Abstract
Water-extractable organic carbon (WEOC) is considered as the most important carbon (C) source for denitrifying organisms, but the contribution of individual organic matter (OM) fractions (i.e., particulate (POM) and mineral-associated (MOM)) to its release and, thus, to denitrification remains unresolved. Here we tested short-time effects of POM and MOM on potential denitrification and estimated the contribution of POM- and MOM-derived WEOC to denitrification and CO2 production of three agricultural topsoils. Suspensions of bulk soils with and without addition of soil-derived POM or MOM were incubated for 24 h under anoxic conditions. Acetylene inhibition was used to determine the potential denitrification and respective product ratio at constant nitrate supply. Normalized to added OC, effects of POM on CO2 production, total denitrification, and its product ratios were much stronger than those of MOM. While the addition of OM generally increased the (N2O + N2)-N/CO2-C ratio, the N2O/(N2O + N2) ratio changed differently depending on the soil. Gas emissions and the respective shares of initial WEOC were then used to estimate the contribution of POM and MOM-derived WEOC to total CO2, N2O, and N2O + N2 production. Water-extractable OC derived from POM accounted for 53–85% of total denitrification and WEOC released from MOM accounted for 15–47%. Total gas emissions from bulk soils were partly over- or underestimated, mainly due to nonproportional responses of denitrification to the addition of individual OM fractions. Our findings show that MOM plays a role in providing organic substrates during denitrification but is generally less dominant than POM. We conclude that the denitrification potential of soils is not predictable based on the C distribution over POM and MOM alone. Instead, the source strength of POM and MOM for WEOC plus the WEOC’s quality turned out as the most decisive determinants of potential denitrification.
Highlights
Agricultural soils vary in terms of the content and composition of organic matter (OM), which may affect their denitrification potential
The organic carbon (OC)/ON ratios of POM fractions showed no relations to the Water-extractable organic carbon (WEOC) contents and WEOC/Water-extractable organic N (WEON) ratios of bulk soils
Since the Ro POM was more efficiently used than the other OM fractions (Figures 2B,C), we suggest that microorganisms in WEOC-poor soils are better adapted to POM-derived water-extractable OM (WEOM)
Summary
Agricultural soils vary in terms of the content and composition of organic matter (OM), which may affect their denitrification potential. Considering the role of N2O in atmospheric processes, understanding the factors controlling denitrification and its N2O/ N2 product ratio is crucial for developing strategies to minimize emissions of greenhouse gases. Previous studies have shown that addition of fresh plant biomass or well-defined low-molecular-weight compounds affects denitrification rates, product ratios, and denitrifier populations (e.g., Beauchamp et al, 1989; Miller et al, 2008; Palmer et al, 2012). When comparing different C substances, soluble low-molecular-weight compounds are much more effective in promoting denitrification and related bacterial populations (e.g., organic acids, glycerol ≫ glucose, methanol) than insoluble polymers, such as cellulose and especially lignin (e.g., Valera and Alexander, 1961; deCatanzaro and Beauchamp, 1985; Rashid and Schaefer, 1988; Akunna et al, 1993). Despite evidence of large differences in release and biodegradability of water-extractable OM (WEOM) (e.g., Don and Kalbitz, 2005; Kalbitz et al, 2005; Mastný et al, 2018), much less information is available on the effects of more complex soil OM fractions, such as particulate and mineral-associated OM (POM, MOM) (Lavallee et al, 2020)
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