A mechanistic approach to modeling saturation and protection mechanisms of soil organic matter
Date
2009
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Abstract
Simulation models have been used extensively as a research tool in the field of soil organic matter (SOM) dynamics and should embody our best understandings of the processes and mechanisms controlling these dynamics. Our objective was to develop and evaluate a SOM model based upon measureable soil organic carbon (SOC) fractions and optimize it against long-term tillage experiments in North America. This model will include (1) soil aggregate dynamics, with direct influence from tillage events; (2); and the mechanisms of SOM stabilization; and (3) explicitly address the concept of potential SOC saturation. The major proposed mechanisms for SOM stabilization-physical occlusion, organic recalcitrance, and organo-mineral interactions-have limited explicit inclusion in current SOM models.
The enhanced soil structure and higher SOC stocks measured under no-till (NT) management have been well documented. However, the roles of residue addition vs. aggregate disruption (i.e. tillage events) have not been compared at different depth increments. Pre-requisite to the major SOM stabilization mechanisms are the availability and continual addition of residues. Therefore, uniformly 13C-labeled wheat residues were added to incubation cores representing soils under NT and tillage management (TM) during a yearlong in situ incubation at a dryland agriculture experiment site. Residue was added directly onto the surface of NT cores, while residues were incorporated into the 0-5, 5-15, and 15-30 cm depth increments of the TM cores. Overall, our results indicate that within a plow depth of 15 cm, limiting the tillage-induced disruption of aggregates has a stronger influence than residue incorporation into the profile via tillage on the efficiency of C stabilization. However, when residues are distributed to a 30 cm depth, the negative impact of aggregate disruption through tillage appears counterbalanced with similar efficiencies of C stabilization between the NT and TM practices, possibly due to slower decomposition of residues deeper in the profile.
Conventional SOM models have been defined by kinetic, rather than functional pools. Further, soil physiochemical processes (e.g. mineral surface bindings) that inhibit the breakdown of organic compounds are generally implicit in the rate constants associated with a particular pool as a rate modifying factor. SATURN, the proposed simulation model, is comprised of functional (or measureable) pools; one of the first attempts at directly 'modeling the measureable'. The model has been initially optimized for total SOC content against seven long-term (> 12 years) agroecosystem experiments; containing contrasts in tillage management (a mechanism for aggregate turnover) and different in crop rotations with a SOC gradient across sites of approximately 5-25 g C kg -1 soil. The final optimized values resulted in a root mean square error for total SOC of 1.2 g C kg-1 soil across all seven sites. This analysis is limited to the total SOC content and individual pool sizes where not optimized against which limits the degree to which we can validate the internal dynamics of the model; an added benefit of measureable pool models.
The enhanced soil structure and higher SOC stocks measured under no-till (NT) management have been well documented. However, the roles of residue addition vs. aggregate disruption (i.e. tillage events) have not been compared at different depth increments. Pre-requisite to the major SOM stabilization mechanisms are the availability and continual addition of residues. Therefore, uniformly 13C-labeled wheat residues were added to incubation cores representing soils under NT and tillage management (TM) during a yearlong in situ incubation at a dryland agriculture experiment site. Residue was added directly onto the surface of NT cores, while residues were incorporated into the 0-5, 5-15, and 15-30 cm depth increments of the TM cores. Overall, our results indicate that within a plow depth of 15 cm, limiting the tillage-induced disruption of aggregates has a stronger influence than residue incorporation into the profile via tillage on the efficiency of C stabilization. However, when residues are distributed to a 30 cm depth, the negative impact of aggregate disruption through tillage appears counterbalanced with similar efficiencies of C stabilization between the NT and TM practices, possibly due to slower decomposition of residues deeper in the profile.
Conventional SOM models have been defined by kinetic, rather than functional pools. Further, soil physiochemical processes (e.g. mineral surface bindings) that inhibit the breakdown of organic compounds are generally implicit in the rate constants associated with a particular pool as a rate modifying factor. SATURN, the proposed simulation model, is comprised of functional (or measureable) pools; one of the first attempts at directly 'modeling the measureable'. The model has been initially optimized for total SOC content against seven long-term (> 12 years) agroecosystem experiments; containing contrasts in tillage management (a mechanism for aggregate turnover) and different in crop rotations with a SOC gradient across sites of approximately 5-25 g C kg -1 soil. The final optimized values resulted in a root mean square error for total SOC of 1.2 g C kg-1 soil across all seven sites. This analysis is limited to the total SOC content and individual pool sizes where not optimized against which limits the degree to which we can validate the internal dynamics of the model; an added benefit of measureable pool models.
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saturation
soil aggregates
soil organic matter
soil sciences