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United States Department of Agriculture

Agricultural Research Service

Research Project: DRYLAND CROPPING SYSTEMS MANAGEMENT FOR THE CENTRAL GREAT PLAINS

Location: Central Plains Resources Management Research

Title: Relating Soil Organic Matter Dynamics to its Molecular Structure

Authors
item Paul, E -
item Mellor, N -
item Haddix, M -
item Magrini, K -
item Calderon, Francisco
item Morris, S -

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: September 9, 2010
Publication Date: September 19, 2010
Citation: Paul, E.A., Mellor, N., Haddix, M., Magrini, K., Calderon, F.J., Morris, S. 2010. Relating Soil Organic Matter Dynamics to its Molecular Structure. Meeting Abstract. Presented at the Soil Organic Matter 2010 Organic Matter Stabilization and Ecosystem Functions. 19 to 23 September 2010. Presquile de Giens (Cote D'Azur)France.

Technical Abstract: Our understanding of the dynamics of soil organic matter (SOM) must be integrated with a sound knowledge of it biochemical complexity. The molecular structure of SOM was determined in 98% sand soils to eliminate the known protective effects of clay on the amount and turnover rate of the SOM constituents clay. Cedar growth for 55yr increased SOM contents above that of the native prairie. The 13C content of the field soils showed that 82% of the original native soil C still remained after 55 yr cedar growth. The first order decay rate constant for the change in native soil C under cedars represented a mean residence time of 278 yr for the surface SOM and subsurface but less for an intermediate depth. Long term incubation Long –term incubation and kinetic analysis of the CO2 evolution curves indicate an active fraction of 10% in the surface, one of the highest we have encountered. Pines accumulated extensive litter but caused a 65% loss of the native C over 70 yr resulting in a calculated MRT of 66 to 90 yr for the native prairie C depending on the depth. The mobilization of Ca at depth by the cedar and reincorporation in its litter is thought to have been a major factor responsible for in stability of the SOM under cedars but not under pine. The non hydrolysable fraction, considered to be resistant, represented 39% in the surface soil for both cedar and pine but increases to 72% in the subsurface of the pine relative to 57% in the cedars showing that the loss in SOM, due to pines, came largely from the active and slow pools. The molecular structure was determined with pyrolysis –molecular beam mass spectrometry (py-MBMS) and diffuse reflectance mid infra red spectroscopy at 400-4000 cm-1 (MiDIR) Seventy percent of the sandy soil C was pyrolyzed compared to 50 to 55% in medium textures soils. The m/z diagrams of the pyrolysis products showed significant differences in the composition of plant inputs and large soil difference with depth. Carbohydrate and phenolic-derived peaks dropped significantly with depth. Nitrogenous compounds, considered to be mostly amino compounds, changed during decomposition showing the effects of microbial transformation of the original plant materials that in general conserved the soil N as decomposition progressed with time. Sterols were highest in the litter and at depth. The high molecular weight signals, that were often unidentified, rose from 12% of the sum of the total ionization in the biomass to 35% in the subsurface soil indicating production of soil specific compounds that differed from the original vegetation. Mid infrared (MiDIR) spectroscopy, complemented the pyrolysis data. It also clearly differentiated the vegetation effects. The MiDIR absorbance by the functional groups more clearly differentiated the plant derived from microbial products than did the breakdown products produced by py-MBMS. It was especially sensitive to changes brought about by the cedar growth when comparing the top to lower soil layers. py-MBMS was more useful in identifying the soil specific high molecular weight breakdown products. The small amount of clay in this soil contained more SOM than any other fraction and it was also oldest. Although clay protection is probably more important than biochemical complexity in most soils, the large molecular weight compounds not found in plant residues do contribute to the inherent resistance to decomposition attributable to the molecular structure independent of clay effects. The combination of SOM dynamics measurements by tracers such as 13C and 14C, fractionation and long-term incubation with molecular structure analysis is giving us information on the role of biocomplexity in soil dynamics that cannot be obtained by individual analysis. Multiple analysis of the molecular structure also is proving exceptionally useful.

Last Modified: 9/2/2014
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