The effects of nutrient enrichment on the decomposition of belowground organic matter in a Sagittaria lancifolia - dominated oligohaline marsh

Wetlands improve water quality through sedimentation and the uptake of excess nutrients. As human population increases in the coastal zone, wetlands receive greater nutrient inputs. These additional nutrients may accelerate microbial activity, leading to faster decomposition rates. This decomposition could exceed belowground organic matter production, resulting in a net reduction in soil organic matter accumulation and vertical marsh accretion. The effects of nutrient enrichment on belowground organic matter decomposition in subtropical marshes have received little attention. As such, this research examined the effects of four levels of nitrogen combined with two levels of phosphorus enrichment on belowground decomposition through the use of cotton strip and litter bag assays in a Sagittaria lancifolia dominated marsh in Madisonville, Louisiana. Litter bags contained S. lancifolia root or shoot tissues; roots were of uniform tissue quality while shoots were from unenriched or enriched soils. Soil nitrogen and phosphorus applications both significantly increased belowground decomposition rates of cotton strips. The effect of tissue quality on shoot decomposition was dependent on nitrogen soil enrichment level. At low nitrogen enrichment levels, low quality shoot tissues decomposed more slowly than high quality tissues; this relationship was reversed at high nitrogen soil enrichments. Also, the effect of phosphorus enrichment on shoot decomposition was dependent on the level of nitrogen enrichment. Phosphorus soil enrichment only increased decomposition at the high nitrogen levels. Similarly, phosphorus enrichment combined with moderate nitrogen enrichments raised the decomposition rate of labile root tissue components. However, neither nitrogen nor phosphorus enrichments affected the decomposition rate of recalcitrant root components. Cellulose decomposition was positively correlated with interstitial pH. Shoot decomposition and the recalcitrant root decomposition rate also positively correlated with interstitial pH. This research demonstrated that nitrogen and phosphorus soil enrichments affect the decomposition of roots, shoots, and cotton strips, though in different ways. Variations in the nutrient and carbon quality of the individual tissues, as well as abiotic factors such as pH, modify the effects of soil nutrient enrichments on the decomposition of different tissues in the study marsh