Soil Carbon and Nutrient Responses to Woody Encroachment in Alpine Grasslands

Mountain pastures in the Alps are cultural landscapes that have been shaped over centuries by traditional grazing practices. These pastures harbour unique biodiversity and provide multiple ecosystem services, such as forage production and soil carbon sequestration. However, in recent decades, socio-economic changes have led to a widespread decline in mountain agriculture, resulting in pasture abandonment and woody encroachment. This study assesses how early successional woody encroachment affects soil carbon storage and nutrient dynamics along an elevation gradient.

To this end, 15-metre transects were randomly placed within non-encroached and encroached areas (with >20% cover of juvenile trees) of eight mountain pastures, which ranged in altitude from 680 to 1270 meters above sea level in the Berchtesgaden National Park in the Northern Limestone Alps in Germany. Soil samples were taken to a depth of 30 cm and analyzed for total organic carbon (TOC), total nitrogen, and available phosphorus (Olsen-P). Bulk density was also measured, and nutrient stocks were calculated.

TOC and nitrogen concentrations, as well as Olsen-P, were significantly higher with encroachment, while carbon and nitrogen stocks showed no significant differences between encroached and non-encroached transects. The effect on TOC was more pronounced in the upper soil layer; in the lower layer, elevation and aspect also significantly affected TOC levels. The magnitude of the TOC increase in encroached sites could be partially explained by soil pH. These results highlight the variable effects of woody encroachment on nutrient and carbon dynamics, which depend on the successional stage, elevation, aspect and parent material.