New paradigms in nitrogen cycling

Background/Question/Methods

A trademark of Terry Chapin’s science is its integrative approach; his early work focused on combining physiological ecology with community dynamics to understand ecosystem biogeochemistry. He was working in tundra and taiga study systems, in which N is a critically limiting resource. These systems didn’t follow the patterns predicted by simple conceptions of resource dynamics that had developed from thinking focused on agricultural monocultures and the mineral nutrition theory. Addressing the limits of those ideas led to two directions that are now prevalent in ecosystem science.

Results/Conclusions

The first direction was recognizing that plants that evolved under low nutrients allocated the bulk of their resources to nutrient acquisition and so had low maximum growth rates; if they are fertilized, total ecosystem production increases but at the expense of the native communities. Integrating plant physiology and the individualistic plant responses to environmental drivers has become a critical component of linking community and ecosystem ecology and creating a more unified vision of ecosystem N-cycling. It has also become an important aspect of invasive species biology.

The second major development was to recognize that plants might not use only inorganic nutrients. During the Arctic growing season tussock tundra plants demonstrably take up N, yet soil measurements showed net N-immobilization. Plants were taking up N in places, times, or forms that were not being captured by soil mineralization assays. Chapin launched studies to evaluate whether tundra plants could use organic forms of N. Eriophorum vaginatum can use amino acids for N and actually grows better with them than with mineral sources of N. Other researchers have picked up on that theme to begin developing an integrated picture of soil N cycling that incudes the role of extracellular enzymes in breaking down N-containing polymers, plant-microbial competition for multiple forms of N, and the role of plant life-history strategies in controlling ecosytem N-cycling. We still do not know how much of any plant’s total N uptake is as organic vs. inorganic forms, but the new more integrative paradigm of N-cycling continues to grow and evolve, tying in plant and microbial physiology, community dynamics, and the physical environment that regulates the interactions among them.