Monthly δ15N-NH3 values are reported for 9 AMoN sites over a year. δ15N-NH3 values ranged from −42.4 to +7.1‰ with an average of −15.1 ± 9.7‰. US agricultural regions have low and seasonally variable δ15N-NH3 values. Rural nonagricultural areas have higher and seasonally consistent δ15N-NH3 values. US spring agricultural activity peak is accompanied by a decrease in δ15N-NH3 values.