Impact of Groundcover Management Systems (GMSs) on Tree Growth and Yields, and Soil Erosion and Runoff in a Hillside Avocado Orchard

Potential environmental problems associated with hillside avocado (Persea americana) production in Chile include increased erosion and runoff when native vegetation is removed from hillsides, and herbicide applications eliminate groundcover vegetation. Herbicide and fertilizer residues in runoff water may also contaminate water resources in downslope areas. We evaluated four GMSs in a steep hillside orchard typical of new avocado plantings in central Chile: 1) bare soil (BS), a combination of pre- and post-emergence herbicides; 2) vegetation strip (VS), post-emergence herbicide applied in a 1-m wide strip centered on the tree row plus a groundcover mixture seeded between tree rows;  3) a complete groundcover (GC), the same groundcover as in the VS treatment covering the entire surface of the plots. Three years after tree establishment, trees in the BS plots were significantly bigger and produced more fruit than trees in the two groundcover treatments. Soil bulk density was significantly higher, and soil macroporosity and aggregate stability were lower in BS than the VS and GC systems. Total soil nitrogen (N) and carbon (C) content, C : N ratios, and essential plant nutrient availability were higher in the GC soil than in other GMSs by the end of our study. Runoff volumes, soil losses, PO₄-P, total N (TN), and dissolved organic carbon (DOC) losses were consistently higher in runoff from the BS than VS and CG treatments during the rainy seasons; no detectable runoff was observed in the GC and VS plots during subsequent years. The use of groundcovers is a viable management practice to residue soil erosion and degradation, runoff, and nonpoint-source pollution of drinking water sources in avocado hillside orchards. However, negative effects on tree growth and productivity during orchard establishment explain the reluctance of avocado growers to adopt these GMS practices.  Groundcover establishment between tree rows, combined with non-residual herbicide applications within tree rows during the growing season, might provide an optimal combination of soil conservation and tree performance during establishment years in these orchards.