Measuring the Enigma of Nitrogen Balances.

Agronomists, ecologists, oceanographers, and atmospheric scientists have been grappling for many decades with the challenges of measuring the numerous forms of nitrogen (N) in the environment and attempting to balance estimates of inputs and outputs at scales ranging from pot studies to the farm gate to the globe. While confident estimates of N balance remain rare, advances in the application of new measurement systems have promoted better understanding of processes. For example, combining soil chamber flux measurements of nitric oxide (NO) and nitrous oxide (N2O), which had been addressed mostly independently in the early 1980s by atmospheric chemistry and agronomy communities, respectively, enabled articulation of the “hole-in-the-pipe” model that helped integrate and synthesize process-level controls on emissions of each. More recently, lasers are enabling real-time, high frequency, high precision N2O measurements that improve both chamber and micrometeorological flux estimates. Use of Membrane Inlet Mass Spectrometry (MIMS) has enabled aquatic ecologists to estimate dinitrogen gas (N2) production and consumption in sediments, and new application of MIMS to groundwater and streams draining agricultural areas may be promising. Stable isotopic analyses of 13C and 15N in plant tissue and soil organic matter are not new, but novel applications have helped constrain field-scale budgets. New nitrate sensors could revolutionize the monitoring of this key mobile N species. Large uncertainties remain in the global N budget, but these new technologies and innovative approaches offer opportunities to target key knowledge gaps.