Bibliography - L.O. Hedin

1. Brookshire, E. N. Jack, L.O. Hedin, J. Denis Newbold, Daniel Sigman, and John K. Jackson, January 2012: Sustained losses of bioavailable nitrogen from montane tropical forests. Nature Geosciences, Nature Publishing Group, doi:10.1038/ngeo1372
   [ Abstract ]

   Tropical forests account for one third of terrestrial primary production and contribute significantly to the land carbon sink^{1, 2}. The future of this sink relies critically on forest interactions with nutrient cycles^{3, 4, 5}. Humid montane tropical forests are often thought to be rich in phosphorus, but to contain low levels of bioavailable nitrogen⁶. Here, we examine the concentration of dissolved nitrogen compounds and the isotopic composition of nitrate in streams in six well-characterized and phosphorus-rich montane forests⁷ in Costa Rica, and in 55 montane forests across Central America and the Caribbean, using data collected between 1990 and 2008. We found high levels of nitrate in these streams, indicative of large losses of bioavailable nitrogen from these forests. We detected no trend in the concentration and isotopic signature of nitrate over the measurement period, implying that high levels of export are neither recent nor episodic. An analysis of the oxygen isotopic signature of stream nitrate showed that exports are sourced from the plant-soil system, rather than from atmospheric deposition that bypasses forest biota. Our findings indicate that nitrogen-rich conditions can develop irrespective of phosphorus availability at the ecosystem scale. We suggest that nitrogen availability may not limit plant growth, or its response to increasing atmospheric carbon dioxide levels, in many montane tropical forests.

2. Gerber, S., L.O. Hedin, Michael Oppenheimer, Stephen W. Pacala, and E. Shevliakova, 2009: Nitrogen Cycling and Feedbacks in a Global Dynamic Land Model. Global Biogeochemical Cycles, http://www.agu.org/journals/pip/gb/2008GB003336-pip.pdf, doi:10.1029/2008GB003336
   [ Abstract ]

   Global anthropogenic changes in carbon (C) and nitrogen (N) cycles call for modeling tools that are able to address and quantify essential interactions between N, C, and climate in terrestrial ecosystems. Here, we introduce a prognostic N cycle within the Princeton-GFDL LM3V land model. The model captures mechanisms essential for N cycling and their feedbacks on C cycling: N limitation of plant productivity, the N dependence of C decomposition and stabilization in soils, removal of available N by competing sinks, ecosystem losses that include dissolved organic and volatile N, and ecosystem inputs through biological N fixation.
   Our model captures many essential characteristics of C-N interactions, and is capable of broadly recreating spatial and temporal variations in N and C dynamics. The introduced N dynamics improves the model’s short term NPP response to step changes in CO₂. Consistent with theories of successional dynamics, we find that physical disturbance induces strong C-N feedbacks, caused by intermittent N loss and subsequent N limitation. In contrast, C-N interactions are weak when the coupled model system approaches equilibrium. Thus, at steady state many simulated features of the carbon cycle, such as primary productivity and carbon inventories are similar to simulations that do not include C-N feedbacks.

3. Houlton, B. Z., Daniel Sigman, E.A.G. Schuur, and L.O. Hedin, 2007: A climate-driven switch in plant nitrogen acquisition within tropical forest communities. Proceedings of the National Academy of Sciences of the United States of America, 104(21), doi:10.1073/pnas.0609935104 8902-8906
   [ Abstract ]

   The response of tropical forests to climate change will depend on individual plant species’ nutritional strategies, which have not been defined in the case of the nitrogen nutrition that is critical to sustaining plant growth and photosynthesis. We used isotope natural abundances to show that a group of tropical plant species with diverse growth strategies (trees and ferns, canopy, and subcanopy) relied on a common pool of inorganic nitrogen, rather than specializing on different nitrogen pools. Moreover, the tropical species we examined changed their dominant nitrogen source abruptly, and in unison, in response to precipitation change. This threshold response indicates a coherent strategy among species to exploit the most available form of nitrogen in soils. The apparent community-wide flexibility in nitrogen uptake suggests that diverse species within tropical forests can physiologically track changes in nitrogen cycling caused by climate change.

Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=3589