Tillage and herbicide decrease soil biodiversity in olive orchards

Olive growing is a centenarian activity in Andalusia, Southern Spain. Andalusia holds the largest olive tree growing area in the world. In spite of the relevance of olive growing for the Mediterranean economy, the influence of soil biodiversity on olive crops has been seldom studied. We hypothesized that soil diversity must be well preserved because the Andalusian olive groves are low-input and no-till systems. Soil depth should also help to structure soil diversity. We tested the effect of site features and soil management on nematode diversity, soil properties and soil food webs. Site features included use, position, orientation, radiation, slope, altitude and regional scale. Soil management included tillage and herbicide use. Results show that at the large scale, nematode abundance and soil food web structure were reduced under the tree canopy affected by herbicides by 57.9 and 14.2 %, respectively, in comparison to areas not treated with herbicides. Nematode abundance decreased by 47.7 %, taxa richness by 12.4 % and soil food web structure by 23.4 % in areas where herbicides are applied, in comparison to surrounding oak woodlands. The absence of vegetation in bare soils impacted the lower levels of the soil food web, depleting bacterial and fungal-mediated decomposition channels. This depletion spread up in turn to microbivore nematodes and nematodes in the upper level. We also found that nematode abundances decreased with soil depth on average by 73.2 % from the top soil layer (0–2 cm) to the deepest one (10–20 cm), irrespectively of the tillage practice.