Soil and fine root chemistry at a tropical Andean timberline

- Nitrogen, phosphorus, and base cations decreased from forest to grassland.
- Stable carbon isotope and shrub patterns suggest that this timberline may be advancing.
- Patterns of soil and fine root chemistry provide evidence of land-use legacies.

Tropical timberlines are dynamic ecotones influenced not only by climate but often also by human activity. Understanding differences in soils across these boundaries therefore requires explicit consideration of land use. We examined patterns of soil and fine root chemistry at a tropical timberline ecotone in northern Peru nearly three decades after grazing and fire reduction. Soils were collected in forest, edge, and grassland habitat on east- and west-facing slopes and evidence of recent grazing, past fire, and shrub presence was recorded in alpine grasslands. We determined concentrations and mass (to 10 cm) of base cations, phosphorus (P), and inorganic nitrogen (N) in soil, as well as total carbon (C), total N, δ13C, and δ15N in soils and fine roots. Calcium and magnesium concentrations and mass, and soil P and fine root N concentrations decreased from forest to grassland. Lower soil P, soil N, and fine root N concentrations were associated with enriched soil δ15N, indicating enhanced nutrient losses from edge and grassland soils. The stable carbon isotopic composition of soils and fine roots was more enriched in grassland (soil δ13C = − 24.1‰; fine root δ13C = − 25.1‰) compared to forest (soil δ13C = − 26.1‰; fine root δ13C = − 28.2‰), except where shrubs were present. In grasslands, topography appeared to influence grazing and shrub expansion; shrubs were more prevalent on the steeper west-facing slopes while cattle dung was found only on the shallower east-facing slopes. The spatially heterogeneous patterns of shrub establishment and surface soil and fine root chemistry across these forest-grassland boundaries provide evidence of legacies of grazing and burning, underscoring the need for further research on land-use legacy effects on belowground processes in tropical timberline ecosystems.