Abstract:

Total nitrogen (N) and organic carbon ? in soil, forest floor and litterfall, and N mineralization rates were compared between pure and mixed Pinus luchuensis plantations on similar soils in the northern Okinawa Island, Japan. Mean accumulations of N and organic C on the forest floor were 133 kg·hm^-22 and 7 199 kg·hm^-2 for the pure pine stand, 105 kg·hm^-2 and 6 143 kg·hm^-2 for the mixed stands. However, in the surface 10 cm mineral soil, the mixed stands had an average of 493 kg·hm^-2 and 5 554 kg·hm^-2 more than the pure pine stand (p<0.001). Amount of N mineralized from soil during 30-day incubation was 18 % greater in the mixed stands than in the pure stand (p<0.01). Average seasonal soil mineralizable N (NH₄⁺+NO₃^-) concentration was 22% greater in the mixed stands than in the pure pine stand (p<0.01). The mixed stands returned an average of 43.7 kg·hm^-2a^-1 (about 70%) and 1 655 kg·hm^-22a^-2 (about 39%) in litterfall more than the pure stand did. The greater litterfall N input in the mixed stands was due to higher concentration in the pine needle litter and the presence of high-N broadleaf litter as well. The results from this study suggest that conifers have different N cycling when mixed with broad-leaved trees from when in pure stand.

Key words: Mixed-species forest, Litterfall, Pinus luchuensis, Nitrogen mineralization, Organic carbon