TILLAGE-INDUCED SOIL CARBON-DIOXIDE LOSS FROM DIFFERENT CROPPING SYSTEMS

Tillage of soils often decreases soil organic matter content and incre ases the flux of carbon dioxide (CO2) from soils. Our objectives were (1) to measure short-term, tillage-induced soil CO2 flux from differen t cropping systems using two instruments, a soil chamber (about 10(-3) m(3)) and a canopy chamber (3.25 m(3)), and (2) to examine the intera ctions between cumulative short-term soil CO2 flux and soil N transfor mations. Measurements were made on 6 and 7 May 1994 for three cropping systems (coastal bermudagrass (Cynodon dactylon (L.) Pers.), continuo usly cultivated sorghum (Sorghum bicolor (L.) Moench), and no-till sor ghum) that had three different tillage practices (moldboard plow, chis el plow, and untilled as the control) imposed on a vertisol at the Bla ckland Research Center, Temple, Texas, USA. The soil CO2 flux was calc ulated from the rate of CO2 concentration increase inside each chamber . Soil inorganic N content (NO2-N, NO3-N, and NH4-N) was measured from soil cores collected immediately preceding tillage and at 8, 24, and 102h thereafter. The CO2 flux over a 24h period measured by both metho ds was greatest immediately after tillage, but maximum soil chamber fl uxes were only about 10% of those measured by the canopy chamber. The large differences between chambers are a concern, and probably were re lated to the inability of the soil chamber to make a representative me asurement for tilled surfaces and to the increased turbulence and poss ible associated pressure effects inside the canopy chamber. Increased soil surface area under the canopy chamber caused by increased soil su rface roughness may also explain observed chamber differences. Fluxes were greatest in the bermudagrass and least in the continuously cultiv ated sorghum. Fluxes in the moldboard plow treatment were usually the greatest, and fluxes in the untilled treatment were considerably small er than fluxes from either tillage treatment. For the first 24h after tillage, there was no relationship between cumulative CO, flux and the change of inorganic N. Thus, to the extent that these inorganic N con tent changes reflect microbial activity, the short-term CO2 flux from tilled soils is controlled more by mass flow processes related to a ti llage-induced change in porosity than to immediate microbial activity.