Age, turnover and molecular diversity of soil organic matter in aggregates of a Gleysol

Abstract

Monreal, C. M., Schulten, H.-R. and Kodama, H. 1997. Age, turnover and molecular diversity of soil organic matter in aggregates of a Gleysol. Can. J. Soil Sci. 77: 379-388. We used an integrated approach to describe soil organic matter (SOM) dynamics through known inorganic and organic components in aggregates of adjacent forested and cultivated Gleysolic soil. Mineral and SOM components were examined in water stable macroaggregates (>250 µm), microaggregates 1 (50-250 µm) and microaggregates 2 (<50 µm) fractions. SOM was characterized by pyrolysis-field ionization mass spectrometry (Py-FIMS), and soil minerals by X-ray diffraction analysis. The mean residence time of organic-C (OC) was determined using radiocarbon dating. OC turnover was determined using the natural abundance of native 13 C and that derived from corn residue. We found that OC in macroaggregates was young (<100 yr), turned over in 14 yr, and consisted of OM typical of that found in tissues of plants and soil organisms. Chemical classes of compounds in macroaggregates consisted mainly of carbohydrates, lignin monomers and phenols, lignin dimers, lipids (alkanes, alkenes, n-alkyl esters), fatty acids, sterols, suberin and aliphatic and aromatic N compounds. The fast turnover time of OC in larger size aggregates supports the hypothesis that the initial decline in SOM after breaking native land is associated with losses of SOM stored in macroaggregates. OC in microaggregates 1 was young (<100 yr) and turned over in 61 yr. OC in microaggregates 2 was old, turned over in 275 yr, and consisted of highly humified macromolecules. Pyrolyzable SOM products representing plant and microbial components like lignin dimers, sterols, suberin and fatty acids were absent from microaggregates 2 containing old OC. The turnover time of OC correlated directly with the amount of smectite and Al extracted with ammonium oxalate, inversely with non-expandable phyllosilicates, and weakly with the total clay content of aggregates. Thermolabile and thermostable molecular components in aggregates indicated degree of association between SOM and clay minerals. Carbohydrates, peptides and alkylaromatics appeared to be less affected by abiotic stabilization reactions. The distribution, allocation and dynamics of soil OC and chemical structures in SOM within aggregate fractions is not well understood. Carbon dynamics were characterized in humic acid fractions, whole soil and particle size fractions (Campbell et al. 1967;Hsieh 1992;Gregorich et al. 1995); kinetically described in simulation models for rapidly cycling (seconds to months) and more stable pools of organic matter (years to thousands of years) (Monreal and McGill 1989;Jenkinson and Rayner 1977;Parton et al. 1987 For personal use only.

Soil aggregate fraction-based 14C analysis and its application in the study of soil organic carbon turnover under forests of different ages

“…Chemical protection of soil organic matter arises from the interaction of different types of organic carbon with metal oxides and clay minerals, and thus, the decomposition rate of SOC is generally slower in smaller particle-size aggregate [36,[50][51][52][53]. By contrast, occlusion is the dominant form for stabilizing soil organic matter in larger particle-size aggregates, so the decomposition rate of SOC is relatively faster [51][52][53].…”

“…Different particle-size aggregates protect organic carbon in different ways, which will lead to differences in the turnover times of organic carbon [26]. Therefore, soil aggregate fractions separation method can help us understand the soil carbon cycle from the perspective of the heterogeneity of soil organic matter, and when combined with the 14 C tracer technique, it can help us better understand the SOC turnover mechanism and predict its response to climate change [34][35][36][37][38].…”

Interpretation of soil enzyme activities in a comparison of tillage practices along a topographic and textural gradient

“…As such it may be a particularly useful measurement to study dynamics of the labile C fraction within the Ap horizon, given the observation from long-term studies on the Canadian prairies that changes in OC content occur predominately in the "young" or labile fractions (Janzen et al 1998). Studies conducted in eastern Canada indicated that corn residue C is incorporated into the domains of stable macroaggregates (>250 mm) containing kinetically active C pools (Monreal et al 1997). …”

Carbon Stocks in Compartments of Soil Organic Matter 31 Years after Substitution of Native Cerrado Vegetation by Agroecosystems

“…In the clayey Oxisol (Typic Haplustox) of our study, the reduction of over 30 % in microbial biomass in conventional tillage systems can be attributed to the lack of readily available substrate in the <53 µm fractions, representing more than 85 % of OM. In this silt-clay fraction, the high energy adsorption to minerals in the soil matrix can promote long-term stabilization, leading to turnover rates of more than 200 years in microaggregates (Monreal et al, 1997).…”