Greenhouse Gas Fluxes following Tillage and Wetting in a Wheat‐Fallow Cropping System

Abstract

AbstractLittle is known about the relative contributions of episodic tillage and precipitation events to annual greenhouse gas emissions from soil. Consequently, we measured carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) fluxes from soil in wheat‐fallow cropping system in western Nebraska using vented surface chambers, before and immediately after tillage and wetting with 5.1 cm of water, during the fallow period 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, electrical conductivity (EC1:1), nitrate (NO3), and pH within the top 30.5 cm soil, and soil temperature at 0 to 7.6 cm were measured to assess their correlation with variations in gas flux and tillage and wetting. Atmospheric concentrations above the soil (at ~40 cm) increased by 15% for CO2 and 9 to 31% for N2O and 6 to 16% for CH4 within 1 min after tillage and returned to background concentrations within 2 h. Except immediately after tillage, net CH4 flux was negative, from the atmosphere into soil, and is referred to as CH4 uptake. Overall, increases (1.5–4‐fold) in CO2 and N2O losses from soil, and CH4 uptake by soil 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 background emissions, respectively, for 24 h following wetting, while CH4 uptake declined by about 60% for 3 to 14 d after wetting. Water‐filled pore space in the surface soil fell below 60% within 24 h after saturation and exhibited an inverse relationship (R2 = 0.66) with CH4 uptake. A significant decline in soil NO3 and EC1:1 in the top 7.6 cm occurred following wetting. Under our experimental conditions, and the expected frequency of tillage and wetting events, failure to include these short‐lived episodic gas pulses in annual flux estimations may underestimate annual CO2 and N2O loss up to 13 and 24%, respectively, and overestimate CH4 uptake by up to 18% in this cropping system.