Estimating food nitrogen and phosphorus footprints and budgeting nitrogen and phosphorus flows of Rwanda's agricultural food system during 1961–2020

Nitrogen (N) and phosphorus (P) are limiting factors for crop production in Rwanda where food security is susceptible to inadequate agricultural techniques, especially fertilization. Understanding N and P footprints for food and their budgets under different fertilized scenarios may help to improve the nutrient use efficiency and crop yield in Rwanda, however, with little information available yet. Here, we estimated food N and P footprints and their budgets for agri-food system in Rwanda using adjusted N-P-Calculator model under fertilized, unfertilized and combined scenarios during 1961–2020. The total food N footprint per capita increased from 4.2, 3.8 and 6.4 (1960s) to 6.8, 4.9 and 9.9 kg N cap−1 yr−1 under combined, unfertilized and fertilized scenarios, respectively (2011−2020). The total food P footprint per capita increased from 0.19, 0.18 and 0.23 (1960s) to 0.31, 0.25 and 0.40 kg P cap−1 yr−1 under combined, unfertilized and fertilized scenarios, respectively (2011–2020). The total N input to croplands increased from 13.9 (1960s) to 37.0 kg N ha−1 yr−1 (2011–2020), while the total crop N uptake increased from 18.1 (1960s) to 32.5 kg N ha−1 yr−1 (2011–2020), resulting in N use efficiency decline from 99.1 % (1960s) to 74.6 % (2011–2020). Gaseous N losses of NH3, N2O, and NO increased from 0.9, 0.1 and 0.0 (1960s) to 7.5, 0.8 and 0.1 kg N ha−1 yr−1 (2011–2020). The total P removal in harvested crops increased from 2.9 (1960s) to 5.1 kg N ha−1 yr−1 (2011–2020). The results revealed large room for crop yield expansion; and low N and P inputs are major agricultural production limitations. We suggest N and P fertilizer improvement by focusing on better management of organic animal manure and ensuring high biologically N fixed through crop rotation of legumes and cereals; lastly to increase in moderation the use of synthetic N and P fertilizers in Rwanda.