Through the introduction of a new fertilizer regulation [1], the European Union has laid the foundation for closing the continental loop of nutrient cycling in agriculture, creating an opportunity for Europe to promote sustainable growth, and benefit both the environmental and the economic sector. Nevertheless, assessing nitrogen (N) use efficiency to describe the potential of organic fertilizers as N source remains challenging. In particular, the dynamics along the manure cascade (from animal excretion until and with application to the field) and the N net-mineralization process in the soil are strongly influenced by environmental factors, soil characteristics and agricultural practices [2]. Underestimating the long-term N recovery of organic fertilizers can consequently have negative environmental impacts, such as nitrate leaching. In Switzerland, N-emission and N use efficiency (NUE) factors are used for the yearly calculation of the Swiss-Balance, a regulatory instrument used in the Swiss agricultural subsidies system to assess, whether the whole farm N and P fertilizing budget is balanced (crop N requirement ≈ N supply) [3]. In this context, N-emission and NUE factors have to be standardized to meet the average national agricultural conditions. However, the factors implemented in the Swiss- Balance have not been reviewed since its introduction in the mid-1990s - early 2000s. The aim of this project was to re-evaluate the potential N availability of manure by combining two models. The first one describes the ammonia losses from manure excretion along the manure cascade [4], and is used to assess how much of excreted NH4-N and Norg is finally applied to the soil. The second model describes the N net-mineralization dynamic in the soil, in order to estimate how much N can be recovered in the long-term for potential plant uptake and is based on the model concepts described in [5] and [6]. The defined standardized values were based on information available from national and international field and pot experiments. By using and combining these two models, it was possible to depict more realistic organic fertilizer dynamics from excretion until complete mineralization of the applied Norg fraction. This approach enables a re-evaluation of the organic fertilizers potential as nutrient source on the national level and simultaneously gives the possibility for continuous fine-tuning and integration of farm and location-specific characteristics for improved fertilizer management.
