Human activities have doubled annual reactive nitrogen inputs to the biosphere with major implications for water and air quality. Recent studies show that most inorganic nitrogen is rapidly converted to organic form and efficiently retained in soils for decades. This proposal will measure the rate and magnitude of nitrogen retention in stable soil organic matter of forest, urban, and agricultural ecosystems. The research is guided by a conceptual model for the nitrogen cycle that explicitly includes a stable organic pool and its dynamics. The proposal focuses on three questions: What are the rate and magnitude of nitrogen retention in stable soil organic matter? What is the relative contribution of the microbial pathway in stable nitrogen formation? What are the molecular structures of the organic nitrogen forms?
Results will be integrated into high school curricula by a participant with expertise in experiential learning and high school science instruction, linking to Pennsylvania State Standards of Learning for the Environment and Ecology, the newest learning standards in the state. Undergraduate students from underrepresented groups will be recruited using established programs. The work will also link three junior level scientists from diverse fields of ecology, chemistry and education.
The global nitrogen cycle is accelerating as human-derived reactive nitrogen enters the biosphere at unprecedented rates. Forest soils can be rapid and large sinks for these new nitrogen inputs, while agricultural and urban ecosystems are often sources of nitrogen pollution to the atmosphere and aquatic ecosystems. The main goal of this research project was to advance ecosystem nitrogen retention theory by measuring the rate and magnitude of nitrogen retention in stable soil organic matter of forest, urban, and agricultural ecosystems. The research improved our mechanistic understanding of how nitrogen becomes stabilized in soils and why soil nitrogen retention varies among land-use types. We found that human dominated (urban and agricultural) ecosystems have lost a key mechanism of N retention: rapid transfer of inorganic N into stable soil organic matter. In forests, this process accounts for 30% of inorganic N additions to O horizons and 15 % of N added to mineral soils. In addition, transfer of inorganic N into labile (easily decomposed by soil microorganisms) soil organic matter is also an important short-term (within 2 days) N retention mechanism in forests. While urban ecosystems retain less N in stable soil organic matter than forests, they have very rapid exchanges of N between soil inorganic N, labile soil N, soil microorganisms, and roots. Thus plant and microbial uptake and recycling of N may be a particularly important, and perhaps effective, mechanism of fertilizer N retention in heavily fertilized turfgrass systems, even when soil N is not stabilized in organic matter. In contrast, agricultural ecosystems had low N recovery in soils (any pool) and roots, even at the height of the corn growing season. Our research findings were disseminated via presentations and peer reviewed publications. In addition, we developed and distributed a high school nitrogen cycle curriculum that is linked to state learning standards in the area of ecology and the environment. Our objective for the curriculum is to provide teachers with a suite of tools to teach students about interactions among land-use, soils, the nitrogen cycle, and environmental and human health.
