Difficulties in Using the Nutrient Budget Approach to Rate Site Sensitivity to Residue Harvesting at the Landscape Level.

Several indicators are being considered to rate site suitability to biomass residue harvesting. The nutrient budget approach consists of estimating nutrient losses in harvested products with nutrient gains through geochemical inputs such as atmospheric deposition and mineral weathering. This approach was used at the landscape scale. A high degree of correlation between nutrient inputs and outputs was observed among ecosystem types because nutrients in harvest products where higher on more fertile sites that supported forest stands of better growth and often containing tree species with higher nutrient contents. These correlations made the evaluation of sensitive ecotypes difficult where major nutrient deficits could be observed on rich sites but rarely on poor sites. The error of estimates of different parameters (both inputs and outputs) being higher also for richer sites.  A better assessment of nutrient weathering rates, of its variability with ecosystem development and of its potential stimulation by the biota are required to make geochemical models performant for nutrient management. In the meantime, relying on soil characteristics may be a better option.