Abstract
We evaluated N dynamics on a climate gradient on old (> 4 million year) basaltic substrate on the Island of Kaua’i, Hawai’i, to evaluate the utility of pedogenic thresholds and soil process domains for understanding N cycling in terrestrial ecosystems. Studies of nitrogen dynamics on the climate gradient on a younger basaltic substrate (~ 150,000 year) had found a good match between soil process domains and N cycling processes. Here we measured net N mineralization and nitrification by incubation, and δ15N of total soil N, to determine whether the soil process domains on the older gradient were equally useful for interpreting N cycling and thereby to explore the general utility of the approach. Net N mineralization varied from 0 to 1.7 mg kg−1 d−1 across the old Kaua’i gradient, and δ15N varied from + 3 to + 11 ο/οο, both ranges similar to those on the younger substrate. However, while the pattern of variation with climate was similar for δ15N, the highest rates of mineralization on the old gradient occurred where forests were dominated by the native N fixer Acacia koa. This occurred in sites wetter than the process domain associated with high net N mineralization on the gradient on younger substrate. We conclude that soil process domains based on rock-derived nutrients are not always useful for evaluating N dynamics, especially where the distribution of biological N fixers is controlled by factors other than rock-derived nutrients.
Highlights
Nitrogen (N) is the element that most often limits primary production and other ecosystem processes in little-managed ecosystems, either alone or in combination with other elements (Elser et al 2007; Hogberg et al 2017)
Weathering and loss of P was nearly complete by an average annual rainfall of 1500 mm year−1 for the Kaua’i gradient, while some parent material P was retained in Kohala sites to rainfall of > 2000 mm year−1 (Vitousek and Chadwick 2013)
Our focus here was to evaluate whether pedogenic thresholds and soil process domains (Chadwick and Chorover 2001; Vitousek and Chadwick 2013) provide useful information for understanding N dynamics
Summary
Nitrogen (N) is the element that most often limits primary production and other ecosystem processes in little-managed ecosystems, either alone or in combination with other elements (frequently phosphorus) (Elser et al 2007; Hogberg et al 2017). N is rarely an independent control over ecosystems, in the sense that (absent fertilization, anthropogenic atmospheric deposition, or other interventions) the pools and availability of N reflect the influence of other controlling processes such as biological N fixation and its controls (often including the availability of phosphorus), and the losses of N by pathways that are Communicated by Richard P. Recent research in the Hawaiian archipelago and elsewhere has identified pedogenic thresholds and soil process domains that emerge along continuous gradients of environmental forcing. Despite the importance of N to biological systems, analyses of thresholds and domains have not generally included N—at least in part because the approach derives from pedology, and pedologists generally consider N to reflect other, independent controlling processes rather than itself being an independent control of soil and ecosystem dynamics
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