Plant community mediated vs. nutritional controls on litter decomposition rates in grasslands

Grasslands are one of the major biomes on earth and can serve as important soil carbon sinks. Nutrient enrichment of these grasslands can have a significant impact on carbon losses through the decomposition process. We investigated the effects of long-term (12-yr) experimentally increased N and/or P supply on litter production and on the chemistry and decomposition of bulk litter from two grasslands differing in soil nutrient status. Potential aboveground litter production in the controls of a P-rich low-productivity riparian grassland was lower than that in a N-rich high-productivity peat grassland and increased with enhanced N supply. Nutrient treatments did not enhance litter production in the high-productivity peat grassland. The concentrations of phenolics in bulk litter from the peat grassland, dominated by sedges, were 2-3 times higher than those in the riparian grassland that was dominated by herbs and grasses. At both sites increased nutrient supply had no detectable effect on phenolics concentrations. All P-related litter chemistry parameters reflected the higher soil P status of the riparian grassland. P fertilization had a greater effect on litter chemistry in the P-deficient peat grassland than in the P-rich riparian grassland. At both sites, there was no change in overall litter chemistry in response to N fertilization, except for higher lignin concentrations in the peat grassland litter. Both short-term (8 wk) litter incubations in the laboratory and a 3-yr litter bag study in the field showed that riparian grassland litter decomposed faster than litter from the peat grassland. The long-term nutrient additions had no significant effects on the decomposition of the bulk litter of each grassland type. Regression analysis on the combined data of the two sites showed that phenolics, and to a lesser extent, P-related litter chemistry parameters, exerted a strong control on both litter respiration and litter mass loss. Our study shows that long-term experimental nutrient additions do not lead to increased decomposition rates in grasslands and that the initial plant community litter quality is the main determinant of carbon losses through the decomposition process. The results of this study suggest that nutrient enrichment will likely affect ecosystem carbon balance more by affecting litter production than by affecting litter decomposition rates.