Salt marsh fertilization increases plant decomposition rate and the abundance of a detritivorous snail

Background/Question/Methods

Salt marshes provide important ecosystem services that may be impacted by anthropogenic activity. Because they are nitrogen-limited, salt marshes readily take up nitrogen from agricultural and land developmental runoff. The salt marsh grass Spartina alterniflora grows in natural monocultures and responds to such inputs via increased biomass and eventually increased litter quality. Long-term loading (a nitrogen press) and short-term inputs (a nitrogen pulse) have the potential to alter salt marsh ecosystem function and food web dynamics. Specifically, decomposition rate and associated detritivore abundance may be altered through changes in S. alterniflora biomass, litter quality, external nitrogen environment, and microbial activity. To test how nitrogen inputs at varying temporal scales (presses and pulses) affect decomposition and detritivore abundance, plots within a salt marsh were experimentally fertilized for two years. Press plots were fertilized both years, pulse plots were fertilized either during the first or second year, and control plots were not fertilized. We conducted a litterbag decomposition experiment in which bags of high quality and of low quality S. alterniflora were placed in each fertilization regime. Additionally, we measured the abundance of the detritivorous salt marsh snail Melampus bidentatus in each plot.

Results/Conclusions

For both low and high quality S. alterniflora litter, decomposition rates significantly increased in press plots, and moderately increased in both pulse plots, compared to the control. High quality litter decomposed significantly faster than low quality litter under each fertilization regime. Both elevated litter quality and nitrogen levels in the environment therefore increased decomposition rate. M. bidentatus abundance was significantly higher in press plots, and moderately higher in both pulse plots, compared to the control. These results suggest that this detritivorous snail responds positively to qualities associated with elevated nitrogen inputs, such as increased shade that results from elevated S. alterniflora biomass, and increased detritus quality. Since M. bidentatus not only shreds detritus but also stimulates fungal colonization with its feces, it is possible that increased snail abundance and a subsequent increase in fungal biomass will further accelerate decomposition rates within salt marshes that receive elevated nitrogen inputs. Thus, multiple channels are responsible for the different rates of decomposition in plots fertilized for one year versus two years. The complexity of these channels demonstrates the importance of conducting long-term experiments to examine the effects of nitrogen inputs on decomposition.