Plant-Regulated Soil N Retention in Temperate Forest Ecosystems:
an Experimental Test
Project Summary
Human alteration of global nitrogen (N) cycling calls for the mechanistic
understanding of N retention at different spatial and temporal scales. This proposal
addresses the plant-soil-microbial ecosystem and terrestrial N retention. The proposal
builds on the current ecological information that soil is the largest sink for N deposition,
that plants exert pivotal controls on soil N transformations and transfers, and that large
spatial and temporal variations exist in terrestrial N retention that require mechanistic
explanation. It is hypothesized in this proposal that 1) by driving upward hydrological
movement in soil, plants reduce downward water movement and therefore N output via
leaching, 2) by regulating soil microbial activities through rhizodeposition, plants modify
processes of microbial N immobilization, mineralization, and nitrification, and 3) by
providing recalcitrant carbon as structural materials for humus formation, plants regulate
abiotic N assimilation into soil organic matter. Thus in addition to plants’ direct
assimilation of N through biomass increment, plants can indirectly regulate N retention in
soil. Each of the above hypotheses will be experimentally tested in greenhouse, with
field-monitoring studies planned to provide a framework to interpret the results of the
greenhouse-based studies.
This work will contribute to the broader effort of understanding the hierarchy of
controls on N retention in temperate forest ecosystems. The experimental approach
proposed herein contrasts with the more common approach involving large-scale
comparative studies for understanding patterns of forest N retention. The experimental
approach should identify the factors and conditions which control complex N
transformations, transfers and retention in coupled plant-soil-microbial systems. The
mass balance approach and isotope pulse-labeling approach proposed in this study should
enhance each other’s findings and better explain not only which pools serve as N sinks
and at what scale, but also how N enters into a particular pool and the controlling
mechanisms involved. A mechanistic understanding of N retention will contribute to our
general knowledge of terrestrial N biogeochemistry and allow us to predict N transfers
and transformations under globally elevated N conditions.
The fact that temperate forests are broadly distributed throughout the
industrialized world (as well as developing countries in Asia), subject to large inputs of
atmospheric N deposition, and are the most familiar and accessible ecosystems to the
general public, makes the proposed study having broader impacts beyond purely
academic interest. The proposed research projects will be integrated into the PI’s
education efforts in the affiliated institution, and various student involvement can
simultaneously benefit both research and education goals of this study. This project will
demonstrate the ecosystem value of secondary growth forests in controlling non-point
source N pollution, countering global CO2 increase, and regulating hydrological and
nutrient outputs from the local terrestrial ecosystems.