Managing soil biological function vs. chemical fertility inputs to enhance productivity in the Colombian Llanos

Background/Question/Methods:

The Llanos region of eastern Colombia is currently undergoing a rapid agricultural expansion, from semi-natural savanna to large-scale, intensive crop and livestock systems.  Implementation of these modernized cropping systems has relied upon high inputs of fertilizers, lime and tillage to correct deficiencies in the highly acid and compacted soils of the region.  While these inputs have proven effective in supporting short-term yields, this form of management has been shown to greatly impact soil biological functioning and an array of interrelated soil-based ecosystem services.  To better understand the impact of reduced soil biological activity and explore potential interactions with soil fertility inputs, we tested the effect of earthworms (Pontoscolex corethrurus), lime (CaCO₃), and P fertilizer in a full factorial experiment using soil mesocosms within a greenhouse setting.  In these microcosms we measured treatment and interaction effects on a range of soil properties as well as the growth and nutrient (N and P) uptake of two plant types, Brachiaria decumbens (a common introduced pasture species) and the common bean (Phaseolus vulgaris). 

Results/Conclusions:

Preliminary results suggest that realistic field densities of earthworms yielded greater impacts than standard applications of traditional fertility inputs (P fertilizer or lime) for a range plant growth and soil properties.  The presence of P. corethrurus nearly tripled the aboveground biomass the B. decumbens and increased that of P. vulgaris by more than 50%.  Similar effects were observed for N uptake, while effects on P uptake were less dramatic.  Meanwhile, the combined application of lime and P fertilizer increase aboveground biomass of both plant species by an average of about 40%.  Earthworms were also found to have large impacts on soil aggregation, increasing mean weight diameter by some 44%, but only in the presence of plant roots.  These results suggest that earthworms are potentially important regulators of productivity and soil processes in agroecosystems of the Llanos and that management interventions need to better consider the role of soil biological functioning and the long-term implications for sustainability in the region.