Jan Willem van Groenigen1, Dorien M. Kool2, Vanessa Palermo2, Ellis Hoffland3, and Peter J. Kuikman2. (1) Alterra Soil Sciences Center, Alterra Soil Sciences Center, Po Box 47, Wageningen, 6700AA, NETHERLANDS, (2) Alterra, Soil Sciences Center, PO Box 47, Wageningen, 6700AA, Netherlands, (3) Dept. of Soil Quality, Wageningen UR, P.O. Box 8005, 6700 EC, Wageningen, Netherlands
Urine patches in grazed pastures are a major source of the
greenhouse gas nitrous oxide (N2O). It is well-documented that the
relative concentration of the various nitrogenous urine constituents varies
significantly with diet. The effect of these variations on N2O
emissions from urine patches, however, has never been documented. In three
incubation studies we (i) quantified the differences
in N2O emissions between urines of different nitrogenous
composition, (ii) identified pathways leading to different emissions, and (iii)
determined implications for future N2O emissions studies. All three
incubation studies used topsoil from a sandy pasture in the Netherlands, and measured N2O
and CO2 emissions over 65 days. Occasionally, destructive analyses
of actual denitrification, soil mineral N content and
pH were performed. In the first study, we found a decrease in N2O
emission from 8.4% to 4.4% of urine-N when the hippuric
acid (HA) content of (artificial) urine was increased from 3% to 9% of urine-N
(consistent with a difference in diet). The second study confirmed our
hypothesis that this was due to an inhibitory effect of the HA breakdown
product benzoic acid (BA) on denitrification. Both N2O
emission and actual denitrification were reduced by
approximately 50% when HA/BA levels increased. The third study quantified how
accurately artificial urine of different composition reproduced N2O
emissions from real cattle urine of known-composition. Only the (often-used)
replacement of HA with Glycine resulted in
significantly higher emissions compared to real cattle urine. We conclude that
(i) HA in urine acts as a natural inhibitor of N2O
emission through its breakdown product BA; (ii) this may be the basis of a
novel diet-based mitigation strategy; and (iii) that artificial urine without
HA is not a good replacement for real cattle urine.