As an essential plant nutrient, nitrogen (N) is important for crop yield and quality. Nitrogen is supplied to crops with mineral N fertilizer and animal manure. However, a considerable part of the N uptake of crops originates from N mineralized from the soil N pool and is a residual effect of fertilizers applied in the past. Nitrogen mineralization from the soil pool happens both during the growing season of crops but also outside of the growing season, and N mineralized during autumn and winter is at risk of leaching. The turnover of N from fertilizer application is therefore an important process both from the perspective of fertilizer use efficiency and risk of N losses. The aim of this PhD project was to look at the N fertilizer effect of different animal manures for three years in addition to the effect of mineral fertilizer applied more than 24 years ago. Three experiments were conducted to do so.

In Study I the N fertilizer effect of a cattle deep litter was investigated over three barley growing seasons in a lysimeter study. Along with crop response, N leaching was measured for two years and mitigation effects of a fodder radish cover crop were also investigated. In Study II the N fertilizer effect of cattle and pig slurry mixtures was measured in a spring barley followed by a grass crop for three years. The slurry mixtures were prepared by centrifuging a cattle and a pig slurry and separating the liquid and solid fraction. The two fractions were mixed based on their ammonium content, resulting in slurries where 0, 25, 50, 75 and 100 % of the ammonium N originated from the solid fraction. Study III was also a lysimeter study based on a set of lysimeters with 15N-labelled soil. The soil was supplied with 15N-labelled fertilizer from 1989 to 1993 and added to the lysimeters in 2004, replacing the top soil layer. A grass crop was grown for 13 years with no material being removed. In 2018, four treatments were established. The grass crop continued in one treatment, one treatment was kept vegetation-free after ploughing, one treatment was grown with spring barley after ploughing and one treatment was grown with spring barley followed by a cover crop after ploughing. In this study the turnover of mineral N incorporated more than 24 years ago was investigated.

Studies I and II revealed a variable N fertilizer effect for the animal manures. The first-year effect was closely related to the ammonium to total N ratio, and on average the fertilizer effect was equal to the ammonium content in the manures. This covered a slightly higher effect than the ammonium content in the deep litter and pig slurry mixtures and a lower effect in the cattle slurry mixtures. This translates to a mineralization of organic N in the pig slurry and deep litter and an immobilization of N in the cattle slurry if nitrogen uptake efficiency (NUE) is assumed to be the same in mineral N fertilizer and mineral N in animal manures.

The correlation between N uptake from manures in the two following years and total N application in 2017 was not strong and the fertilizer effect was not correlated to quality parameters such as C/N ratio when averaged across manure types. The NUE of deep litter was 5.3 % of total manure N applied in the year after amendment and 0.9 % in the second year after amendment. The same values were on average 3.6 and 1.5% in cattle slurry mixtures and 9.9 and 2.8 % in pig slurry mixtures. There was no systematic difference in the residual effect of the liquid and solid fraction of the pig and cattle slurry mixtures. Mineralization of residual soil N after the first barley harvest as estimated by the nitrogen fertilizer replacement value showed the same picture with the pig slurry showing the highest residual value. For the pig and cattle slurry mixtures a linear relationship was seen between growing degree days and mineralization of residual manure N from the first grass cut in 2018 and throughout the experimental period.

In Study I leaching of N was also measured. The proportion of N applied which was leached was the same from mineral N fertilizer and manure and averaged 8.1 % of total N applied in the first leaching season. In the second season, 4.8 % of total N applied in 2017 was leached. Nitrogen uptake and leaching accounted for a similar proportion of N in the second year. Introducing a cover crop in the first leaching season reduced the proportion of N leached to 1.4% of the N applied. The residual effect of the cover crop on the following barley crop was variable, ranging from no effect to 22 kg N ha-1 and averaging 10 kg N ha-1. In the second leaching season there was a tendency for higher N leaching in treatments with a cover crop in the previous year.

Study III revealed that 24 years after application, 15N-labelled mineral N fertilizer still had a higher availability than bulk soil organic N. This was seen from a higher 15N atom fraction in the crop than in the soil. Assuming that the 15N is still concentrated in the topsoil, an average of 1.7 % of the 15N present in 2004 was lost from the soil every year, the majority presumably by leaching. In the continued grass crop, approximately 1.8 % of the 15N content in 2017 was taken up by the grass crop during 2018 and 0.18 % was leached in the following season with the same assumption. Ploughing of the grass crop caused a high N leaching, but the partitioning between leaching and N uptake in the treatment with spring barley was similar to Study I. Leaching was reduced with 66 % by introducing a cover crop to the system.

The turnover of N applied in previous years contribute a significant amount of N to crop uptake in addition to the fertilizer N added in a given year. N mineralized outside of the growing season may also be leached, and the turnover of residual N should be taken into account when planning fertilizer applications and crop rotations.