Pine afforestation alters rhizosphere effects and soil nutrient turnover across a precipitation gradient in Patagonia, Argentina

Pine species, when planted outside their native range, can have profound impacts on soil carbon (C) and nitrogen (N) pools, which may be related in part to their association with ectomycorrhizal fungi. We investigated the effects of exotic planting of Pinus ponderosa on C and N cycling in the rhizosphere along a precipitation gradient in Patagonia, Argentina. We measured C mineralization, microbial biomass-C, and soil enzyme activity in rhizosphere and bulk soil collected from pine plantations and native vegetation. Rhizosphere effects were calculated as the percent difference between paired rhizosphere and bulk soil samples. In afforested sites, we found enhanced rhizosphere effects for C mineralization and microbial biomass relative to native vegetation, but not for enzymatic activity. The absolute values of all evaluated variables were significantly reduced in pine plantations compared to native vegetation. We also observed strong correlations between enzymatic activity, and soil organic matter and microbial biomass. For both pine and native trees species, rhizosphere effects for N-degrading enzymes were positively correlated with precipitation. Pine afforestation reduced overall N turnover and microbial activity in these ecosystems. Our data suggest that these reductions may be driven by reductions in soil organic C pools. Additionally, ecosystem water availability may directly or indirectly regulate the magnitude of rhizosphere effects on N cycling independent of plant species. The negative impacts of afforestation on N cycling should be considered in evaluating the long-term potential for C sequestration in these human modified ecosystems.