Frank Douglas A, Groffman Peter M
Biological Research Laboratories, Syracuse University, Syracuse, NY 13244-1220, USA e-mail:
Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545-0129, USA, , , , , , US.
Oecologia. 1998 Dec;117(4):564-569. doi: 10.1007/s004420050693.
The effect of large herbivores on gaseous N loss from grasslands, particularly via denitrification, is poorly understood. In this study, we examined the influence of native migratory ungulates on denitrification in grasslands of Yellowstone National Park in two ways, by (1) examining the effect of artificial urine application on denitrification, and (2) comparing rates inside and outside long-term exclosures at topographically diverse locations. Artificial urine did not influence denitrification 3 and 12 days after application at hilltop, mid-slope, and slope-bottom sites. Likewise, grazers had no effect on community-level denitrification at dry exclosure sites, where rates were low. At mesic sites, however, ungulates enhanced denitrification by as much as 4 kg N ha year, which was double atmospheric N inputs to this ecosystem. Denitrification enzyme activity (DEA, a measure of denitrification potential) was positively associated with soil moisture at exclosure sites, and herbivores stimulated DEA when accounting for the soil moisture effect. Glucose additons to soils increased denitrification and nitrate additions had no influence, suggesting that denitrification was limited by the amount of labile soil carbon, which previously has been shown to be enhanced by ungulates in Yellowstone. These results indicate that denitrification can be an ecologically important flux in portions of semi-arid landscapes, and that there is a previously unsuspected regulation of this process by herbivores.
大型食草动物对草原气态氮损失的影响，尤其是通过反硝化作用造成的损失，目前还知之甚少。在本研究中，我们通过两种方式研究了黄石国家公园草原上本地迁徙有蹄类动物对反硝化作用的影响：一是通过研究人工施加尿液对反硝化作用的影响，二是比较地形多样地点长期围栏内外的反硝化速率。在山顶、山坡中部和山坡底部位点施加人工尿液3天和12天后，反硝化作用并未受到影响。同样，在干燥的围栏位点，食草动物对群落水平的反硝化作用也没有影响，这些位点的反硝化速率较低。然而，在湿润位点，有蹄类动物使反硝化作用增强了多达4千克氮/公顷·年，这是该生态系统大气氮输入量的两倍。反硝化酶活性（DEA，一种反硝化潜力的衡量指标）与围栏位点的土壤湿度呈正相关，在考虑土壤湿度影响时，食草动物刺激了DEA。向土壤中添加葡萄糖增加了反硝化作用，而添加硝酸盐则没有影响，这表明反硝化作用受土壤中不稳定碳量的限制，此前研究已表明黄石地区的有蹄类动物会增加不稳定碳的含量。这些结果表明，反硝化作用在半干旱景观的部分区域可能是一个重要的生态通量，而且食草动物对这一过程存在此前未被怀疑的调控作用。
