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Dietary Nitrate for Methane Mitigation Leads to Nitrous Oxide Emissions from Dairy Cows
Author(s) -
Petersen S. O.,
Hellwing A. L. F.,
Brask M.,
Højberg O.,
Poulsen M.,
Zhu Z.,
Baral Khagendra R.,
Lund P.
Publication year - 2015
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq2015.02.0107
Subject(s) - nitrous oxide , rumen , zoology , dry matter , latin square , chemistry , nitrate , excretion , urea , methane emissions , respiration , methane , environmental chemistry , agronomy , food science , biology , fermentation , botany , biochemistry , organic chemistry
Nitrate supplements to cattle diets can reduce enteric CH 4 emissions. However, if NO 3 − metabolism stimulates N 2 O emissions, the effectiveness of dietary NO 3 − for CH 4 mitigation will be reduced. We quantified N 2 O emissions as part of a dairy cow feeding experiment in which urea was substituted in nearly iso‐N diets with 0, 5, 14 or 21 g NO 3 − kg −1 dry matter (DM). The feeding experiment was a Latin square with repetition of Period 1. Each period lasted 4 wk, with CH 4 emission measurements in Week 4 using respiration chambers. During Period 3, N 2 O concentrations in chamber outlet air were monitored semicontinuously during 48 h. High, but fluctuating, N 2 O concentrations were seen at the two highest NO 3 − levels (up to between 2 and 5 μL L −1 ), and dynamics were linked with recent feed intake. In Periods 4 and 5, N 2 O concentrations and feed intake were determined from all four respiration chambers during two 7‐h periods. Emissions of N 2 O coincided with feed intake, again with N 2 O concentrations in the microliter per liter range at the two highest NO 3 − intake levels. Neither feed nor excretion of NO 3 − via urine were significant sources of N 2 O, indicating that emissions came from the animals. Leakages due to rumen fistulation could also not account for N 2 O emissions. The possibility that N 2 O is produced in the oral cavity is discussed. Nitrous oxide emission factors ranged between 0.7 and 1.0% except in one case at 21 g NO 3 − kg −1 DM, where it was 3.4%. When accounting for N 2 O emissions at the highest NO 3 − intake level, the overall GHG mitigation effect in two different animal‐diet combinations changed from −47 to −40%, and from −19 to −17%, respectively, due to N 2 O emissions.