CO2 EFFLUX IN VERTISOL UNDER DIFFERENT LAND USE SYSTEMS

Determinations of CO2 efflux, soil temperature and soil-water content in vertisols were monitored at least twice a week between July 2001 and January 2002. At each sampling date, two daily measurements (at 08:00 and 14:00 h local time, named as morning and afternoon, respectively) were carried out. A dynamic closed chamber with a portable system EGM employing a infrared gas analyzer (IRGA) and a soil chamber (SRC-1) were used to assess soil CO2 efflux throughout the experimental period from vertisols under different land uses in northeastern Mexico: Pasture (Dichanthium annulatum), Leucaena leucocephala in an alley cropping system, a native and undisturbed shrubland plot, a Eucalyptus microtheca plantation, and a Sorghum bicolor field. Results showed for the Eucalyptus and Pasture plots a highly significant and positive linear relationship between morning and afternoon soil respiration rate and soil temperature, while no significant relationship was found between soil temperature and soil respiration for the Leucaena, Sorghum nor the Shrubland plots. Soil temperature alone explained 68% of the variation in the CO2 efflux rate in Eucalyptus and 33% in Pasture. During the study period, average morning soil respiration rates for all land uses ranged from 0.7 (October) to 8.4 mmol CO2 m-2 s-1 (August), while afternoon soil respiration rates ranged from 0.6 to 14.4 mmol CO2 m-2 s-1. Average morning and afternoon soil respiration rates showed the following decreasing CO2 efflux order among the five investigated land uses: Pasture>Shrubland>Leucaena>Eucalyptus>Sorghum; thus, the pasture plot showed the highest average morning and afternoon soil respiration rates; 3.5 and 5.0 mmol CO2 m-2 s-1, respectively. In contrast, the Sorghum plot showed the lowest average morning (1.9) and afternoon (2.5 mmol CO2 m-2 s-1) soil respiration rates. The Pasture and Shrubland, which are common livestock management practices in this region, contribute to more CO2 emissions than agriculture and forestry systems. The dry period had a significant influence in the vertisol structure, since the soil shrinks and swells noticeably in response to soil moisture content and this affects the reliability of the CO2 efflux measurements by the close dynamic chamber. Our field observations have also illustrated the need of research efforts in vertisols under dry periods, especially when soil water content drops below 15%, in order to explain the dynamics of the characteristic CO2 balance in different land uses.