B095-0002
Allocation and Dynamics of Recently Fixed Plant Carbon in a California Rangeland Soil

Tuesday, 15 December 2020
Poster
Christina Fossum1, Ilexis X Jacoby1, Mengting Yuan1, Katerina Estera-Molina2, Jennifer Pett-Ridge3 and Mary K Firestone4, (1)University of California Berkeley, Berkeley, CA, United States, (2)University of California Berkeley, Berkeley, United States, (3)Lawrence Livermore National Laboratory, Livermore, CA, United States, (4)University of California Berkeley, Environmental Science, Policy, and Management, Berkeley, CA, United States
Abstract:
This study examines the impacts of reduced precipitation, time and season on soil organic carbon allocation and persistence in a California annual grassland under a Mediterranean-type climate. Over several growing seasons in annual grasslands, most of the carbon introduced to the soil by plant roots and litter decomposes; a small amount, however, persists as soil organic matter in large part due to interactions with soil minerals and protection within soil aggregates.

Using a field experiment with controlled precipitation, plant-derived carbon was followed through soil using the stable isotope 13C. Samples were collected at 3 weeks, 5 months (after the summer dry period), 12 months, and 2 years following the 13CO2 labeling in order to explore the dynamics of newly assimilated carbon over multiple annual seasonal cycles. At each harvest, samples were collected from both 13CO2 and ambient CO2-treated plots under two precipitation treatments—50% and 100% of average annual precipitation.

Soil samples underwent density fractionation using a sodium polytungstate (ρ = 1.75 g- cm-3) density gradient to isolate three pools of SOM: the Free-Light Fraction (FLF) which includes relatively undecomposed litter, the Occluded-Light Fraction (OLF) which is thought to be physically protected by occlusion in aggregate structures, and the Heavy Fraction (HF) which includes carbon that is putatively protected from decomposition through interaction with surfaces of “heavy” soil minerals. Each of the three fractions was analyzed for total C and 13C by isotope ratio mass spectrometry to determine how allocation of carbon to these three pools changes over time under different precipitation regimes.

Our precipitation manipulation achieved significantly lower soil water potential in 50% treatment plots relative to 100% treatment plots over the course of both the 2017-2018 and 2018-2019 growing seasons. 13C allocation differed significantly between the three density fractions at all of the sampling timepoints, with significantly lower 13C enrichment measured in the physically protected Occluded-Light Fraction than in either the Free-Light or Heavy fractions. C, N, and C:N ratio also differed significantly between the three density fractions and likely reflects extent of microbial processing.

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