Studies of the mechanisms of stabilization of organic matter in semiarid soils
Date
1985
Authors
Payne, Bryce F., author
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Abstract
The effectiveness of three mechanisms of stabilization of soil organic matter (OM) were studied. These mechanisms are (1) chemical recalcitrance, (2) physical stabilization by adsorption onto soil particles, and (3) physical stabilization by occlusion within microaggregates. Mechanisms 1 and 2 were studied by incubation (mineralization) experiments using soil particle size fractions as sources of native soil OM of different chemical quality. These experiments examined microbial utilization and transformations of the C and P associated with the particle size fractions. The characteristics of particle size fractions and the effects of ultrasonic dispersion on them were examined during fractionations using different combinations of sedimentation and ultrasonic dispersion of aqueous soil suspensions. The chemical compositions (C, N, and P) were consistently different for the ultrasonically dispersible and readily water-dispersible fine soil particle-sizefractions. Carbon contents of the fine fractions ultrasonically dispersed from water-stable aggregates of >50 μm equivalent spherical diameter (ESD) were greater than those from smaller (<50 μm ESD) water-stable aggregates. Evidence was obtained that ultrasonic dispersion causes a redistribution of the organic matter associated with different particle size fractions. This has important implications for interpretations of information obtained using ultrasonic dispersion techniques. Several observations indicated the existence of anaerobic microsites inside the larger (>50 μm ESD) water-stable aggregates. Alight blue-gray clay sediment indicating reduced forms of Fe and/or Mn appeared whenever ultrasonic dispersion was used. The fine silt from water-stable aggregates had DTPA-extractable Mn levels 53 times higher than the water-dispersible fine silt. The consistently higher C contents and different qualitative chemical character of the fractions from water-stable aggregates indicated that the decomposition of organic matter in these aggregates is limited by oxygen stress. Occlusion of organic matter in microaggregates is an important mechanism of organic matter stabilization. The concepts of chemical recalcitrance and clay adsorption as mechanisms of organic matter stabilization were studied using incubations of three particle size fractions obtained from two semiarid grassland soils. Microbial transformations of C and P were assessed as indices of relative availability of native forms of these elements. Chemical recalcitrance was hypothesized to increase with increasing particle size. Stabilization by clay adsorption was hypothesized to increase with decreasing' particle size. Three levels of cellobiose-C amendment were used to establish different levels of microbial activity. No utilization of soil organic C occurred without the cellobiose amendment. Net mineralization of P to plant-available forms was found after 124 hours at the lowest C amendment level for all particle size fractions. Net utilization of unextractable P was apparent during the period of increasing biomass. Among the fractions equivalent microbial P was not held in equivalent extractable forms. Native soil C was extensively utilized for all fractions and at all C amendment levels. Although sorptive phenomena were active for all three fractions, there was no greater stabilization of soil organics by clay-sized than by fine-silt-sized particles. It was concluded that chemical recalcitrance was of relatively little importance in stabilizing native soil organic matter. The utilization of native organic nutrients was most closely related to the amount of readily available energy (cellobiose C) present. From the results of the fractionation and incubation studies it was concluded that occlusion within microaggregates is the dominant mechanism of stabilization of soil OM.
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Subject
Plant-soil relationships
Growth (Plants)
Soils