WIND AND LUNAR TIDES ^ 



0, EXCHANGE, 



SEDIMENT AND 

 NUTRIENT 



INFLUX 



SOIL REDOX 

 POTENTIAL 



J MICROBIAL 



^ ANAEROBIC 



"nIO" METABOLISM 



J 



'■"flONS 



Figure 48. Marsh soil transformations 

 that result from tidal flooding. 



inland, but for shorter periods of time 

 (Table 7), and the inland floodwaters are 

 more slowly exchanged. Furthermore, the 

 streamside marshes drain better on falling 

 tides because their sediments are coarser. 

 They also contain more reducible mineral 

 ions to buffer redox changes. All these 

 factors lead to stronger reducing 

 potentials in inland marshes than 

 streamside. 



The 

 strongly 

 potential . 

 nutrient, 

 availabl e 



chemistry of many minerals is 



influenced by the redox 



Phosphorus, a key plant 



is much more soluble (and hence 

 to plants) under reduced than 

 oxidized conditions (Delaune et al. 1981). 

 Inorganic nitrogen, the primary limiting 

 nutrient in marshes, is reduced to the 

 aimonium ion which is readily absorbed by 

 plant roots. More nutrients are delivered 

 to streamside than to inland sites; this 

 should favor streamside plant growth 

 rates. Organic nitrogen is also more 

 rapidly mineralized to ammonium in 

 streamside sites (Brannon 1973). 



Other minerals may be transformed to 

 toxins or accumulate in toxic concentra- 

 tions (for example, sulfide) (Hollis 1967). 

 Toxic byproducts of anaerobic microbial 

 metabolism may accumulate. In general, the 



levels of these potential toxins are 

 higher in inland marshes than streamside 

 marshes, increasing the stress on inland 

 plants. Finally, referring again to 

 Figure 48, the direct flushing of marsh 

 soils and the leaching of olant leaves can 

 dilute toxic materials, reducing their 

 activity. Flushing occurs more readily in 

 streamside sites, reducing the potential 

 for accumulation of toxins. With all 

 these potential effects it is not surpris- 

 ing that plant production is higher along 

 streams than inland. 



Soil analyses can, at times, mislead. 

 For example, it has been found that 

 ammonium in marsh soil interstitial water 

 is more concentrated inland than stream- 

 side. This is not expected, considering 

 the higher rates of ammonium production in 

 streamside areas. Apparently, however, 

 the interstitial water concentration is 

 controlled by the rate of plant root up- 

 take. The concentration is maintained at 

 low levels by streamside plants; it accu- 

 mulates in inland sites because the less 

 robust inland plants are unable to use all 

 the ammonium available to them. 



Figure 49 summarizes typical seasonal 

 patterns for various physical and biologi- 

 cal processes in marsh soils. Soil water 

 salinity is highest during the summer but 

 probably does not reach levels that ax-& 

 biologically limiting for the euryhaline 

 marsh species. The low winter and early 

 spring salinities correspond with winter 

 rains and low transpiration rates, indi- 

 cating flushing of the marsh by rainwater. 



Soil-reducing potential (Eh) is least 

 negative (least anaerobic) during the 

 winter, but even during this period it is 

 too low to support any free oxygen. The 

 seasonal Eh curve is the inverse of the 

 tenperature curve - the soil becomes more 

 and more reduced as temperatures rise and 

 biological activity increases. Soils 

 begin to become less anoxic in late summer 

 as temperature drops, even though the 

 marsh is flooded almost all the time 

 during these months. Free sulfide follows 

 the redox curve closely. It is generally 

 highest when the Eh is lowest. Extract- 

 able manganese is an example of a 

 metal ion that is fairly easily reduced. 

 The substrate is always anoxic enough to 

 reduce the manganic ion and the reduced 



53 



