Degradation of grass-derived pyrogenic organic material, transport of the residues within a soil column and distribution in soil organic matter fractions during a 28 month microcosm experiment

The microbial recalcitrance of char accumulated after vegetation fires and its transport within a soil column were studied in microcosms using C-13- and N-15-enriched pyrogenic organic material (PyOM). The PyOM from rye grass (Lolium perenne L.) was produced by charring at 350 degrees C under oxic conditions for 1 and 4 min to examine the impact of the charring degree. After 28 months, C-13 recovery decreased to values between 62% and 65%, confirming that this material can be attacked by microorganisms and that the degradation occurs rapidly after accumulation of PyOM at the soil surface. The respective N-15 recovery followed the same trend but tended to be higher (between 67% and 80%). Most of the added PyOM isotopic labels were recovered in the particulate organic matter (POM) fraction, containing between 84% and 65% of the added C-13 and N-15 after the first 2 months, being reduced by half at the end of the experiment. After 1 month, up to 13.8% of the C-13 label and 12.4% of the N-15 label were detected in the POM-free mineral fractions. This fast association of PyOM with the mineral phase indicates that physical soil properties have to be considered for the elucidation of PyOM stability. Addition of fresh unlabelled grass material as co-substrate resulted in comparable trends as for the pure PyOM but the total recovery of the isotopic labels clearly increased with respect to the amount of mineral-associated PyOM. Between 73% and 82% of the mineral-associated PyOM occurred in the clay separates (<2 mu m) for which the highest values were obtained for the experiment with the more intensively charred PyOM and co-substrate addition. In summary, the study demonstrates the degradability of grass-derived PyOM. The addition of fresh plant material as an easily degradable co-substrate promoted the formation of partially decomposed PyOM and subsequently its association with the mineral phase, but did not increase the respective mineralisation rates. Detection of C-13 and N-15 content at different depths of the microcosm column demonstrated an additional loss of PyOM from top soil by way of mobilisation and transport to deeper horizons. All these processes have to be taken into account in order to obtain a more realistic view about the behaviour of PyOM in environmental systems and for estimation of the C and N sequestration potential. (C) 2010 Elsevier Ltd. All rights reserved.