Impact of plant litter on nonprotonated aromatics and aromaticity of organic matter in some Cerrado Ferralsols

Abstract

In fire-prone ecosystems such as the Brazilian Cerrado, pyrogenic C (PyC) can be a relevant contributor to soil C pools. However, the extent to which increasing additions of fresh plant litter can affect the relative contribution of PyC to soil organic matter (SOM) remains unclear. To address this question, we used a 12-month laboratory incubation experiment arranged according to a complete factorial scheme including: three Ferralsols, two depths i.e. topsoil (0–10 cm) and subsoil horizons (60–100 cm), and two levels of a doubly isotope-labeled (13C and 15N) plant litter (0 and 40 mg g−1 soil), with three blocks as replicates. After the incubation, the samples were physically fractionated to isolate SOM within the particle-size fractions (PSF) greater and smaller than 53 µm. Subsequently, we quantified the amount of litter-derived C and N remaining in both PSF and used hydrofluoric (HF) acid 10% (v/v) to demineralize the PSF smaller than 53 µm, yielding the HF-insoluble SOM. This fraction was evaluated with 13C/15N–nuclear magnetic resonance (NMR) spectroscopy by applying the multiple cross-polarization (multiCP) pulse sequence with magic-angle spinning (MAS), yielding semiquantitative solid-state NMR spectra. Averaged across the Ferralsols, the PSF smaller than 53 µm retained about 22.8% of the C and 32.6% of the N added via plant litter. Our NMR results indicated that the incorporation of litter-C and -N led to an increase in aliphatic C (0–110 ppm) coupled to a marked decrease in aromatic moieties (115–154 ppm), particularly nonprotonated aromatics in subsoil samples. Consequently, there was a significant decrease in the aromaticity of SOM. Therefore, with increasing additions of fresh plant material, soil C dynamics in Cerrado Ferralsols may become less reliant on the relative contribution of nonprotonated aromatics to SOM chemistry.