Ecosystem responses to nitrogen deposition in the Colorado Front Range

We asked whether 3-5 kg N y(-1) atmospheric N deposition was sufficient to have influenced natural, otherwise undisturbed, terrestrial and aquatic eco systems of the Colorado Front Range by comparing ecosystem processes and pr operties east and west of the Continental Divide. The eastern side receives elevated N deposition from urban, agricultural, and industrial sources, co mpared with 1-2 kg N y(-1) on the western side. Foliage of east side old-gr owth Englemann spruce forests have significantly lower C:N and lignin:N rat ios and greater N:Mg and N:P ratios. Soil % N is higher, and C:N ratios low er in the east side stands, and potential net N mineralization rates are gr eater. Lake NO, concentrations are significantly higher in eastern lakes th an western lakes. Two east side lakes studied paleolimnologically revealed rapid changes in diatom community composition and increased biovolumes and cell concentrations. The diatom flora is now representative of increased di sturbance or eutrophication. Sediment nitrogen isotopic ratios have become progressively lighter over the past SO years, coincident with the change in algal flora, possibly from an influx of isotopically light N volatilized f rom agricultural fields and feedlots. Seventy-five percent of the increased east side soil N pool can be accounted for by increased N deposition comme nsurate with human settlement. Nitrogen emissions from fixed, mobile, and a gricultural sources have increased dramatically since approximately 1950 to the east of the Colorado Front Range, as they have in many parts of the wo rld. Our findings indicate even slight increases in atmospheric deposition lead to measurable changes in ecosystem properties.