Climatic controls on the isotopic composition and availability of soil nitrogen in mountainous tropical forests

Abstract:

Tropical forests in mountainous regions are often assumed to be nitrogen (N) limited, yet N dynamics across rugged terrain can be complex due to gradients in climate and topography. Elucidating patterns of N availability and loss across such gradients is necessary to predict and manage tropical forest response to environmental changes such as increasing N deposition and rising temperatures. However, such data is currently lacking, particularly in remote locations that are of high conservation value. To address this gap, a research expedition organized by the American Climber Science Program recently made a coast-to-coast journey across a remote region of Costa Rica, travelling over the Cordillera Talamanca and through La Amistad International Park. Numerous biological, chemical and hydrologic measurements were made en-route across montane to premontane wet tropical forests, spanning nearly 2,000 m in elevation and 200 km. Surface soil samples collected at regular intervals along this transect illuminate environmental drivers of N dynamics across the region. The dataset reveals strong links between soil natural abundance N isotopic composition (δ¹⁵N) and elevation and temperature parameters, and weaker links to precipitation and topography. This is in general agreement with global scale observations, but divergence from some previously published works is apparent and will be discussed. δ¹⁵N mass balance models suggest that N isotope patterns reflect differences in forms of N loss and the relative importance of fractionating and non-fractionating pathways. When combined with data on several other edaphic properties, especially C:N stoichiometry, the results points toward notable variation in soil N availability and N constraints across the transect. This study illustrates large, but predictable, variation in key N cycle traits across the premontane to montane wet tropical forest transition. These findings have management-relevant implications for tropical regions.