Investigating the fate of nitrogen from leaf litter into soil, soil organisms, and plants at a tall grass prairie, by the use of 15N enrichment

Background/Question/Methods

Decomposition is an essential process for nutrient recycling. The decomposition of dead plant material is the largest source of nitrogen (N) for plants, and N is the most limiting nutrient for primary productivity in the Northern Hemisphere. The general fate of N during decomposition is well understood in a broad sense, due to extensive studies of litter mass loss and litter N dynamics. However, the relative contribution of litter N to a) mineral N available for plants and b) immobilized N in microbial biomass and further stabilized in soil organic matter pools remains unknown. This understanding is important in the context of reactive N dynamics and global change. Additionally, the effect of soil fauna on the fate of N is not well understood, and has historically been overlooked in studies of litter decomposition. We present here results on litter-N dynamics from an ongoing study conducted in the context of a larger NSF-DEB funded experiment. Isotopically labeled (e.g., ¹³C and ¹⁵N enriched) big bluestem (Andropogon gerardii) litter, the dominant native species at the site, was incubate in October of 2010 inside raised collars, at the Konza tall-grass prairie, located  in Kansas. A fauna suppression treatment (naphthalene) was applied to half of the collars. Litter and soils were collected in May and October of 2011. Soils were separated by depths (0-2, 2-5, 5-10, and 10-20 cm) and soil organic matter separated by size and density (e.g., LF, free-OM, mineral associated-OM), and analyzed for N and ¹⁵N.

Results/Conclusions

The amount of litter-derived N entering the soil was not influenced by soil fauna; however, there were significantly more exogenous N incorporation in the litter of the suppressed fauna treatment, indicating the importance of fauna to litter N dynamics. Additionally, there were significantly higher amounts of N in the LF and silt associated-OM, in both the 0-2 and 2-5 cm depths, and in the clay associated-OM, in the 0-2 cm depth, in the fauna suppression treatment as compared to the control. Finally, the amount of N recovered in the first 20 cm of soil did not account for the total litter-derived N release , suggesting additional N sinks (e.g. plant matter, and deep soil), which will be the focus of future research. Our study demonstrates that a large fraction (approximately 40%) of N lost through decomposition is stored in the mineral soil and emphasizes the importance of studying soil fauna as a modifier of decay dynamics.