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Biogeochemical Investigation at Prairie Ridge, NC Prairie Ridge Soil Profile Amy Keyworth Jovi Saquing November 2006 Prairie Ridge Soil Profile Outline • What we expect to see… and why? • What we do see… and how come? • What can we conclude? Soil Profile Description Litter (undecomposed) Organic layer, fermented Organic layer, humified Mineral layer with organic carbon and leached minerals Mineral layer with precipitation of oxides/hydroxides and/or carbon Unaltered parent substrate Source: Gleixner, G. 2005. Stable isotope composition of soil organic matter. In Stable isotopes and biosphere-atmosphere interactions. ed. Flanagan, L.B., E.J. Ehleringer and D.E. Patake. Prairie Ridge Soil Profile What we expect to see.. • • • • • 13C – increase with depth C/N – decrease with depth % C – decrease with depth % N – increase/decrease with depth Carboxylic and aromatic groups – present in organic layers, increasing aromaticity with depth Prairie Ridge Soil Profile Organic Compounds Cellulose Monosaccharide (e.g. glucose) Lipid Lignin Lignin monomers Intermediates Protein Amino acid Ammonium (e.g. acetic acid) CO2 Source: Gleixner, G. 2005. Stable isotope composition of soil organic matter. In Stable isotopes and biosphereatmosphere interactions. ed. Flanagan, L.B., E.J. Ehleringer and D.E. Patake. Humic Substances Nitrites/Nitrates N2, N2O What we expect to see - 13C Carbon isotopic composition profiles. Undisturbed site Disturbed (agricultural) site (Fig 2 middle, J.G. Wynn, et al., 2006) What we expect to see – [C] Carbon concentration profiles. Undisturbed site Disturbed (agricultural) site “Kink” in the Cz curve reflects root depth or productivity zone (Fig 2. Top, J.G. Wynn, et al., 2006) What we expect to see – C/N Source: C/N of soil organic matter from different depth intervals (Gleixner, 2005) Why do we expect to see it ? 1. 2. 3. 4. Suess effect Soil carbon mixing Preferential microbial decomposition Kinetic fractionation Why we expect to see it ? 1. 2. 3. 4. Suess effect Soil carbon mixing Preferential microbial decomposition Kinetic fractionation Why we expect to see it • Suess effect – – Older, deeper SOM originated when atmospheric 13C was more positive (CO2 was heavier) – From 1744 to 1993, difference in 13C app 1.3 ‰ – Typical soil profile differences = 3 ‰ 1. Suess effect Mixing of SOC derived from the modern atmosphere versus that derived from a pre-Industrial Revolution atmosphere. (Fig. 1A, J.G. Wynn, et al., 2006) Why we expect to see it ? 1. 2. 3. 4. Suess effect Soil carbon mixing Preferential microbial decomposition Kinetic fractionation 2a. Soil carbon mixing - Surface litter (depleted) vs. root derived (enriched) SOM Mixing of leaf litter-derived SOC and root-derived SOC. (Fig. 1B, J.G. Wynn, et al., 2006) 2b. Soil carbon mixing - Variable biomass inputs (C3 vs. C4 plants) Mixing of SOC formed under two different vegetation communities, e.g. C3 vs C4. Slope could vary from positive to negative depending on direction of shift. (Fig. 1C, J.G. Wynn, et al., 2006) 2. Soil carbon mixing c. Some of the carbon incorporated into SOM by these critters has an atmospheric or soil gas, not SOM, source. d. Atmospheric C is heavier. Atmospheric CO2 in the soil is 4.4 ‰ heavier than CO2 metabolized by decomposition (Wedin, 1995) Why we expect to see it ? 1. 2. 3. 4. Suess effect Soil carbon mixing Preferential microbial decomposition Kinetic fractionation 3. Preferential microbial decomposition – Lipids, lignin, cellulose - 13C depleted with respect to whole plant – Sugars, amino acids, hemi-cellulose, pectin - 13C enriched – Lipids and lignin are preferentially accumulated in early decomposition – Works against soil depth enrichment – More C than N are lost from soil as SOM decomposes due to internal recycling of N. Why we expect to see it ? 1. 2. 3. 4. Suess effect Soil carbon mixing Preferential microbial decomposition Kinetic fractionation 4. Kinetic fractionation – Microbes choose lighter C – Microbial respiration of CO2 – 12C preferentially respired – Frequently use Rayleigh distillation analyses (Wynn 2006) – No direct evidence for this (Ehleringer 2000) – Preferential preservation of 13C enriched decomposition products of microbial transformation 4. Kinetic fractionation 13C distillation during decomposing SOM. The gray lines show the model with varying fractionation factors from 0.997 to 0.999. (Fig. 1D, J.G. Wynn, et al., 2006) 4. Kinetic fractionation Rayleigh distillation 13Cf 1 1 F 1000 13Ci 1 e t 1 1 1 1000 e 1t 1 t • • • • • • fraction of remaining soil organic matter (SOC) – approximated by the calculated value of fSOC 13Cf isotopic composition of SOC when sampled 13Ci isotopic composition of input from biomass α fractionation factor between SOC and respired CO2 e efficiency of microbial assimilation t fraction of assimilated carbon retained by a stabilized pool of SOM F Assumptions by Wynn etal • Open system – All components decompose – Contribute to soil-respired CO2 at same rate with depth • FSOC fSOC Anthropogenic mixing (agriculture) Various reasons that disturbed land might not conform to nice regression curve in fig 1D (Wynn fig 9 ) A – natural B – introduce C4 plants, enriched in 13C C – Cropping – removes new, low 13C material, leading to surface enrichment D – Erosion – removes upper layer, moving the whole curve up E – Reintroduce soil organic carbon (better management practices) – reverses the trends in C, D, and E What we do see - results δ13C %C %N Mean C:N Mole O- horizon PRS-15 Bulk -19.11 1.49 0.12 14.38 A- horizon (0-6 cm) PRS-16 Bulk -18.95 2.01 0.18 13.36 AP horizon (6-11 cm) PRS-17 Bulk -15.92 0.81 0.05 17.28 B horizon (11+ cm) PRS-18 Bulk -22.84 0.73 0.05 15.99 O- horizon PRS-15 Plant Fragment -21.27 36.77 1.37 31.43 A- horizon (0-6 cm) PRS-16 Plant Fragment -29.63 39.13 1.93 23.68 AP horizon (6-11 cm) PRS-17 Plant Fragment -27.01 18.71 0.64 34.07 B horizon (11+ cm) PRS-18 Plant Fragment O- horizon PRS-15 Heavy Fraction -19.00 1.50 0.11 15.42 A- horizon (0-6 cm) PRS-16 Heavy Fraction -18.71 1.19 0.10 14.66 AP horizon (6-11 cm) PRS-17 Heavy Fraction -15.60 0.71 0.05 17.66 B horizon (11+ cm) PRS-18 Heavy Fraction What we do see - results • 13C – increase 3 ‰ to 8 cm (PRS 18 = anomaly) • C/N – increases to 8 cm, then decreases • % C – decrease with depth (PRS 15 = anomaly) • % N – decrease with depth (PRS 15 = anomaly) What we do see - 13C Depth vs delta 13C delta 13C -25 -20 -15 0 2 Depth (cm) 4 6 8 10 12 14 16 Increase of 3 ‰ to 8 cm (PRS 18 = anomaly) What we do see - C/N Depth vs C/N C/N 0 5 10 15 20 0 2 Depth (cm) 4 6 8 10 12 14 16 Increases to 8 cm, then decreases What we do see - % C Depth vs %C %C 0 2 4 0 2 Depth (cm) 4 6 8 10 12 14 16 Decrease with depth (PRS 15 = anomaly) What we do see - % N Depth vs %N %N 0 0.2 0.4 0 2 Depth (cm) 4 6 8 10 12 14 16 Decrease with depth (PRS 15 = anomaly) 3000 2500 2000 1500 - Normalized 1000 500 Soil FTIR wave number 0 Soil FTIR (normalized) 1 15 16 17 18 7 0.6 0.4 0.2 0 4000 3500 3000 2500 2000 1500 1000 500 0 Wave number (cm-1) 15 16 17 18 7 • PRS 7 and PRS 15, both surface soils, have similar absorbencies • All soils have peak at wavelength 1032 • All 5 spectra have similar peaks, though not necessarily similar absorbencies • In our bulk and heavy samples, are the mineral spectra masking the organics, as in Poirier’s M-SOM? Absorbance 0.8 Wavenumbr Description Possible functional groups 3700 sharp peak O-H stretching region (3800-3400 for clay mineral) 3622 sharp peak O-H stretching region (3800-3400 for clay mineral) Bands due to Si-O-O-OH vibration. 3464 broad, strong intensity O-H , N-H Since it's broad and strong intensity, this is due to O-H bond rather than N-H bond. 2935 tiny broad C-H (3150-2850) The peak is below 3000, so it is an aliphatic C-H vibration. Medium intensity absortions at 1450 and 1375 cm-1 will indicate -CH3 bend. strectching. 1655 medium intensity C=C (1680-1600 for aromatic and alkenes); C=O vibrations (1680-1630 for amide), C=N (16901630) and also of N-H bend (1650-1475) Some soil literature assigned this to C=O vibratios of carboxylates and aromatic. Vibrations involving most polar bonds, such as C=O and O-H have the most intense IR absorptions. This peak has medium intensity and most likely due to N-H bending. 1450 & 1400 weak C-H, alkanes, -CH3 (bend, 1450 and 1375), -CH2 (bend, at 1465), Most likely CH3 bending. 1099-1034 sharp & strongest peak Si-O vibration of clay minerals Consistent with FTIR spectra of soil in the literarture 800 medium intensity, saw tooth NH2 wagging and twisting, =C-H bend, alkenes 696 medium intensity, sharp 540 medium intensity, sharp N-C=O bend for secondary amides 472 strong intensity, sharp C-C=O bend for secondary amides, SiO3 Intense absorption at 460-475 -2 corresponds to SiO3 vibration. In the literarture, bands at 800,780,650,590,530 and 470 are attributed to inorganic materials, such as clay and quartz minerals. cm Comments -1 -2 Problems with Methods – Random protocol on soil sampling at the site (i.e. depth interval, mass of soil) – Inconsistent sample preparation procedure (i.e. different mass, subjective sorting) – Poor implementation of IRMS protocols (i.e. sample size, standard calibration) – Insufficient samples for statistical accuracy