Denitrification in a subtropical, semi-arid North American savanna: field measurements and intact soil core incubations
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Information on denitrification (particularly N2) losses from dry ecosystems is limited despite their large area. Here, we present the first direct denitrification measurements for a northern hemisphere savanna, a Prosopis-dominated grassland/grove matrix in south Texas. We used the gas-flow intact soil core method to quantify N2, N2O and CO2 losses and compared these with field measurements of N2O, NOy, NH3 and CO2. Under field-realistic soil moisture and O2 conditions (average 17.5–20 % O2, minimum 15 %) incubated soils produced no measurable N2 flux (detection limit 52.2 µg N m−2 h−1). Only in a subset of grove soils were fluxes of 70–75 µg N m−2 h−1 recorded after 102 h of incubation at 5–10 % O2 following wetting of very dry soils. Making the assumption that potential N2 production falls just below the detection limit (likely an overestimate given the conditions needed to generate measurable fluxes), N2 flux rates would fall on the low end of that recorded for a tropical Australian savanna (45–110 µg N m−2 h−1) under comparable abiotic conditions. Assuming maximum possible production rates, N2 could comprise <32–76 % of total soil N gas flux following soil wetting in summer. Lack of flux response to soil wetting in winter suggests that cold-season N2 fluxes are negligible. N2O fluxes for core incubations were significantly higher than for field chambers; thus it is likely that incubations may overestimate N2O flux by reducing soil column consumption. Overall, results indicate that soil N2 fluxes are less dominant in this savanna than in other ecosystems investigated.
KeywordsGrassland N-FARM di-nitrogen Nitrous oxide Nitric oxide Prosopis
We thank Shauntle Barley for assistance with sample collection, Kimberlee Sparks, John Pollak, Collin Edwards, David Lewis and Lisa Martel for technical support, and David and Stacy McKown for field logistics. This work was supported a National Science Foundation Doctoral Dissertation Improvement Grant to JPS (Award #1309124), Sigma Xi (Cornell chapter), the Cornell University Betty Miller Francis’47 Fund for Field Research, the Andrew W. Mellon Foundation and the Cornell University Program in Cross-Scale Biogeochemistry and Climate (supported by NSF-IGERT and the Atkinson Center for a Sustainable Future).
FMS, JPS, and PMG formulated the original idea and developed methodology, FMS and JPS conducted field sampling, FMS performed sample analysis and statistical analysis and FMS, PMG and JPS interpreted data and wrote the manuscript.
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