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. Reversion to near-original conditions with respect to plant populations and productivity requires more than 50 years following plowing. The impact of more subtle management changes like small, annual applications of N fertilizer or changing cattle stocking rates, which alters N redistribution caused by grazing and cattle urine deposition, is not known. To investigate the long-term effects of N addition to the Colorado shortgrass steppe we made weekly, year-round measurements of N2O and CH4 from the spring of 1990 through June 1996. Fluxes of NOx (NO plus NO2) were measured from October 1995 through June 1996. These measurements illustrated that large N applications, either in a single dose (45 g N m(-2)), simulating cattle urine deposition, or in small 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, unfertilized site. The long-term impact of these N additions on CH4 uptake was soil-dependent, with CH4 uptake decreased by N addition only in the coarser textured 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 during the summer of 1996, There was little short-term effect of N addition on 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 soil, but did significantly increase N2O flux from the clay loam soil. The reverse 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.