Determination of Sources of Soil Nitrate in a Long-Term Irrigated Rotation Using Ion Exchange Membranes and Isotope Ratio Measurements.

Nitrate is the dominant form of N taken up by crops or lost to the hydrosphere in most agricultural soils. The isotopic composition of nitrate in soils can often reveal the sources of this nutrient and the biogeochemical history of nitrate in the pedosphere. Laboratory and field studies were conducted to test the potential use of anion exchange membranes (Plant Root Simulator [PRS] probes) for isotopic tracing of sources and fate of soil nitrate. A series of laboratory experiments in sandy media containing nitrate of known isotopic composition revealed that soil nitrate was adsorbed onto PRS probes in sufficient quantities for isotope analysis within days without significant nitrogen and oxygen isotope fractionation.  Field measurements on nitrate collected with PRS probes in a long-term irrigated experiment were used to evaluate differences in nitrate isotope compositions in cropping systems that varied in the proportion of N inputs from manure, synthetic fertilizer and biological di-nitrogen fixation. These preliminary results suggest that PRS probes are an effective tool to collect soil nitrate for isotope analyses with the goal to elucidate different sources of nitrate in soils.