Soil and microbial nutrient status are heterogeneous within an elevational belt on a neotropical mountain

Abstract

Tropical mountains support a variety of vegetation types that are threatened by global change, but our understanding of these threats is limited by our scarce knowledge of nutrient constraints on belowground processes in these sensitive ecosystems. To address this, we quantified chemical and microbiological properties at five sites associated with four vegetation types at high elevations (>3000 m a.s.l.) on Volcán Barú, the highest mountain in Panamá. At each site we quantified total and extractable soil nutrients, microbial biomass nutrients, and the activities of hydrolytic enzymes involved in nutrient cycling. We found high heterogeneity among vegetation types in soil and microbial nutrients. Phosphorus (P) was mainly in organic form (>70%) in high organic matter soils under forest and grassland, but mostly in inorganic form (≥68%) in shrublands. Inorganic nitrogen (N) concentrations were low to moderate in all soils (0.7–8.7 mg N kg-1), whereas extractable P measured by anion-exchange resins was relatively high (5–54 mg P kg-1) compared with tropical lowlands. We found marked variation in microbial N (51–199 mg N kg-1), microbial P (40–170 mg kg-1), and hydrolytic enzyme activities (e.g. ß-glucosidase from 202 to 1399 nmol product g-1 h-1). Stoichiometric ratios in microbial biomass and between enzymes provided further evidence of shifts in the relative investment in N vs P acquisition among vegetation types. For example, the enzymatic N:P ratio (N-acetyl-ß-D-glucosaminidase (NAG) to phosphomonoesterase (PME)) pointed to greater investment in microbial acquisition of N relative to P in bamboo-oak forest (NAG:PME = 0.55) compared to grassland and shrublands (NAG:PME ≤0.30). In conclusion, our results show that tropical mountains can be highly heterogeneous within a relatively narrow elevation range, indicating the likelihood of contrasting responses of plant and microbial communities to global change drivers such as cloud base, precipitation, and nitrogen deposition in tropical mountains.