Albaladejo, Juan (CEBAS-CSIC, Campus de Espinardo, Murcia, MU, 30100, Spain; Phone: ++34 968 396338; Fax: ++34 968 396213; Email: jalba@cebas.csic.es)

Carbon Fraction and C-CO₂ Released in Abandoned Agricultural Soils

C. Garcia, J. Albaladejo*, J.A. Pascual, J.L. Moreno, F. Bastida, T. Hernandez

Soil is an important natural resource that needs to be preserved. However, the need to maintain increasing levels of production have led to inappropriate soil management and the consequent loss of soil fertility. When a soil is exposed to degradative processes (for example, abandonment after intensive agricultural practices, devegetation), its biological state is the first to be affected, diminishing its productive capacity. This fact is of paramount importance since, as it is widely accepted, metabolic activity is essential to the suitable functioning of ecosystems. The microbial metabolic activity of soil is responsible for the mineralization and humification of any organic components, including recalcitrant ones, reaching the soil. One important characteristic of the soils of the Mediterranean regions is that they are indeed submitted to erosion and desertification processes and that they have a low organic matter content. Intensive cultivation, continual ploughing and forest fires, allied to years of unsuitable agricultural practices, have had an important effect on humification processes and on properties associated with degradation. All the above has led to a great diminution in the quantity of vegetal remains provided to the soil, while the humus is undergoing a process of accelerated mineralization as a result of tilling. The inevitable result is a progressive diminution of the organic matter content and the negative consequences entailed. Changes in soil organic matter and several carbon fraction in abandoned semiarid agricultural soils, at different times after abandonment in comparison with natural soils exposed to the same climate have been studied. Total organic C and water soluble C showed a decrease in the abandoned soils. The scarce quantity of plant residues in these soils can be responsible for this effect. Microbial Biomass C (MBC) can be considered to be a more sensitive indicator of soil quality than organic matter or total organic C, since it responds more rapidly and to a greater extent to changes (e.g. degradation). The value of MBC detected in the abandoned soils varied greatly. It declined when agricultural soils were abandoned and decreased with time elapsed, presumably as a consequence of the loss of the capacity to protect soils against erosion processes. Soil respiration (CO₂ loss) is a good indicator of soil microbial activity. CO₂ losses in all the abandoned soils were significantly lower than in natural soils, particularly in the soils abandoned for the longest period of time, the importance of the duration of abandonment on microbial activity being again demonstrated.