Simulating nitrate retention in soils and the effect of catch crop use and rooting pattern under the climatic conditions of Northern Europe

Summary

This model analysis of catch crop effects on nitrate retention covered three soil texture classes sand, loamy sand, sandy loam) and three precipitation regimes in a temperate climate representative for northern Europe (annual precipitation 709-1026 mm) for a period of 43 years.
Simulations were made with two catch crops (ryegrass and Brassica) with different rooting depths, and soil N effects in the next spring were analysed to 0.25, 0.75 and 2.0 m depth to represent the catch crop effect on following crops with different rooting depths. Nitrate retained without a catch crop was generally located in deeper soil layers. In the low precipitation regime the overall fraction of nitrate retained in the 0-2.0 m soil profile was 0.23 for the sandy soil, 0.69 for the loamy sand and 0.81 for the sandy loam. Ryegrass reduced leaching losses much less efficiently than Brassica, which depleted nitrate in the 0-0.75 m soil layer more completely, but also in the deeper soil layer, which the ryegrass could not reach. A positive N effect (Neff, spring mineral N availability after catch crop compared with bare soil) was found in the 0-0.25 m layer (i.e. shallow rooting depth of a subsequent main crop) in all three soil texture classes, with on average 10 kg N ha-1 for ryegrass and 34 kg N ha-1 for Brassica. Considering the whole soil profile (0-2.0 m deep rooting of next crop), a positive Neff was found in the sandy soil, whereas generally a negative Neff was found in the loamy sand and especially the sandy loam. The simulations showed that for shallow-rooted crops, catch crop Neff values were always positive, whereas Neff for deeper-rooted crops depended strongly on soil and annual variations in precipitation conditions. These results are crucial both for farmers crop rotation planning and for design of appropriate catch crop strategies with the aim of protecting the aquatic environment.