High Daily and Year-Round Variability in Denitrification and Nitrogen Fixation in a Northern Temperate River

Abstract

Rates of nitrogen (N) cycling processes like denitrification and dinitrogen (N2) fixation, which together are the primary contributors to N2 flux rates from surface waters, may change at different time scales from seasons to weeks to days. Yet, we know little about the magnitude, mechanisms or drivers of these temporal changes, especially at shorter daily and weekly timescales. Quantifying variation in rates and drivers across temporal scales is essential to understand how nutrient cycling processes operate in aquatic ecosystems and predict how they may respond to shifting seasonal dynamics caused by global change (i.e., earlier snowmelt and extreme weather events). This study quantified denitrification and N2 fixation rates seasonally and daily in a northern temperate river, and explored how environmental conditions such as discharge, light, and N and phosphorus (P) concentrations were related to that variation at different time scales. We measured denitrification and N2 fixation rates on biweekly and daily intervals at a single 20-m long sampling reach in the Pilgrim River in Michigan's Upper Peninsula from May 2017 through May 2019. We found high rates of daily change (difference in rate from one day to the next) for both processes in all seasons (maximum daily change 5,690 μg N/m2/h for denitrification and 38 μg N/m2/h for N2 fixation). No detectable differences in rates among seasons were detected using Multiple Response Permutation Procedure (MRPP). Day-to-day variation did not change before and after elevated discharge events, including a 1,000-year flood that occurred in June 2018. Partial least squares (PLS) regression identified total dissolved N, dissolved organic N, and ammonium as important predictors of denitrification and N2 fixation, but explained only 15–28% of the variation in all measured rates. The unexpectedly high daily variation and lack of seasonal difference in rates found in this study demonstrate the need to use caution when studying these processes and/or extrapolating rates across time scales, as discrete and infrequent measurements may be misleading.

Comparing 15N2 uptake and acetylene reduction methods for measuring stream N2-fixation: release of N2 fixed

“…C2H4 production allows for estimates of gross nitrogenase activity, and examinations of relative rates of N2-fixation in an experiment (Arango et al 2009). However, in many cases it is imperative to have accurate estimates of N2-fixation e.g., if comparing N2-fixation to other N fluxes (Kunza andHall 2014, Nevorski andMarcarelli 2022) or when scaling to larger areas (Grobkopf et al 2012, Mullholand et al 2019. When accurate estimates are needed, 15 N2 uptake, or at a minimum, well-calibrated C2H4:N2 ratios are required (Soper et al 2021), and paying attention to the potential for temperature to affect this calibration ratio.…”

“…The flux of new nitrogen (N) entering ecosystems via N2-fixation can greatly influence N cycling (Newell et al 2016, Marcarelli et al 2021) and vary temporally (Nevorski and Marcarelli 2022). For instance, as much as 82% of total N input may derive from N2-fixation in some aquatic ecosystems (Howarth et al 1988).…”

“…Despite the range of mechanisms that may control the fate of fixed N and thus the C2H4:N2 ratio, most stream N2-fixation studies assume a fixed 3:1 ratio (Grimm and Petrone 1997, Horne 1975, Marcarelli and Wurtsbaugh 2006, Eberhard et al 2018, Nevorski and Marcarelli 2022. The large deviations in recorded C2H4:N2 ratios for other ecosystems (Seitzinger and Garber 1987, Liengen 1999, Saiz et al 2021 suggest the need for using 15 N2 to both calibrate the C2H2 reduction method in streams and examine the immediate fate of this fixed N. Our goals for this study were to 1) measure N2fixation using a combination of the C2H2 reduction method and 15 N2 uptake methods to examine the C2H4:N2 ratio, 2) estimate potential controls on variation in the C2H4:N2 ratio, and 3) to assess the immediate fate of 15 N2 fixed by examining the net release of 15 N2 fixed as total dissolved N (TDN) and NH4 + .…”