Estimating the gross nitrogen budget under soil nitrogen stock changes: A case study for Turkey

The method to estimate the Gross Nitrogen Budget proposed by Eurostat and the OECD was developed under the assumption of no changes in soil nitrogen stock, due to the lack of available data. We estimated the national and regional nitrogen budgets of agriculture in Turkey, calculated according to the recommended methodology at the level of administrative regions. Results suggest that changes in soil nitrogen stocks are likely for some regions. In such cases, the method warns that its estimated indicators (gross nutrient surplus and nitrogen use efficiency (NUE)) are not valid. We propose two different approaches to improve the Eurostat/OECD method, based on assumptions of minimum and maximum NUE, and on regressing the N-input and N-output relationship for regions without obvious soil nitrogen stock changes. Our results show that both approaches give reasonable results for all regions, including those for which the Eurostat/OECD method failed. The results also suggest that soil nitrogen accumulates in some regions and depletes in others. Results give a range of 6-93 kg N ha−1 yr−1 (mean 35 kg N ha−1 yr−1) for the Gross Nitrogen Surplus, and a range of 49-82% (mean 62%) for the NUE.