Testing Fertilizer Dependency to assess sustainability in Agriculture and Food Systems using Circularity Indicators in three different scales

The nutrient demand is increasing due to the growing world population as well as the use of fertilizers in agricultural systems. To minimize the impact of applying fertilizers the strategy is nutrient reuse and recycling to achieve sustainability in agricultural systems. This study aims to evaluate and compare the use of various indicators such as fertilizer dependency, nitrogen use efficiency, phosphorus use efficiency, cycle count and cycle count returned, in agricultural systems at different spatial scales. The data from this study were taken from the longest and continuous winter wheat experiment at Broaldbalk (field scale), the dairy farm at De Marke, and the Mozambique agriculture sector. At the field scale, results showed a nitrogen use efficiency (NUE) of 66% when accounting with atmospheric deposition, and a phosphorus use efficiency (PUE) of 66%, both quite high. The fertilizer dependency was strong, 89% for nitrogen accounting with atmospheric deposition and 100% for phosphorus. The farm scale exhibited a low NUE (28%) and high PUE (72%), although lower nitrogen fertilizer dependency. Biological nitrogen fixation plays a significant role in reducing dependence on external nitrogen sources. As biological nitrogen fixation increases, the reliance on external fertilizers decreases. However, if the nitrogen use efficiency of feed production becomes excessively high, it may lead to an unexpected increase in fertilizer dependency. In the Mozambican agricultural sector, NUE showed values >100% in 1961 and decreased to ≈40% nowadays, revealing a pathway from soil mining to a higher nitrogen fertilizer dependency. Circularity indicators such as Cycle Count and Cycle Count returned also highlighted limited nutrient reuse at the national level. Overall, this study demonstrates that fertilizer dependency, when used alongside with NUE and circularity metrics, offers a more nuanced and comprehensive view of nutrient sustainability. These indicators can guide strategies to reduce dependency on external inputs and improve nutrient recycling, especially in low-input systems such as those in sub-Saharan Africa.