On the tracks of Nitrogen deposition effects on temperate forests at their southern European range - an observational study from Italy

We studied forest monitoring data collected at permanent plots in Italy over the period 2000-2009 to identify the possible impact of nitrogen (N) deposition on soil chemistry, tree nutrition and growth. Average N throughfall (N-NO3 +N-NH4 ) ranged between 4 and 29 kg ha(-1) yr(-1) , with Critical Loads (CLs) for nutrient N exceeded at several sites. Evidence is consistent in pointing out effects of N deposition on soil and tree nutrition: topsoil exchangeable base cations (BCE) and pH decreased with increasing N deposition, and foliar nutrient N ratios (especially N : P and N : K) increased. Comparison between bulk openfield and throughfall data suggested possible canopy uptake of N, levelling out for bulk deposition >4-6 kg ha(-1) yr(-1) . Partial Least Square (PLS) regression revealed that - although stand and meteorological variables explained the largest portion of variance in relative basal area increment (BAIrel 2000-2009) - N-related predictors (topsoil BCE, C : N, pH; foliar N-ratios; N deposition) nearly always improved the BAIrel model in terms of variance explained (from 78.2 to 93.5%) and error (from 2.98 to 1.50%). N deposition was the strongest predictor even when stand, management and atmosphere-related variables (meteorology and tropospheric ozone) were accounted for. The maximal annual response of BAIrel was estimated at 0.074-0.085% for every additional kgN. This corresponds to an annual maximal relative increase of 0.13-0.14% of carbon sequestered in the above-ground woody biomass for every additional kgN, i.e. a median value of 159 kgC per kgN ha(-1) yr(-1) (range: 50-504 kgC per kgN, depending on the site). Positive growth response occurred also at sites where signals of possible, perhaps recent N saturation were detected. This may suggest a time lag for detrimental N effects, but also that, under continuous high N input, the reported positive growth response may be not sustainable in the long-term.