Soluble phenolics extracted from Larrea divaricata leaves modulate soil microbial activity and perennial grass establishment in arid ecosystems of the Patagonian Monte, Argentina

Abstract

Sheep grazing induces the reduction of perennial grass cover and the increase of shrub cover with high concentration of chemical defences. We analysed the effects of secondary metabolites released from green and senesced leaves of the shrub Larrea divaricata on soil microbial activity and the establishment of perennial grasses in arid ecosystems of the Patagonian Monte. We carried out microcosm experiments with soil from plant patches without and with L. divaricata and inert substrate seeded with the perennial grasses Poa ligularis and Nassella tenuis, which are characteristic of the Patagonian Monte. Microcosms were subjected to three watering treatments: distilled water and aqueous extracts of green and senesced leaves of L. divaricata with high concentration of soluble phenolics. We assessed the microbial N-flush and net-N mineralization in soil, andseed germination, survival, and biomass of both perennial grass species. Aqueous leaf extracts led toa 29% increase in microbial N-flush and a 20% reduction in the net-N mineralization. Seed germination was less negatively affected by aqueous leaf extracts in P. ligularis (18% reduction) than in N. tenuis (2?69% reduction). Survival of P. ligularis was not affected by aqueous leaf extracts while that of N. tenuis was 21?45% reduced only in the soil from plant patches without L. divaricata. Biomass accumulation of both perennial grass species was negatively affected by aqueous extracts of senesced leaves. We concluded that soluble metabolites extracted from L. divaricata may have positive or negative effects on microbial activity and potential allelopathic effects on perennial grass regeneration depending on species.