Nitrogen and Carbon Mineralization from Green and Senesced Leaf Litter Differ between Cycad and Angiosperm Trees.

Simple Summary: The addition of plant leaf litter to soils represents a critical component of the global carbon (C) and nitrogen (N) cycles. Tropical cyclones known at typhoons in the western Pacific islands transfer an enormous amount of green leaf litter from the forest canopy to the soil. We have shown on the island of Guam that this green litter releases carbon and nitrogen extremely rapidly compared with senesced leaf litter. Moreover, species such as Guam's native cycad Cycas micronesica produce slowly decomposing litter, while other species such as Guam's native angiosperm Morinda citrifolia produce rapidly decomposing litter. Soil priming is an important process in the global C cycle, and occurs when pre-existing organic matter releases more C as a result of new organic C additions. The green litter generated soil priming that was almost double that of the senesced litter. The release of N from the green litter into forms that are available to plants required only 14 d for the M. citrifolia litter but required more than 90 d for the C. micronesica litter. Scientists predict more frequent intense typhoons in the future, and we have shown one of the ecosystem-level processes that these typhoons will change. Plant leaf litter decomposition is directly influenced by the identity of the source plants and the leaf age. Defoliation of forests by tropical cyclones (TC) transfers copious amounts of high-quality green leaf litter to soils. We used a soil amendment approach with the incubated buried bag method to compare carbon (C) and nitrogen (N) mineralization dynamics of green and senesced leaf litter from cycad Cycas micronesica and angiosperm Morinda citrifolia trees on the island of Guam. Soil priming increased the decomposition of pre-existing organic C, and were greater for green leaf litter additions than senesced leaf litter additions. Available N content increased by day 14 and remained elevated for the entire 117-d incubation for soils amended with green M. citrifolia litter. In contrast, available N content increased above those in control soils by day 90 and above those in soils amended with senesced litter by day 117 for green C. micronesica litter. The net N mineralization rate was higher than control soils by 120% for the senesced litter treatments and 420% for the green litter treatments. The results reveal a complex but predictable interplay between TC defoliation and litter quality as defined by tree identity. We have illuminated one means by which increased frequency of intense TCs due to climate change may alter the global C and N cycles. [ABSTRACT FROM AUTHOR]