Determining ecological indicators of anthropogenic nitrate deposition in a desert ecosystem using stable isotopes of nitrogen and oxygen

Background/Question/Methods

Anthropogenic nitrogen emissions created in the Los Angeles air basin are incorporated into the atmosphere and are transported east via dominant wind patterns before being deposited in the western Sonoran Desert. In an ecosystem naturally low in nitrogen, this deposition can alter ecosystem level cycling. Increasing nitrogen loads to the soil increases the presence of invasive grasses and leads to annual biomass accumulation capable of carrying fire. The distinct isotopic signature of oxygen, δ¹⁸O, within anthropogenic nitrate allows atmospheric inputs to be differentiated from natural sources. Our hypotheses are that the isotopic signature of deposited nitrate will be measurable on the soil surface in the summer due to an extended dry period and that early germinating invasive annuals, and perennial vegetation will have access to the pulse of nitrogen mobilized by the initial winter rain events and will be likely bioindicators of deposition.  We conducted isotopic analysis of surface soil nitrate extracts along the west to east (high to low) deposition gradient to determine the influence of anthropogenic nitrogen on plant-available nitrate. Leaves from common annual plant species were then collected in early spring along with leaves from the dominant shrub, Larrea tridentata and were analyzed for δ¹⁵N.

Results/Conclusions

The δ¹⁸O signature of atmospheric nitrate peaked at 70‰ in areas exposed to high summer atmospheric nitrogen deposition (12 kg h^-1 yr^-1), indicating a high contribution of anthropogenic nitrogen. The signature decreased from west to east along the deposition gradient dropping to 30‰ in areas experiencing low amounts of deposition (3 kg ha^-1 yr^-1). The concentration of soil nitrate was positively, linearly correlated with atmospheric nitrate concentration, while increasing soil nitrate availability was positively, linearly correlated with its δ¹⁸O value, suggesting that the increase in soil nitrogen across the gradient is the result of anthropogenic loading. Inverse correlations between soil nitrate δ¹⁸O and δ¹⁵N of the late spring annuals and perennial shrub leaf tissue suggest that the leaf tissue δ¹⁵N may be influenced by stomatal uptake of HNO₃ rather than fractionation of soil nitrate pools. Early germinating species were not correlated with anthropogenic inputs of nitrate which cycled out of the soil after precipitation began as is evident by low δ¹⁸O values associated with winter soil samples. Summer surface soil proves to be the most effective ecoindicator of anthropogenic deposition, while ubiquitous desert shrubs offer a potential bioindicator of local variations in deposition.