4. EXAMPLES FOR RECENT MULTIMETHODOLOGICAL STUDIES OF SOIL N ORG CHEMISTRY
4.3. Impact of Fire and Soil Cultivation
Fire is an important environmental factor that alters soil properties and causes changes in SOM composition (González-Pérez et al., 2004). Heat- induced formation of pyrroles (major) and pyridines (minor) from peptides was observed in 15N-NMR studies of HF-treated soils and extracted humic substances (Knicker, 2009). The term “black nitrogen” was coined for the heterocyclic N in pyrogenic organic material (PyOM), and it was hypothe- sized that this fraction contributes to the recalcitrance of charcoal (Knicker, 2010). However, methodological limitations led to unsatisfactory quantifi- cation and understanding of the processes occurring at the molecular level.
N-XANES spectra for a soil heated under aerobic conditions exhibited a predominant diagnostic feature for amide-N (feature (d) in Fig. 2.16) in the original soil and in soil heated to 100 °C or 200 °C (Kiersch et al., 2012b).
This agreed with previous N-XANES spectra of soil and soil-related materials (Gillespie et al., 2009, 2011a; Leinweber et al., 2010a, 2010b; Vairavamurthy and Wang, 2002). However, in soils heated to 300 °C, diagnostic features for non-peptide C]N (feature (a); as in aliphatic imines and/or aromatic pyridines or pyrazines) and nitriles and/or C–NH–C (feature (b); as in aromatic purines, pyrazoles and/or imidazoles) became more prominent—reaching a maximum in the range 400–500 °C. As well, in all soils heated above 200 °C, feature (b) exceeded feature (a). These spectral changes suggest the formation of unsatu- rated and/or heteroaromatic N compounds from other N species at tempera- tures >200 °C. The overlap of feature (c) by (d) in all spectra for soil heated
to <500 °C indicates an increasing contribution of pyrrolic N, which can interfere with the high energy side of the amidic N feature in the N-XANES spectra (Abe and Watanabe, 2004; Kelemen et al., 2002). This interference complicates compound differentiation in mixtures of amidic and pyrrolic N at the present spectral resolution of N-XANES. Finally, at 600–700 °C, a large amount of Norg was lost and the vibrational manifold at 400.8 eV indicated the release of N2 gas from the remaining interlayer NH4+ (Leinweber et al., 2010b). The predominance of nitrilic and heteroaromatic N compounds at 300–500 °C agreed with enrichments in C]C and substituted aromatic C in
Figure 2.16 Stacked N K-edge X-ray absorption near edge structure spectra of the original and the heated samples; (a) 398.7 eV, (b) 399.6 eV, (c) 400.8 eV, (d) 401.2 eV, (e) 406.0 eV, (f) 407.4 eV. Values at the left side indicate proportions of peak areas. (Repro- duced from Kiersch et al. (2012b) with permission from Elsevier).
the corresponding C-XANES spectra, and with further confirmation obtained using Py-FIMS (Kiersch et al., 2012b). Although the temperature range of intensive alterations in Norg compounds (300–600 °C) agreed with an evolved gas analysis performed by coupling thermal analysis (thermogravimetry and differential scanning calorimetry) with a fast quadruple mass spectrometer, De la Rosa et al. (2008) did not detect Norg compounds released from the control (unburned) and burned (340 °C) soils.
Fire impacts on natural soils are generally much less pronounced than those produced in high-temperature laboratory experiments. Neverthe- less, N-XANES spectra of samples from two long-term field experiments showed that peak areas of the diagnostic features assigned to N in aro- matic compounds and nitriles increased as a result of burning at both sites (Kiersch et al., 2012a). Textural differences between the two sites were reflected in the alterations in Norg compounds observed due to burning, and were more pronounced at the sandy clay loam site than the silty clay site.
This was thought to be a reflection of the greater water holding- and heat capacities of the silty clay soil. Furthermore, alterations in compound classes were accompanied by increases in thermal stability, as revealed by Py-FIMS.
Decreases in the abundance of low-molecular-weight compounds due to the release of functional groups and/or formation of higher molecular weight compounds were detected by Py-FIMS and pyrolysis-single photon ionization-time-of-flight mass spectrometry (Py-SPI-ToF-MS).
Soil from a wheat field near Rostock, Germany showed dark-colored aggregates at the surface a few days after an accidental fire. Stratified sampling followed by N-XANES analyses revealed larger peak areas corresponding to pyridinic N (29%) and nitrilic N (20%) at the surface, as opposed to the interior (19% pyridinic N and 17% nitrilic N), of a fire-affected aggregate (Table 2.4). These results were confirmed by C-XANES spectroscopy and, at least partly, by Py-FIMS.
Data compiled in Table 2.4 show that in the abovementioned sample sets and another laboratory-scale experiment (Lab experiment SL-2N), burning increased the proportions of the peaks corresponding to pyridinic N (398.9 eV) and nitrilic N (399.9 eV) in the N-XANES spectra at the expense of the peak corresponding to amide N (401.4 eV). In the labo- ratory and plot experiments, this corresponded to an increase in hetero- aromatic N at the expense of proteinaceous (aliphatic) N in the Py-FIM spectra. In summary, combining N- and C-XANES with pyrolysis-mass spectrometry revealed that burning produced a multitude of nonpeptidic
N compounds in soil. Whereas this involved formation of pyridines and nitriles, there were no strong indications for the formation of substituted pyrroles. In this respect, our results differ considerably from those reported for 15N-NMR-based studies (e.g. Knicker, 2009).
Cultivation of the Canadian prairies has resulted in depleted levels of Norg in the soil (Monreal and Janzen, 1993). We investigated this impact
Table 2.4 Proportions of Integrated Peak Areas in N-XANES Spectra and the Corresponding Compound Classes (% TII assigned to Norg) Obtained from Py-FI Mass Spectra of Control Samples and the Corresponding Samples Affected by Heat from Burning. The Samples Originate from Two Laboratory-scale Heating Experiments (Lab experiment LS-6N: Kiersch et al., 2012b; Lab experiment SL-2N: Kiersch, unpublished data), a Plot-scale Field Experiment with Periodic Vegetation Burning (Kiersch et al., 2012a) and an Accidentally Burned Wheat Field (Kiersch, unpublished data)
Sample origin
Peak position for binding energy in N K-edge XANES spectra
398.9 eV 399.9 eV 401.4 eV
Control Burned Control Burned Control Burned Lab experiment
LS-6N* 18‡ 33§ 15‡ 21§ 66‡ 43§
Lab experiment
SL-2N† 13 21 10 15 77 64
Plot: periodic burning
18 20 11 13 71 67
Wheat field 19 29 17 20 64 51
Proportions of compound classes in Py-FIMS Aromatic N heterocyclic
compounds and nitriles Amides, free amino acids
Control Burned Control Burned
Lab experiment
LS-6N* 15‡ 66§ 85‡ 34§
Lab experiment
SL-2N† 31 42 69 58
Plot: periodic
burning 16 18 84 82
Wheat field 23 15 77 85
*LS-6N means texture loamy sand and Nt concentration 6.5 g kg−1 soil.
†SL-2N means texture sandy loam and Nt concentration 1.6 g kg−1 soil.
‡Averaged for temperature ranges 20–100 °C.
§300–400 °C.
of cultivation by examining three native vs cultivated sample pairs along a pedoclimatic gradient in Saskatchewan (Leinweber et al., 2007). N-XANES and Py-FIMS revealed that cultivation altered the composition of the Norg and that susceptibility to cultivation-induced losses varied among the dif- ferent compound classes. Py-FIMS analysis revealed that thermally labile peptides and, to a lesser extent, other labile N-containing compounds were preferentially lost from the cultivated sites. The magnitude of these losses decreased in the order: Lethbridge (80-year cultivation) > Macklin (85-year cultivation) > St. Denis (57+ year cultivation). Relative gains in thermally stable N-containing compounds (all sites) and peptides (Macklin only) fol- lowed the same order. The weaker cultivation effect at St. Denis could be explained by a greater inherent stability of the Norg compounds in the native soil—as indicated by the thermal volatilization curves of the peptides.
N-XANES confirmed enrichment of nitriles and N heterocycles, at the expense of amide N, in the cultivated Lethbridge soil.
In the hummocky landscape of Saskatchewan, Canada, variations in SOM characteristics at different topographic positions have been linked to tillage-induced translocation of soil (Pennock et al., 1994) and to water redistribution toward downslope positions (Verity and Anderson, 1990).
Using a novel application of nonmetric multidimensional scaling ordination to XANES data, Gillespie et al. (2011a) observed more heteroaromatic N in cultivated vs uncultivated soils, thus confirming the results of Leinweber et al. (2007). However, the XANES data also showed the presence of unique oxidized N-bonded aromatics, which predominated at calcareous divergent slope positions. These types of Norg compounds are rarely reported in the literature, except in samples obtained from acidic or anaerobic environ- ments (e.g. Olk, 2008; Schmidt-Rohr et al., 2004). These examples show that C- and N-XANES and Py-MS techniques complement one another very well, allowing detection of even small differences in the composi- tion of Norg in whole (nonextracted) bulk soil samples and providing new insights into how the chemistry of soil Norg is impacted by changes in envi- ronment and management.