Search

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 for 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), and occluded microaggregates for 43% (95% CI = 33, 52), consistent relationships that have potential to be used as benchmarks for fraction-based SOC models. Image 1 • We quantitatively reviewed 41 studies reporting macroaggregate-occluded fractions. • SOC increase was associated with gains in macroaggregate mass. • Macroaggregate-occlusion increased C concentration of microaggregates, silt + clay. • Macroaggregates may enable stabilization of POM to microaggregates, silt + clay. • 83% of SOC accumulated in macroaggregates; 43% occurred in occluded microaggregates. [ABSTRACT FROM AUTHOR]

Individual molecular components of soil organic matter (SOM) have been studied since the early 19th century. Their characterization is essential, because knowledge of the molecular structures constituting SOM allows for the detailed understanding of its origin and the processes implicated in soil C sequestration. This provides the basis for target-oriented development of management practices to optimize ecosystem services provided by soil. The aim of this review is to give an overview about the major analytical developments and the information gain that could be achieved by application of molecular methods in SOM research. Up to the 1980s, studies concentrated on the characterization of the chemical nature of SOM. Analyses were mainly based on soluble OM compounds, which were fractionated chemically into humic and fulvic acids. During the 1990s, the focus changed and scientists were more interested in biological processes controlling SOM formation and its dynamics. Introduction of physical fractionation and combination of molecular and isotopic techniques allowed assessment of composition, origin, and turnover of SOM within specific localizations in the mineral soil matrix. Analyses of the dynamics of single molecules led to a massive change of paradigms. Long residence times of SOM are no longer explained by chemical recalcitrance but by microbial products being stabilized by the interaction with soil minerals and microbial inaccessibility. In recent years, techniques yielding results with high molecular and spatial resolution were introduced, which will allow the acquisition of much more detailed information, moving a step further toward elucidating the nature and properties of SOM. [ABSTRACT FROM AUTHOR]

Plant roots contribute substantially to the formation of stable soil organic matter (SOM), and there is evidence that species differ in their contribution to SOM stabilization. However, it remains unclear what specific root traits contribute to the three SOM stabilization mechanisms: recalcitrance against decomposition, occlusion in soil aggregates and interaction with soil minerals and metals. This is likely because research is highly fragmented and hampered by disciplinary barriers. By reviewing both plant functional ecology and soil science literature, we identified 18 different traits: architectural, morphological, physiological, symbiotic and chemical root characteristics, influencing the three SOM stabilization mechanisms. We found that traits increasing root recalcitrance promote short term stabilization by slowing decomposition, but that traits reducing recalcitrance contribute to long term stabilization by reaction of microbial products with mineral surfaces. Root length density, mycorrhizal association and rhizodeposition contribute to microaggregation. These and other traits, such as hemicellulose, soluble compounds, and high root branching index, favor macroaggregation. For stabilization by minerals and metals, those root traits promoting higher microbial activity: root nitrogen, hemicellulose and soluble compound concentrations are fundamental, while polyphenols, and litter Al and Mn also contribute to complexification and stabilization. Root depth distribution is the most important trait to control root C storage and stabilization in the subsoil; once roots have reached deeper soil layers, other traits, such as rhizodeposition and root chemistry, influence interaction with minerals and metals. Both mycorrhizal presence and root suberin promote SOC stabilization by means of all three mechanisms, indicating that these are important targets for continued work. Surprisingly, morphological traits commonly measured, namely specific root length and root diameter, poorly relate to stabilization mechanisms. Alternative traits such as chemical composition of the different root orders, root apex characteristics, quantity and quality of rhizodeposits as well as mycorrhizal fungal traits, should be further investigated. For future research, this review highlights the need to evaluate root decomposition and root-C stabilization concomitantly over the long-term, considering simultaneously root litter quality, estimated by root traits, the microbial products and properties of the soil matrix. The information accrued in this review can be used to evaluate the potential of plant species and cultivars to promote SOM stabilization, based on their root traits. [ABSTRACT FROM AUTHOR]

Abstract: Stabilization offered by physicochemical protection in hierarchical soil aggregates is critical for building and maintaining soil C and N stocks. However, it is unclear if complex stabilization mechanisms can completely recover when native plant communities are re-established on soils depleted of C and N by agriculture. We isolated particulate organic matter (POM) and silt- and clay-sized fractions from four structurally defined locations within soil collected from an agricultural field, prairies restored for 3–33 years, and a never-cultivated remnant prairie. We used aggregate hierarchy to define our four soil locations: non-aggregated material, free microaggregates, macroaggregates (excluding encapsulated microaggregates), and microaggregates-within-macroaggregates. We found that the duration of linear soil C and N accumulation differed among aggregate-occluded pools in relation to the combined influences of soil mass redistribution and increases in C and N concentrations. Silt in microaggregates isolated from within macroaggregates contributed the greatest quantities of C and N to whole soil, yet reached steady state C and N contents that were only 59% (C) and 56% (N) of those observed in the remnant prairie soil. Although the C and N contents of most pools were still well below the amounts in the reference remnant prairie, the overall distribution of C among pools was similar to the remnant within 33 years of restoration, suggesting that SOM stabilization mechanisms do largely recover in the first decades after cessation of tillage and restoration of the plant community. Thus, the pools that fell short of pre-cultivation C and N contents within the timespan of the chronosequence might continue to build C and N even though they appeared to be at steady state at the time of sampling, possibly because not enough time has passed at the current input rate or because of lags in SOM transfer among pools. We hypothesize that several “transient steady states” could occur in some SOM pools along the way to an overall whole-soil steady state that could take centuries to achieve. [Copyright &y& Elsevier]