Predicting Nutrient Incontinence in the Anthropocene at Watershed Scales

Abstract

Quantifying nutrient attenuation at watershed scales requires long-term water chemistry data, water discharge, and detailed nutrient input chronicles. Consequently, nutrient attenuation estimates are largely limited to long-term research areas or modeling studies, constraining understanding of the ecological characteristics controlling nutrient attenuation and complicating efforts to protect or restore water quality in developed and developing regions. Here, we combined long-term data and a broad suite of biogeochemical parameters from 49 watersheds in northwestern France to test how well instantaneous measurements can predict nitrogen (N) and phosphorus (P) attenuation at watershed scales. We evaluated 13 biogeochemical and 12 hydrological proxies of hydrological flowpaths, residence time, and biogeochemical transformation. Across the 49 watersheds, nutrient attenuation ranged from 88 to −2% for N and 99-96% for P. The strongest biogeochemical proxies of N attenuation were NO − 3 isotopes, rare earth elements (REEs), radon, and turbidity, together explaining 75% of observed variation. For P attenuation, REEs, NO − 3 isotopes, molecular weight of dissolved organic matter, and radon were the strongest proxies, but only explained 27% of observed variation. However, a single hydrological parameter-annual runoff-explained 91% of N attenuation and the relative abundance of schist bedrock explained 56% of P attenuation. We discuss how runoff both controls and reflects watershed hydrology, biogeochemistry, and nutrient attenuation. For example, runoff was correlated with long-term decreases in nutrient concentration, demonstrating how leakier watersheds recover more quickly from nutrient saturation. Given the immense fertilization capacity of modern society, we propose that eutrophication can only be solved by reducing nutrient inputs, though hydrochemical proxies can provide valuable information on where to carry out essential food production activities.

Stream Dissolved Organic Matter in Permafrost Regions Shows Surprising Compositional Similarities but Negative Priming and Nutrient Effects

“…These feedbacks (Figure 11) create spatially and temporally dynamic relationships between DOM composition and expressed reactivity (Catalán et al, 2016; Helton et al, 2015; Wymore et al, 2015). Consequently, it is the interaction between inherent DOM composition and ecosystem conditions that determines the relative importance of reaction rates and exposure times for a particular compound (Abbott, Baranov, et al, 2016; Frei et al, 2020; Kolbe et al, 2019; Oldham et al, 2013). For example, in our dark incubations, the effect of added nutrients or BDOC depended on the decomposability of the background DOM (Figure 8), potentially due to kinetic and nutrient constraints (Guenet et al, 2010; Wymore et al, 2015).…”

“…Rates of DOM mineralization depend on the intrinsic properties of the DOM such as chemical composition as well as external conditions such as temperature, microbial community structure, and interactions with other elements (Abbott, Baranov, et al, 2016; Arnosti, 2004; Frei et al, 2020; Marín‐Spiotta et al, 2014; Nalven et al, 2020; Zarnetske et al, 2011). Even DOM that has low inherent reactivity because of its source or prior processing may undergo further mineralization and transformation when mixed with BDOC or inorganic nutrients (Bianchi, 2012; Guenet et al, 2010; Kuzyakov et al, 2000; Mutschlecner et al, 2018; Rosemond et al, 2015).…”

“…While priming and nutrient effects have been considered independently, even in the few studies where both were measured (Guenet et al, 2014; Hotchkiss et al, 2014; Jenkinson et al, 1985), they are likely functionally linked, at least in oligotrophic and mesotrophic ecosystems (Figure 1). In the absence of external nutrient inputs from humans or other sources (Brahney et al, 2014; Frei et al, 2020), DOM mineralization is the proximate source of nutrients in terrestrial and aquatic ecosystems (Fork et al, 2020; McDowell et al, 2006; Mutschlecner et al, 2017), meaning that increased DOM biodegradability may increase nutrient availability. In turn, nutrient‐rich environments support higher rates of primary productivity, root exudation, and production of more decomposable organic matter (Carey et al, 2019; Guenet et al, 2010; Hewitt et al, 2018; Mutschlecner et al, 2017; Salmon et al, 2018), increasing BDOC availability (Figure 1).…”

Tundra wildfire triggers sustained lateral nutrient loss in Alaskan Arctic

“…One approach to quantify catchment-scale nutrient balance is to use streams as sensors of ecosystem processes (Brookshire et al, 2009;Carey et al, 2019;Shogren et al, 2019;Zarnetske et al, 2018). Stream nutrient concentrations and fluxes can indicate changes in terrestrial nutrient sources and sinks, providing a metric that integrates nutrient demand and uptake efficiency of terrestrial plants and microorganisms (Abbott et al, 2018;Frei et al, 2020;Wymore et al, 2019).…”

Synergistic Impacts of Rainfall Variability and Land Use Heterogeneity on Nitrate Retention in River Networks: Exacerbation or Compensation?

“…To improve the estimates of nitrate retention under different land use planning scenarios, we need to obtain a better understanding of nitrate retention during different seasons or under different hydrological conditions. This calls for field experiments on nitrate retention processes in rivers during different times of the year under different levels of flow variability (Frei et al, 2020; O'Donnell & Hotchkiss, 2020). In spite of using a coupled hydrological‐biogeochemical model to study synergistic impacts of temporal variability of precipitation under potential future climate changes and heterogeneous spatial land use changes using data in an actual catchment, this remains a theoretical study and is not meant to study the effects of climate change per se.…”