Tillage-induced soil carbon dioxide loss from different cropping systems

Tillage of soils often decreases soil organic matter content and increases the flux of carbon dioxide (CO2) from soils. Our objectives were (1) to measure short-term, tillage-induced soil CO2 flux from different 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 interactions between cumulative short-term soil CO2 flux and soil N transformations. Measurements were made on 6 and 7 May 1994 for three cropping systems (coastal bermudagrass (Cynodon dactylon (L.) Pers.), continuously cultivated sorghum (Sorghum bicolor (L.) Moench), and no-till sorghum) that had three different tillage practices (moldboard plow, chisel plow, and untilled as the control) imposed on a vertisol at the Blackland Research Center, Temple, Texas, USA. The soil CO2 flux was calculated 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 methods was greatest immediately after tillage, but maximum soil chamber fluxes were only about 10% of those measured by the canopy chamber. The large differences between chambers are a concern, and probably were related to the inability of the soil chamber to make a representative measurement for tilled surfaces and to the increased turbulence and possible associated pressure effects inside the canopy chamber. Increased soil surface area under the canopy chamber caused by increased soil surface roughness may also explain observed chamber differences. Fluxes were greatest in the bermudagrass and least in the continuously cultivated sorghum. Fluxes in the moldboard plow treatment were usually the greatest, and fluxes in the untilled treatment were considerably smaller 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 content changes reflect microbial activity, the short-term CO2 flux from tilled soils is controlled more by mass flow processes related to a tillage-induced change in porosity than to immediate microbial activity.