AVAILABLE AND REFRACTORY NITROGEN IN DETRITUS OF COASTAL VASCULAR PLANTS AND MACROALGAE

The chemical nature of nitrogenous compounds in detritus both reflects decay processes and influences rates of detrital decomposition. We followed changes in total, available and refractory nitrogen and amino acids during decay of 6 different species of marine plants and macroalgae. Immobilization of nitrogen occurred only in detritus derived from Spartina patens, probably because the decay rates of all other species were too rapid to allow mechanisms fostering nitrogen incorporation into detritus to exceed nitrogen losses. In vascular plants, nitrogen in whole detritus and detrital cell walls was lost more slowly than detrital carbon; in marine algae, there was little difference between rates of nitrogen and carbon loss. Ratios of amino acids: total nitrogen changed very little during decay, suggesting that the chemical transformation of amino nitrogen into heterocyclic rings did not occur. Pools of available nitrogen, as determined by lability to a bacterial protease and extraction in a neutral detergent, generally declined at a faster rate than pools of refractory nitrogen. In the detritus with the slowest overall rate of organic matter loss, there was little difference between rates of available and refractory nitrogen loss, suggesting that carbon rather than nitrogen was more limiting to decomposers in this species and other vascular plants. In S. alterniflora detritus, the pool of N associated with lignocellulose actually increased in size during decay, probably because nitrogen becomes bound to lignin. A comparison of the decay rates, nitrogen dynamics, and detrital cell wall chemistry of the plants and algae support the central role of detrital chemistry in controlling rates of decomposition and nitrogen turnover.