Evaluating critical loads of nutrient nitrogen and acidity for terrestrial systems using ecosystem-scale experiments (NITREX)

Abstract

Critical loads (CL) for nitrogen as a nutrient (CL nut (N)) and total acidity (CL(S + N)) for the terrestrial ecosystem have been calculated and evaluated for all NITREX sites. NITREX constitutes a series of N addition or removal experiments in coniferous forest stands and one alpine site across a European deposition gradient (2–60 kgN ha −1 yr −1 ). Experimental manipulation of S and N inputs at these sites provides a valuable opportunity to test the validity of the mass balance approach to computing CLs at the ecosystem scale. CL nut (N) values calculated using the sustainability (nutrient limitation) approach were 3–8 kgN ha −1 yr −1 for sites on acidic geology, and 19 kgN ha −1 yr −1 for sites on basic geology. Where N uptake was estimated using N removed in bolewood, CL values increased to 11–15 kgN ha −1 yr −1 at acidic sites. These values will increase further if whole-tree rather than stem-only harvesting is employed and will be greater in areas where stand rotation lengths are relatively short (e.g., UK). Mean annual uptake rates of N into bolewood could not be sustained by available P at any of the sites indicating the probable need for fertiliser applications in the long-term. Problems associated with estimating values for other parameters in the CL equation (N i(crit) , N de(crit) , N le(crit) ) are discussed and preferred approaches identified. CL(S + N) values were computed as 1.7–6.9 keq ha −1 yr −1 reflecting differences in weathering rates, deposition and uptake of base cations, and the (Ca + Mg + K) Al crit ratio in soil solution for the dominant vegetation. Comparison of ambient deposition loadings and calculated CL values indicated that the sites were in three categories: (1) sites with deposition below both CL nut (N) and CL(S + N) (Sogndal and Alptal), (2) sites where deposition was equal to or exceeded CL nut (N) but not CL(S + N) (Gårdsjön, Aber and Klosterhede), and (3) sites in which both CL nut (N) and CL(S + N) were exceeded (Speuld, Solling and Ysselsteyn). Response of soil solution (Ca + Mg + K) Al ratio following changes in deposition at the sites is discussed with respect to observed ecological changes. Nitrate leaching was positively related to exceedance of CL nut (N) if the larger CL nut (N) values were used, highlighting the importance of this removal pathway for excess N in coniferous forest ecosystems. The study has demonstrated the value of long-term, ecosystem scale experiments if current approaches to calculating CLs are to be rigorously tested.