Are differences in root growth of nitrogen catch crops important for theirability to reduce soil nitrate-N content, and how can this be measured?

An experiment was made to measure root growth of nitrogen catch crops, to i nvestigate whether differences in root growth among plant species are relat ed to their ability to deplete the soil nitrate-N pool. Large differences w ere observed in root growth parameters. Monocot species had rooting depth p enetration rates in the range of 1.0 to 1.2 mm d(-1) degreesC(-1), whereas the non-legume dicot species had rates between 1.5 and 2.3 mm d(-1) degrees C(-1). Substantial differences were also found in the lag time from sowing until significant root growth was observed. The estimated temperature sum n eeded for the crops to reach a rooting depth of 1.0 m varied from 750 d deg reesC for fodder radish to 1375 d degreesC for Italian ryegrass. The depth distribution of the root system varied strongly, and at a depth of 1.0 m th e non-legume dicot species generally had root intensities (number of root i ntersections m(-1) line on the minirhizotrons) 12 times as high as the mono cot species. The amount of nitrate left in the topsoil (0-0.5 m) was only weakly correla ted to a few of the measured plant and root parameters, whereas nitrate lef t in the subsoil (0.5-1.0 m) was clearly correlated to several root paramet ers. Subsoil nitrate residues were well correlated to root intensity, but s howed even stronger correlations to more simple estimates of rooting depth. In the deepest soil layer measured (1.0-1.5 m), the soil water nitrate con centration was reduced from 119 mug L-1 without a catch crop to 61 mug L-1 under Italian ryegrass and to only 1.5 mug L-1 under fodder radish. The results show that to identify the important differences in root growth among catch crops, root growth must be measured in deep soil layers. In thi s study, none of the measurements made aboveground or in the upper soil lay ers were well related to subsoil nitrate depletion.