The aimed of our research project was to study the effect of sewage sludge land application on soil-plant N cycling. To achieve this objective two field experiments were conducted at the Arazuri Research Station (Navarra, Spain). In the first experiment, we established nine plots (3x9 m²) that were sown with barley ( Hordeum vulgare L.) in November-94, and pea (Pisum sativum L.) in November-95. The plots were amended with a) sludge and NH₄NO₃ only, b) NH₄NO₃ only, c) or left unamended. With respect to the control, the barley yield increase for the N-fertilizer treatment was 85% and 45% for the sludge + N-fertilizer treatment. The decrease of N-fertilizer efficiency in the barley sludge amended treatment was presumably due to a decrease in spring plant available N, caused by an enhancement of microbial immobilisation and denitrification. No significant differences in production or plant N uptake were observed between pea treatments.

In the second experiment, new plots (6x9 m²) were established in an adjacent field, and were sown with wheat (Triticum aestivum L.) in November-96. The plots were amended with a) sludge, b) sludge plus dicyandiamide (DCD), or c) left unamended. At the end of winter, plots were splitted in two, and half of them were fertilized with NH₄NO₃. The wheat yield increase for the N-fertilizer treatment was 22% and 35% for the sludge + N-fertilizer treatment. Therefore, there was not a decrease in N-fertilizer efficiency in the sludge amended treatment, presumably because denitrification losses in spring were very low due to climatological conditions. Addition of DCD to sewage sludge increased soil inorganic N content and reduced denitrification losses in autumn, resulting in a higher crop N uptake and wheat yield.