Organic Matter Characteristics and Eroded Soils Nutrient Content in

Twenty-one severely eroded soils of SE Spain (Torriorthent xeric soils) were studied. These soils form a fragile system characterized by soils with a low density of plant cover (<5%), are loamy and occur in a semiarid climate. The soils formerly were used for agricultural purposes but were abandoned at least 15 years ago. These eroded soils had a low total organic carbon content, and their humic substances, humic acid carbon, and carbohydrates were lower compared with soils that had never been cultivated (natural soils). The variables in which the effects of erosion were particularly noted were those related with the active organic matter (respiration and water- soluble organic matter). Those eroded soils with higher salt content showed lower organic matter and carbohydrate contents. Only total nitrogen was correlated with the carbon fractions in the eroded soils. Given the long period of time necessary for its forma- tion, soil might be considered as a slowly renewable natural resource. A soil reaches a steady state with envi- ronmental factors and acquires properties that maintain its productive capacity (L6pez-Bermudez and Albala- dejo 1990). However, not all factors are favorable to the maintenance of the steady state, and sometimes human intervention (unsuitable agricultttral practices, the abandonment of agricultural soils, etc.) can contribute to the deterioration of a soil's natural productivity, as can unfavorable natural factors such as climate (Arnold and Jones 1989, Dick 1992). As examples, one can point to the large land area abandoned many years ago for agricultural use. A key factor in soil degradation in this area is the destruction of plant cover, which means that water erosion and salinization processes also play a role, aggravating still further the effects of the semiarid cli- mate. In these soils, the loss or diminution of organic matter as a consequence of plant cover degradation is an added effect (Francis and Thornes 1990). Larson (1981) cites soil erosion as the principal cause of the loss of organic matter and nutrients in soils, which un- doubtedly contributes to the loss of productivity. According to Follet and Westfall (1986), organic mat- ter is concentrated at the surface where it is extremely susceptible to loss through erosion. Because organic