Long-term nitrogen fertilization rates affect crop micronutrient concentrations but not soil micronutrient availability

Authors: MINER, GRACE - Colorado State University; Delgado, Jorge; IPPOLITO, JAMES - Colorado State University; BARBARICK, KEN - Colorado State University; Stewart, Catherine; Manter, Daniel; Del Grosso, Stephen - Steve; Halvorson, Ardell; Floyd, Bradley; D Adamo, Robert

Submitted to: Field Crops Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2018
Publication Date: 9/17/2018
Citation: Miner, G.L., Delgado, J.A., Ippolito, J.A., Barbarick, K.A., Stewart, C.E., Manter, D.K., Del Grosso, S.J., Halvorson, A.D., Floyd, B.A., D Adamo, R.E. 2018. Long-term nitrogen fertilization rates affect crop micronutrient concentrations but not soil micronutrient availability. Field Crops Research. 228:170-182. https://doi.org/10.1016/j.fcr.2018.08.017.
DOI: https://doi.org/10.1016/j.fcr.2018.08.017

Interpretive Summary: Alongside yield goals, agricultural production must concurrently consider the impacts of agronomic practices on crop quality, human and animal nutrition, and soil fertility. While the effects of management practices on crop quality and nutritional value are dependent on soil and environmental conditions, we found that stover [Cu] and [Mn] increased substantially with increasing N fertilization, whereas grain [Cu] and [Mn] did not change in response to N fertility. Agronomic ‘biofortification’ of stover with increasing N rate was evident for Cu and Mn, which would increase the nutrient concentration of crops harvested for silage or other purposes. Stover and grain [Fe] varied considerably between growing years, highlighting the inter-annual variability within genotype that can occur due to growing environment. Grain [Fe] increased in response to N fertilization, suggesting that Fe nutrition is improved with N fertilization. Conversely, [Zn] declined in all plant fractions with increasing N. It is possible that declines in stover and grain [Zn] with increasing N rate were due to insufficient supply of soil Znavail under the increased biomass of high N fertility, but other studies have also found dilutions in stalk and grain [Zn] under increased N fertility, suggesting that maize stover and grain [Zn] are susceptible to yield dilution. Our results underscore the importance of agronomic management on crop nutritional quality. While multiple studies have attributed long-term declines or increases in total or available soil micronutrients to removal via crop harvest or addition via fertilization, we found no difference in available or total micronutrients as a function of N fertilization. This was even in despite of widely differing rates of yield and export. Instead, our results underscore that changes in available micronutrients under long-term production cannot be attributed solely to export via crop harvest or import via fertilization without considering changes in soil organic carbon. Our results suggest that declines or increases in available micronutrients were not due primarily to nutrient export or addition. Changes in available Cu, Mn, and Zn closely followed changes in SOC, underscoring the complex role of SOC in sustaining long-term soil fertility.

Technical Abstract: Sustaining or increasing micronutrient concentrations in food and feed is essential for human and animal health, yet little research has quantified the relative importance of nitrogen (N) fertilization on crop micronutrient concentrations. Nitrogen fertilization could dilute some micronutrients while increasing others (i.e., biofortification). Nitrogen fertilization also influences the amount of residue returned to the soil, potentially altering soil organic carbon (SOC) and micronutrients. While changes in soil micronutrients under long-term production are sometimes attributed to export via harvest or import via fertilization, available micronutrients are also influenced by soil organic matter changes in SOC could impact available micronutrients. Our objectives were to (1) examine the long-term effects (14 years) of different N rates (0, 68, 120,173, 232 kg ha-1) on the concentration ([ ]), uptake, and cycling of N and micronutrients (Fe, Zn, Cu, Mn) in no-till continuous maize and (2) assess whether nutrient removal via harvest, or residue return have resulted in depletion or accumulation of SOC and available and total micronutrients. Results showed a positive impact of fertilization on stover [N], [Cu] and [Mn], and increased grain [N] and [Fe]. However, fertilization decreased [Zn] in all plant fractions, indicating that maize [Zn] is susceptible to yield dilution. Total and available soil micronutrients were not impacted by fertilization, despite widely differing rates of nutrient harvest and return. Available Cu and Mn followed changes in SOC, which declined in all depths except the surface 0 - 7.5 cm. Available Zn increased in the surface 0 - 7.5 cm, with slight increases in deeper depths. Micronutrients appear to be closely regulated by SOC in this system. These findings give additional insights into how N fertilization impacts micronutrient concentrations, and suggest that SOC contributes to micronutrient availability, underscoring the role of SOC in sustaining long-term soil fertility.