Identifying new deep-rooted plant species suitable as undersown nitrogen catch crops

Abstract

A five-year field study was conducted in order to identify and test plant species suitable as undersown nitrogen (N) catch crops. With the aim of maximizing the catch crop effect against N leaching loss, the main focus was on identifying species with deeper rooting than ryegrass (Lolium perenne L.), which is typically used for this purpose. More than 20 species from 9 plant families were studied as undersown catch crops in spring barley (Hordeum vulgare L.). The root and shoot biomass production, N uptake, carbon (C) to N ratio, root distribution, rooting depth, and reduction of soil nitrate-N (NO₃-N) were measured, and observations of general performance as undersown catch crops were made. During barley growth only small differences in root growth among the catch crops was observed. Later, during autumn, striking differences in root growth appeared, and while ryegrass had few roots below 0.8 m (31.5 in) depth, a number of species showed high root intensities in the soil layer from 0.8 to 1.6 m (31.5 to 63.0 in). Dyer's woad (Isatis tinctoria L.) and viper's bugloss (Echium vulgare L.) showed high root intensities even at 1.6 to 2.4 m (63.0 to 94.5 in). The measured differences in root growth were significantly related to the ability of the different species to deplete soil NO₃-N from deeper soil layers. Dyer's woad, which was the most efficient, reduced soil NO₃-N content in November to 15 kg N ha⁻¹ (13 lb N ac⁻¹) in the 0 to 2.5 m (0 to 98.4 in) soil layer compared to 53 kg N ha⁻¹ (47 lb N ac⁻¹) under ryegrass and 62 kg N ha⁻¹ (55 lb N ac⁻¹) without a catch crop, and most of this difference occurred below 1 m (39.4 in) depth. There were many other differences among the species in terms of growth, N uptake, root/shoot ratio, and C/N ratio—differences that may be of interest for their use as catch crops for N management. One major effect was that legumes took up far more N than the other species and also produced more biomass than most others. However, none of the four legumes were among the deep-rooted species identified; they were among the shallowest rooted species with a root distribution close to that of ryegrass.