Biochars are potentially effective sorbents for NH₄⁺ and NO₃⁻ in water treatment and soil applications. Here we compare NH₄⁺ and NO₃⁻ sorption rates to acid-washed biochars produced from red oak (Quercus rubra) and corn stover (Zea mays) at three pyrolysis temperatures (400, 500 and 600 °C) and a range of solution pHs (3.5–7.5). Additionally, we examined sorption mechanisms by quantification of NH₄⁺ and NO₃⁻ sorption, as well as Ca²⁺ and Cl⁻ displacement for corn stover biochars. Solution pH curves showed that NH₄⁺ sorption was maximized (0.7–0.8 mg N g⁻¹) with low pyrolysis temperature (400 °C) biochar at near neutral pH (7.0–7.5), whereas NO₃⁻ sorption was maximized (1.4–1.5 mg N g⁻¹) with high pyrolysis temperatures (600 °C) and low pH (3.5–4). The Langmuir (r² = 0.90–1.00) and Freundlich (r² = 0.81–0.97) models were good predictors for both NH₄⁺ (pH 7) and NO₃⁻ (pH 3.7) sorption isotherms. Lastly, NH₄⁺ and NO₃⁻ displaced Ca²⁺ and Cl⁻, respectively, from previously CaCl₂-saturated corn stover biochars. Results from the pH curves, Langmuir isotherms, and cation displacement curves all support the predominance of ion exchange mechanisms. Our results demonstrate the importance of solution pH and chemical composition in influencing NH₄⁺ and NO₃⁻ sorption capacities of biochar.

Biochar application to soil has been proposed as a means for reducing soil greenhouse gas emissions and mitigating climate change. The effects, however, of interactions between biochar, moisture and temperature on soil CO₂ and N₂O emissions, remain poorly understood. Furthermore, the applicability of lab-scale observations to field conditions in diverse agroecosystems remains uncertain. Here we investigate the impact of a mixed wood gasification biochar on CO₂ and N₂O emissions from loess-derived soils using: (1) controlled laboratory incubations at three moisture (27, 31 and 35%) and three temperature (10, 20 and 30 °C) levels and (2) a field study with four cropping systems (continuous corn, switchgrass, low diversity grass mix and high diversity grass-forb mix). Biochar reduced N₂O emissions under specific temperatures and moistures in the laboratory and in the continuous corn cropping system in the field. However, the effect of biochar on N₂O emissions was only significant in the field and no effect on cumulative CO₂ emissions was observed. Cropping system also had a significant effect in the field study, with soils in grass and grass-forb cropping systems emitting more CO₂ and less N₂O than corn cropping systems. Observed biochar effects were consistent with previous studies showing that biochar amendments can reduce soil N₂O emissions under specific but not all, conditions. The disparity in N₂O emission responses at the lab and field scales suggests that laboratory incubation experiments may not reliably predict the impact of biochar at the field scale.

Both soil and biochar properties are known to influence greenhouse gas emissions from biochar‐amended soils, but poor understanding of underlying mechanisms challenges prediction and modeling. Here, we examine the effect of six lignocellulosic biochars produced from the pyrolysis of corn stover and wood feedstocks on CO₂ and N₂O emissions from soils collected from two bioenergy cropping systems. Effects of biochar on total accumulated CO₂‐C emissions were minimal (<0.45 mg C g⁻¹ soil; <10% of biochar C), consistent with mineralization and hydrolysis of small labile organic and inorganic C fractions in the studied biochars. Comparisons of soil CO₂ emissions with emissions from microbially inoculated quartz–biochar mixtures (‘quartz controls’) provide evidence of soil and biochar‐specific negative priming. Five of six biochar amendments suppressed N₂O emissions from at least one soil, and the magnitude of N₂O emissions suppression varied with respect to both biochar and soil types. Biochar amendments consistently decreased final soil NO₃⁻ concentrations, while contrasting effects on pH, NH₄⁺, and DOC highlighted the potential for formation of anaerobic microsites in biochar‐amended soils and consequential shifts in the soil redox environment. Thus, results implicated both reduced substrate availability and redox shifts as potential factors contributing to N₂O emission suppression. More research is needed to confirm these mechanisms, but overall our results suggest that soil biochar amendments commonly reduce N₂O emissions and have little effect on CO₂ emissions beyond the mineralization and/or hydrolysis of labile biochar C fractions. Considering the large C credit for the biochar C, we conclude that biochar amendments can reduce greenhouse gas emissions and enhance the climate change mitigation potential of bioenergy cropping systems.

A recent study in Carbon Management, by Bach et al., argues that biochar amendments’ positive impacts on crop yields and soil carbon sequestration have been overestimated, and biochar amendment to soil is hence unlikely to be an economically viable technique for cropping system management or C abatement. We question the data selection and analysis techniques that the authors used to assess the effect of biochar on crop yield, biochar stability in soil, and biochar production cost. Although the research article was not intended as a meta-analysis – and hence the data reported need not be analyzed with the full rigor of a systematic review – we assert that the employed data set, while containing a sufficient quantity of data, requires closer inspection and a more careful interpretation to avoid significant bias in the conclusions. Furthermore, we assert that proper implementation of biochar and inclusion of non-yield benefits in the analysis would render it more economically viable for both cropping system enhancement and C sequestration than portrayed by Bach et al.

This textbook introduces readers to the basics of soil science, including: the physical, chemical, and biological properties of soils; soil formation, classification, and global distribution; soil health, soils and humanity, and sustainable land management.