Soil respiration induces co-emission of greenhouse gases and methylated selenium from cold-region Mollisols: Significance for selenium deficiency.

Mollisols rich in natural organic matter are a significant sink of carbon (C) and selenium (Se). Climate warming and agricultural expansion to the cold Mollisol regions may enhance soil respiration and biogeochemical cycles, posing a growing risk of soil C and Se loss. Through field-mimicking incubation experiments with uncultivated and cultivated soils from the Mollisol regions of northeastern China, this research shows that soil respiration remained significant even during cold seasons and caused co-emission of greenhouse gases (CO and CH) and methylated Se. Such stimulus effects were generally stronger in the cultivated soils, with maximum emission rates of 7.45 g/m/d C and 1.42 μg/m/d Se. For all soil types, the greatest co-emission of CO and dimethyl selenide occurred at 25 % soil moisture, whereas measurable CH emission was observed at 40 % soil moisture with higher percentages of dimethyl diselenide volatilization. Molecular characterization with three-dimensional fluorescence and ultra-high resolution mass spectrometry suggests that CO emission is sensitive to the availability of microbial protein-like substances and free energy from organic carbon biodegradation under variable moisture conditions. Predominant Se binding to biodegradable organic matter resulted in high dependence of Se volatilization on rates of greenhouse gas emissions. These findings together highlight the importance of dynamic organic carbon quality for soil respiration and consequent Mollisol Se loss risk, with implications for science-based management of C and Se resources in agricultural lands to combat with Se deficiency.