Marshall D. McDaniel

Background Labile carbon (C labile ) limits soil microbial growth and is critical for soil functions like nitrogen (N) immobilization. Most experiments evaluating C labile additions use laboratory incubations. We need to field‐apply C labile 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 13 C‐depleted source of C labile —crude glycerol carbon (C glyc ), a by‐product from biodiesel production—to agricultural soils under typical crop rotations in Iowa, USA. Methods We broadcast‐applied C glyc 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 C glyc in microbial biomass carbon (MBC), salt‐extractable organic C, and potentially mineralizable C pools. We paired these measurements with nitrate N (NO 3 − –N) and potential net N mineralization to examine short‐term effects on N cycling. Results C glyc was found to at least 45‐cm depth with the majority in MBC (18%–23% of total C glyc added). The δ 13 C values of the other measured C pools were too variable to accurately track the C labile fate. NO 3 − –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 C glyc (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, C labile dynamics with soil depth—analogous to how mobile, dissolved organic C might behave in soils—and can be applied to rapidly immobilize NO 3 − –N.

Managing soils to retain new plant inputs is key to moving toward a sustainable and regenerative agriculture. Management practices, like diversifying and perennializing agroecosystems, may affect the decomposer organisms that regulate how new residue is converted to persistent soil organic matter. Here we tested whether 12 years of diversifying/perennializing plants in agroecosystems through extended rotations or grassland restoration would decrease losses of new plant residue inputs and, thus, increase retention of carbon (C) and nitrogen (N) in soil. We tracked dual‐labeled ( 13 C and 15 N), isotopically enriched wheat ( Triticum aestivum ) residue in situ for 2 years as it decomposed in three agroecosystems: maize–soybean (CS) rotation, maize–soybean–wheat plus red clover and cereal rye cover crops (CSW2), and spring fallow management with regeneration of natural grassland species (seven to 10 species; SF). We measured losses of wheat residue (C wheat and N wheat ) in leached soil solution and greenhouse gas fluxes, as well as how much was recovered in microbial biomass and bulk soil at 5‐cm increments down to 20 cm. CSW2 and SF both had unique, significant effects on residue decomposition and retention dynamics that were clear only when using nuanced metrics that able to tease apart subtle differences. For example, SF retained a greater portion of C wheat in 0–5 cm surface soils (155%, p = 0.035) and narrowed the C wheat to N wheat ratio ( p < 0.030) compared to CS. CSW2 increased an index of carbon‐retention efficiency, C wheat retained in the mesocosm divided by total measured, from 0.18 to 0.27 (49%, p = 0.001), compared to CS. Overall, we found that diversifying and extending the duration of living plants in agroecosystems can lead to greater retention of new residue inputs in subtle ways that require further investigation to fully understand.