CH4 AND N2O FLUXES IN THE COLORADO SHORTGRASS STEPPE .1. IMPACT OF LANDSCAPE AND NITROGEN ADDITION

A weekly, year-round nitrous oxide (N2O) and methane (CH4) flux measur ement program was initiated in nine sites within the Central Plains Ex perimental Range in the Colorado a shortgrass steppe in 1990 and conti nued through 1994. This paper reports the observed intersite, interann ual, and seasonal variation of these fluxes along with the measured va riation in soil and air temperature and soil water and mineral nitroge n content. We found that wintertime fluxes contribute 20-40% of the an nual N2O emissions and 15-30% of CH4 consumption at all of the measure ment sites, Nitrous oxide emission maxima were frequently observed dur ing the winter and appeared to result from denitrification when surfac e soils thawed. Interannual variation of N2O maximum annual mean fluxe s was 2.5 times the minimum during the 4-year meas urement period, whi le maximum annual mean CH4 uptake rates were 2.1 times the minimum ann ual mean uptake rates observed within sites. Generally, site mean annu al flux maxima for CH4 uptake corresponded to minimum N2O fluxes and v ice versa, which supports the general concept of water control of diff usion of gases in the soil and limitations of soil water content on mi crobial activity. We also observed that pastures that have similar use history and soil texture show similar N2O and CH4 fluxes, as well as similar seasonal and annual variations. Sandy loam soils fertilized wi th nitrogen 5-13 years earlier consumed 30-40% less CH4 and produced m ore N2O than unfertilized soils, In contrast, the N addition 13 years ago does not affect CH4 uptake but continues to increase N2O emissions in a finer-textured soil, Our long-term data also show that soil mine ral N concentration is not a reliable predictor of observed changes, o r lack of changes, in either N2O efflux or CH4 uptake. Finally, from o ur data we estimate that annual global N2O emission rates for native, temperate grasslands are about 0.16 Tg N2O-N yr(-1), while CH4 consump tion totals about 3.2 Tg CH4-C yr(-1).