The role of catch crops in the ecological intensification of spring cereals in organic farming under Nordic climate

Catch crops can contribute to nitrogen supply of following crops through uptake of excess soil mineral nitrogen (N) and through biological N fixation. The contribution of catch crops to the ecological intensification of organic arable systems was investigated using data of a 12-year field experiment carried out at three sites in Denmark. This study focused on the yields of spring oats and spring barley in systems with and without manure in two different cropping systems (O2 and O4) that differed in the proportion of legume-based catch crops (O2 lower and O4 higher) and in the rotation composition (grass-clover green manure in O2 and pulse crops in O4). Three consecutive four-year crop cycles were established at three locations representative of different soil types (loamy sand, sandy loam and coarse sand) and climatic conditions. Crop management and soil operations were performed following common practices in organic farming. Measurements of dry matter (DM) and N content of grain cereals at harvest, above-ground biomass in catch crops and green manure crops in autumn and of the green manure crop at the first cutting were performed. The effect of catch crops on grain yield varied with cereal and catch crop species, soil and rotation type, and the application of N in manure. Higher yield increases from previous catch crops were obtained for spring oat than for spring barley with mean estimates of the apparent N recovery efficiency of N in above-ground catch crops of 69% and 46%, respectively. However, lower autumn N in catch crops undersown in high yielding cash crop was also observed. For spring oats mean grain yield benefits of including catch crops varied from 0.2 to 2.4 Mg DM ha(-1) depending on location, manure use and cycle of the rotation. In spring barley mean grain yield benefits from catch crops varied from 0.1 to 1.5 Mg DM ha(-1). There was a tendency for the effect of catch crop on grain yield to increase over time. These results indicate that in Nordic climates catch crops can contribute to the ecological intensification of spring cereals, not only by reducing the nitrate leaching and increasing N retention, but also by improving yields. Management practices in relation to catch crops must be adapted to the specific soil and cropping systems. (C) 2012 Elsevier B.V. All rights reserved.