Pasture diversification affects soil macrofauna and soil biophysical properties in tropical (silvo)pastoral systems

The diversification of tropical pastures with legumes (trees) for increased forage and animal productivity has been advocated. Nevertheless, effects on soil quality and belowground biodiversity, and the implications for sustainable intensification remain poorly documented, particularly when cattle grazing is included in the study. We evaluated the impact of forage system diversification with herbaceous and woody legumes on soil properties and soil macrofauna communities and their spatial heterogeneity in a three-year-old field trial in Cauca Valley, Colombia. Three forage-based systems were compared: (i) a conventional monoculture-species grass pasture system of Brachiaria hybrid cv. Cayman (CP); (ii) a mixed pasture system consisting of Brachiaria grass with the leguminous herb Canavalia brasiliensis (LP); and (iii) a silvopastoral system with rows of the legume tree Leucaena diversifolia planted within LP pastures (SPS). The experiment was arranged in a complete randomized block design with three replicates and grazing cattle rotating across blocks. Plots were grazed by three (treatments CP and LP) or four bulls (SPS) aiming to reflect the expected cattle intensification in SPS systems. Physico-chemical soil properties and macrofauna abundance and their spatial heterogeneity as affected by the distance from the tree rows in SPS, were assessed. Herbaceous legumes positively affected the abundance and diversity of soil macrofauna and soil physical properties in LP and the alleys between tree rows in SPS, as compared to CP. In the SPS, the highest soil quality and macrofauna abundance occurred at the edge of the tree lines, while the highest soil compaction and the lowest abundance of soil macrofauna occurred in the tree rows, probably due to the behavioral change of the grazing cattle in combination with the higher stocking rate in SPS. Soil properties in LP and in the alleys between the tree rows of SPS were comparable despite higher stocking rate in SPS. Overall, the SPS and LP systems, proved to be suitable alternatives to CP allowing for sustainable intensification of pastures although careful evaluation of possible trade-offs associated with increased spatial heterogeneity in SPS is recommended to avoid localized soil compaction. Soil macrofauna, particularly functional groups (classified by feeding habits) proved to be a sensitive soil quality indicator in response to contrasting pasture systems.