EVALUATING CRITICAL LOADS OF NUTRIENT NITROGEN AND ACIDITY FOR TERRESTRIAL SYSTEMS USING ECOSYSTEM-SCALE EXPERIMENTS (NITREX)

Critical loads (CL) for nitrogen as a nutrient (CLnut,,(N)) and total acidity (CL(S +; N)) for the terrestrial ecosystem have been calculate d and evaluated for all NITREX sites. NITREX constitutes a series of N addition or removal experiments in coniferous forest stands and one a lpine site across a European deposition gradient (2-60 kgN, ha(-1) yr( -1)). Experimental manipulation of S and N inputs at these sites provi des a valuable opportunity to test the validity of the mass balance ap proach to computing CLs at the ecosystem scale. CLnut,,,(N) values cal culated 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 wh ere stand rotation lengths are relatively short (e.g., UK). Mean annua l uptake rates of N into bolewood could not be sustained by available P at any of the sites indicating the probable need for fertiliser appl ications in the long-term. Problems associated with estimating values for other parameters in the CL equation (N-i(crit), N-de(crit), N-le(c rit)) are discussed and preferred approaches identified. CL(S + N) val ues were computed as 1.7-6.9 keg ha(-1) yr(-1) reflecting differences in weathering rates, deposition and uptake of base cations, and the (C a + Mg + K)/Al-crit,,, ratio in soil solution for the dominant vegetat ion. Comparison of ambient deposition loadings and calculated CL value s indicated that the sites were in three categories: (1) sites with de position below both CLnut,,,(N) and CL(S + N) (Sogndal and Alptal), (2 ) sites where deposition was equal to or exceeded CLnut (N) but not CL (S + N) (Gardsjon, Aber and Klosterhede), and (3) sites in which both CLnut,,,(N) and CL(S + N) were exceeded (Speuld, Selling and Ysselstey n). Response of soil solution (Ca + Mg + K)/Al ratio following changes in deposition at the sites is discussed with respect to observed ecol ogical changes. Nitrate leaching was positively related to exceedance of CLnut,,,(N) if the larger CLnut,,,(N) values were used, highlightin g the importance of this removal pathway for excess N in coniferous fo rest ecosystems. The study has demonstrated the value of long-term, ec osystem scale experiments if current approaches to calculating CLs are to be rigorously tested. (C) 1998 Elsevier Science B.V.