Nitrogen accumulation by Ulex gallii (Planch.) in a forest ecosystem.

Abstract

Accumulation of nitrogen was assessed under naturally occurring, 5 year old Ulex gallii (Planch.), dwarf furze, on impoverished soils formed from Old Red Sandstone. Nitrogen accumulation in U. gallii and non-Ulex vegetation (including litter), in roots and soil (25 x 25 x 20cm cores) was estimated within 5 x 2m2 subplots chosen subjectively on the basis of legume occurrence within a single, phosphate-fertilised plot of a forest experimental site at Kilworth, Co. Cork. The influence of U. gallii on carbon and nitrogen mineralisation in litter and soil (A1 horizon) was also evaluated. A close correlation (r=O.99; p<0.05) was found between U. gallii cover (2.4 to 19.9 t DM ha-1) and the nitrogen content of the total vegetation (220 to 495kg N ha- 1, resp.). The relationships between U. gallii occurrence and soil nitrogen (2325 to 2588kg N ha-1, resp.) or soil plus vegetation N (2545 to 3085kg N ha-1, resp.) were weaker (r=O.72 and 0.86, resp.). The correlation between legume cover and the nitrogen content (% DM) of the non-Ulex vegetation was r=0.83. The annual accumulation of nitrogen within the vegetation was estimated to be 55kg N ha-1. Only Ulex litter released appreciable quantities of nitrogen after 80 days incubation at 20°C - 5.89 compared to 1.17 and 2.28mg N g-l total-N by senescent Ulex spines and non-Ulex litter, respectively. Mineralisation of soil nitrogen after 112 days incubation at 20°C was considerably greater under U. gallii than under other vegetation (Calluna-Molinia spp) - 3.3% and 0.4% of soil - N, respectively. These results were corroborated by CO evolution studies. The results are discussed in the context of the nitrogen requirements of Sitka spruce stands on these soils and the distribution and exploitation of U. gallii as a nurse crop.