Predicting soil N supply and yield parameters in peat grasslands

Considerable nitrogen (N) mineralization occurs in drained peat soils in use for dairy grassland, due to aerobic decomposition of soil organic matter (SOM). N losses may be limited by matching grass N uptake with N mineralization and by adapting on-farm fertilization schemes to soil N supply (SNS) and apparent N recovery (ANR). Previous attempts to predict SNS of peat grasslands from soil parameters have been unsuccessful, partly due to high variation in SNS between sites and years. In this paper, we present field data from twenty dairy grasslands on drained peat (29-65% SOM; Terric Histosols). Grass yield parameters (e.g. SNS and ANR) were compared with a comprehensive data set of soil biotic and abiotic properties measured at the start of the growing season, and with N mineralization calculated from this data. SNS ranged between 171 and 377 kg N ha(-1) (mean: 264 kg N ha(-1)) during the growing season. Soil N mineralization estimated by laboratory incubation and by foodweb-based production ecological calculations gave similar mean values with slightly higher coefficients of variation, but correlations with SNS were not significant. Regression analysis with soil properties showed a positive correlation between SNS and soil Ca:Mg ratio and a negative correlation between fertilized grass yield and soil C:SOM ratio. No significant models were found for ANR. Based on our data and on literature, we conclude that these parameters indicate linkages between grass yield and soil physical-hydrological properties such as soil structure and water availability. In particular, the C:SOM ratio in these soils with high organic matter content may be an indicator of water repellency, and our results suggest that grass growth was limited by drought more than by nutrient availability.