Formation of aggregates and carbon sequestration in ameliorated tepetates in the Río Texcoco basin, Mexico
In many states of central Mexico, hardened volcanic materials called tepetate (hardened layers from tuff) cover large proportions of the land surface, and are therefore of economic and social importance. Under natural conditions, tepetates are unproductive due to their hardness and low porosity, but they can be used agriculturally after the hardened layer is broken up and fertilizer and organic matter are added. Just broken up tepetate consists of angular clods of the original consolidated material of various sizes and they are devoided of aggregates. Because of the scarce presence of organic matter, these fragmented tepetates present low availability of N and P, limited water infiltration, and small water holding capacity. Additions of manure and cultivation of Leguminosae aim to improve the physical properties of tepetates and particularly to promote the formation of aggregates. Such ameliorated tepetates alleviate the demand for agricultural land. The aim of this study was to evaluate the formation of aggregates from initially broken up and fragmented tepetates, their dry and wet stability, and the distribution of organic carbon in aggregates and fragments of different sizes in tepetates cultivated between 1 and 100 years. The tepetates were arbitrarily classified as (I) recently broken up; (II) affected by severe erosion; (III) cultivated mostly under monoculture; (IV) cultivated with Leguminosae for at least 20 years and receiving approximately 2 Mg·ha-1 of farm yard manure (FYM) per year; (V) cultivated under no-tillage and few external inputs; (VI) cultivated with cereals and Leguminosae and receiving more than Mg·ha-1 per year of FYM; (VII) Used as greenhouse bed; (VIII) cultivared with no-tillage and approximately 5 to 10 Mg·ha-1 of FYM; (IX) used as greenhouse bed and addition of approximately 2 Mg·ha-1 of composted wood residues per year. In addition to the above mentioned tepetates, two agricultural soils: one managed with traditional tillage (X) and a second with no tillage (IX) were included as checks. We collected 97 samples of tepetates and four soils in the Basin of the Río Texcoco and neighboring communities. The aggregate stability test overestimated stable aggregates due to the presence of remaining hard fragments of tepetates of different size; the hardness of the latter was one to three times greater than that of the aggregates. The relative amount of stable aggregates increased with the time of cultivation, approaching 80% after 100 years. The recently broken up tepetates contained only little of C, whereas stable aggregates showed one to three times larger concentration. The accumulation of C with time in tepetate fragments was small, in contrast to the aggregates formed in the same period. The accumulation of organic C was larger in smaller aggregate particles and followed logarithmic or potential models. The results of this study show that ameliorated tepetates have the potential to store carbon. However, the accumulated amount of C depends on the aggregate formation, which in turn is related with the agricultural management. Classes VII, VIII y IX produced more stable aggregates and thus accumulated more carbon.