LINX I and II: Lessons Learned and Emerging Questions

Abstract

The Lotic Intersite Nitrogen eXperiments (LINX I and II) were a series of replicated in situ manipulations of 15 N across biomes and land-uses designed to assess the factors that control the removal, retention, and ultimate fate of inorganic nitrogen in stream ecosystems. By studying streams at the continental scale, the lessons learned provide some of the best data available to understand the functional role of streams across the landscape, the management implications of nitrogen uptake in streams and rivers, and the value of small streams in elucidating fundamental principles of ecosystem science. Because small streams are characterized by high throughput and low standing stocks of primary producers and stored carbon and nutrients, they provide unique opportunities to assess the fundamental drivers of nitrogen cycling that would be difficult to conduct in other ecosystems. In addition to the water quality implications of understanding controls on nitrogen delivery to downstream systems, LINX I and II also provide unique insights for ecosystem science and stream ecology more broadly. Here we review a series of ecosystem and network-scale lessons that can be inferred from LINX I and II. We then propose three emergent research questions motivated by the LINX projects which are amenable to future continental-scale work. LINX I and II also demonstrate the value of a highly collaborative, distributed approach to ecosystem science that requires each member of the team to conduct a standardized protocol while simultaneously encouraging team member to improve protocols and develop cross-site projects in his or her specific area of expertise.

Gradients of Anthropogenic Nutrient Enrichment Alter N Composition and DOM Stoichiometry in Freshwater Ecosystems

“…We develop a set of testable hypotheses that describe interactions between the N and C cycles and patterns in stream chemistry and biogeochemical processes associated with the observed changes in the availability of DON and the stoichiometry of DOM. Systems with low DOC:DON ratios may be hot‐spots of nitrification, not because C is necessarily limiting (Strauss et al., 2002), but rather because DON provides sufficient N to reduce competition for NH 4 + between heterotrophs and nitrifiers (Wymore et al., 2019). Availability of DON could also reduce rates of assimilatory demand for NO 3 − in systems where the ion is found in low abundance, resulting in a positive effect on rates of denitrification by increasing substrate availability.…”

“…Biotic uptake of some forms of DON can be rapid (Brailsford et al., 2019a; Brookshire et al., 2005; Mackay et al., 2020), and both the concentration and the stoichiometry of dissolved organic matter (DOM; the molar ratio of DOC:DON) can be highly responsive to changes in inorganic N concentrations (Lutz et al., 2011; Wymore et al., 2015; Yates et al., 2019). Changes in the composition of both TDN and DOM along gradients of N availability will have implications for how C and N interact in freshwater ecosystems, influencing biogeochemical processes and rates of nutrient uptake by aquatic microbial communities (Brailsford et al., 2019b; Del Giorgio & Cole, 1998; Wymore et al., 2019).…”

Shifting stoichiometry: Long‐term trends in stream‐dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition

“…N‐enriched DOM may provide additional nutrients to microbial communities making more NH 4 + available through mineralization. In turn, competition for dissolved inorganic N may decline with higher rates of nitrification and increased NO 3 − production and export (Wymore et al, 2019 ), while increasing DON concentrations instream may provide an alternative nutrient resource for uptake by the primary producers (Mackay et al, 2020 ). In contrast, streams with increasing DOC:DON ratios may reflect increasing watershed N demand from greater retention in soils and increasing vegetative growth, possibly from CO 2 enrichment (Craine et al, 2018 ; Groffman et al, 2018 ; Huang et al, 2015 ).…”

“…A broad assessment of how DOM stoichiometry changes concurrently with changes in concentrations of DOC and DON could provide insights into how the energy and nutrient balance of one of the larger pools of organic matter in freshwater ecosystems is changing with potential impacts on other biogeochemical reactions (e.g., Strauss & Lamberti, 2002;Wymore et al, 2019).…”

“…DOC:DON ratios provide a relatively simple quantification of bulk DOM characteristics, which serves as an indicator of bioavailability (del Giorgio & Cole, 1998 ) and of changing OM sources within catchments (Yates et al, 2019 ). A broad assessment of how DOM stoichiometry changes concurrently with changes in concentrations of DOC and DON could provide insights into how the energy and nutrient balance of one of the larger pools of organic matter in freshwater ecosystems is changing with potential impacts on other biogeochemical reactions (e.g., Strauss & Lamberti, 2002 ; Wymore et al, 2019 ).…”