Direct and indirect effects of nitrogen inputs on the decomposition rate of the saltmarsh grass Spartina alternifora
Coastal marshlands are an important global carbon sink. However, nitrogen inputs from urban and agricultural runoff can turn these carbon sinks into a carbon source by directly and indirectly affecting the rate of decomposition of the coastal marshland grasses. With a high rate of decomposition, detritus does not have time to be buried in the anoxic conditions of the peat soils before decomposition releases the carbon in the grass into the atmosphere. Past experiments have found that nitrogen inputs increase decomposition of Spartina alterniflora, one of the dominant grasses found in Eastern coastal marshlands. Specifically, higher quality plant litter decomposes faster, and both current and past levels of fertilization can alter rates of decomposition. However the mechanisms responsible for these results are uncertain. We therefore examined the factors that directly and indirectly affect decomposition under high nitrogen conditions. Nitrogen fertilization increases plant height, the quality of dead material (thatch) and the amount of thatch. Plant height and thatch biomass could alter the microclimate while elemental composition of the thatch (thatch quality) could influence biotic and abiotic losses. We therefore designed a factorial experiment to examine the influence of these three factors, thatch quality, thatch quantity and plant height, on decomposition.
Our results indicate that two mechanisms increase the rate of decomposition under high nutrient conditions: increased plant height in fertilized plots, and the quantity of surrounding litter. Litter in the fertilized plot decomposed faster than litter in the control plot. This suggests that taller vegetation resulting from high nutrient conditions may create a more favorable microclimate for decomposition. Decomposition rate also increased with an increase in the quantity of surrounding litter, but this effect was weaker than the effect of grass height. Litter quantity could increase decomposition by increasing the contact between the litter and decomposers in the soil. The quality of the surrounding litter did not have a significant effect on decomposition. These results indicate that the primary mechanism by which nutrient inputs increase the decomposition of the aboveground vegetation is through the height of the surrounding vegetation. Furthermore, there may be a negative feedback loop in the salt marsh that limits the accumulation of thatch in nitrogen-enriched systems because as thatch builds up, decomposition rate increases. Thus, an increased rate of decomposition under nutrient-enriched conditions may limit carbon accumulation and storage in salt marsh ecosystems.