Nutrient Cycling and Limitation: Hawai'i as a Model System. Peter M. Vitousek. Princeton University Press, Princeton, NJ, 2004. 232 pp. $35.00 (ISBN 069111580X paper).
Peter Vitousek's Nutrient Cycling andLimitation: Hawai'i as a Model System makes an important contribution to the field of biogeochemistry. In this excellent book, Vitousek draws on examples from research he and others have conducted on Hawai'i since the early 1980s to explore general principles about the processes controlling nutrient availability, cycling, and limitation in terrestrial ecosystems. He begins by introducing four fundamental biogeochemical questions that apply to all terrestrial ecosystems. He then moves from the general to the specific, sharing with his readers the “stories that the data have to tell” about nutrient dynamics along a “substrate age gradient” that spans more than four million years and includes six core sites distributed across the Hawaiian archipelago. In the final chapter, Vitousek puts the Hawai'i studies in a global context as he returns to the four fundamental questions raised in the book's introduction.
The four questions that open and close the book have challenged ecosystem ecologists for the past quarter-century: How do biological and geochemical processes that operate on very different timescales interact to cause, sustain, or offset nutrient limitation? How are element inputs to and losses from terrestrial ecosystems regulated, and what are the implications for nutrient cycling and limitation? How do the cycles of different elements interact? Finally, how do genotypes, species, and communities of organisms affect nutrient cycling and limitation in ecosystems? Vitousek argues that these questions can be answered more straightforwardly in Hawai'i than anywhere else. Along with reviewing the natural history of Hawai'i in chapter 2, he describes the attributes of this Pacific archipelago that make it a model system that facilitates research on these questions.
Often the systems chosen as models for research are relatively simple in structure, a factor that facilitates observation and experimentation. Simplicity is an essential feature of the six core sites at which Vitousek and his colleagues have done much of their field research. Substrate age is the dominant variable across their sites, with the oldest site being 4.1 million and the youngest being 300 years old. The six core sites have developed on rock that varies relatively little in chemistry. They are all forested, with canopies dominated by the tree Metrosideros polymorpha and subcanopies dominated by native tree ferns of the genus Cibotium. All occur between 1120 and 1210 meters in elevation and share a common climate—each has a mean annual temperature near 15.5 degrees Celsius and receives about 2500 millimeters of precipitation yearly. Additional details on the characteristics of the gradient are given in chapter 3.
Over more than two decades of research, Vitousek and his colleagues, including a cadre of graduate students, have used observational studies and experimental manipulations along the substrate age gradient to test theories and gain new insights about nutrient dynamics during ecosystem development. They use observational studies to reveal patterns, and experimental manipulations to identify the mechanisms responsible for the patterns.
Many of their studies focus on nitrogen (N) and phosphorus (P), two elements required in relatively large quantities by all living organisms. In the book's fourth chapter, Vitousek writes about how he and his colleagues have used observational studies along the gradient to test conceptual models such as the one developed by Walker and Syers (1976) that describes how total P stocks and the bioavailability of P change during long-term soil development. The studies on P done across the Hawai'i gradient generally confirm this model and, when combined with observational studies of N pools and fluxes across the gradient, indicate that young sites have low levels of available P and N, intermediate-aged sites have relatively high levels of P and N availability, and the oldest sites have low P but high N. In chapter 4, Vitousek also uses observational studies to develop the concept of a plant–soil–microbe positive feedback that has different implications for infertile versus fertile sites. The idea is that biological responses to a geochemically driven shortage of nutrients can further reduce nutrient availability and set in motion a positive feedback. The feedback functions to reinforce nutrient deficiency in infertile sites and greater nutrient availability in fertile sites.
While observations along the substrate age gradient in Hawai'i illustrate the feedback, they do not explain why the feedback works as it does. To get at the “why” requires experimental manipulations. In the book's fifth chapter, Vitousek describes several factorial fertilization experiments carried out across the gradient, and uses the results to explain nutrient limitations and controls of plant–soil–microbe feedbacks. With respect to nutrient limitations, responses to fertilization across the substrate age gradient strongly support predictions based on the conceptual model of Walker and Syers—N supply limits tree growth at the young site, N and P supply have equilibrated at a relatively high level at the intermediate-aged site, and P limits growth at the oldest site. On the topic of plant–soil–microbe feedbacks, the effects of P fertilization on P-limited (older) sites were different from those of N fertilization on N-limited (younger) sites. Overall, fertilization with P caused the P-limited 4.1-million-year-old site to function more like a forest on a naturally fertile site, at least with regard to P cycling. In contrast, N fertilization did not cause the forest on the N-limited 300-year-old-site site to function like a forest on a naturally more fertile site. Vitousek sets out several plausible explanations for these differences and explores them further in the book's eighth and final chapter.
Nutrient inputs and outputs are the topics of the sixth and seventh chapters, respectively. I found the discussion of atmospheric transport of P in dust from Asia particularly well presented as an example of an important “teleconnection” operating in the Earth system. Vitousek argues that one of the most striking results of the Hawaiian study is the significance of this continental dust to the P economy of old soils in the archipelago. Near the end of chapter 6, Vitousek states that “were it not for the trickle of P that travels more than 6000 km in dust from Asia, much of it during full glacial periods, I'm not sure it would be possible to maintain forests on the oldest substrates.” He goes on to point out the general applicability of this phenomenon to older soils across the globe.
The last chapter is titled “Issues and Opportunities.” The first issue that Vitousek explores is the interaction of nutrient cycling processes operating at different timescales—supply and demand for nutrients over minutes to years; feedbacks between nutrient availability, plant growth, and nutrient supply over months to decades; nutrient sources and sinks over years to centuries; and inputs and outputs of nutrients over centuries to millions of years. This is done with a simple model that is available on the Web. One interesting result of this model is that the three fastest sets of processes—nutrient supply and demand, plant–soil–microbe feedback, and nutrient sources and sinks—all reinforce each other in positive feedbacks that reduce nutrient availability in nutrient-poor sites and enhance it in rich sites. An equally interesting model result is that nutrient input–output balances provide a long-term negative feedback to nutrient limitation that can be large enough to offset the three sets of faster processes. I am still pondering the robustness of these insights into the interactions of biogeo-chemical feedbacks across time scales; that is, do they operate in the same way in most other terrestrial ecosystems? I also find myself wondering about the degree to which Vitousek's interpretations are dependent on the structure of his simple model.
Another issue that Vitousek explores in the final chapter is the influence of changes in populations, species, and diversity on nutrient dynamics over different time scales. He structures his discussion of this issue with two questions: Could relatively subtle differences in biota across the Hawaiian age gradient affect the functioning of ecosystems there? And would the functioning of Hawaiian ecosystems across the gradient differ if biological diversity in Hawai'i were substantially greater? While neither question is definitively answered, I found Vitousek's considerations of them stimulating.
Nutrient Cycling and Limitation is essential reading for students and scientists interested in terrestrial biogeo-chemistry. It is a model of good science writing and a crisp and clear introduction to some of the big ideas that intrigue ecosystem ecologists.
