Long-term quantification of the intensity of clay-sized particles transfers due to earthworm bioturbation and eluviation/illuviation in a cultivated Luvisol

As a result of the limited knowledge on eluviation/illuviation and bioturbation rates, these two processes of soil particles translocation are qualitatively described either as synergic or competing processes. Here we take the opportunity of the recent development of an image analysis procedure to quantify illuvial clay and earthworm's porosity to quantify the intensity of illuviation and bioturbation cumulated over soil formation in a temperate cultivated Luvisol. The key objectives of the study are i) to quantify the total intensity of illuviation and bioturbation and their depth distributions and ii) to assess the possibility for bioturbation to limit or compensate the depletion of the clay-sized fraction in topsoil horizons due to eluviation. The total quantity of illuvial clay is 1,100 t.ha(-1) while the estimated annual amount of clay-sized fraction translocated by eluviation is between 0.08 and(-1) t ha(-1) yr(-1). This is comparable to the annual loss of land by water erosion (between 1 and 5 t ha(-1) yr(-1)) or by arable erosion (3.3 t ha(-1) yr(-1)). Eluviation/illuviation is thus a discrete and active form of soil loss. With approximately 1,900 t.ha(-1) of clay-sized fraction, the amount of fine particles displaced at least once by bioturbation is higher than the one related to eluviation/illuviation. At first sight, it therefore seems possible for biological activity to compensate for vertical transfers of the clay-sized fraction by eluviation/illuviation. However, our study shows that a considerable amount of the clay-sized fraction will never be brought up by the bioturbation and will remain definitively lost for the surface horizons as bioturbation decreases non-linearly with depth. Consequently, a preventive management of the depletion of the clay-sized fraction in topsoil horizons by eluviation/illuviation should be preferred to the curative management of its consequences by bioturbation.

Automated summary

We used image analysis to quantify the intensity of illuviation and bioturbation in a cultivated Luvisol. Although bioturbation can compensate for clay-sized fraction loss caused by eluviation, a significant amount of clay will still be permanently lost. Thus, preventive management of clay-sized fraction depletion is preferable to curative management through bioturbation.