Citation:

Hill, T., N. Sommer, C. Kanaskie, E. Santos, AND A. Oczkowski. Nitrogen uptake and allocation estimates for Spartina alterniflora and Distichlis spicata. JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY. Elsevier Science Ltd, New York, NY, 507:53-60, (2018). https://doi.org/10.1016/j.jembe.2018.07.006

Impact/Purpose:

Coastal wetlands contribute to local water quality by intercepting and storing nitrogen that would otherwise affect coastal waters. Plants play an especially important role, serving as nutrient sponges during the growing season, when they require large amounts of nitrogen to grow. Because natural N pools are large, nitrogen uptake and movement between plant parts can be difficult to trace, leading to substantial uncertainty in our understanding of nitrogen uptake by wetland plants. To address this uncertainty, we used a heavy isotope of nitrogen to measure rates of nitrogen uptake and allocation to aboveground and belowground plant tissues. This study was conducted using two dominant salt marsh plants, Spartina alterniflora and Distichlis spicata, and we repeated the study over a range of time periods to understand how conclusions about nitrogen uptake rates can change depending on the duration of the experiment. As it turns out, experiment duration had a larger effect on estimates of nitrogen uptake than other more commonly-measured processes, like net primary production or stem growth rates. Uptake rates varied by a factor of three depending on experiment duration, although each experiment shared the qualitative conclusion that Distichlis scavenged nitrogen approximately twice as rapidly as Spartina. This work offers insights into the applications of 15-N, a critical tool for measurement of nitrogen cycling in the environment. Our research is also of direct interest to the management community, suggesting that changes in wetland plant communities lead to substantial differences in the provision of ecosystem services such as nitrogen uptake.

Description:

Salt marshes have the potential to intercept nitrogen that could otherwise impact coastal water quality. Salt marsh plants play a central role in nutrient interception by retaining N in above- and belowground tissues. We examine N uptake and allocation in two dominant salt marsh plants, short-form Spartina alterniflora and Distichlis spicata. Nitrogen uptake was measured using 15N tracer experiments conducted over a four-week period, supplemented with stem-level growth rates, primary production, and microbial denitrification assays. By varying experiment duration, we identify the importance of a rarely-measured aspect of experimental design in 15N tracer studies. Experiment duration had a greater impact on quantitative N uptake estimates than primary production or stem-level relative growth rates. Rapid initial scavenging of added 15N caused apparent nitrogen uptake rates to decline by a factor of two as experiment duration increased from one week to one month, although each experiment shared the qualitative conclusion that Distichlis roots scavenged N approximately twice as rapidly as Spartina. We estimate total N uptake into above- and belowground tissues as 154 and 277 mg N·m-2·d-1 for Spartina and Distichlis, respectively. Driving this pattern were higher N content in Distichlis leaves and belowground tissue and strong differences in primary production; Spartina and Distichlis produced 8.8 and 14.7 g biomass·m-2·d-1. Denitrification potentials were similar in sediment associated with both species, but the strong species-specific difference in N uptake suggests that Distichlis-dominated marshes are likely to intercept more N from coastal waters than are short-form Spartina marshes. The data and source code for this manuscript are available as an R package from https://github.com/troyhill/NitrogenUptake2016.

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