Sewage sludge affects pasture productivity and nitrogen use efficiency in a grassland field experiment

Aim of study: This study seeks to evaluate the use of different sewage sludge doses combined or not with different inorganic fertilisers (N and K) on pasture production, N Use Efficiency (NUE) through the Crop Recovery Efficiency (CRE) index estimator, and soil chemical properties. Area of study: Northwest of the Iberian Peninsula. Material and methods: A total of 88 experimental units (4 x 2 m2), including 11 fertilisation treatments: two controls (no fertilisation and traditional mineral fertilisation 500 kg ha-1 8:24:16 (N:P2O5:K2O), sewage sludge (SS) at three different N doses: 40, 80, and 120 kg N ha-1; and SS combined with K and NK fertilisation. Pasture productivity, agronomic efficiency (AE) on the use of N, and CRE were measured over four years and soil properties over three years. Main results: SS significantly boosted pasture yields, with maximum production increase (7 t ha-1) using SS120_NK, while mineral fertilisers produced lower increases, below 2 t ha-1. The intermediate SS dose (80 kg N ha-1) was the most effective overall. Agronomic efficiency declined with increasing SS doses, and adding NK fertilisers further reduced AE, especially in the third year (from 36% in SS80 to 18% in SS80NK). Mineral fertiliser and the SS120_NK treatment both showed NUE below 50%, indicating risk of N pollution. In the other treatments, CRE performance evolved over time: during the first year, most treatments fell into the pollution-risk zone, while after four years, only SS120_NK showed a risk of leaching. Research highlights: While SS treatments improved pasture production, the highest dose and the combination with NK fertilisers reduced N efficiency, indicating a potential risk of N pollution, highlighting the need for balanced fertilisation strategies.