δ 13 C Variation from Plants to Soil Jonathan Harris MEA 760 NCSU

δ 13 C within Plants and Soil C3 Plants depleted ~ 21 – 30 per mil C3 Plants depleted ~ 21 – 30 per mil C4 Plants depleted ~ 10 – 15 per mil C4 Plants depleted ~ 10 – 15 per mil

Plants at Prarie Ridge, NC Current Plot currently dominated by bermuda grass, a C4 grass. Current Plot currently dominated by bermuda grass, a C4 grass. Historically maintained as a cow pasture dominated by fescue, with some orchard grass, both C3 grasses. Historically maintained as a cow pasture dominated by fescue, with some orchard grass, both C3 grasses. Area underwent manipulation starting 2 years ago. Area underwent manipulation starting 2 years ago.

The Plant to Soil Transition How δ 13 C changes from plant to soil How δ 13 C changes from plant to soil Disequilibrium factor Disequilibrium factor Difference in atmospheric CO 2 between time of soil carbon production and current plant utilized CO 2 Difference in atmospheric CO 2 between time of soil carbon production and current plant utilized CO 2 Modern CO 2 averages -8 per mil while years ago CO 2 averaged -6.5 per mil. Modern CO 2 averages -8 per mil while years ago CO 2 averaged -6.5 per mil. Decomposition is likely not a factor Decomposition is likely not a factor

How δ 13 C changes with depth Many studies have indicated that δ 13 C becomes enriched with depth. (Nisselbaum et. al 1974, Deines 1980, Ehleringer et. al 2000) Many studies have indicated that δ 13 C becomes enriched with depth. (Nisselbaum et. al 1974, Deines 1980, Ehleringer et. al 2000) Also that soil organic matter decreases in size and amount with depth. Also that soil organic matter decreases in size and amount with depth.

What causes this? 4 hypotheses from Ehleringer et. al hypotheses from Ehleringer et. al 2000 Atmospheric CO 2 change over time Atmospheric CO 2 change over time Microbial fractionation during decomposition Microbial fractionation during decomposition Preferential microbial decomposition of litter Preferential microbial decomposition of litter Mixing of C in the soil Mixing of C in the soil

Plant δ 13 C Relative to Surface The mean δ 13 C of all the plant samples collected was per mil The mean δ 13 C of all the plant samples collected was per mil The mean δ 13 C of all the surface soil samples (PRP-5,7) was The mean δ 13 C of all the surface soil samples (PRP-5,7) was Clearly an enrichment has occurred from just the transition of living plant material to soil organic matter. Clearly an enrichment has occurred from just the transition of living plant material to soil organic matter.

δ 13 C change in the Soil Profile Ignoring the isotopic values or large size plant fragments removed from soil the δ 13 C changed as follows Ignoring the isotopic values or large size plant fragments removed from soil the δ 13 C changed as follows O horizon per mil O horizon per mil A horizon per mil A horizon per mil A p horizon per mil A p horizon per mil B horizon per mil B horizon per mil

Do our results match the model? All of the data collected matched the idea of soil organic matter becomes enriched in δ 13 C with an increase in depth. All of the data collected matched the idea of soil organic matter becomes enriched in δ 13 C with an increase in depth. The one exception was the B horizon which in both samples showed a more depleted δ 13 C than any other soil. The one exception was the B horizon which in both samples showed a more depleted δ 13 C than any other soil.

Conclusions δ 13 C decreases with soil depth at Prairie Ridge, NC δ 13 C decreases with soil depth at Prairie Ridge, NC Likely a combination of processes Likely a combination of processes Soil mixing Soil mixing Disequilibrium effect Disequilibrium effect A great deal of information can be learned from this site if coupled with a definite time of transition from C3 to C4 dominated. A great deal of information can be learned from this site if coupled with a definite time of transition from C3 to C4 dominated.

References Balesdent et. al, Site-related δ 13 C of Tree Leaves and Soil Organic Matter in a Temperate Forest. Ecology 74: Balesdent et. al, Site-related δ 13 C of Tree Leaves and Soil Organic Matter in a Temperate Forest. Ecology 74: Ehleringer et. al, Carbon Isotope Ratios in Belowground Carbon Cycle Processes. Ecological Application 10: 412–422. Ehleringer et. al, Carbon Isotope Ratios in Belowground Carbon Cycle Processes. Ecological Application 10: 412–422. Keyworth, Amy J. An Investigation into the Fate of Carbon, Nitrogen, and their Isotopes in a Former Cow Pasture in the Piedmont Region of North Carolina Keyworth, Amy J. An Investigation into the Fate of Carbon, Nitrogen, and their Isotopes in a Former Cow Pasture in the Piedmont Region of North Carolina Yamakura,T. and Sahunalu, P Soil Carbon/nitrogen Ratio as a Site Quality Index for Some South-east Aasian Forests. Journal of Tropical Ecology. 6: Yamakura,T. and Sahunalu, P Soil Carbon/nitrogen Ratio as a Site Quality Index for Some South-east Aasian Forests. Journal of Tropical Ecology. 6: