Interactions of organic matter and microbial dynamics in pasture soils along management chronosequences

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In the South Ecuadorian Andes natural forests have often been converted to pastures by slash-and-burn. Frequently, the pasture grass is displaced by tropical bracken leading to the abandonment of this unproductive pastures. The main goals of the proposed project are 1) to assess the point in time of pasture fertility decline, as indicated by soil organic matter and microorganism activity and structure, and the respective reasons to be able to develop effective measures for the sustainable use of already existing pasture land and 2) to gain insight into land-use induced changes of factors and mechanisms responsible for the mobilization and sequestration of soil organic matter. The first goal will be achieved by connecting nutrient availability, microbial activity (SOC, gross and net N-mineralization) and community structure (PLFA analysis) in topsoils with vegetation characteristics depending on pasture age, fertilization, management and fire regime. To reach our second goal the biochemical composition of different fractions of soil organic matter (particulate, “heavy”, H2O2- resistant and water extractable organic matter) will be determined using a diverse array of molecular characterisation methods (Py-GC/MS, NMR, BPCA, SEC-MALLS, δ13C-signature, degradability). Interactions between organic matter and microbial dynamics will be identified with multivariate statistical analysis. The increased density of data along the pasture management chronosequence and an improved knowledge of stabilization processes of SOM in the tropical mountain soils will improve modelling of SOM stock changes with the CENTURY model.