Biological Nitrogen Fixation Detected through Stable Isotope Ratios across a Climate Gradient in Hawai'i

I evaluate nitrogen (N) cycling across a rainfall gradient in leeward Kohala on the island of Hawaiʻi, focusing on a large set of vegetation samples collected between 2015 and 2017. Vegetation was sampled at a series of points where soils had been previously characterized across the climate gradient. An array of isotopic and elemental abundance analyses, combined with biomass data from July 2015, reveal relationships between climate, biological N fixation (BNF), and the soil N supply: δ15N of non-leguminous plants predictably mirrors soil δ15N, while legume δ15N values are closer to that of the atmosphere (indicative of BNF), especially towards the wet end of the gradient. Herbaceous legumes have elevated δ15N in the fertile region previously occupied by the Leeward Kohala Field System, indicating a lesser reliance on BNF at moderate rainfall levels. Herbaceous legume biomass also peaks in this region, suggesting the relative decrease in the fraction of N from BNF is not hindering plant growth, but that plants are instead able to derive greater amounts of N from the soil system. As leaf C:N ratios sharply increase in non-leguminous plants below 1,000 mm of annual rainfall while legumes show very consistent C:N across the gradient, BNF appears especially important for legumes in the arid zone while non-legumes face N deficiencies across many rainfall levels.Interestingly, a sharp soil δ15N drop-off in the very arid sites of the leeward Kohala gradient is accompanied by a peak in leguminous tree δ15N (more positive values closer to those of soil), which indicates increased BNF is not responsible for the soil δ15N signal. Leaf nutrient data reveal trends in calcium and phosphorus uptake by woody legumes, suggesting dryland BNF may be limited by the abundance of these elements, although these trends differ distinctly by species and are likely influenced by other nutrients and environmental factors. Herbaceous legumes and non-leguminous grasses all show similar trends in foliar phosphorus, potassium, and iron levels, but legumes show elevated manganese, magnesium, and calcium concentrations versus the grasses. Foliar phosphorus concentrations suggest that the element is plentiful in the soils of leeward Kohala’s fertile zone—formerly the center of the field system—perhaps accounting for high total amounts of BNF that led to the high N levels in the soil there. Potassium concentrations across all plant species also peak in the middle of leeward Kohala’s fertile zone, indicating it could possibly constrain plant growth at the extremes of the gradient.While this study demonstrates the impressive ability of ancient Hawaiians to locate and exploit the most fertile soils of leeward Kohala, the results could also have important implications for the future of the region. For instance, the post-contact replacement of native dryland forests and Hawaiian croplands with exotic pasture plants (including legumes) could have resulted in increased N inputs from BNF and altered micronutrient regimes throughout the soil system that persist to this day. These drastic ecological changes will almost certainly influence ongoing efforts to restore agricultural productivity and connectedness to Kohala’s slopes, as the revitalization of traditional Hawaiian agriculture today will face different challenges than those met by the ancient agriculturalists of the Leeward Kohala Field System.

Keywords:  Earth Systems Program
School of Earth Energy &amp; Environmental Sciences
Biological Nitrogen Fixation
Kohala
Stable isotopes
Earth Systems
Hawaii
Text
Thesis
Advisor: Vitousek Peter
Fendorf Scott