Soil N mineralization in a dairy production system with grass and forage crops

This paper describes the dynamics of soil N mineralization in the experimental intensive dairy farming system ‘De Marke’ on a dry sandy soil in the Netherlands. We hypothesized that knowledge of the effects of crop rotation on soil N mineralization and of the spatial and temporal variability of soil N mineralization in a farming system can be used to better synchronize N application with crop N requirements, and hence to increase the recovery of applied N and to reduce N losses. Soil N mineralization was recorded continuously in the soil layer 0–0.30 m, from 1992 to 2005, using a sequential in situ coring technique on six observation plots, of which two were located in permanent grassland and four in crop rotations with a 3 year grassland phase and an arable phase of 3 or 5 years, dominated by maize. Average annual soil N mineralization was highest under permanent grassland: 381 kg ha-1 and lowest under =3rd years arable crops: 184 kg ha-1. In temporary grassland, soil N mineralization increased in the order: 1st year, 2nd year, 3rd year grassland and in arable crops after grassland mineralization decreased in the order: 1st year, 2nd year, =3rd years. Total mineral N input, i.e. the sum of N mineralization and mineral N supply to soil, exceeded crop N requirements in 1st year maize and was lower than the requirements in 1st year temporary grassland. N mineralization in winter, outside the growing season, was 77 kg ha-1 in maize and 60 kg ha-1 in grassland. This points at the importance of a suitable catch crop to reduce the susceptibility to N leaching. Temporal and spatial variability of soil N mineralization was high and could not be related to known field conditions. This limits the extent to which N fertilization can be adjusted to soil N mineralization. Variability increased with the magnitude of soil N mineralization. Hence, situations with high soil N mineralization may be associated with high risks for N losses and to reduce these risks a strong build-up of soil organic N should be avoided. This might be achieved, for instance, by fermenting slurry before application on farmland to enhance the fraction mineral N in slurry at the expense of organic N.