The response of decomposing leaf litter to dissolved organic nitrogen additions was studied during two years in an acid blackwater stream located in South Carolina. Decomposition, transformation and colonization of sweetgum (Liquidambar styraciflua L.) and water oak (Quercus nigra L.) leaves were studied in situ in floating microcosms placed in Upper Three Runs Creek. Physical-chemical decomposition parameters measured during the first year included: remaining leaf biomass, carbon (C), nitrogen (N), lignin, cellulose, soluble material and ash. Additions of 0.25 and 0.50 mg/l urea did not alter decomposition rates of leaf C, N, soluble material or ash content. However, urea additions did result in lesser relative cellulose content and greater relative lignin content. Greater microbial utilization of cellulose was judged to be responsible for the lesser relative cellulose content. Nonenzymatic artifact lignin formation and, to a lesser degree, decreased microbial utilization contributed to the greater relative lignin content. Urea addition appeared to increase variability associated with each of the physical-chemical parameters.; Simultaneous validation studies were conducted to determine how accurately the microcosms represented natural processes. Leaf litter bags, identical to the ones used in the microcosms, were placed directly into the stream in a macrophyte bed and stretch of sandy bottom. These two sites represented the different types of stream microhabitats. Litter bags placed in the microcosms were more similar to those on the homogeneous sandy substratum than in the macrophyte bed.; Effects of urea additions on biotic decomposition processes were studied during the second year. Carbon and nitrogen were again determined for leaf packs placed in microcosms and at validation sites. Changes in microbial biomass were estimated by measuring ATP content and colonizing macroinvertebrate populations were censused. Total numbers of macroinvertebrate organisms and taxa were determined for each litter bag as well as number of organisms and taxa in each functional feeding group. Urea additions did not affect C or N content of sweetgum material in the second year and microbial and macroinvertebrate populations were also unaffected. Results of urea additions to water oak leaves were not as easily interpreted. Leaf C and N was not altered. However, both concentrations of urea caused nonsignificant increases in microbial biomass while addition of 0.50 mg/l caused nonsignificant increases in total numbers of macroinvertebrate organisms and taxa. Again, urea additions increased variability associated with the measured parameters.; Validation studies identified slight differences in both microbial and macroinvertebrate colonization patterns between microcosms and validation sites. Microbial biomass was greater on water oak than sweetgum leaves at the validation sites; however, the increases were not statistically significant. Microbial biomass was similar on both leaf species in the control microcosms. Macroinvertebrate colonization of litter bags was initially slower in the microcosms but population densities were similar to the validation site densities midway through the experiment.; Urea additions had subtle effects on the leaf material at the concentrations studied. Changes in leaf C and N content and microbial and macroinvertebrate colonization patterns could alter energy transfer to higher trophic levels. Such changes could have profound effects on the structure and function of lotic ecosystems.
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