GREENHOUSE-GAS FLUXES FOLLOWING TILLAGE AND WETTING IN A WHEAT-FALLOWCROPPING SYSTEM

Little is known about the relative contributions of episodic tillage a nd precipitation events to annual greenhouse gas emissions from soil. Consequently, we measured carbon dioxide (CO2), nitrous oxide (N2O), a nd methane (CH4) fluxes from soil in a wheat-fallow cropping system in western Nebraska using vented surface chambers, before and immediatel y after tillage and wetting with 5.1 cm of water, during the fallow pe riod in 1995/1996. Replicated fallow management treatments included no -tillage, subtillage, and plow representing a wide range in degree of soil disturbance. Soil bulk density, water-filled pore space, electric al conductivity (ECh(1:1)), nitrate (NO3), and pH within the top 30.5 cm soil, and soil temperature at 0 to 7.6 cm were measured to assess t heir correlation with variations in gas nux and tillage and wetting. A tmospheric concentrations above the soil (at similar to 40 cm) increas ed by 15% for CO2 and 9 to 31% for N2O and 6 to 16% for CH4 within 1 m in after tillage and returned to background concentrations within 2 h. Except immediately after tillage, net CH4 nux was negative, from the atmosphere into soil, and is referred to as CH4 uptake. Overall, incre ases (1.5-4-fold) in CO2 and N2O losses from soil, and CH4 uptake by s oil were short Lived and returned to background levels within 8 to 24 h after tillage. Losses of CO2 and N2O increased to 1.7 and 5 times ba ckground emissions, respectively, for 24 h following wetting, while CH 4 uptake declined by about 60% for 3 to 14 d after wetting. Water-fill ed pore space in the surface soil fell below 60% within 24 h after sat uration and exhibited an inverse relationship (R-2 = 0.66) with CH4 up take. A significant decline in soil NO3 and EC1:1 in the top 7.6 cm oc curred following wetting. Under our experimental conditions, and the e xpected frequency of tillage and wetting events, failure to include th ese short-lived episodic gas pulses in annual nux estimations may unde restimate annual CO2 and N2O loss up to 13 and 24%, respectively, and overestimate CH4 uptake by up to 18% in this cropping system.