Soil-dependent short-term transfer of fertilizer nitrogen into microbial biomass and mineral associated organic matter

Authors: REKHI, MANJOT - University Of New Hampshire; DALY, AMANDA - University Of New Hampshire; GARDNER, BRIAN - Ag Spectrum Company; FREY, SERITA - University Of New Hampshire; HILDEBRAND, LEE - University Of New Hampshire; Schmer, Marty; GRANDY, STUART - University Of New Hampshire

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2026
Publication Date: 8/11/2026
Citation: Rekhi, M., Daly, A.B., Gardner, B.B., Frey, S., Hildebrand, L., Schmer, M.R., Grandy, S.A. 2026. Soil-dependent short-term transfer of fertilizer nitrogen into microbial biomass and mineral associated organic matter. Geoderma. 473. Article 117959. https://doi.org/10.1016/j.geoderma.2026.117959.
DOI: https://doi.org/10.1016/j.geoderma.2026.117959

Interpretive Summary: Starter nitrogen fertilizer is commonly applied to agricultural soils in the U.S. Midwest in the spring, when crop demand is low and precipitation is high. This timing increases the risk of environmental nitrogen loss and reductions in fertilizer nitrogen use efficiency. Here, we tested a potential mechanism for retention of this nitrogen fertilizer. Our results show that microbial fertilizer nitrogen uptake and subsequent conversion to mineral associated soil organic matter can move nitrogen fertilizer from inorganic to organic soil pools, reducing its vulnerability to loss and potentially increasing nitrogen availability for crops later in the growing season.

Technical Abstract: Starter nitrogen (N) fertilizer is commonly applied to agricultural soils in the U.S. Midwest in the spring, when crop demand is low and precipitation is high. This timing increases the risk of environmental N loss and reductions in fertilizer N use efficiency. Here, we tested a potential mechanism for retention of this fertilizer N: microbial N interception and subsequent transfer into mineral-associated organic matter (MAOM). We also tested how strongly this pathway depends on soil type and microbial properties. We conducted an incubation with two contrasting agricultural soils amended with 15N-labeled urea ammonium nitrate (UAN) fertilizer, a biostimulant-containing starter fertilizer (InFurrow), or their combination at low and high rates, and quantified fertilizer-derived N in total dissolved N (TDN), microbial biomass N (MBN), and MAOM at 14 and 48 days. TDN was the largest sink, accounting for ~47–66% of applied N, and scaled linearly with fertilization rate. Despite the absence of plants, fertilizer N moved quickly into microbial and MAOM pools. MBN recovered ~5–17% of applied 15N, and fertilizer N comprised >25% of MBN by day 14 and 45–100% by day 48. Recovery in MAOM at day 14 ranged from ~5–12% in NH soils and ~20–30% in Nebraska soils, reaching up to 32%. Fertilizer-derived MAOM was moderately to strongly related to fertilizer-derived MBN (R2 = 0.31-0.68), and an elevated qCO2 (metabolic quotient) under high-rate UAN further suggests microbial physiology helped regulate the conversion of added N into organic pools. Our results show that microbial fertilizer N uptake and subsequent conversion to MAOM can move fertilizer N from inorganic to organic pools, reducing its vulnerability to loss and potentially increasing N availability for crops later in the growing season.