Stability of Soil Carbon Fractions  Aggregation Versus Mineral Association
Abstract
Models that seek to describe the dynamics of soil organic C typically distinguish between two or more C fractions according to differences of biochemical and microbial degradation. The rates are a consequence of recalcitrance, accessibility and interactions. Soil aggregation is an important mechanism controlling the accessibility of substrates by microbes and enzymes and thus the dynamics of minerals bound C are interacting with soil aggregate dynamics. In this study we focused on C fractions isolated by particle size fractionation. The main objective of our study was to differentiate between C stabilization of soil fractions due to accessibility/aggregation or to association with minerals. For a detailed understanding of these processes and the sources of respired soil CO2 we combined the measurement of heterotrophic respiration, CO213C analyses and radiocarbon dating of the respired CO2 in a longterm laboratory mineralization experiment. For the experiment we took soil material from the A horizon of an Albic Luvisol under Norway spruce forest (Picea abies) in southern Germany. The air dried bulk soil (< 2000 µm) was subjected to ultrasonication (1st step 60 J ml1; 2nd step 440 J ml1) and separated according to particle size in three fractions: > 63 µm to 2000 µm  sand, > 6.3 µm to 63 µm  silt and silt/clay fraction < 6.3 µm  clay. Solidstate 13CCPMAS NMR spectroscopy was used to analyze the composition of bulk soil and fractions. The incubation of the three fractions and the bulk soil was done for 250 days in triplicate at 20 degree Celsius and 70% of maximal water holding capacity. A relative enrichment of alkyl C and an increase of the alkyl / O/Nalkyl C ratios in the order of sand < silt < clay were observed by 13CNMR. On a long term the sand fraction and the bulk soil showed a sustained C bioavailability. For the silt and clay fraction similar respiration rates and a low C bioavailability were detected. The recombined fractions (by calculation) showed 35% higher amounts of respired CO2C than the bulk soil. This difference accounts for the absence of restricted accessibility due to soil aggregation. Because of the high amounts of mineral bound C the main source of CO2C (70%) in the recombined fraction is the clay fraction. Nevertheless the recalcitrance of mineral bound C is restricting the positive effects of aggregate disruption on the C turnover. The small fast decomposing C pool of the sand fraction is of minor importance to the total soil respiration balance. CO213C signatures showed higher values of the silt and clay fractions in contrast to the sand fraction, indicating a lower bioavailability of 13Cdepleted carbon sources in the small fractions. The analyses of CO214C showed a shift to the utilization of older C sources with time.
 Publication:

AGU Fall Meeting Abstracts
 Pub Date:
 December 2007
 Bibcode:
 2007AGUFM.B11G..02M
 Keywords:

 0428 Carbon cycling (4806);
 0486 Soils/pedology (1865)