Monday, November 13, 2006

A Natural Inhibitor of Nitrous Oxide Emission from Urine Patches.

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.