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

Assessing poorly crystalline and mineral-organic species by extracting Al, Fe, Mn, and Si using (citrate-) ascorbate and oxalate

Poorly crystalline and mineral-organic species are commonly assessed by oxalate (Ox) extraction of aluminium (Al), iron (Fe), manganese (Mn), and silicon (Si). Drawbacks regarding selectivity towards analytical targets led to test alternatively, but mostly with topsoils, citrate-ascorbate (CA), combined with a separate citrate extraction. Here, we tested the extraction by Ox, CA, and citrate with 61 carbonate-free subsoils characterized by pedogenic element accumulation, and with 17 minerals, including primary silicates, clay minerals, and (Al, Fe) oxides. Iron extraction from the minerals, apart from pyrite, by citrate was negligible (<= 2% of total Fe), like from Fe oxides (<= 1.5% of total Fe), while oxalate-extractable Al, Fe, and Mn contents were larger than those extracted by CA, the relative fraction of which was below 9%, except for allophane. In subsoils, extractable element contents spanned several orders of magnitude whatever the extractant. With several samples, Mn was entirely solubilized by any extractant. The Al-Ox, Fe-Ox, and Mn-Ox amounts were very similar among soils lacking allophanic minerals. However, the ratio Fe-CA:Fe-Ox increased with Bl and Bg horizons, which received Fe2+ by ground- or stagnic water. Citrate-extraction data revealed, depending on the soil type, a potentially large contribution of organic forms to CA- and Ox-extractable Al and Fe. Thus, we recommend combining Ox and CA extraction with citrate extraction to approximate the contents of poorly crystalline pedogenic Al and Fe species, rather than using CA- or oxalate-extraction data alone. Our results challenge the appropriateness of oxalate-extraction data for species identification and mineral quantification