Building bottom‐up aggregate‐based models (ABMs) in soil systems with a view of aggregates as biogeochemical reactors

Saving the face of soil aggregates

“…Concluding, a dual pore-aggregate perspective is necessary for holistic understanding of soil structure and functions. Three recent papers: "Aggregates as biogeochemical reactors" (Wang, Brewer, Shugart, Lerdau, & Allison, 2019a), "Soil aggregates as biogeochemical reactors: Not a way forward …" (Kravchenko, Otten, Garnier, Pot, & Baveye, 2019), and "Building bottom-up aggregate based models..." (Wang, Brewer, Shugart, Lerdau, & Allison, 2019b) presented contrasting views on the importance of aggregates for C and N turnover, with a main focus on greenhouse gas (GHG) emissions. Wang et al (2019a) Whereas we fully support the view (Kravchenko et al, 2019) that the "soil is not a simple sum of its individual components" and agree on the uselessness of process modelling at the aggregate scale for global upscaling, we disagree with the insignificance of aggregates and the absence of their existence.…”

“…Wang et al (2019a) Whereas we fully support the view (Kravchenko et al, 2019) that the "soil is not a simple sum of its individual components" and agree on the uselessness of process modelling at the aggregate scale for global upscaling, we disagree with the insignificance of aggregates and the absence of their existence. This Letter to the Editor is essential to "save the aggregates": to show their relevance as a functional and methodological basis for understanding soil structure and processes, but to moderate the opinion of Wang et al (2019b) about their overbearing role for biogeochemical modelling and upscaling.…”

“…The statement that nearly all biochemical processes are generally ongoing in aggregates (Wang et al, 2019b) is incorrect because nearly all processes in soil are ongoing (a) in the liquid phase or (b) on the interfaces between liquid and solid phases (Totsche et al, 2018).…”

“…The "Aggregate reactors" concept (Wang et al, 2019b) Soil is not merely a porous medium composed of four phases (solid, liquid, gaseous, living) but also represents a hierarchically organized system (Yudina, Fomin, Kotelnikova, & Milanovskii, 2018). Any soil is characterized by structure, which is one of the main properties distinguishing the soil from parent material.…”

“…Nevertheless, no clear mechanistic separation between processes of macro-and microaggregation is available. Therefore, a uniform protocol for aggregate fractionation as called for in Wang et al (2019b) requires clear definitions of aggregates.…”