rganic matter is vital for understanding P supply in forest ecosystems . Carbon (C) and phosphorus (P) fluxes were determined for forest floo r samples from three Pinus radiata plots which had received no P (Cont rol), 62.5 kg P ha(-1) (Low P) and 125 kg P ha(-1) (High P) 20 years b efore sampling. The P concentration of the forest floor samples had in creased with fertilizer application: and the C:P ratio ranged between 585 and 1465. During a 9-week laboratory incubation 8.2-19.0% of the f orest floor C was evolved as CO2-C. The amount of CO2 evolved from the forest floor of the Control plot was more than twice the amounts from the Low P and High P plots. There was little change in net P minerali zation in the Control and Low P treatments throughout the incubation, but it increased slightly for the High P samples, suggesting a critica l forest floor C:P ratio of 550 for net P mineralization. Changes in t he P-32-specific activities of the P-i and microbial P pools during in cubation, and concurrent changes in microbial-P-32 and P-32(i), indica ted internal P cycling between these pools. The rate of internal P cyc ling Varied with forest floor quality, and was highest in the High P f orest floor. The High P samples had microbial C:P ratios of 22:1 which remained constant during the incubation, suggesting the microorganism s had adequate P levels.