Monitoring nitrogen deposition in global forests

Monitoring the status of nitrogen (N) deposition is a prerequisite for evaluating its ecological and environmental impacts. This chapter reviews the monitoring strategies to measure N deposition to forest ecosystems in six different regions (Europe, North America, East Asia, South Asia, Africa, Central and South America). The current status of monitoring is at very different stages for these regions, ranging from sophisticated and coordinated monitoring networks devoted to the quantification of fluxes to both forested and non-forested sites to sparse intensive studies with variations in methodologies and limited quality assurance protocols. Methods for routine measurements and inferential estimates are described and include: (1) wet and dry deposition, (2) throughfall, stemflow, and bulk deposition, and (3) biomonitoring of lichens and mosses. Key uncertainties and limitations to the routine monitoring methods are described. Low total N deposition rates (<5 kg N ha−1 yr−1) were observed in remote areas and the highest rates of N deposition (>20 kg N ha−1 yr−1) to forests occurred in East Asia, parts of the western U.S., central and western Europe, and areas of Africa influenced by biomass burning. Throughfall studies in forests in China reported a substantial increase in N deposition with closer distance to urban areas. Globally, the monitoring of N deposition to forests is limited and current methods are subject to large uncertainty. Suggestions are made to improve the amount of monitoring sites, the accuracy and the standardization of the monitoring methods, and the accessibility and standardization of the monitoring data. An improved connection between forest monitoring and deposition model development is also needed to improve the utilization of existing monitoring data, along with the coupling of new technologies (e.g., satellite data, measurement model fusion) that can be complementary to monitoring strategies.