Abstract
We hypothesised that, during occlusion inside granular aggregates of oxide-rich soils, the light fraction organic matter would undergo a strong process of decomposition, either due to the slow process of aggregate formation and stabilisation or due to digestion in the macro- and meso-fauna guts. This process would favour the accumulation of recalcitrant materials inside aggregates. The aim of this study was to compare the dynamics and the chemical composition of free and occluded light fraction organic matter in a natural cerrado vegetation (woodland savannah) and a nearby pasture (Brachiaria spp.) to elucidate the transformations during occlusion of light fraction in aggregates of a clayey Oxisol. Nuclear Magnetic Resonance of the 13C, with Cross Polarisation and Magic Angle Spinning (13C-CPMAS-NMR), and 13C/12C isotopic ratio were combined to study organic matter composition and changes in carbon dynamics, respectively. The occluded light fraction had a slower turnover than the free light fraction and the heavy fraction. Organic matter in the occluded fraction also showed a higher degree of decomposition. The results confirm that processes of soil organic matter occlusion in the typical "very fine strong granular" structure of the studied oxide-rich soil led to an intense transformation, selectively preserving stable organic matter. The small amount of organic material stored as occluded light faction, as well as its stability, suggests that this is not an important or manageable sink for sequestration of atmospheric CO2.
Highlights
Soil organic matter (SOM) represents only a small portion of the total soil matrix, it is a very active component linked to fundamental soil processes, such as biological activity, nutrient cycling, and aggregation (Christensen, 2001)
For the density fractions (Table 2), the largest difference was observed in the free light fraction (F-LF), followed by the haevy fraction (HF)
Compared to the light fractions, a marked increase in δ13C values was observed in the HF (Table 2), which is attributed to isotopic discrimination during decomposition (Balesdent & Mariotti, 1996)
Summary
Soil organic matter (SOM) represents only a small portion of the total soil matrix, it is a very active component linked to fundamental soil processes, such as biological activity, nutrient cycling, and aggregation (Christensen, 2001). SOM can be free or only weakly associated to soil particles (free light fraction – F-LF), or strongly associated to mineral particles as organomineral complexes (OMC) (Golchin et al, 1997; Christensen, 2001) These complexes are called primary, when formed by direct interaction between primary mineral particles and organic compounds. At a higher hierarchical level, primary complexes may bind together into secondary complexes or aggregates In this process of aggregation, free organic material can be encapsulated, forming the intra-aggregate particulate organic matter or occluded light fraction (O-LF). For O-LF, the occlusion inside aggregates limits the access of decomposers (micro-organisms) and extra-cellular enzymes In addition to these two mechanisms, the haevy fraction (HF) is protected by direct complexation with clay particles. The turnover rate of the three fractions is expected to decrease in the order: F-LF > O-LF > HF (Golchin et al, 1997; Christensen, 2001)
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