very slowly. After 2 years, about 5% of 

 the initial litter still remains. These 

 remnants are indistinguishable from the 

 organic matter of the sediments and are 

 presumably what accumulates as marsh peat. 



During both the leaching and decom- 

 position phases, the more luxuriant the 

 marsh that produces the litter, the more 

 nitrogen the litter will contain and the 

 more rapid will be its decomposition (Fig- 

 ure 19). In the Great Sippewissett Salt 

 Marsh, the rate of decomposition also 

 increased if the extra nitrogen was added 

 to the marsh soil rather than being within 

 the leaf. The same effect was produced if 

 nitrogen was enriched in the soil water 

 either experimentally or by pollution of 

 the marsh (Valiela et al . 1984) (Figure 

 20). The fact that nitrogen enhances 

 decomposition whether it is in the plant 

 tissue or the environment of the decom- 

 poser organisms implies that there is a 

 nitrogen limitation to decomposition as 

 well as to primary production in the 

 marsh. 



Incubated in 

 F Plot C Plot 



Litter { 



400 



Figure 20. Decay of Spartina litter under 

 different conditions of nitrogen presence. 

 Litter from control plots (C Plot) was 

 incubated in the plots in which they grew 

 and in fertilized plots (F Plot) where 

 there was higher nitrogen in the litter's 

 environment. Litter from fertilized plots 

 was similarly treated. Only control 

 litter in unfertilized plots decayed more 

 slowly than the others. (Teal and 

 Valiela, unpubl. data, Great Sippewissett 

 Salt Marsh, MA). 



Since the initial losses of nitrogen 

 from litter are soluble, it is not 

 surprising that over time, the remaining 

 nitrogen in the detritus is less and less 

 soluble. After 1 year, amino acids still 

 constitute about 20% of the remaining 

 nitrogen, but they are almost entirely 

 bound to insoluble compounds in the litter 

 and are presumably resistant to decay. 



It has been known for some time that 

 as detritus ages, its relative 

 concentration of nitrogen increases (Odum 

 and de la Cruz 1967; Figure 19). 

 Scientists initially believed that this 

 increase represented the nitrogen in 

 microbes on the detritus and that aged 

 detritus was improved as a food source for 

 marsh animals. Later, researchers found 

 that bacteria contribute only a small 

 percent of the total nitrogen in decaying 

 Spartina (Rublee at al. 1978). Fungi may 

 contain about one-fifth of the non-protein 

 nitrogen in detritus (Odum et al. 1979a). 

 In any event, there is not sufficient 

 microbial biomass to account for all of 

 the nitrogen in the detritus (Lee et al. 

 1980). A portion of this unaccounted-for 

 nitrogen is certainly in the form of 

 extracellular compounds produced by 

 microbes; many of these compounds are 

 probably resistant to decomposition. Some 

 nitrogen is also bound as proteins to 

 oxidized phenolic compounds that come from 

 the degradation of lignin (a structural 

 component of plants) or that are present 

 in the plant as so-called "secondary 

 products" (compounds which may protect the 

 plant from being eaten). Aside from the 

 microbial biomass itself, most of the 

 nitrogenous compounds in detritus are not 

 readily available as food for the 

 detritivores. Therefore, relative 

 increases of nitrogen in detritus do not 

 necessarily enhance its food value for 

 animals. 



Animals are able to harvest microbes 

 from detritus (Jeffries 1972; Welsh 1975; 

 Wetzel 1975, 1976). Microbes will 

 recolonize the particles and grow at the 

 expense of compounds like cellulose that 

 are not readily digested by animals. 

 Animals can then reprocess detritus and 

 harvest the microbes again. Algae also 

 grow on processed detritus and may 

 significantly enhance its food value. 

 Apparently, pure detritus is not as 



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