Carbon and nitrogen degradation on molecular scale of grass-derived pyrogenic organic material during 28 months of incubation in soil

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Abstract

The present study focuses on the microbial recalcitrance of pyrogenic organic material (PyOM) on a molecular scale. We performed microcosm incubation experiments using 13C- and 15N-enriched grass-derived PyOM mixed with a sub soil material taken from a Haplic Cambisol. Solid-state 13C and 15N NMR studies were conducted to elucidate the humification processes at different stages of PyOM degradation. The chemical structure of the remaining PyOM after incubation was clearly different from the initial pyrogenic material. The proportion of O-containing functional groups was increased, whereas that of aryl C and of N-containing heterocyclic structures had decreased, probably due to mineralisation and conversion to other C and N groups. After 20 months of incubation the aryl C loss reached up to 40% of the initial amount and up to 29% of the remaining PyOM C was assigned to carboxyl/carbonyl C and O-aryl C. These reactions alter the chemical and physical properties of the char residue and make it more available for further microbial attack but also for adsorption processes. Our study presents direct evidence for the degradation of N-heterocyclic domains in charred plant remains adding new aspects to the understanding of the N cycling in fire-affected ecosystems.

Research highlights

► Modification of chemical structure of PyOM during biotic incubation. ► Increasing proportion of O-containing functional groups. ► Loss of aryl C and N-containing heterocyclic structures.

Introduction

Black carbon (BC) is a general term used to describe products derived from incomplete combustion of vegetation and fossil fuels (Schmidt and Noack, 2000). It comprises a continuum of combustion products, ranging from slightly charred, degradable biomass to highly condensed, refractory soot. All components of this continuum are carbon-rich, chemically heterogeneous, and dominated by aryl structures (Masiello, 2004). With respect to this continuum, the present study focuses on charred plant residues and humic material, which in the following is termed pyrogenic organic material (PyOM).

PyOM is a quantitatively important carbon (C) and nitrogen (N) pool in the global C and N cycle. With an average turnover rate in soil of thousands of years, PyOM is considered to be recalcitrant. This assumption is based on charcoal findings in soils whose age was determined to be thousands of years old by radiocarbon dating (Saldarriaga and West, 1986, Glaser et al., 2001), and the fact that the turnover of PyOM in marine sediments is very slow (several 1000 yrs; Masiello and Druffel, 1998). However, recent field and incubation studies indicated that PyOM can be degraded within several decades time scale (Bird et al., 1999, Hammes et al., 2008, Hilscher et al., 2009, Steinbeiss et al., 2009; Hilscher and Knicker, in press). A further aspect, which presently receives much attention, is the amendment of so-called biochar to incorporate additional photosynthetically fixed carbon into the soil. The presence of biochar in soil could contribute to a long-term C storage and thus to the mitigation of increasing atmospheric CO2 concentrations (Lehmann, 2007). The extent of such a C sequestration effect on a long-term scale as well as the influence of biochar addition on the quality of soil organic matter (SOM) is still not clear.

We still lack knowledge about the degradation and humification processes and about the stability of different PyOM structures to fully understand the C sequestration potential of these materials in the environment. In particular, knowledge concerning the chemical structure of PyOM is also important for the establishment of more accurate PyOM quantification methods because common degradative techniques developed on the basis of the chemical recalcitrance of polycondensed aryl structures (Hammes et al., 2007, Knicker et al., 2008) are characterised by low specify. Chemical modification of these aryl domains during the degradation process may decrease their chemical recalcitrance and thus may be responsible for an underestimation of the PyOM content assessed by traditional methods.

Recent studies (Covington and Sackett, 1992, Prieto-Fernandez et al., 2004) reported an increase of inorganic N immediately after fire. During the post-fire phase, this inorganic N can be rapidly lost by erosion due to the missing of a plant cover and/or leakage with seepage water. A considerable part of the remaining fire-affected organic N was shown to occur in heterocyclic N structures derived from heat-transformed proteins (Knicker et al., 1996, Knicker et al., 2008, Almendros et al., 2003). On the other hand, this “black nitrogen” (Knicker, 2007) is probably characterised by an increased resistance to biological degradation, leading to a preferential accumulation of heterocyclic N compounds in fire-affected soils. A recent study indicated a low chemical stability of the N-fraction in char against acid digestion with potassium dichromate (Knicker, 2010). To which extent this may also be true for biochemical stability is still not known, because at the moment no studies are available, in which degradation and humification processes of black N have been addressed.

We conducted microcosms incubation experiments using 13C- and 15N-enriched grass-derived PyOM to elucidate its degradability. In a previous study, focusing on the mineralisation potential of this material (Hilscher and Knicker, in press), the recovered 13C-PyOM label decreased to values between 62% and 65% during the incubation. This confirmed that PyOM can be attacked by microorganisms at an initial phase of the degradation process. The respective 15N-PyOM recovery followed the same trend but tended to be larger. Already after one month of incubation, PyOM was associated with the POM (particular organic material)-free mineral soil fraction.

Our study focuses on the chemical alteration of PyOM during degradation by applying solid-state 13C and 15N NMR spectroscopy on samples taken at various stages of the PyOM degradation process. With the obtained data, we intend to elucidate some important aspects of the C and N sequestration potential and the fate of aged PyOM in soil and sediments.

Section snippets

Sample material

For the production of the PyOM we used rye grass (Lolium perenne L.), as a representative plant fuel consumed by grassland fires. Seeds of rye grass were cultured on quartz sand in a closed plexiglass chamber, located in a phytotron that allowed the automatic control of climatic and light conditions. The grass was grown with 13C-enriched CO2 gas (13C: 99 atom%) and 15N-labelled potassium nitrate nutrient solution (15N: 98 atom%). After two weeks the grass shoot stems were harvested. The pots

Efficiency of HF treatment of PyOM-enriched mineral fractions on NMR sensitivity

The HF treatment of the fine silt (6.3–2 μm) and clay fraction (<2 μm) resulted in high C-enrichment factors (Ec = 4 ± 1; Table 1), leading to C concentrations between 20 and 40 mg C g−1 for the clay fraction and between 6 and 13 mg C g−1 for the fine silt fraction. During the demineralisation step 83 ± 2% of the mineral phase was removed. High recoveries of the 13C label (88 ± 7%) underline that HF treatment did not change the chemical composition of the PyOM samples. For the 15N-labelled and

Structural alteration of PyOM by degradation

Compared to fresh plant material, the PyOM revealed a reduced microbial availability of O/N-alkyl C and alkyl-C residues, which can be explained by the chemical alteration induced by charring, as e.g. formation of anhydrosugars (Elias et al., 2001). Alternatively, some of those compounds may have been physically protected by entrapment of more charred domains (Knicker et al., 1996). However, as indicated in the present study, those alkyl C and O/N-alkyl C residues will be primarily decomposed

Conclusion

Fire events generate PyOM that is typically characterised by high aromaticity and small abundance of O-containing functional groups. It is expected that the contribution of PyOM to the soil organic matter leads to an increase of the recalcitrant C and N pool on a long-term scale. However, the present study indicates that this assumption may be oversimplified. The heterogeneous chemical structure of PyOM results in different degradation and humification dynamics. This aging process includes

Acknowledgements

We are grateful to X. Chen, P. Müller and J. Fischer for technical assistance and help with laboratory work. We thank F. Steinbacher from the Lehrstuhl für Zierpflanzenbau of the TU München for providing a phytotron camber for cultivation of the labelled grass. The constructive comments of Dr. A. Kölbl, Dr. S. Spielvogel, Dr. C.W. Müller, Prof. Dr. J. Prietzel, Dr. M. Steffens, the editor and anonymous reviewers on earlier drafts are gratefully acknowledged. The project was financially

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