Semi-Arid Grassland Nitrogen Addition Experiment: New Mexico, 2018-2021

This dataset contains field and laboratory incubation measurements from a four-year nutrient addition experiment (2018–2021) conducted in three adjacent (<5 km apart) Chihuahuan Desert grasslands near Carlsbad Caverns National Park, New Mexico, USA (32°10′31″N, 104°26′38″W). The three replicate grassland sites were dominated by different grass species: Bouteloua gracilis (“Native Grama” site), Muhlenbergia setifolia (“Native Muhly” site), and Eragrostis lehmanniana (“Invasive Lovegrass” site). The Native Grama and Invasive Lovegrass sites were located on recently deposited alluvial soils classified as Entisols (Ustic Torrifluvents), formed from gravelly alluvium derived from limestone. Soils at the Native Muhly site were classified as shallow Aridisols formed from colluvium and residuum weathered from limestone and dolomite. Plot soils at the Native Grama and Invasive Lovegrass sites were sandy loam, while soils at the Native Muhly site were loam. Pre-treatment soil chemistry (collected May 2018 at 0–5 cm depth) was relatively consistent across sites, with pH ranging from 7.6 to 7.8 and similar inorganic N concentrations. Experimental field plots at each site received annual additions of nitrogen (+2 or +4 kg N ha⁻¹ yr⁻¹ as ammonium nitrate), carbon (+6 g m⁻² as sucrose), or no additions (ambient control). In 2020, a supplemental water treatment was applied only at the Native Grama site to simulate an additional 55 mm of rainfall during the monsoon season. Field data include measurements of soil chemistry (pH, inorganic nutrients, extractable organic C, total N), microbial biomass (C, N, P), extracellular enzyme activities, vegetation cover by functional group, species richness, Shannon diversity indices, and foliar chemistry (%C, %N, C:N ratios). Measurements were collected seasonally (pre-monsoon, monsoon, winter) or annually at peak biomass from 2018 through 2021. Laboratory incubations were conducted to complement field measurements. In 2019, a 30-day nitrogen transformation incubation measured net mineralization, nitrification, and ammonification in soils collected from all field treatments, incubated at constant 60% water holding capacity at room temperature. In 2021, a nitric oxide efflux incubation was performed using soils from Ambient, +2N, and +4N field plots. These soils were subjected to factorial laboratory treatments: (1) deionized water (DI control) and (2) an additional nitrogen input equivalent to +4 kg N ha⁻¹, both under standardized incubation moisture (20% water holding capacity). Nitric oxide flux was measured after 1 and 24 hours to capture short-term gaseous responses. Note on precipitation context: Precipitation data are not provided as part of this dataset, but we characterized rainfall conditions during the study to contextualize field measurements. Annual and summer (June–September) precipitation for 2018–2021 were obtained from a NOAA-COOP weather station located within 3 km of the sites (Station ID #291480). Thirty-year normal values (1981–2010) for mean annual precipitation (MAP = 399 mm) and mean summer precipitation (252 mm) were also derived from the same station. Relative to these long-term averages, 2018–2020 experienced major deficits in summer rainfall (–50%, –54%, –71%, respectively), while 2021 delivered 25% above-average summer rainfall. Because summer rainfall is largely governed by the North American Monsoon (July–September), these anomalies indicate strong variability in monsoon strength across the study years.