Fate of synthetic fertilizer nitrogen in a maize system depends on dairy manure type: insights from an isotopic tracing field study

Abstract

Understanding the fate of fertilizer nitrogen (N) in cropping systems is critical to improving agricultural sustainability and minimizing environmental losses. However, efforts to trace fertilizer N are often complicated by manure application in dairy cropping systems. We used a stable isotope tracing approach to investigate how different manure products influence the fate of synthetic fertilizer N in a Midwestern maize silage system by tracking 15 N-labled ammonium sulfate into crop biomass, soil organic matter fractions (particulate (POM) and mineral associated (MAOM)), and gaseous loss as nitrous oxide (N 2 O). Treatments included unprocessed liquid manure (LM), solid manure derived through chemical (CS) or physical (PS) separation, and a no-manure control (NM), combined with fertilizer rates of 90, 180, and 270 kg N ha −1 . Cumulative N 2 O emissions increased significantly with both manure application and higher fertilizer rates, with PS having lower emissions than LM following fertilizer application. Fertilizer contributed similar proportions to cumulative N 2 O emissions regardless of manure treatment. Total fertilizer N uptake by crops did not differ among manure treatments, but crops took up more fertilizer N at higher fertilizer application rates. Notably, application of CS and PS enhanced fertilizer N retention in the 0–15 cm soil MAOM pool compared to LM and NM. These findings highlight that although manure type does not significantly affect fertilizer N uptake by crops or fertilizer-derived N 2 O emissions, solid manures (i.e., CS and PS) can significantly increase fertilizer N retention in soils, offering potential benefits for long-term soil fertility and reduced N losses.

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