Crude glycerol, a biodiesel byproduct, used as a soil amendment to temporarily immobilise and then release nitrogen

Abstract

AbstractLoss of nitrate‐nitrogen (NO3−–N) from Midwestern U.S. agricultural fields can impair water quality and be an economic loss to farmers. Winter cover crops have shown promise as a remedy, but low adoption illustrates the need for alternatives. Here, we tested whether adding a carbon (C)‐rich soil amendment (i.e., crude glycerol, a biodiesel byproduct) can increase soil microbial biomass (MB) and promote N immobilisation under various conditions and then determined whether and when immobilised N would be released. We conducted a laboratory incubation with a full factorial combination of four glycerol rates (0, +117, +468 and +1872 mg C kg−1 soil), three supplemental NO3−–N rates (0, +10 and +40 mg N kg−1) and two soils (Clarion clay loam and Sparta loamy sand). Soil inorganic N (NH4+–N and NO3−–N) and MB were measured at seven and three time points, respectively, across the 98 days incubation period. Across all treatments, glycerol increased MBN in both short term (7 days; 4%–1137% compared to no glycerol addition) and long term (98 days; 10%–169%) and decreased NO3−–N with increasing rate of glycerol. Adding glycerol caused net N immobilisation of 21%–61% (+117 mg C kg−1 addition) and ~100% (+468 and +1872 mg C kg−1 addition) compared to the control. Some of that immobilised inorganic N was likely released through MB turnover, but timing and rate of release depended on the soil and added N rate. Adding 40 mg N kg−1 with no glycerol showed nearly twice the net N mineralisation rate than with the low or no applied N – providing evidence for soil N priming. Overall, glycerol has the potential for use as a soil amendment to increase MB and temporarily immobilise NO3−–N and then make some of that N crop available through MB turnover.Highlights Crude glycerol, a biodiesel by‐product, was evaluated as a soil amendment to reduce soil nitrate. Glycerol strongly increased soil microbial biomass and decreased nitrate under all conditions. N immobilisation was temporary, and N was mineralised at a lower glycerol rate. Glycerol rates, N rates and soil type affected N immobilisation‐mineralisation dynamics.