Biogeochemical and ecosystem properties in three adjacent semi‐arid grasslands are resistant to nitrogen deposition but sensitive to edaphic variability

Abstract

1. Drylands have low nitrogen stocks and are predicted to be sensitive to modest increases in reactive nitrogen availability, but direct evidence that atmospheric nitrogen deposition will have sustained effects on dryland ecosystems is sparse and conflicting.2. We used three long-running in situ nitrogen deposition simulation experiments and a complementary laboratory incubation experiment to address fundamental questions about how nitrogen inputs affect drylands: (1) What are the long-and short-term consequences of nitrogen inputs for biogeochemical and ecosystem properties?; (2) Do these consequences depend on soil moisture availability?; and (3) Does soil texture modify the effects of nitrogen inputs and/or soil moisture availability? 3. In 2011, we established three study sites along a soil texture gradient in Arches National Park with plots receiving 0, 2, 5 or 8 kg N ha −1 annually (n = 5 per treatment per site). We assessed a suite of biogeochemical metrics over the long term and short term. To assess longer term effects, we sampled annually (2013)(2014)(2015)(2016)(2017)(2018)(2019), just prior to spring nitrogen fertilization. To assess short-term effects, we sampled immediately before and after spring nitrogen fertilization in 2013.Additionally, we compared foliar chemistry, soil extracellular enzyme activities, heterotrophic respiration rates, and nitrogen trace gas fluxes at select intervals during the study period (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019). Finally, we conducted a laboratory incubation to measure the individual and interacting effects of soil moisture and nitrogen additions on soil microbial activity. 4. We identified some short-term effects in situ, but no lasting consequences of added nitrogen for any of the metrics measured. In the incubation, soil moisture treatments independently increased heterotrophic respiration rates but did not modify the effects of added nitrogen. In contrast to nitrogen treatments, soil texture was associated with large differences in biogeochemical cycling.

Long-term fertilization increases soil but not plant or microbial N in a Chihuahuan Desert Grassland

“…Because the N added each year is in an inorganic form (NH4NO3), this increase in organic or total N over time suggests that the N had been converted to an organic form (through uptake and assimilation of N into microbial and plant biomass, as observed by Harrison et al (2007)), and then deposited into the soil. This is supported by short-term studies which have reported only increases in plant-N, but no increases in soil N after five (Zeng et al, 2010) or eight (Osborne et al 2022) years of N addition, although other experiments with similar amounts of N added for six years were sufficient to increase total soil N content (Sha et al, 2021). Thus, over the duration of the experiment there were at least some years where there was higher plant or microbial biomass produced in the fertilized plots, such as when production increases during high precipitation years (Hall et al, 2011;Ladwig et al, 2012).…”

Restoration research actions to address rapid change in drylands: insights from the Colorado Plateau

“…Current ecological states dominated by invasive species, which are often legacies of global change drivers such as grazing, are now experiencing co‐occurring global change drivers (e.g. nitrogen deposition; Osborne et al 2022b and aridification; Phillips et al 2022). This degree of novelty leaves managers with limited information about potential future ecological transitions.…”

Bacterial denitrification drives elevated N 2 O emissions in arid southern California drylands

“…Nitrogen concentrations in the wetting solutions corresponded to a range in annual N deposition rates observed in Southern California drylands, so that each shrub received a different amount of N: 0, 10, 20, 30, 40, 50, 60, or 70 kg of N ha −1 (8,57,58). While these N addition amounts increased soil inorganic N in the top 10 cm of the soil by between ~1.5 and ~4 times, lower N addition amounts (between 2 and 15 kg of N ha −1 ) have not stimulated N trace gas emissions in other desert soils (7,8,59). Thus, we used higher N amounts to maximize our ability to predict changes in N emissions from soil N availability.…”