CH4 and N2O fluxes in the Colorado shortgrass steppe: 1. Impact of landscape and nitrogen addition

A weekly, year‐round nitrous oxide (N 2 O) and methane (CH 4 ) flux measurement program was initiated in nine sites within the Central Plains Experimental Range in the Colorado shortgrass steppe in 1990 and continued through 1994. This paper reports the observed intersite, interannual, and seasonal variation of these fluxes along with the measured variation in soil and air temperature and soil water and mineral nitrogen content. We found that wintertime fluxes contribute 20–40% of the annual N 2 O emissions and 15–30% of CH 4 consumption at all of the measurement sites. Nitrous oxide emission maxima were frequently observed during the winter and appeared to result from denitrification when surface soils thawed. Interannual variation of N 2 O maximum annual mean fluxes was 2.5 times the minimum during the 4‐year measurement period, while maximum annual mean CH 4 uptake rates were 2.1 times the minimum annual mean uptake rates observed within sites. Generally, site mean annual flux maxima for CH 4 uptake corresponded to minimum N 2 O fluxes and vice versa, which supports the general concept of water control of diffusion of gases in the soil and limitations of soil water content on microbial activity. We also observed that pastures that have similar use history and soil texture show similar N 2 O and CH 4 fluxes, as well as similar seasonal and annual variations. Sandy loam soils fertilized with nitrogen 5–13 years earlier consumed 30–40% less CH 4 and produced more N 2 O than unfertilized soils. In contrast, the N addition 13 years ago does not affect CH 4 uptake but continues to increase N 2 O emissions in a finer‐textured soil. Our long‐term data also show that soil mineral N concentration is not a reliable predictor of observed changes, or lack of changes, in either N 2 O efflux or CH 4 uptake. Finally, from our data we estimate that annual global N 2 O emission rates for native, temperate grasslands are about 0.16 Tg N 2 O‐N yr −1 , while CH 4 consumption totals about 3.2 Tg CH 4 ‐C yr −1 .