Differences in labile soil organic matter explain potential denitrification and denitrifying communities in a long-term fertilization experiment

Content and quality of organic matter (OM) may strongly affect the denitrification potential of soils. In particular, the impact of soil OM fractions of differing bioavailability (soluble, particulate, and mineral-associated OM) on denitrification remains unresolved. We determined the potential N₂O and N₂ as well as CO₂ production for samples of a Haplic Chernozem from six treatment plots (control, mineral N and NP, farmyard manure - FYM, and FYM + mineral N or NP) of the Static Fertilization Experiment Bad Lauchstädt (Germany) as related to OM properties and denitrifier gene abundances. Soil OM was analyzed for bulk chemical composition (¹³C-CPMAS NMR spectroscopy) as well as water-extractable, particulate, and mineral-associated fractions. Soils receiving FYM had more total OM and larger portions of labile fractions such as particulate and water-extractable OM. Incubations were run under anoxic conditions without nitrate limitation for seven days at 25 °C in the dark to determine the denitrification potential (N₂O and N₂) using the acetylene inhibition technique. Abundances of nirS, nirK, and nosZ (I + II) genes were analyzed before and after incubation. The denitrification potential, defined as the combined amount of N released as N₂O + N₂ over the experimental period, was larger for plots receiving FYM (25.9–27.2 mg N kg^{− 1}) than pure mineral fertilization (17.1–19.2 mg N kg^{− 1}) or no fertilization (12.6 mg N kg^{− 1}). The CO₂ and N₂O production were well related and up to three-fold larger for FYM-receiving soils than under pure mineral fertilization. The N₂ production differed significantly only between all manured and non-manured soils. Nitrogenous gas emissions related most closely to water-extractable organic carbon (WEOC), which again related well to free particulate OM. The larger contribution of N₂ production in soils without FYM application, and thus, with less readily decomposable OM, coincided with decreasing abundances of nirS genes (NO₂ ⁻ reductase) and increasing abundances of genes indicating complete denitrifying organisms (nosZ I) during anoxic conditions. Limited OM sources, thus, favored a microbial community more efficient in resource use. This study suggests that WEOC, representing readily bioavailable OM, is a straightforward indicator of the denitrification potential of soils.