In Silico Evaluation of Plant Nitrification Suppression Effects on Agroecosystem Nitrogen Loss

AO_SCPLOWBSTRACTC_SCPLOWNitrification regulates potential for nitrogen (N) loss from ecosystems because it converts ammonium to nitrate, which is susceptible to leaching and gaseous emissions. Crops can suppress the microbes that perform nitrification by exuding nitrification-inhibiting compounds from their roots and taking up available ammonium, the substrate for nitrification. However, the effect of nitrification suppression on agroecosystem N losses remains poorly characterized, and a lack of temporal synchrony between nitrification, N losses, and nitrification suppression by plants could limit the effect of nitrification suppression. We used the DayCent-CABBI model to evaluate the effectiveness of the suppression of nitrification by sorghum to reduce N2O emissions and nitrate leaching in an energy sorghum/soybean rotation at the Energy Farm in Urbana-Champaign, IL. We simulated nitrification suppression at the measured levels (measNS) and at the maximum measured level applied to the entire growing season (maxNS), and we also explored ways to better utilize nitrification suppression by altering the timing of UAN fertilizer applications. Model experiments showed that most nitrification occurred immediately after fertilizer was applied, whereas nitrification suppression begins to ramp up more than a month after planting. On an annual basis, measNS experiments showed a 1-2% reduction in annual N2O emissions relative to no nitrification suppression (noNS), and maxNS experiments showed a 4-9% reduction in annual N2O emissions relative to noNS. Both nitrification suppression levels showed < 1% reduction in nitrate leaching. Altering the timing of fertilizer applications to better synchronize nitrification suppression with high soil ammonium levels had mixed effects on annual N2O emissions and nitrate leaching and sometimes resulted in increased N losses. The timing of simulated N2O emissions shifted with the timing of fertilization, and N2O emissions from denitrification increased when N2O emissions from nitrification decreased. Increasing N retention during the non-growing season may be more effective and growing-season nitrification suppression for reducing annual N losses in the rainfed Midwest, particularly for NO3- leaching in the early spring. Optimizing the timing of nitrification suppression alongside off-season N retention strategies would best improve the N sustainability of agroecosystems.