Low retention of added nitrogen at realistic levels of simulated nitrogen deposition in the Northern Chihuahuan Desert

Authors: Holguin, Jennifer; MCLAREN, JENNIE - The University Of Texas-Pan American; DARROUZET-NARDI, ANTHONY - The University Of Texas-Pan American

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2025
Publication Date: 9/23/2025
Citation: Holguin, J., McLaren, J., Darrouzet-Nardi, A. 2025. Low retention of added nitrogen at realistic levels of N deposition in the Northern Chihuahuan Desert. Soil Biology and Biochemistry. 211:109977. https://doi.org/10.1016/j.soilbio.2025.109977.
DOI: https://doi.org/10.1016/j.soilbio.2025.109977

Interpretive Summary: Anthropogenic nitrogen (N) deposition is one of the most prominent factors driving environmental change. Aridlands are expected to be sensitive to even low levels of N deposition. However, most studies apply N at moderate to high levels (>20 kg N ha -1 yr-1). Thus, limiting our understanding of the effects at lower, yet possibly ecologically relevant, N deposition levels. Here, we performed a comprehensive assessment of realistic levels of N enrichment in a northern Chihuahuan Desert grassland experiencing N deposition associated with oil and gas activities. In the field, we examined plant, microbial, and soil ecosystem property responses to four years of treatments, including ambient (i.e., modeled ~3.5–4 kg N ha-1 yr-1), carbon addition (to assess N saturation) and two N addition treatments (+2 and +4 kg N ha-1 yr-1). Drought during the first two years led us to provide supplemental water to determine if water availability affects N input responses. We also performed two short-term complementary laboratory incubation experiments, one measuring soil N transformations and another measuring nitric oxide (NO) emissions. Field N additions had minimal detectable effects on our plant, microbial, and soil ecosystem property measures, including plant, microbial, and soil N pools. In our incubation experiments, added N reduced net N-mineralization and nitrification and enhanced NO emissions. While the minimal responses observed in our field experiment may have been associated with water limitation, effects were still minimal during years when summer monsoon rainfall was well above average. Together, our field and laboratory findings highlight that although detectable ecosystem-level responses may have been minimal, significant N losses (e.g., gaseous losses) likely occur rapidly and intermittently and thus suggest overall low retention of added N in our study ecosystem.

Technical Abstract: Aridlands are expected to be sensitive to even low levels of nitrogen (N) deposition, yet most experiments apply N in amounts that greatly exceed deposition estimates. We performed a low, but realistic N addition (i.e., based on N deposition estimates) at three adjacent Chihuahuan Desert semi-arid grassland sites. For four years, we applied 2 and 4 kg N ha-1 yr-1. We also included two additional treatments: a labile carbon (C) addition to test whether pre-existing N deposition had already altered our study ecosystem, and a supplemental water treatment prompted by drought during the first two years. In the field, we measured plant responses (cover, community composition, diversity, foliar C and N), soil ecosystem properties (pH, C, N, and P pools), microbial biomass, and extracellular enzyme activities. Additionally, we performed two short-term laboratory incubations: one measuring net N mineralization, net nitrification, and net ammonification, and another measuring nitric oxide (NO) emissions. Overall, our N inputs rarely drove significant changes in our field measures (including plant and soil N pools). Supplemental water did not induce any N addition effects, presumably due to a concurrent drought. However, despite above-average summer rainfall in the final year, N addition responses were still not apparent. In contrast, our incubations demonstrated that N enrichment can suppress net N mineralization and nitrification, while increasing NO emissions. Our field and incubation findings suggest that aridlands may be insensitive to low N deposition levels. This insensitivity may be associated with N losses, which could limit N retention in these ecosystems.