Terrestrial denitrification:challenges and opportunities

Denitrification is a process of great environmental importance but is difficult to study in terrestrial ecosystems.Methods for quantifying the process are problematic,variability in activity is high,and temporal and spatial scaling challenges are extreme.Available methods are problematic for a variety of reasons;they change substrate concentrations,disturb the physical setting of the process,lack sensitivity or are prohibitively costly in time and expense.Most fundamentally,it is very difficult to quantify the dominant end-product(N_(2))of denitrification given its high background concentration in the atmosphere.Spatial and temporal variation in denitrification is high due to control of the process by multiple factors(oxygen,nitrate,carbon,pH,salinity,temperature etc.)that each vary in time and space.A particular challenge is that small areas(hotspots)and brief periods(hot moments)frequently account for a high percentage of N gas flux activity.These phenomena are challenging to account for in measurement,modeling and scaling efforts.The need for scaling is driven by the fact that there is a need for information on this microscale process at the ecosystem,landscape and regional scales where there are concerns about nitrogen effects on soil fertility,water quality and air quality.In this review,I outline the key challenges involved with denitrification and then describe specific opportunities for making progress on these challenges including advances in measurement methods,new conceptual approaches for addressing hotspot and hot moment dynamics,and new remote sensing and geographic information system–based scaling methods.Analysis of these opportunities suggests that we are poised to make great improvements in our understanding of terrestrial denitrification.These improvements will increase our basic science understanding of a complex biogeochemical process and our ability to manage widespread nitrogen pollution problems.