Long-term large N and immediate small N addition effects on trace gas fluxes in the Colorado shortgrass steppe

Land use changes in semiarid grasslands have long-lasting effects. Reversio n to near-original conditions with respect to plant populations and product ivity requires more than 50 years following plowing. The impact of more sub tle management changes like small, annual applications of N fertilizer or c hanging cattle stocking rates, which alters N redistribution caused by graz ing and cattle urine deposition, is not known. To investigate the long-term effects of N addition to the Colorado shortgrass steppe we made weekly, ye ar-round measurements of N2O and CH4 from the spring of 1990 through June 1 996. Fluxes of NOx (NO plus NO2) were measured from October 1995 through Ju ne 1996. These measurements illustrated that large N applications, either i n a single dose (45 g N m(-2)), simulating cattle urine deposition, or in s mall annual applications over a 15-year period (30 g N m(-2)) continued to stimulate N2O emissions from both sandy loam and clay loam soils 6-15 years after N application. In sandy loam soils last fertilized 6 years earlier, average NOx emissions were 60% greater than those from a comparable, unfert ilized site. The long-term impact of these N additions on CH4 uptake was so il-dependent, with CH4 uptake decreased by N addition only in the coarser t extured soils. The short-term impact of small N additions (0.5-2 g N m(-2)) on N2O, NOx emissions and CH4 uptake was observed in field studies made du ring the summer of 1996, There was little short-term effect of N addition o n CH4 uptake in either sandy team or clay loam soils. Small N additions did not result in an immediate increase in N2O emissions from the sandy loam s oil, but did significantly increase N2O flux from the clay loam soil. The r everse soil type, N addition interaction occurred for NOx emissions where N addition increased NOx emissions in the coarser textured soil 10-20 times those of N2O.