Field-applying an inexpensive, 13C-depleted, labile carbon source to study in situ fate and short-term effects on soils

Abstract

Background: Labile carbon (Clabile) limits soil microbial growth and is critical for soil functions like nitrogen (N) immobilization. Most experiments evaluating Clabile additions use laboratory incubations. We need to field-apply Clabile to fully understand its fate and effects on soils, especially at depth, but high cost and logistical difficulties hinder this approach. Aims: Here, we evaluated the impact of adding an in situ pulse of an inexpensive and 13C-depleted source of Clabile—crude glycerol carbon (Cglyc), a by-product from biodiesel production—to agricultural soils under typical crop rotations in Iowa, USA. Methods: We broadcast-applied Cglyc at three rates (0, 216, and 866 kg C ha−1) in autumn after soybean harvest, tracked its fate, and measured its impact on soil C and N dynamics to four depths (0–5, 5–15, 15–30, and 30–45 cm). Nineteen days later, we measured Cglyc in microbial biomass carbon (MBC), salt-extractable organic C, and potentially mineralizable C pools. We paired these measurements with nitrate N (NO3−–N) and potential net N mineralization to examine short-term effects on N cycling. Results: Cglyc was found to at least 45-cm depth with the majority in MBC (18%–23% of total Cglyc added). The δ13C values of the other measured C pools were too variable to accurately track the Clabile fate. NO3−–N was decreased by 13%–57% with the 216 and 866 kg C ha−1 rates, respectively, and was strongly related to greater microbial uptake of Cglyc (i.e., immobilization via microbial biomass). Crude glycerol application had minor effects on soil pH—the greatest rate decreased pH 0.18 units compared to the control. Conclusions: Overall, glycerol is an inexpensive and effective way to measure in situ, Clabile dynamics with soil depth—analogous to how mobile, dissolved organic C might behave in soils—and can be applied to rapidly immobilize NO3−–N.