Interactions between soil mineral composition and particulate organic matter mediate bioavailable nitrogen

Soil organic matter (SOM) can supply critical quantities of nitrogen (N) to plants, but fundamental questions remain regarding from what pools and by which processes plants and microbes obtain N. The source of bioavailable N is generally assigned to polymeric or particulate organic matter (POM), but in mineral soils, POM is often a minor pool of N and can be a poor predictor of bioavailable N or plant N uptake. Mounting evidence suggests mineral-associated organic matter (MAOM) is significantly more dynamic than previously assumed and may serve as an important source of N for plants and microbes. The abundance and chemical traits of POM (e.g., C/N ratio) are likely key controls on N mineralization, but bioavailable N is also regulated by the capacity for minerals to intercept, immobilize and release DON via sorption and desorption.

Our objective was to assess how the relative abundance of and chemistry of MAOM controls N bioavailability. The conceptual underpinnings for this project will be presented as will the result of an associated lab incubation. We assembled mixtures of POM and MAOM and manipulated the MAOM type and the POM:MAOM ratio to achieve varying levels of POM-N supply and mineral sorption capacity. MAOM was isolated with a particle-size based fractionation technique from four soil types that differed in mineralogical composition as determined via X-ray diffraction. Wheat residue was partially decomposed in the lab to 60% of original mass to generate POM, which was then mixed in varying ratios with a fixed amount of MAOM to achieve 5, 10, 15, and 20% of POM-N out of total soil N. Mixtures were brought to 40% of water-holding capacity and incubated for seven days. Destructive sampling occurred on days 0, 2, and 7 to assess KCl-extractable inorganic N (ammonium + nitrate), pH, and gross ammonification and nitrification rates. Greater abundance of POM-N was associated with increased inorganic N. However, when normalized to total N, we observe notable differences by MAOM type where ammonium was more bioavailable in soils with greater 2:1 clays and iron oxide minerals. We expect gross mineralization results to shed more light on how MAOM-N quantity and mineral surface properties interact with POM-N supply to control the supply of bioavailable N.