Nitrogen cycling through senescent leaves and litter in swards of Ruanui and Nui ryegrass with high and low nitrogen inputs

The nitrogen (N) flux from green herbage to the soil via leaf senescence was determined for monoculture swards of 2 perennial ryegrass cultivars ( Lolium perenne L.) at 2 rates of applied N, with minimal animal return. Recovery of added N in the above ground herbage varied between 240 and 40% for the ryegrass cultivars Ruanui and Nui with annual fertiliser applications of 155 and 2360 kg N/ha respectively. High N treatment increased the quality and quantity of green and dead herbage, and the higher N content of leaves in the high nutrition plots was reflected in the higher N content of the senescing leaves and litter. Reabsorption of N from senescing leaves varied from 30 to 50%. Reabsorption was more efficient in low nutrition plots. About 25% of the N remaining in dead leaves appeared to be readily leached. Seasonal changes in litter yields resulted mainly from seasonal differences in leaf death rate, as litter disappearance rates per unit area were relatively constant. Leaf death and subsequent N return was sometimes greater for Ruanui than Nui at high nutrition, but there were no cultivar differences in litter yields. N return in dead leaves varied from 10 to 20% of the annual N uptake in herbage. The rates of N cycling to the litter via leaf senescence peaked at 3.5 kg N/ha per day in plots where insect damage was severe. Rates of N cycling to the soil via litter disappearance varied from 35 to 140 kg N/ha per year.