Successional dynamics of phytomass and nutrients in successional plots of a paramo agroecosystem

Abstract

In some areas of the Venezuelan paramos (3000-3800 m) a long fallow agriculture system is practised, which alternates long fallow periods of 5 to more than 10 years with short crop periods (1 to 3 years). During the fallow phase a secondary succession take place, allowing a progressive reestablishment of the paramo natural vegetation. The phytomass cumulated during the succession is used as a green manure when the plots are cultivate again. In this study, the successional and seasonal changes of the above and belowground phytomass and of the C and plant nutrients are analysed in plots with 1, 3, 6 and 12 years in fallow after potato cultivation. Several soil physic-chemical properties are also analysed in the same plots (texture, pH, C, N and exchangeable bases). The results show that during the succession there are not significant changes in any of these soil properties, but a lineal increase in the total phytomass and plant nutrients. This increase take place because of the augmentation of the aboveground biomass, though the belowground biomass remains the same during the succession. As a consequence there is a progressive transformation from a predominantly belowground system to a more aerial system, as is showed by the aboveground/belowground ratio, which pass from 0.25 during the first year of succession to almost 4 after 12 years. Clear successional trends in the concentration of plant nutrients are also observed, with a decrease in N and P and an increase in Ca and K. Based on the results, possible successional mechanisms are proposed. For example, a system that becomes progressively more aerial implies that the species would be more efficient in the uptake and/or utilisation of water and nutrients as succession proceeds, requiring fewer roots per unit of biomass.