Biogeochemical Investigation at Prairie Ridge, NC Prairie Ridge Soil Profile Amy Keyworth Jovi Saquing November 2006

Outline What we expect to see… and why? What we do see… and how come? What can we conclude? Prairie Ridge Soil Profile

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 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.

What we expect to see..  13 C – 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 LipidLignin Protein Monosaccharide (e.g. glucose) Source: Gleixner, G 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 Intermediates (e.g. acetic acid) CO 2 Amino acid Ammonium Nitrites/Nitrates N 2, N 2 O Lignin monomers Alkanes

Order of decay of compounds (Melillo 1989) 1.Loss of C fractions depleted in 13 C (p 192) –tannins –non-polar components –water-soluble compounds –lignin, also depleted in 13 C is conserved 2.Cellulose – C fraction enriched in 13 C 3.Lignin –Recalcitrant –Can be enhanced by addition of simple sugars –N may slow lignin decay (Fenn etal 1981, Keyser etal 1978) – not proven

Fig 2 middle. Carbon isotopic composition profiles. Undisturbed siteDisturbed (agricultural) site (J.G. Wynn, et al., 2006) What we expect to see -  13 C

Fig 2. Carbon concentration profiles. Undisturbed siteDisturbed (agricultural) site “Kink” in the Cz curve reflects root depth or productivity zone (J.G. Wynn, et al., 2006) What we expect to see – [C]

What we expect to see – C/N Source: C/N of soil organic matter from different depth intervals (Gleixner, 2005)

Why we expect to see it ? 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 ‰ –Typical soil profile differences = 3 ‰

Suess effect Fig. 1A. Mixing of SOC derived from the modern atmosphere versus that derived from a pre-Industrial Revolution atmosphere. (J.G. Wynn, et al., 2006)

Why we expect to see it Soil carbon mixing –Surface litter (depleted) vs. root derived (enriched) SOM –Variable biomass inputs (C3 vs. C4 plants) –Some of the carbon incorporated into SOM by these critters has an atmospheric, not SOM source. –Atmospheric C is heavier. Atmospheric CO 2 in the soil is 4.4 ‰ heavier than CO 2 metabolized by decomposition (Wedin, 1995)

Soil carbon mixing - Surface litter (depleted) vs. root derived (enriched) SOM Fig. 1B. Mixing of leaf litter-derived SOC and root-derived SOC. (J.G. Wynn, et al., 2006)

Soil carbon mixing - Variable biomass inputs (C3 vs. C4 plants) Fig. 1C. 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). (J.G. Wynn, et al., 2006)

Why we expect to see it 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 –Organic C is mineralized

Why we expect to see it Kinetic fractionation –Microbes choose lighter C –Microbial respiration of CO2 – 12 C 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

Kinetic fractionation Fig. 1D. 13 C distillation during decomposing SOM. The gray lines show the model with varying fractionation factors from to (J.G. Wynn, et al., 2006)

Kinetic fractionation Rayleigh distillation Ffraction of remaining soil organic matter (SOC) – approximated by the calculated value of fSOC  13C f isotopic composition of SOC when sampled  13C i isotopic composition of input from biomass αfractionation factor between SOC and respired CO2 eefficiency of microbial assimilation tfraction of assimilated carbon retained by a stabilized pool of SOM Assumptions by Wynn etal Open system –All components decompose –Contribute to soil-respired CO2 at same rate with depth FSOC  fSOC

Anthropogenic mixing (agriculture) Wynn fig 9 – various reasons that disturbed land might not conform to nice regression curve in fig 1D 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

Controls on decay Melillo, et al, 1989 Temperature Moisture Soil texture Availability of labile C and N

δ13C% C% NC:N Mean Mole O- horizonPRS-15 Bulk A- horizon (0-6 cm)PRS-16 Bulk AP horizon (6-11 cm)PRS-17 Bulk B horizon (11+ cm)PRS-18 Bulk O- horizonPRS-15 Plant Fragment A- horizon (0-6 cm)PRS-16 Plant Fragment AP horizon (6-11 cm)PRS-17 Plant Fragment B horizon (11+ cm)PRS-18 Plant Fragment O- horizonPRS-15 Heavy Fraction A- horizon (0-6 cm)PRS-16 Heavy Fraction AP horizon (6-11 cm)PRS-17 Heavy Fraction B horizon (11+ cm)PRS-18 Heavy Fraction What we do see - results

 13C – increase 0.4 ‰ with 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 Increase 0.4 ‰ with to 8 cm (PRS 18 = anomaly)

What we do see - C/N Increases to 8 cm, then decreases

What we do see - % C Decrease with depth (PRS 15 = anomaly)

What we do see - % N Decrease with depth (PRS 15 = anomaly)

Soil FTIR (normalized) Wave number (cm -1 ) Absorbance

FTIR results 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?

WavenumbrDescriptionPossible functional groupsComments cm sharp peakO-H stretching region ( for clay mineral) 3622sharp peakO-H stretching region ( for clay mineral)Bands due to Si-O-O-OH vibration. 3464broad, strong intensityO-H, N-HSince it's broad and strong intensity, this is due to O-H bond rather than N-H bond. 2935tiny broadC-H ( )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. 1655medium intensityC=C ( for aromatic and alkenes); C=O vibrations ( for amide), C=N ( ) and also of N-H bend ( ) 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 & 1400weakC-H, alkanes, -CH 3 (bend, 1450 and 1375), -CH2 (bend, at 1465), Most likely CH3 bending sharp & strongest peakSi-O vibration of clay mineralsConsistent with FTIR spectra of soil in the literarture 800medium intensity, saw toothNH2 wagging and twisting, =C-H bend, alkenesIntense absorption at corresponds to SiO 3 -2 vibration. In the literarture, bands at 800,780,650,590,530 and 470 are attributed to inorganic materials, such as clay and quartz minerals. 696medium intensity, sharp 540medium intensity, sharpN-C=O bend for secondary amides 472strong intensity, sharpC-C=O bend for secondary amides, SiO 3 -2

Sampling 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)