Effect of beech (Fagus sylvatica L.) rhizosphere on phosphorous availability in soils at different altitudes (central Italy)

Phosphorus (P) is an important nutrient for plant growth but its availability in soil is limited. Although plants are able to respond to the P shortage, climatic factors might modify the soil-plant-microorganisms system and reduce P availability. In this study we evaluated the rhizosphere effect of beech (Fagus sylvatica L.) in forest soils of Apennines mountains (central Italy) at two altitudes (800 and 1000 m) and along 1° of latitudinal gradient, using latitude and altitude as proxies for temperature change. Specifically, we tested if 1) soil organic C, total N, and organic and available P decrease with increasing latitude and altitude, and 2) the rhizosphere effect on P availability becomes more pronounced when potential nutrient limitations are more severe, as it happens with increasing latitude and altitude. The results showed that the small latitudinal gradient has no effect on soil properties. Conversely, significant changes occurred between 800 and 1000 m above sea level, as the soils at higher altitude showed greater total organic C (TOC) content, organic and available P contents, and alkaline mono-phosphatases activity than the soils at lower altitude. Further, at the higher altitude, a marked rhizosphere effect was detected, as indicated by greater concentration of TOC, water extractable organic C, and available P, and its fulfillment was mainly attributed to the release of labile organics through rhizodeposition. The availability of easy degradable compounds in the rhizosphere should foster the mineralization of the organic matter with a consequent increase of available P. Hence, we speculate that at high altitude the energy supplied by the plants through rhizodeposition to the rhizosphere heterotrophic microbial community is key for fuelling the rhizospheric processes and, in particular, P cycling.