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The stability of soil organic matter (SOM) as it relates to resistance to microbial degradation has important implications for nutrient cycling, emission of greenhouse gases, and C sequestration. Hence, there is interest in developing new ways to quantify and characterise the labile and stable forms of SOM. Our objective in this study was to evaluate SOM under widely contrasting management regimes to determine whether the variation in chemical composition and resistance to pyrolysis observed for various constituent C fractions could be related to their resistance to decomposition. Samples from the same soil under permanent pasture, an arable cropping rotation, and chemical fallow were physically fractionated (sand: 2000-50 μm; silt: 50-5 μm, and clay: K -edges. Relative to the pasture soil, SOM in the arable and fallow soils declined by 30% and 40%, respectively. The mineralization bioassay showed that SOM in whole soil and soil fractions under fallow was less susceptible to biodegradation than that in other management practices. The SOM in the sand fraction was significantly more biodegradable than that in the silt or clay fractions. Analysis by XANES showed a proportional increase in carboxylates and a reduction in amides (protein) and aromatics in the fallow whole soil compared to the pasture and arable soils. Moreover, protein depletion was greatest in the sand fraction of the fallow soil. Sand fractions in fallow and arable soils were, however, relatively enriched in plant-derived phenols, aromatics, and carboxylates compared to the sand fraction of pasture soils. Analytical pyrolysis showed distinct differences in the thermal stability of SOM among the whole soil and their size fractions; it also showed that the loss of SOM generally involved preferential degradation of H-rich compounds. The temperature at which half of the C was pyrolyzed was strongly correlated with mineralizable C, providing good evidence for a link between the biological and thermal stability of SOM.

Decomposition of soil organic matter (SOM) and plant litter has been shown to be affected by high solar radiation; this could partly explain why biogeochemical models underestimate decomposition in arid and semi-arid ecosystems. We set out to test the effect of using traditional PVC chambers for measuring soil gas fluxes versus quartz chambers that allowed passage of light during field measurements in a dry-land field in Davis, CA. Results showed that fluxes from quartz-top chambers were on average 29% higher than from opaque chambers. We also studied the effect of solar light exposure on decomposition of native grass litter and SOM in a field experiment where plots were shaded or left exposed for 157 days during summer; litter did not seem to be affected by exposure to light. However, we concluded that SOM decomposition was affected by light exposure since shaded soil had similar respiration to sunlight-exposed soil indicating that microbial respiration occurred under the shade while photo-degradation likely occurred under the sun. Additionally, 15N-labeled grass was placed in litter bags in the field with either clear filters to allow light or aluminum covers to block light; 3-month exposure caused a change in lignin degradability as indicated by the change in the Ad/Al ratio. Incubation of that litter showed 9.3% more CO2 produced from litter in clear and aluminum bags than unexposed litter. This showed that photo-facilitation occurred although to a small degree and was a result of light exposure and/or heat degradation. We attributed the similar respiration from clear- and aluminum-exposed litter to heat degradation of the aluminum-exposed litter. In conclusion, our results show that in hot dry ecosystems conventional PVC chambers underestimate measured CO2 flux rates; sunlight exposure changes litter chemistry and appears to affect the degradation of soil organic matter, but the magnitude of degradation depends on an interaction of factors such as soil temperature and moisture.

As plants are major contributors to soil organic matter (OM), the chemical composition of plant tissues entering the soil should be related to the soil OM composition. We examined the OM composition of maize leaves, stems, and roots, to determine if differences in plant tissue composition altered the soil OM composition during a 36-week degradation experiment. Lignin phenols were measured by gas chromatography/mass spectrometry (GC/MS), and soil OM and humic substances were characterized using solid-state 13 C and solution-state 1 H nuclear magnetic resonance (NMR) spectroscopy respectively. Lignin phenol composition, oxidation, and depletion relative to cutin-derived OH-acids changed less in leaf- compared to stem- and root-amended soils over time, and may be due to greater vanillyl concentrations in leaves. Soil amended with stems had higher concentrations of carbohydrates in soil OM. Humic substances from leaf-amended soils had higher concentrations of aliphatic components, likely due to higher concentrations of aliphatic compounds in leaf tissues, which suggests that compounds derived from leaves are potential contributors to the stable pool of soil OM. After 36-weeks of incubation, the contribution of microbial-derived OM was greatest in humic extracts from root-amended soils, and increased contribution from these compounds was detected earlier in these soils than stem- and leaf-amended soils. This indicates that root amendment may enhance contributions from microbial-derived OM. Our study suggests that changes in soil OM composition over time was related to the chemical composition of the plant tissue, and demonstrates the important link between plant chemistry and soil OM turnover.

Bt (Bacillus thuringiensis) corn (Zea mays L.) accounted for 74.5% of the corn acreage in eastern Canada in 2009. Reports that Bt corn has greater yield and lignin concentrations than unmodified corn have raised questions about its effect on the soil ecosystem. Our objectives were to evaluate the biomass of field-grown Bt and non-Bt corn, the chemical composition of different corn components that remain as residues in the field after harvest, and the effect of the Bt modification on residue decomposition. Nine Bt corn hybrids and their near isolines were field-grown in 2008 and 2009. Grain and stover yields were measured and leaves, stems, and roots were collected and analyzed for lignin, C, and N concentrations. Stem sections from a Bt/non-Bt corn pair were buried in the field and sampled periodically during 1 yr. No difference in yield or lignin concentrations due to the Bt gene was noted; however, N concentration in Bt stems was significantly greater than in non-Bt stems in 1 yr of the 2-yr study. Leaves had less lignin and a lower C/N ratio than stems and roots in both years. In buried field litterbags, the decline in C/N ratio and mass loss suggests that Bt stems were decomposing more rapidly than non-Bt stems. We conclude that the Bt gene does not affect the agronomic performance or the chemical composition of corn in fields without herbivory, and that Bt corn residue may be more susceptible to decomposition than non-Bt corn residue.

Bt ( Bacillus thuringiensis ) corn is reported to produce lignin-rich residues, compared to non-Bt (NBt) corn, suggesting it is more resistant to decomposition. As the Bt gene is expressed selectively in stem and leaf tissue, it could affect lignin distribution in corn, which naturally has greater lignin content in roots than in stems and leaves. Our objective was to evaluate the effects of corn plant components, the Bt gene and elevated-lignin inputs on decomposition. Roots, stems and leaves from Bt corn and NBt corn isolines enriched with 13 C and 15 N were finely ground and mixed separately with soil, then incubated at 20 °C for 36 weeks. The effect of elevated lignin on decomposition was tested by adding a commercial lignin source (indulin lignin) to half of the samples. In addition to weekly CO 2 analysis and regular measurement of N mineralization, the degree of lignin degradation was evaluated at 1 and 36 weeks from the acid to aldehyde ratio (Ad/Al) of vanillyl and syringyl lignin-derived phenols. The CO 2 production and N mineralization was lower in root-amended soils than stem- and leaf-amended soils. The Bt genetic modification increased CO 2 production from stem-amended soils ( P 13 C and 15 N results also showed more residue-C and -N retained in soils mixed with NBt stem residues. After 36 weeks leaf- and stem-amended soils with indulin lignin had a lower Ad/Al ratio and were less degraded than soils without exogenous lignin. In conclusion, plant lignin and nitrogen contents were good predictors of CO 2 production and N mineralization potential. Corn roots decomposed more slowly than aboveground components emphasizing the importance of recalcitrant root residues in sustaining the organic matter content of soil.