Glomalin and carbon sequestration in cultivated tepetates
Résumé
The tepetates are hardened volcanic tuffs that outcrop on the surface due to erosive processes; some of them can be ameliorated for agriculture and thus sequester carbon ©. This element accumulates in fragile aggregates that are formed as part of their developing physical structure after being cultivated, but the mechanism by which it is sequestered in the mineral fraction in these volcanic substrates is unknown. It is assumed that the biological activity contributes to the stabilization of the organic C (COS), specifically that of the arbuscular mycorrhizal fungi (AMF). It has been postulated that glomalin (a glycoprotein produced by the AMF) influences both processes. To contribute to the comprehension of this phenomenon, the present study was made with the following objectives: 1) to evaluate the accumulation of COS, glomalin and glomalin carbon (GC) in cultivated tepetates; 2) to measure the contribution of C made by the glomalin to the COS; and 3) to evaluate the tendency of accumulation of GC with years of cultivation. Based on interviews with producers of the Texcoco River basin, 83 plots were selected with 10 to 20 years of cultivation, and 10 classes of agronomic management were defined (I to X). As reference four Phaesozem soils of the region were used, grouped in two classes of management. Another 21 plots of tepetate had from 4 to over 100 years of cultivation. Tepetate samples were collected (0-20 cm) and COS, glomalin and GC were determined. The relationship between COS and the latter variables was close (R>0.91). The agronomic management had significant influence (p≤0.05) on the accumulation of COS, glomalin and GC. As function of the maximum concentration of COS observed, it was calculated that the tepetates ameliorated for agricultural production after two decades of cultivation, with an agronomic management with constant incorporation of organic residues, have capacity for storing approximately 90 t ha-1 of C in the first 20 cm of depth, with a contribution of 15 % of stabilized C from glomalin (p≤0.05). The accumulation of GC as a function of time followed a logarithmic tendency.