SPATIAL AND TEMPORAL VARIABILITY OF SOIL RESPIRATION IN AGRICULTURAL FIELDS

Chamber measurements of CO2 evolution were made on bare soil, and in maize (1988) and wheat (1989) crops in order to study the spatial and temporal variability of soil respiration (Rsoil). Semivariograms showed no definite structure of spatial auto-correlation on bare soil when measurements were made along transects. Spatial variability was shown to occur at a scale smaller than 15 cm. In a maize crop, Rsoil in the row was significantly higher (P = 0.05) than in the interrow when the soil surface was dry. Under wet soil conditions, Rsoil in the interrow compacted by the tractor wheel was lower (P = 0.05) than on noncompacted soil and no significant difference was found between rows and interrows. These observations were attributed to the contribution of plant roots to Rsoil in dry conditions. In wetter soil, the role of microbial activity was dominant except in the compacted interrow where lower airfilled porosity caused lower Rsoil. Random measurements in a wheat crop over the growing season showed that the number of samples needed to estimate the Rsoil of a 1 ha area within 10% (P = 0.05) of its mean value decreased from 190 at the time of seeding to 30 at the end of the season. The maximum Rsoil during the growing season coincided with the period of maximum growth of both crops. A post-rainfall Rsoil burst is also described. Immediately after a 2-h rainfall event, when soil was at field capacity, Rsoil was nine times higher than its level prior to the rainfall and gradually decreased with time.