Relationship between greenhouse gas emissions and changes in soil gas diffusivity in a field experiment with biochar and lime

Reducing greenhouse gas emissions from arable soil while maintaining productivity is a major challenge for agriculture. Biochar is known to reduce N₂O emissions from soil, but the underlying mechanisms are unclear. This study examined the impact of green waste biochar (20 Mg ha^?1) and lime (CaCO₃; 2 Mg ha^?1) application on soil gas transport properties and related changes in these to soil N₂O and CO₂ emissions measured using automated chambers in a field experiment cropped with maize. In situ soil water content monitoring was combined with laboratory measurements of relative soil gas diffusion coefficient (D_p/D₀) at different matric potentials, to determine changes in D_p/D₀ over time. Cumulative N₂O emissions were similar in the control and lime treatment, but much lower in the biochar treatment. Cumulative CO₂ emissions decreased in the order: lime treatment > biochar treatment > control soil. When N₂O emissions were not driven by excess N supply shortly after fertilisation, they were associated with D_p/D₀ changes, whereby decreases in D_p/D₀ corresponded to N₂O emissions peaks. No distinct pattern was observed between CO₂ emissions and D_p/D₀. Cumulative N₂O emissions were positively related to number of days with D_p/D₀ < 0.02, a critical limit for soil aeration. These results indicate that improved soil gas diffusivity, and hence improved soil aeration, may explain the effect of biochar in reducing N₂O emissions. They also suggest that knowledge of D_p/D₀ changes may be key to explaining N₂O emissions.     

Physical carbon‐sequestration mechanisms under special consideration of soil wettability

The protective impact of aggregation on microbial degradation through separation has been described frequently, especially for biotically formed aggregates. However, to date little information exists on the effects of organic‐matter (OM) quantity and OM quality on physical protection, i.e., reduced degradability by microorganisms caused by physical factors. In the present paper, we hypothesize that soil wettability, which is significantly influenced by OM, may act as a key factor for OM stabilization as it controls the microbial accessibility for water, nutrients, and oxygen in three‐phase systems like soil. Based on this hypothesis, the first objective is to evaluate new findings on the organization of organo‐mineral complexes at the nanoscale as one of the processes creating water‐repellent coatings on mineral surfaces. The second objective is to quantify the degree of alteration of coated surfaces with regard to water repellence. We introduce a recently developed trial that combines FTIR spectra with contact‐angle data as the link between chemical composition of OM and the physical wetting behavior of soil particles. In addition to characterizing the wetting properties of OM coatings, we discuss the implications of water‐repellent surfaces for different physical protection mechanisms of OM. For typical minerals, the OM loading on mineral surfaces is patchy, whereas OM forms nanoscaled micro‐aggregates together with metal oxides and hydroxides and with layered clay minerals. Such small aggregates may efficiently stabilize OM against microbial decomposition. However, despite the patchy structure of OM coating, we observed a relation between the chemical composition of OM and wettability. A higher hydrophobicity of the OM appears to stabilize the organic C in soil, either caused by a specific reduced biodegradability of OM or indirectly caused by increased aggregate stability. In partly saturated nonaggregated soil, the specific distribution of the pore water appears to further affect the mineralization of OM as a function of wettability. We conclude that the wettability of OM, quantified by the contact angle, links the chemical structure of OM with a bundle of physical soil properties and that reduced wettability results in the stabilization of OM in soils.