Latitudinal variation in the availability and use of dissolved organic nitrogen in Atlantic coast salt marshes

Abstract

North American Atlantic salt marshes are generally considered to be nitrogen (N) limited systems, and plants within these marshes were historically thought to use only inorganic forms of N, such as NH4+. Recent research has suggested that Spartina alterniflora may take up some organic nitrogen compounds directly. To determine the availability of dissolved organic nitrogen (DON) to S. alterniflora in marshes along the North American Atlantic coast, porewater was sampled at eight field sites along the North American Atlantic Coast, ranging from Maine (44° N) to Florida (30° N). To determine if a latitudinal gradient existed in the assimilation of either DON or NH4+, replicated mesocosm experiments were conducted with S. alterniflora plants at three field sites (Massachusetts, Virginia, South Carolina), which represented three distinct ecotypes in the latitudinal range. To determine how microbial activity may influence DON availability, we also assessed rates of microbial activity. We found that porewater DON availability increased linearly with latitude. While S. alterniflora assimilated DON at all sites, plants from the southernmost site had 50% greater NH4+ assimilation rates than the other two sites, suggesting that low‐latitude S. alterniflora may rely more strongly on NH4+ as an N source. In contrast, high‐latitude salt marshes contain greater pools of available DON, and high‐latitude S. alterniflora plants exhibit similar uptake rates of DON and NH4+, suggesting a greater reliance upon DON as an N source. We also found that microbial activity decreased by an order of magnitude with increasing latitude along our latitudinal gradient. This suggests that DON uptake is more important at high‐latitude locations, where temperature constrains microbial mineralization of organic N. Our analysis also suggests that N limitation of S. alterniflora increases with decreasing latitude. We suggest that other detritus‐based ecosystems may exhibit similar latitudinal patterns regarding DON availability and use.