Effect of land use on the density of nitrifying and denitrifying bacteria in the Colombian Coffee Region

Soil microbial communities involved in the cycling of nitrogen (N) are essential to maintaining and improving soil fertility, productivity and functionality of natural and agricultural ecosystems. However, some compounds generated during the metabolic processes performed by nitrifying (NB) and denitrifying (DB) bacteria are associated with the production of greenhouse gases, groundwater pollution and acidification. Therefore, the study of these bacteria is essential for economic and environmental sustainability. This study evaluated the effect of different land uses in two river basins (La Vieja and Otun) on NB and DB densities. Two sampling events (SE) were conducted by selecting the most representative land uses. Physicochemical (T °, pH, moisture and nitrate) and microbiological properties (NB and DB densities) were evaluated. In both SEs, significantly higher densities of NB and DB were observed in the land uses: pasture, guadua (DB only) and unshaded coffee (La Vieja) and onion (Otun). These land uses, excluding guadua, are dependent on nitrogen fertilizers, which together with the activities of grazing livestock on pastures may lead to greater availability of substrates for the NB. The use of agricultural machinery and overgrazing in pasture and onion uses generate compacted soil and other physical disturbances, encouraging the growth of DB. Forests had the lowest densities of NB and DB possibly due to a reduced availability of N and the releasing of allelopathic compounds from certain plants. Finally, the densities of ammonium-oxidizing bacteria had the greatest differences between the land uses evaluated, demonstrating its high sensitivity to agricultural management practices and livestock. We suggest that changes in the abundance of this community could serve as a relevant and cost-effective bioindicator for soil monitoring.