Experimental evidence for N recycling in the leaves of the seagrass Posidonia oceanica

A one-year in situ experiment using 15 N as a tracer was designed to assess the N recycling in the leaves of the seagrass Posidonia oceanica (L.) Delile. P. oceanica was shown to partly recycle the internal nitrogen pool of its leaves in order to contribute to new leaf growth. The leaves sampled in June 1999 contained 20% of the quantity of 15 N found in June 1998. N recycling caused a difference between N and biomass turnover rate (0.8 vs 1.3 y -1 ) of Posidonia leaves. This 40% difference should correspond to the contribution of recycled N to the annual N requirement of Posidonia leaves. The N recycling appears to be insufficient to significantly reduce the quantitative impact of N loss due to autumnal leaf fall. However, new leaf growth between June and October is mainly sustained by this recycling because the tracer concentration in new leaves was the same as in the other leaves. By contrast, tracer concentration decreased drastically between October 1998 and June 1999, showing the more important contribution of N uptake during winter and spring. Nevertheless, recycling occurs throughout the year as demonstrated by the presence of tracer in the youngest leaves of shoots sampled one year after the tracer addition.