Are Pyrenean mixed Scots pine / European beech forests in the path to being N-saturated?

Abstract

Blanco, J.A., San Emeterio, L., González de Andrés, E., Imbert, B.J., Larrainzar, E., Peralta, J., Lo, Y-H., Castillo, F.J. 2017. Are Pyrenean mixed Scots pine / European beech forests in the path to being N-saturated? Ecosistemas 26(1): 66-78. Doi.: 10.7818/ECOS.2017.26-1.08

Human activities are causing high chronic atmospheric N deposition levels that may be disrupting N cycling in forests in the western Pyrenees. To test such hypothesis, we researched the effects of atmospheric N deposition on N cycling at two Scots pine / European beech mixed forests in Navarre. One forest is placed at 1350 m.a.s.l. and has a continental climate, whereas the other forest is placed at 650 m.a.s.l. and has a wet Mediterranean climate. Preliminary tests indicated that biological N fixation was undetectable, as host plants for symbiotic N fixers were very rare and the activity of free N fixers was undetectable. Therefore, it was assumed that the main input of N in these forests was atmospheric deposition. Hence, we estimated the dependence of ecosystem productivity with the FORECAST ecological model, calibrated for these sites. Six scenarios were simulated with N inputs in the range of 5 to 30 kg ha^-1 year^-1. Our results indicated that forest productivity depends on deposition N inputs, but indicators of N saturation (increased leached N, no additional productivity gains) show that they can become N-saturated when deposition is above 20-25 kg N ha^-1 year^-1, about 5-10 kg N ha^-1 year^-1 above current observed deposition levels.