Fitness for Purpose of Reactive Nitrogen Monitoring Methods in Ecosystems: A Multi-Faceted Comparative Assessment

Anthropogenic reactive nitrogen (Nr) production now greatly exceeds natural creation, generating cascading environmental impacts on the environment and human health. Effective monitoring of Nr in ecosystems is essential for early warning and policy response, yet the landscape of available monitoring methods is wide and heterogeneous, varying substantially in accuracy, purpose, cost, and ecological scope. This study evaluates nine-teen Nr-monitoring methods, grouped into chemistry-based (CM), biodiversity-based (BM), and transplant-based (TM) methods, against 36 fitness-for-purpose (FFP) indicator facets. Facets are organized into intrinsic (direct measurement of nitrogen cycle components), projected (ecological effects the method can indicate), and extrinsic (metaproperties such as cost or precision) types. Scores are derived from 89 core papers, ranked using evidence-normalized weighted-sums (WS) and non-metric multidimensional scaling (NMDS) approaches. CM generally outperform BM and TM, with critical loads, total tissue nitrogen, and nitrogen isotopes leading overall. Lichen diversity, ectomycorrhizal fungi, and Ellenberg's N lead among BM, particularly for projected facets. The NMDS reveals a structural divide and monitoring complementarity between CM and BM, with CM serving as early warning indicators whereas BM integrate cumulative, persistent ecosystem impacts. While no single method seems adequate for comprehensive monitoring, the FFP matrix facilitates selecting optimal monitoring combinations.