Detectability of 15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of 15N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if 15N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.)-winter rye (Secale cereale L.) crop rotation. Nitrogen as 15N-depleted (NH4)2SO4 was applied at five rates (0, 84, 168, 252, and 336 kg N ha-1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO3(-)-N fraction following corn harvest (140-d after application). Low levels of exchangeable NH4(+)-N (<2.5 mg kg-1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha-1 N rate. Atom % 15N concentrations in the nonexchangeable NH4(+)-N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO3(-)-N levels below the 10 kg ha-1 level needed for accurate isotope-ratio analysis. Atom % 15 N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha-1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom % 15N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of 15N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.