Soil aggregates as biogeochemical reactors and implications for soil–atmosphere exchange of greenhouse gases—A concept

Abstract

Soil–atmosphere exchange significantly influences the global atmospheric abundances of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These greenhouse gases (GHGs) have been extensively studied at the soil profile level and extrapolated to coarser scales (regional and global). However, finer scale studies of soil aggregation have not received much attention, even though elucidating the GHG activities at the full spectrum of scales rather than just coarse levels is essential for reducing the large uncertainties in the current atmospheric budgets of these gases. Through synthesizing relevant studies, we propose that aggregates, as relatively separate micro‐environments embedded in a complex soil matrix, can be viewed as biogeochemical reactors of GHGs. Aggregate reactivity is determined by both aggregate size (which determines the reactor size) and the bulk soil environment including both biotic and abiotic factors (which further influence the reaction conditions). With a systematic, dynamic view of the soil system, implications of aggregate reactors for soil–atmosphere GHG exchange are determined by both an individual reactor's reactivity and dynamics in aggregate size distributions. Emerging evidence supports the contention that aggregate reactors significantly influence soil–atmosphere GHG exchange and may have global implications for carbon and nitrogen cycling. In the context of increasingly frequent and severe disturbances, we advocate more analyses of GHG activities at the aggregate scale. To complement data on aggregate reactors, we suggest developing bottom‐up aggregate‐based models (ABMs) that apply a trait‐based approach and incorporate soil system heterogeneity.

Sediment addition and legume cultivation result in sustainable, long‐term increases in ecosystem functions of sandy grasslands

“…The main reason that sandy grasslands provide low ecosystem services is that their soils have extremely low silt and clay contents, and thus low water and nutrients holding capacity (Schapel, Marschner, & Churchman, 2018;Wang, Brewer, et al, 2018). In this study, even after 32 years, sediment addition improved soil structure significantly (Figure 1), increasing soil bulk density, clay, and silt contents (Hao & Kravchenko, 2007).…”

“…Sandy grasslands are fragile ecosystems, which have been substantially degraded, and present major challenges for restoration (Chen et al, ; Godefroid, Le Pajolec, Hechelski, & Van Rossum, ). The main reason that sandy grasslands provide low ecosystem services is that their soils have extremely low silt and clay contents, and thus low water and nutrients holding capacity (Schapel, Marschner, & Churchman, ; Wang, Brewer, et al, ). In this study, even after 32 years, sediment addition improved soil structure significantly (Figure ), increasing soil bulk density, clay, and silt contents (Hao & Kravchenko, ).…”

“…However, almost half of the sandy grasslands in China had been substantially degraded in the last 50 years (Li, Zhao, Liu, & Huang, 2008), due to the consequences of both direct human activities and climate change. In particular, livestock overgrazing, overcutting, land-use change (conversion of natural grasslands to croplands), and overuse of groundwater have promoted major loss of ecosystem function (Wang, Brewer, Shugart, Lerdau, & Allison, 2018). Degradation in sandy grasslands has serious negative impacts on many aspects of ecosystem services and, therefore, is considered an important ecological and environmental problem both regionally and globally (Dlamini, Chivenge, Manson, & Chaplot, 2014;Ru et al, 2018).…”

“…In these grasslands, the sandy soils are characterized by low waterand nutrient-holding capacities, limiting primary production (Knapp, Briggs, & Koelliker, 2001). Furthermore, the high sand contents of the soils result in them being particularly susceptible to severe wind and water erosion (De Boer, Deru, & Van Eekeren, 2018), and the amount and quality of new dust inputs is very important for replacing eroded material and maintaining soil microbial diversity and ecosystem productivity (Gupta & Germida, 2015;Hao et al, 2017;Wang, Brewer, et al, 2018). Critically, because the loss of productivity in sandy grasslands has been shown to be mainly caused by soil degradation (Schoenholtz, Van Miegroet, & Burger, 2000), improvement of soil structure may be key for restoring and enhancing the productivity of these ecosystems.…”

Saving the face of soil aggregates

“…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. 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.…”