CHANGE IN THE ORGANIC-CARBON CONTENT OF SOIL IN A LONG-TERM CONTINUOUS NONIRRIGATED MAIZE CROP

In 2 long-term experiments located on loamy sandy soils in southwest France, change in soil carbon status was measured over a 22- and 25-yr period respectively. Soil cropping was as follows: continuous maize crop with stalks returned to the soil (TR), continuous maize crop with stalks removed (TE), continuous maize crop including a winter annual cover as rye-grass (RG) and rotation involving temporary grassland for 3 yr followed by a maize crop for 4 yr (PT). Whatever the treatment applied, the soil organic carbon content decreased during the course of experiment In the plots under continuous maize cropping with returned stalks, soil carbon concentration decreased from 8.30 to 6.92 g/kg (32 to 26 t organic C ha(-1)) over 22 yr and from 15.50 to 9.65 g/kg (57 to 36 t C ha(-1)) over 25 yr respectively in both experiments. No marked change in carbon balance due to the various organic matter treatments occurred during the course of experiment. The amount of stored C decreased by 3 and 4 t organic C ha(-1) respectively in both experiments as a result of continuous removal of plant residues. On the other hand, using a winter cover crop of rye-grass or the establishment of a temporary grassland resulted in an increase of C storage ranging from 2 to 3 t ha(-1) or from 2 to 4 t ha(-1) organic C. On the contrary, introducing a cultivated fallow in 1980 resulted in a marked decay of organic matter averaging 7 t ha(-1) C for both experiments in 1988 and in 1991 respectively. An asymptotic exponential model led to the prediction that the ploughed soil layer would contain 23 and 31 t ha(-1) organic C (6.1 and 8.4 g/kg C content) at the equilibrium stage. Using the Henin-Dupuis relationship, which involves parameters based on field measurements (k(1) = 5.6 and 6.5%; k(2) = 1.16 and 1.63%), calculated equilibrium values of soil carbon level content are much lower. Taking into account data from experiments carried out in other countries, such low values are questionable. The unreliability of the Henin-Dupuis (1945) model could be attributable to ifs single-compartmental conception and to an unsatisfactory approach using the k(1) and k(2) parameters calculation. Indeed, such a calculation does not take into account the marked change in soil mineralization due to the various cultural practices or different organic matter fluxes entering the soil in the field experiment that result in a systematic underestimation of predicted equilibrium values.