Manual Chamber Sampling Strategies to Help Account for Temporal and Spatial Variability of N2O Emissions from Agricultural Cropping Systems.

Despite a strong mechanistic knowledge of the biotic and abiotic processes that generate nitrous oxide (N₂O) in the soil, the inherent heterogeneity of agricultural landscapes, and the complex and interactive nature of the environmental and management factors that influence N₂O emissions, make accurate quantification of N₂O emissions challenging.

Due to multiplicative effects arising from the many interacting processes and factors that govern N₂O in the soil, emissions of N₂O are patchy; the spatial variability (CVs) at multiple scales frequently exceeds 100%. Elevated, short duration fluxes of N₂O often rapidly follow management and environmental events such as nitrogen (N) fertilization, rainfall, tillage, and freeze-thaw cycles. A small number of these sporadic N₂O peaks can contribute substantially to cumulative emissions over annual or longer time periods. Increasing the accuracy of N₂O emissions measurements can help improve estimates of agricultural GHG inventories, better identify N₂O mitigation opportunities, and advance modeling efforts for better prediction of future emissions.

Sampling regimes must therefore be designed to adequately capture the range of conditions conducive to N₂O emissions in the area being investigated. At a field plot scale, typically, N, carbon (C), and oxygen availability (as controlled by water content of the soil) are the key soil attributes that control the pattern and magnitude of N₂O producing processes.

Other than scientific rationale, resource trade-offs for measurement of N₂O fluxes need careful consideration; the financial cost of measurement and analysis, the logistics associated with chamber or other sampling technologies, and the labor required for both. Where multiple treatment comparisons and /or smaller scale field experimentation are required, chamber technology with GC analysis is still the methodology of choice. Although automated systems are becoming more common, manual chamber operators in the field with sample transport to the laboratory for analysis still predominate.

Well-designed, manual chamber sampling strategies can address the temporal and spatial variability of N₂O emissions from agro-ecosystems. In this presentation we will discuss some general scientific principles and logistical constraints that need to be considered before gas sampling begins. We highlight some of these issues, including chamber number and placement, and sampling timing and frequency, using case-studies in Michigan, Maryland, and Puerto Rico.