Foliar ẟ15N patterns in legumes and non-N fixers across a climate gradient, Hawai'i Island, USA

Recent studies from the Hawaiian Islands showed that pedogenic thresholds demarcate domains in which rock-derived nutrient dynamics remain similar across wide variations in rainfall. These thresholds appear related to certain aspects of N cycling, but the degree to which they correspond to patterns of biological N fixation (BNF)-the dominant input of N into less-managed ecosystems-remains unclear. We measured aboveground plant biomass, foliar nutrient concentrations, and foliar δ¹⁵N along a climate gradient on ~ 150,000-year-old basaltic substrate to characterize foliar N sources and spatially relate them to soil nutrients. Patterns in legume δ¹⁵N correspond to known pedogenic thresholds along the rainfall gradient, with low δ¹⁵N values (~ 0 to - 2‰) occurring in the dry, biologically inactive domain and the wet, highly weathered domain. Elevated δ¹⁵N in the middle, fertile domain suggests a greater reliance of legumes on soil N where it has accumulated over time. Non-legume face N deficiencies throughout most of the gradient while legumes maintain low C:N ratios via symbiotic BNF. However, legume abundance declines outside the fertile domain, limiting ecosystem N inputs. Breakpoints in legume δ¹⁵N data suggest that P (and potentially other nutrients) limits BNF and, by extension, legume abundance in wet region. Nutrients may also constrain legume abundance in the dry domain, but pedogenic effects could not be isolated from climatic constraints at the dry sites. We conclude that pedogenic thresholds defined by climate can be informative of foliar δ¹⁵N patterns in cases where legumes are not directly constrained by climate, land use, or other external factors.