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Harvard Forest Symposium Abstract 2012

• Title: Development of a process-based ecosystem model to simulate the dynamics of 15-N in a hardwood forest
• Primary Author: Zaixing Zhou (University of New Hampshire - Main Campus)
• Additional Authors: Changsheng Li (University of New Hampshire - Main Campus); Scott Ollinger (University of New Hampshire - Main Campus)
• Abstract:

  Nitrogen (N) cycling is a fundamental ecological process because N is both a common limiting nutrient and a contributor to atmospheric and aquatic pollution. The isotope ratio of 15N to 14N in various N pools (expressed in δ15N) in terrestrial ecosystems varies with different δ15N sources and/or N transformation rates and can, thus, offer insights into nitrogen cycling at different temporal and spatial scales. In the past two decades, the N cycle has been studied increasingly by using 15N tracers or 15N natural abundances, owing to the general increase in availability of stable isotope technology (e.g., continuous-flow isotope ratio mass spectrometer). δ15N signals in N pools have been used to quantify N allocation, identify N inputs to, and detect N losses from, ecosystems.
  
  
  
  The complexity of the N cycle has limited the development and broad application of mechanistic N cycling models. In fact, detailed process-based models capable of explaining spatial and temporal patterns of natural abundances of 15N have not yet been developed. In this study, we present a new effort designed to improve the ability to simulate N cycling in forests by incorporating isotope 15N fractionation processes into a soil biogeochemistry model known as Forest-DNDC. As a case study, we tested the revised model against observations in a northeastern hardwood forest at Harvard Forest in Massachusetts, U.S.A, which has a long term nitrogen addition experiment to assess the effect of elevated N inputs on temperate forest stands.
  
  
  
  Model interpretations of field-observed 15N recovery show that forest floor soil organic matter and foliage retained 15N rapidly, accounting for about 90% of 15N added, and then released some of this 15N for plant uptake in wood and loss via leaching and gases. In 2010, the simulations still suggested that the soil and tree biomass retained 53% and 25% of 15N amended, respectively, in Low N plots, and 48% and 42%, respectively, in control plots. The simulated mineral soil δ15N values were consistent with field observations, while the simulated δ15N values in foliage and wood and forest floor were overestimated. This discrepancy may be attributed to the lack of the process of abiotic immobilization suggested by the field investigation. The new featured Forest-DNDC with 15N signals prediction could help to interpret 15N abundance in forest systems and therefore N cycling processes.
  
  

• Research Category: Ecological Informatics and Modelling