Quantifying the relationships between soil fraction mass, fraction carbon, and total soil carbon to assess mechanisms of physical protection

Abstract

Relationships between soil fractions (their mass or carbon (C)) and soil organic carbon (SOC) have been used to develop central ideas in SOC research. However, few attempts have been made to quantify the relationship between SOC and all soil fractions, despite the potential of such an effort to address SOC stabilization processes. We identified 41 published studies that used diverse management techniques to cause a change in SOC concentration and disrupted soil into macroaggregates (> 250 µm), free microaggregates (53-250 µm) and free silt + clay (< 53 µm), subsequently disrupting macroaggregates into constituent fractions (coarse particulate organic matter [cPOM] > 250 µm, occluded microaggregates, and occluded silt + clay). We used linear hierarchical models to quantify relationships between mass, C concentration and total C of fractions and SOC. Soil mass redistribution toward macroaggregates was associated with SOC accumulation, however total microaggregate mass (free + occluded) did not increase with macroaggregate mass, as would be expected given de novo microaggregate formation within macroaggregates. Instead, high SOC soils exhibited a greater percent of total microaggregates occluded in macroaggregates. Occlusion in macroaggregates was also associated with increased C concentrations of microaggregates (35% higher, SE = 3.2) and silt + clay (30% higher, SE = 3.9) relative to their free counterparts. Taken together, these relationships suggest reduced macroaggregate turnover promotes SOC accumulation via the stabilization of C into occluded fractions. Rates of SOC increase with silt + clay C concentrations failed to increase with mean site-level SOC concentration, indicating of the studied soils (median SOC concentration = 14 g kg -1 ; max 68), SOC accumulation appears unlikely to be limited by C storage capacity in the silt + clay fraction. For each unit SOC gain, macroaggregates accounted for 83% (95% CI = 74, 91),

Conceptualizing soil organic matter into particulate and mineral‐associated forms to address global change in the 21st century

“…Aggregates of various size classes have been heavily studied as important regulators of SOM turnover (Feller & Beare, 1997;Jastrow & Miller, 1997;King et al, 2019;Six, Bossuyt, Degryze, & Denef, 2004;Tisdall & Oades, 1982) and early indicators of environmental change, in particular land use and soil disturbance (Six & Paustian, 2014). Various aggregate size class definitions exist in the literature, but the most common definitions are shown in Table 2.…”

“…Various aggregate size class definitions exist in the literature, but the most common definitions are shown in Table 2. We recommend large (>50 µm) aggregate dispersion for questions of SOM stock changes related to global changes for three reasons: (a) they are a mixture of POM and MAOM (Table 2) Aggregates of various sizes interact with one another in complex ways, conceptualized as the "aggregate hierarchy" in which aggregates form around smaller aggregates (Tisdall & Oades, 1982), and facilitate formation of smaller aggregates (Jastrow & Miller, 1997;Six et al, 2000) and MAOM (Fulton-Smith & Cotrufo, 2019;Grandy & Robertson, 2007;King et al, 2019) within themselves. As a result, larger aggregates (>50 µm) contain mixtures of POM, MAOM, and smaller aggregates (Figure 2), all of which are subject to different levels of protection from decomposition (Jastrow & Miller, 1997;Tisdall & Oades, 1982) and are difficult to interpret when lumped together.…”

Pathways to persistence: plant root traits alter carbon accumulation in different soil carbon pools

“…This C is mostly derived from dissolved organic C originating from the degradation of above and belowground plant C input (Bird et al 2008;Rubino et al 2010;Sanderman et al 2014), from root exudation of labile rhizospheric compounds and from microbial compounds (Simpson et al, 2007;Mambelli et al, 2011;Cotrufo et al 2013;Vidal et al 2018;Rossi 2019). It is now generally accepted that labile low molecular weight compounds persist in soil longer than chemically recalcitrant C structures, when protected by organo-mineral adsorption (Mikutta et al 2006;Kleber et al 2015;King et al 2019;Robertson et al 2019;Sokol et al 2019). The stability of sequestered C in soil is therefore linked to the fraction of soil to which it is associated, with a greater stability of C pools associated with finer fractions (Torn et al, 2009).…”

“…As C accumulation into the coarse POM pool is related to the amount of recalcitrant matter present, it should therefore be greater in soils containing roots with high cellulose, lignin and carbon:nitrogen ratio (C:N) (Poirier et al 2018). However, it is C-rich exudates produced during fine root elongation that promote long-term C stabilization in the coarse silt and fine silt+clay fractions (Mikutta et al 2006;King et al 2019;Robertson et al 2019;Sokol et al 2019), and together with mucilage and border cells (shed during growth), are important substrates for microbial communities (Dennis et al, 2010). These C substrates that are assimilated by microorganisms close to the root apex are utilized rapidly for respiration and growth, or lost as microbial exudates or exopolysaccharides that are then used as a substrate for subsequent microbial communities.…”

Fertility of slightly saline chernozem under the effect of long-term fertilization system and tillage method in short crop rotation

“…Використання азотних добрив дає можливість підвищити вміст мінерального азоту у ґрунті, сприяє зростанню сполук амонію, підвищується вміст лужногідролізованого азоту [8,9].…”