The mechanisms behind reduced NH and NO accumulation due to litter decomposition in the acidic soil of subtropical forest.

Background and purpose: To date, very few studies have investigated the effects of litter decomposition on gross nitrogen (N) transformations and N retention in subtropical forest soils. In this study, six forest soils were taken from two contrasting forest types (broad-leaved vs. coniferous) in the humid subtropical zones of China to investigate the effects of litter decomposition on gross N transformations. Methods: A N tracing experiment was conducted to investigate the effects of litter decomposition (at a rate of 0.5 % w/w) on soil gross N transformations rates in broad-leaved and coniferous forests. Results: Litter decomposition enhanced gross N mineralization and NH immobilization rates by 45.9-79.0 % and 553-563 %, respectively. There was a shift from net N mineralization to net N immobilization in each of the forest soils, which was due to litter decomposition. In contrast, gross nitrification rates were unaffected by litter decomposition for each of the forest soils, while gross NO immobilization rates were stimulated significantly by litter decomposition in broad-leaved forest soils but not in coniferous forest soils. On average, 39.4 % of the total NO produced was immobilized into organic-N in broad-leaved forest soils due to litter decomposition. Therefore, the decrease in net nitrification rates caused by litter decomposition was attributed to increasing gross NO immobilization rates rather than decreasing gross nitrification rates. During 15 days of incubation, the decomposition of litter increased cumulative NO emissions by 55-76 % and 100-640 % in broad-leaved forest and coniferous forest soils, respectively, but there was no effect on the cumulative NO emissions for both forest soil types. Conclusions: The combined results suggest that the decomposition of litter stimulates NO emissions but reduces NH and NO accumulation in forest soils, and thus a delay in N availability for plant N uptake or leaching. [ABSTRACT FROM AUTHOR]