Sewage sludge showed high agronomic value, releasing nitrogen faster than farmyard manure

The use of fertilizers in agriculture, particularly organic fertilizers such as sewage sludge, is a key research priority due to its impacts on crop productivity, production costs, environmental risks and the push for a more circular economy. This study, conducted in Bragan & ccedil;a, northeastern Portugal, focused on forage maize during the 2022 and 2023 growing seasons. Eight fertilization treatments were established corresponding to the application of mineral nitrogen (N) at rates of 50 (N50), 100 (N100), 150 (N150) and 200 (N200) kg ha(-1), three organic amendments applied at a rate equivalent to 200 kg ha(-1) of N, namely sewage sludge (SS200), cow manure (CM200) and sheep manure (SM200), and a non-fertilized control (N0), with the aim of assessing the agronomic value of these treatments for forage maize (Zea mays L.). Maize dry matter yield (DMY) ranged from 10.8 to 20.3 t ha(-1) in 2022 and 13.7 to 23.6 t ha(-1) in 2023 for N0 and N200, respectively. Organic amendments produced 14.7-17.7 t ha(-1) in 2022 and 20.5-24.4 t ha(-1 )in 2023. Increased mineral N rates resulted in higher soil inorganic N content, N concentration in leaves and N recovery in tissues, with organic amendments showing lower values than crops fertilized with N200. However, due to cumulative nutrient release, organic amendments improved DMY and N use efficiency in the second year. Sewage sludge mineralized rapidly due to its low carbon (C)/N ratio and absence of hard-to-degrade compounds. It is also rich in phosphorus (P), enhancing its fertilizing value. Although nitrate leaching and denitrification potential appeared lower with organic amendments, long-term applications may increase risks, requiring careful monitoring to ensure sustainable and safe practices.