THE EFFECTS OF GRAZING AND BURNING ON SOIL AND PLANT NUTRIENT CONCENTRATIONS IN COLOMBIAN PARAMO GRASSLANDS

The impact of extensive livestock farming on the physical and chemical characteristics of the volcanic soils and on the nutrient status of green plant tissues of neotropical alpine grasslands (paramo) is studied. Soil and plant samples were taken over a one-year period at five sites with different agricultural (grazing and burning) management. In the undisturbed paramo ecosystem, soil moisture (50-250%) and organic matter content are high (7-27%) and decomposition (11-35% yr(-1)) and element concentrations are low. Low temperatures (max < 10 degrees C) and phosphorus fixation by the soil (5 mg P g(-1) soil) determine the low mineralization and turn-over rates. Multivariate analysis of laboratory results indicates that the season of sampling and the agricultural practice are the most important explanatory factors for variation of soil characteristics. After long-term heavy grazing, soils have a higher bulk density and a lower moisture content. The outcome of a litterbag experiment confirms the hypothesis of higher decomposition rates at grazed sites. In the intermediate (wet-dry) season, conditions were somewhat better for plant growth but the system remained nutrient limited. Surprisingly, no relation between soil density, moisture or carbon content and concentrations of available nutrients in the soil is found. This is supported by the rather uniform nutrient concentrations in green plant tissue among the sites. It is concluded therefore that the effect of burning and grazing on paramo soils is principally restricted to physical characteristics, and that differences in chemical characteristics of the soil do not cause differences in vegetation structure between grazed, burned and undisturbed sites.