Effects of ridging on crop performance and symbiotic N₂ fixation of fababean (Vicia faba L.)

The success of organic cropping systems depends on symbiotic N2 fixation by leguminous crops, and it is important to explore new management systems to improve the nitrogen input through N2 fixation. During two growing seasons the possible advantage of growing fababean (Vicia faba L.) in ridges was studied in comparison to the traditional method on flat soil. Differences in soil physical parameters resulted in a significantly greater microbial activity and a deeper root system at the flowering stage when grown in the ridge than on the flat. Consequently, the amount of fixed N at flowering was significantly greater in ridges than in flat soil. However, during the period from flowering until harvest, when the major part of the N uptake and N2 fixation took place, the differences between the treatments disappeared. Average values for the growing season of fluorescein diacetate hydrolysis, arylamidase activity and arylsulphatase activity were significantly greater in the ridge than on the flat, and the microbial biomass-C, derived from substrate induced respiration (SIR), was on average 232 and 223 microgram C g(-1) soil in the ridge and on the flat, respectively. Measured total-N uptake, including root N (0-30 cm depth), ranged from 206 to 247 kg N ha(-1), of which 182-201 kg N ha(-1) was fixed N. From 154 to 173 kg N ha(-1) was removed in grain resulting in a soil-N balance of +28 kg N ha(-1) in both years. However, by including estimates of total root N and rhizodeposition-N the soil-N balance ranged from +52 to +62 kg N ha(-1).