Short-term and residual availability of nitrogen after long-term application of organic fertilizers on arable land

Knowledge on short-term and long-term availability of nitrogen (N) after application of organic fertilizers (e.g., farmyard manure, slurry, sewage sludge, composts) provides an important basis to optimize fertilizer use with benefits for the farmer and the environment. Nitrogen from many organic fertilizers often shows little effect on crop growth in the year of application, because of the slow-release characteristics of organically bound N. Furthermore, N immobilization after application can occur, leading to an enrichment of the soil N pool. However, this process finally increases the long-term efficiency of organic fertilizers. Short-term N release from organic fertilizers, measured as mineral-fertilizer equivalents (MFE), varies greatly from 0% (some composts) to nearly 100% (urine). The most important indicators to be used for predicting the short-term availability of N are total and NH ₄⁺-N contents, C:N ratio (especially of the decomposable organic fraction), and stability of the organic substances. Processing steps before organic fertilizers are applied in the field particularly can influence N availability. Composting reduces mineral-N content and increases the stability of the organic matter, whereas anaerobic fermentation increases NH ₄⁺-N content as well as the stability of organic matter, but decreases the C:N ratio remarkably, resulting in a product with a high content of directly available N. Nevertheless, long-term effects of organic fertilizers rather slowly releasing N have to be considered to enable optimization of fertilizer use. After long-term application of organic fertilizers, the overall N-use efficiency is adequate to a MFE in the range of 40%-70%.