Experimentally derived nitrogen critical loads for northern Great Plains vegetation

Abstract

The critical load concept facilitates communication between scientists and policy makers and land managers by translating the complex effects of air pollution on ecosystems into unambiguous numbers that can be used to inform air quality targets. Anthropogenic atmospheric nitrogen (N) deposition adversely affects a variety of ecosystems, but the information used to derive critical loads for North American ecosystems is sparse and often based on experiments investigating N loads substantially higher than current or expected atmospheric deposition. In a 4‐yr field experiment in the northern Great Plains (NGP) of North America, where current N deposition levels range from ~3 to 9 kg N·ha−1·yr−1, we added 12 levels of N, from 2.5 to 100 kg N·ha−1·yr−1, to three sites spanning a range of soil fertility and productivity. Our results suggest a conservative critical load of 4–6 kg N·ha−1·yr−1 for the most sensitive vegetation type we investigated, badlands sparse vegetation, a community that supports plant species adapted to low fertility conditions, where N addition at this rate increased productivity and litter load. In contrast, for the two more productive vegetation types characteristic of most NGP grasslands, a critical load of 6–10 kg N·ha−1·yr−1 was identified. Here, N addition at this level altered plant tissue chemistry and increased nonnative species. These critical loads are below the currently suggested range of 10–25 kg N·ha−1·yr−1 for NGP vegetation and within the range of current or near‐future deposition, suggesting that N deposition may already be inducing fundamental changes in NGP ecosystems.

How big is big enough? Surprising responses of a semiarid grassland to increasing deluge size

“…We used potable local water (McDonald Farms Enterprises, Frederick, CO) that met US EPA drinking water standards. Therefore, added water was not a significant source of nitrogen relative to typical annual atmospheric inputs (Burke et al, 2002, 2008) and well below nitrogen critical load estimates for regional grasslands (Symstad et al, 2019).…”

The importance of extreme rainfall events and their timing in a semi‐arid grassland

“…Water was applied using a hand‐held watering wand attached to a flow meter and pump to deliver potable water from a local water delivery service (McDonald Farms Enterprises). Added water met US EPA drinking water standards and thus was not a significant source of nitrogen compared to annual atmospheric inputs (Burke et al., 2002, 2008) and was well below nitrogen critical load estimates for regional grasslands (Symstad, Smith, Newton, & Knapp, 2019).…”

“…Added water met US EPA drinking water standards and thus was not a significant source of nitrogen compared to annual atmospheric inputs (Burke et al, 2002(Burke et al, , 2008 and was well below nitrogen critical load estimates for regional grasslands (Symstad, Smith, Newton, & Knapp, 2019).…”

Managing invasive plants on Great Plains grasslands: A discussion of current challenges

“…Invasive species may be better adapted than native species to take advantage of these supplements ( Dukes and Mooney 1999 ). A CO 2 enrichment experiment in a greenhouse showed that five species of invasive plants in the Great Plains followed this pattern ( Ziska 2003 ), and a field experiment in northern mixed-grass prairie suggests that cheatgrass and field brome respond positively to levels of nitrogen enrichment expected from atmospheric deposition ( Symstad et al 2019 ). A special report of IPCC (Intergovernmental Panel on Climate Change) working group for North America highlighted the facilitative effect of elevated CO 2 and regional climate change on the invasion of woody C 3 species into C 4 grasslands ( Watson et al 1998 ).…”