Effects of Positive and Negative Nitrogen Perturbations on an Old-field Ecosystem

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Abstract
The effects of contrasting N stresses on an old-field ecosystem were investigated. A positive stressor (urea fertilizer) and a negative stressor (Amitrole-T herbicide) were each applied to 4 replicate 21.0 .times. 30.5 m plots on 25 July 1975. Four control plots were also established. Standing crop biomass (live), dead plant material (detritus), net primary productivity and species diversity (richness and apportionment) were used to evaluate ecosystem response to positive and negative stressor application. Standing crop biomass was significantly (P .ltoreq. 0.01) reduced in the herbicide-treated plots for approximately 3 mo. following treatment; standing crop biomass in urea-treated plots did not differ significantly from control plots following treatment. No significant differences in dead plant material were found between treatments and control plots at any time during the study. Fertilized plots exhibited peaks in seasonal primary productivity 2 wk prior to control and herbicide-treated areas. Net primary productivity within positive or negative treated plots did not differ significantly from control means; positive and negative treatment plots differed significantly (P .ltoreq. 0.05) from each other for approximately 6 wk following treatment. Species richness and evenness values increased dramatically in herbicide-treated plots as compared to either control or urea-treated plots. Young ecosystems apparently respond more to changes in community structure (e.g., species richness) than to changes in community function (e.g., primary productivity) following negative stress treatment; the reverse response appears true for young systems treated with a positive stressor.