(ipickleweed) as variables. In addition, he 

 repeated the ±urea experiment in an area of 

 pure cordgrass. The field experiments were 

 carried out in 1983 near transects TJE-28 

 and TJE-30 (Chapter 5). Urea was broadcast 

 onto the marsh soils biweekly through the 

 growing season in + urea plots. Biomass was 

 measured in August 1983 and significant 

 treatment effects were found. Urea stimulated 

 only the growth of pickleweed; competitor 

 removal stimulated only the growth of 

 cordgrass (Table 4.6). Pickleweed was the 

 superior competitor for nitrogen uptake; its 

 biomass increased significantly with urea 

 additions, with no significant effect from the 

 presence of cordgrass. 



Nitrogen added to pure stands of cordgrass 

 was readily taken up by the plants--so 

 thoroughly that soil nitrogen concentration did 

 not increase. Instead, the nitrogen went 

 directly into the leaves, and could be measured 

 only as increased nitrogen concentrations in 

 plant tissues. Insects seemed able to locate 

 plants with enhanced tissue nitrogen, and it 

 appeared that local outbreaks did serious 

 damage to cordgrass. Thus, the net effect of 

 urea fertilization on pure stands of cordgrass 

 may be beneficial, by stimulating growth, or 

 detrimental by causing insect damage. 



The experimental results (Table 4.6) 

 were complex, and they led to a new model of 

 nitrogen-marsh dynamics (Covin 1984; 

 Covin and Zedler 1988): Nitrogen is 

 certainly limiting to vascular plant growth. 

 Additions will stimulate cordgrass in pure 

 stands, but herbivores may gain the ultimate 

 benefit from the increased nutritional quality 

 of the plants. In mixed stands, additions 

 stimulate pickleweed, which then outcompetes 

 cordgrass. 



Some interesting questions remain: Why 

 didn't nutrient additions stimulate herbivory 

 on pickleweed? Was it chance that precluded 

 an outbreak in the fertilized plots? Perhaps, 

 but a 1984 experiment performed by Beare 

 and Beezley (unpubl. data) suggests 

 otherwise. Plots with urea added had less 

 herbivory than plots fertilized with 

 Milorganite. In replicate plots of pure 

 pickleweed, urea was added biweekly along 



Table 4.6. Standing crop of cordgrass and 

 pickleweed in August 1983 in replicate (n = 

 2) plots with and without urea additions and 

 with and without competitors removed. All 

 data are aboveground biomass in g/m 2 (from 

 Covin 1984). 



with fresh water to cylinders of 0.33 m 2 area 

 that penetrated 10 cm of soil and protruded 

 30 cm aboveground. Milorganite was added in 

 low, medium, and high concentrations to 

 additional cylinders, with the high treatment 

 adding an amount of nitrogen equal to that in 

 the urea treatment. Other cylinders received 

 fresh water only. Each treatment was 

 replicated four times. Plant growth increased 

 with nutrients added in high concentrations, 

 but results were complicated by insect attack. 



Beetle herbivory became pronounced in 

 August, and the lengths of chew marks were 

 summed for stems sampled from each 

 treatment. Insect damage in plots fertilized 

 with Milorganite averaged 40 mm/branch, 

 while those with urea averaged 4 mm/branch. 

 The growth response to the two fertilizers was 

 similar, but the insect response was not. 

 Likewise, Covin did not find insect damage on 

 urea-fertilized pickleweed, although he did on 

 cordgrass (see above). There may be a 

 differential insect response to different plant 

 species fertilized by the same nitrogen source. 



However, these results actually suggest 

 more questions than they answer. Because the 



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