IN-SITU DECOMPOSITION OF C-14-LABELED WHEAT AS AFFECTED BY SOIL DISTURBANCE

Soil disturbance by tillage was previously considered one of the reaso ns accounting for the depletion of organic matter of virgin lands when subjected to agricultural practices. An experiment simulating cultiva tion was performed in situ to evaluate the effects of soil disruption on wheat decomposition and plant-derived carbon turnover. C-14-labelle d wheat straw was incorporated into soil located inside cylinders whic h were installed in the field. Mechanical disturbance was performed in some of them each time the field was tilled. Cylinders were sampled a t 6-month intervals during four years and residual C-14 determined. De composition intensity was similar in disturbed and undisturbed soil. T he dynamics of the degradation fitted closely to the negative exponent ial model (r(2) = 0.933, P = 0.01). The half-life of labelled carbon w as ca. 1.8 years, indicating an intense turnover of microbial biomass and recently formed soil organic matter. These data suggested that agg regate disruption by tillage is not a primary factor in causing the in crease in carbon loss from soil under cultivation.