Proposing a strict epidemiological methodology for setting empirical critical loads for nitrogen deposition

Currently empirical critical loads are derived from manipulation experiments and field survey data and more recently these data have come under scrutiny as our understanding of how ecosystems respond to reactive nitrogen (Nr) deposition evolves. The importance of background nitrogen (N) deposition and the significance of the starting N capital, cumulative N, are now recognized. This has led to a credibility rating against which experimental data can be evaluated. However, there is still no robust and transparent system in place for setting empirical critical loads for nitrogen deposition. This chapter discusses some of the issues involved in the evaluation of the available data and proposes a testable approach to carry the system forward.