NITROGEN UPTAKE IN BARLEY AFTER SPRING INCORPORATION OF N-15-LABELED ITALIAN RYEGRASS INTO SANDY SOILS

The fertilizer effect of ryegrass plant material in spring barley was studied during two successive growing seasons. The aim of the study was to investigate whether the level of mineral fertilizer could be reduced after incorporation of a catch crop. To simulate standard agronomic practice, reference plots and amended plots were fertilized with mineral N fertilizer just below the recommended levels. N-15-labelled ryegrass roots or shoots were incorporated into a coarse sand and two sandy loam soils in spring before sowing. The amended plots were fertilized with NH4NO3 while the reference plots, without amendment of ryegrass, received (NH4NO3)-N-15-N-15. Half of the plots were harvested at earing in the first growing season, the other plots were harvested at maturity in the first and second growing season. The first barley crop recovered 29%-41% of N-15 added to the soil in ryegrass shoots. First year recovery was significantly higher for barley grown in the coarse sand. In both years, plant uptake of root N-15 was lower than uptake of N-15 derived from shoots and was not related to soil type. For soils fertilized with (NH4NO3)-N-15-N-15, the highest recovery of N-15 was obtained in a sandy loam. Availability of N-15 in the second year was lower for residual (NH4NO3)-N-15-N-15 than for N-15 left in plant residues. Most of (NH4NO3)-N-15-N-15 derived N in mature barley had already been assimilated at earing. However, only 61% to 77% of the total N content found in barley at harvest was assimilated at this growth stage, indicating that the N taken up between earing and maturity was derived primarily from the unlabelled soil N pool. Incorporation in spring of an Italian ryegrass catch crop containing 60 kg N ha-1 may reduce the mineral fertilizer requirement of subsequent barley crops by 17 and 7 kg N ha-1 during the first and second growing season, respectively.