How stable is Black Carbon? - An incubation experiment

Tom Miles

How stable is Black Carbon? - An incubation experiment
A. Hilscher (1), H. Knicker (1)
(1) ehrstuhl fur Bodenkunde, TU Munchen, Germany

Black carbon (BC) and black nitrogen (BN) are produced by incomplete combustion of vegetation and occurs ubiquitously in soils. BC has a highly aromatic structure with few functional groups, and is assumed to be relatively resistant to decay. Thus, the charred plant material may play an important role in carbon sequestration. But assuming that biomass burning occurred at the same rate than nowadays since the last glacial maximum, BC should account for 25 to 125% of the total SOM pool (Masiello, 2004). To be able to estimate the recalcitrance of BC in soils, knowledge is required concerning its degradation rates and mechanisms. Therefore, the objectives of this study are to examine the degradation and humification of BC and its transport within the soil profile. A further interest is the impact of addition fresh plant material as co-substrate (CM) on the BC degradation.

The used BC derived from charred residues of rye grass (Lolium perenne), which were grown with 13C-labelled CO2 (99.6 atom%) and with 15N-enriched nutrient solution (KNO3; 99.4 atom %). After harvesting and drying, the two week old shoots were charred at 350˚C for one (partly burnt) and four minutes (burnt), respectively. The char was put onto a soil column and incubated for up to 18 months at a temperature of 30˚C. The chemical composition of organic matter in the potentially mobile (dis-solved organic matter; DOM, and particulate organic matter; POM) and immobilized fractions (mineral associated organic matter of the mineral phase) was investigated by means of solid-state 13C NMR spectra obtained on a Bruker DSX 200 spectrometer using the cross-polarization (CP) magic-angle spinning (MAS) technique. The 13C and 15N contents of the samples were measured with a quadrupole mass spectrometer (QMS) connected to an Elementar Vario EL.

First results shown that after an incubation time of 6 months, 67% to 75% of the 13C input remained in the POM fraction. The 15N recovery for the POM fraction was comparable. The application of fresh plant litter as a co-substrate after four months of incubation resulted in the development of a fungal mat. With respect to the amount of 13C-labeled carbonyl given to the incubate with the fresh partly charred grass, an enrichment (24%) of the carbonyl 13C was observed for the POM fraction, 2 months after co-substrate addition. All other C groups of the POM decreased which is in accordance with a redistribution of isotopically labelled BC into the mineral fractions, DOM and deeper horizons. This affirmed a recovery of 0.2% 13C and 1.5% 15N in the 3 cm deeper sub layer.

Already after one month of incubation 10% of the added 13C was recovered with the mineral phase. After 6 months this amount increased to 27%. Co-substrate addition reinforced this process. We found an increase of up to 39%. The most of 13C from the added char was associated with the clay fraction (61% -75%). This points to the involvement of the minerals in the immobilisation and stabilisation process of char in soil systems. The fixation of the char to the mineral surfaces requires functional groups on the char surface, developed from biotic oxidation of the char.

Our study demonstrates that BC may not be as recalcitrant as commonly assumed. The char material has a high potential to be oxidised. Such oxidized residues are more accessible to microbial degradation and may also be responsible for a more efficient translocation of char into deeper horizons.

Masiello, C. A. (2004). New directions in black carbon organic geochemistry. Marine Chemistry 92 (1-4): 201-213.