The paper ‘Nitrogen fertilizer value of animal slurries with different proportions of liquid and solid fractions: A 3-year study under field conditions, published in the Journal of Agricultural Science, has been chosen as the latest Editorial Highlight.

Animal manure contains nitrogen (N) in both mineral and organic form. Most mineral N is available to a crop shortly after manure addition, and is responsible for the effects during the season of application. By contrast, most of the organic nitrogen remains in the soil at least six months after application and is released in the years following the application of manure to soil. This condition complicates the utilization of nitrogen in manure and, thus, farmers need to know more about the timing of the nitrogen release, in order to account for it for fertilizer planning.

We studied the release and crop uptake of nitrogen from pig and cattle slurry over three growing seasons after application. Prior to the application, the slurries were separated into solid and liquid fractions by centrifugation, and mixtures with variable ratio between solid and liquid fraction were applied to small plots in the field. The slurry mixtures were injected in the soil before sowing spring barley. Nitrogen uptake was measured for barley and an undersown ryegrass crop for the following two seasons to evaluate the nitrogen fertilizer value compared to the use of mineral fertilizers.

The study showed that the nitrogen fertilizer value in the first year was influenced by the proportion between solid and liquid fractions, whereas the nitrogen effect in the following two years could not be related to the solid-liquid proportion. However, we found that the residual fertilizer value in the following two years was about twice as high for pig manures than for cattle manures, when related to total manure nitrogen applied in the first year.

The mineralization of organic manure nitrogen is driven by microbial activity in the soil and, therefore, also by soil temperature. This can be accounted for by calculating growing degree days after manure application, which is here defined as the air temperature above 5oC for each day, cumulated over a given period. Residual organic manure N present in soil after harvest of the first barley crop was estimated, as was the cumulated mineralization of organic manure N, based on N uptake in the ryegrass crop. We found an almost linear relationship between manure nitrogen mineralization and growing degree days during the period 2000-5000 degree days (here the period ca 1-2.5 years after application). The mineralization of pig manure N was about twice as high as for cattle manure N. During the first period (0-2000 growing degree days), there was no N mineralization from cattle manure but a high mineralization rate from pig manure.

The variable release pattern for organic nitrogen in manures should be incorporated in fertilizer planning. Often computer programs and decision support systems are used in fertilizer planning and legacy effects of manures are normally integrated in the software. The present results can be used for updating these computer programs.

The Journal of Agricultural Science Editorial Highlights are selected by the Editor-in-Chief and are freely available for one month. View the recent selections here.