Confirmation of co-denitrification in grazed grassland

Abstract

Pasture-based livestock systems are often associated with losses of reactive forms of nitrogen (N) to the environment. Research has focused on losses to air and water due to the health, economic and environmental impacts of reactive N. Di-nitrogen (N2) emissions are still poorly characterized, both in terms of the processes involved and their magnitude, due to financial and methodological constraints. Relatively few studies have focused on quantifying N2 losses in vivo and fewer still have examined the relative contribution of the different N2 emission processes, particularly in grazed pastures. We used a combination of a high 15N isotopic enrichment of applied N with a high precision of determination of 15N isotopic enrichment by isotope-ratio mass spectrometry to measure N2 emissions in the field. We report that 55.8 g N m−2 (95%, CI 38 to 77 g m−2) was emitted as N2 by the process of co-denitrification in pastoral soils over 123 days following urine deposition (100 g N m−2), compared to only 1.1 g N m−2 (0.4 to 2.8 g m−2) from denitrification. This study provides strong evidence for co-denitrification as a major N2 production pathway, which has significant implications for understanding the N budgets of pastoral ecosystems.

Contribution of Anammox to Nitrogen Removal in Two Temperate Forest Soils

“…Codenitrification can occur in both fungi (e.g., Fusarium oxysporum) and bacteria (e.g., Streptomyces antibioticus) (37,47,48). This process with N 2 formation has previously been found in grassland and agricultural soils (47,49,50). The present study also demonstrated that codenitrification with N 2 formation coexisted with anammox and denitrification in N removal for forest soils under anaerobic conditions.…”

“…Our results for the soil from the mixed forest showed that codenitrification accounted for 2% to 12% of the total N 2 production ( Table 2). These results were much lower than in previous studies for two pasture soils (92% and 97%, respectively [49,50]) where the N 2 emission from codenitrification, including the part from anammox, was the dominant loss pathway. In addition, based on the original results from Long et al (47) and our equations as given above, the contribution of codenitrification was recalculated to be 0 to 68% of the total N 2 production for the six agricultural soils in the United States (see Table S2 in the supplemental material), assuming that 29 N 2 production with 15 NH 4 ϩ addition was from anammox.…”

Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question

“…These data may be subject to artefacts if antibiotics repress NO 2 − oxidation, leading to NO 2 − accumulation and potential abiotic formation of N 2 and/or N 2 O. Our report also calls into question if codenitrification alone accounts for N 2 and N 2 O emissions following high doses of N as urea (approximately 9 mmol g −1 soil)13 and/or NO 2 − (166 mmol L −1 )10. If abiotic, hybrid N 2 is formed under these conditions, our understanding of soil fungal codenitrification and N trace gas emissions would need to be re-examined.…”

“…Codenitrification occurs when individual atoms of the N 2 O or N 2 molecules are derived from two distinct N sources910, resulting in hybrid N 2 8. Formation of hybrid N 2 is widely reported as evidence of anammox11 or codenitrification121314. Previously, isotope pairing experiments revealed chemodenitrification1516 and denitrification by the fungi Bipolaris sorokiniana used NO 2 − as the sole source for N 2 O formation16.…”

Long-term elevation of temperature affects organic N turnover and associated N<sub>2</sub>O emissions in a permanent grassland soil

“…1). Laughlin and Stevens (2002) confirmed the importance of co-denitrification for N 2 production, a process that may comprise 25 % of the total N balance in pastures (Selbie et al, 2015). Müller et al (2014) found that, for the same grassland soil as used in this study, co-denitrification contributed 17.6 % of the total N 2 O production.…”