Erosion and Non-Steady State Storage of Soil Organic Carbon on Undisturbed Hillslopes in Coastal California

Erosion and Non-steady State Storage of Soil Organic Carbon on Undisturbed Hillslopes in Coastal California Kyungsoo Yooa1, Ronald Amundsona, Arjun M. Heimsathb, William E. Dietrichc aDivision of Ecosystem Sciences, University of California at Berkeley, CA 94720, USA bDepartment of Earth Sciences, Dartmouth College, Hanover, NH 08755, USA cDepartment of Earth and Planetary Sciences, University of California at Berkeley, CA 94720, USA [email protected] Introduction Sites and Measurements Result 2: C Accumulation in Depositional Slope Most upland ecosystems under natural vegetation are not level; land with < 8% slope On the following hillslopes at two sites, we made detailed measurements of the spatial variation in soil organic carbon content and above ground net primary productivity Fig.10 Soil Accumulation in the Tennessee Valley Hollow comprises only 36% of global soil [Staub and Rosenzweig, 1992]. These hilly upland (ANPP). The soil %C was combined with the erosion mechanism on the site to determine the %C of eroding soils. We also made detailed topographic survey with total Eroded soils accumulated in depositional slopes that were estimated to have residence times of (Eq.3) landscapes represent unexplored processes relevant to the global C cycle. In recent laser station. The resulting data was used to calculate the local slope curvature over the entire hillslopes. 13 kyrs at TV and 5.3 kyrs at BD. In the TV hollow (depositional slope within a zero order 200 years, geomorphic research on the processes of soil transport on soil mantled Current Thickness Table 1 watershed), 15-24 kg C m-2 of soil C has accumulated at a long-term rate of hillslopes has provided an empirically constrained mathematical framework to explore Tennessee Valley Black Diamond Fig.3 -2 -1 Tennessee Valley Fig.4 Mediterranean Mediterranean 1.6-1.9 g C m yr . The present rates of C accumulation were calculated to be an unknown aspect of the upland soil C cycle: the rate of soil C erosion and burial Black Diamond Climate -1 -1 MAP: 1200 mm yr MAP: 350 mm yr -2 -1 -2 -1 ) 150 within watersheds. We conducted intense soil C studies on two sites where extensive Summit 0.3 g C m yr at TV and 0.6 g C m yr at BD. During the hollow infilling, the depositional C m Study Area Shoulder Bedrock Sandstone Clay-rich Shale c ( geomorphic research on soil production and transport has been made. In particular, we Backslope inputs outweigh the C accumulation rates, meaning that much of the incoming eroded C was s Erosion Soil creep with s Bioturbation and Rainsplash mechanism Soil swelling and shrinking e attempt to achieve two objectives: (1) quantify the C fluxes associated with soil erosion n Footslope oxidized to CO2. Given the higher C:N ratio of soil organic matter in hollows and the model 100 k K 2 -1* 2 -1 + 50cm yr 360cm yr c and deposition, and (2) integrate these two processes to determine the C accumulation (Eq.2) simulation that suggests the present soil C profile can be formed even in the absence of i Studied Hillslope h T in typical hillslopes in central California. l Convex Slope Transect Case I erosional C inputs, most soil C in depositional areas appears to be from in-situ NPP rather than i (Gopher Induced erosion) o C concentration 2-5% varying with elevation S 50 Convergent Hollow Decreasing from 2% at summit to of eroding soils eroded soil C. 0.7% at footslope (f in Eq.2) Case II Model (Rainsplash induced erosion) In contrast to the traditional soil organic carbon mass balance models that consider 3.5-6% varying with elevation %Carbon Solving soil C profile using Eq,4 and 5 with empirically parameterized plant C input and decomposition rate 0 soil carbon storage (C) as a balance of plant C inputs (I) and C losses through 0 1 2 3 4 5 * Heimsath et al., 1997, +McKean et al., 1993 microbial decomposition (R ), we included erosional loss (positive) or depositional 0 I= (179±5.8)e-z/14.5 g C m-2 yr-1 0 2 4 6 8 10 12 14 16 h Result 1: Soil C Erosion Flux 1ky k=(0.09±0.01 )e-z/19.5 yr-1 gain (negative) of soil C (ε) [Stallard, 1998]: Time since evacuation (Kyr) 1. C erosion flux at Tennessee Valley dC ) 50 = I − R − ε (1) m 30 dt h c Soil Erosion by Gopher Soil Perturbation ( Fig.5 Soil Erosion by Rainsplash or Root Decay 5ky h Fig.6 We measured the %C of eroding soils and slope t On convex land surfaces, soil transport is driven by a combination of biological and physical N p 25 N e 100 ) curvatures over the entire hillslopes. These D -2 -1 processes aided by gravity, which results in curvature-dependent soil C erosion losses: -2 -1 2 gCm yr gCm yr l i m o measurements allowed us to calculate the soil C / 20 S 2 Depth Integration C ε = ρ − ∇ g εf ( K Z) 6 16 150 k (2) erosion flux using Eq.2. 13kyr ( 5 14 e 15 12 g ρε 4 a where is the bulk density of eroded soils, f is the mass fraction of carbon in eroded r 10 o 3 t 2 The average C erosion loss from convex slopes was soils, K is the diffusivity, and -∇ Z is the negative curvature of the slope. 200 S 10 8 2 C 6 -2 -1 between 1.4 and 2.7 g C m yr at TV and O 1 S n 4 5 o i -2 -1 t t 0 2 approximately 8 g C m yr of at BD. The C u t Z a r p n i -1 a n 0 p l i elevation erosional flux was locally as high as 5 (case I)-14% Fig.11 Soil C profile during hollow infilling s 0 P C e -2 -2 R (case II) of ANPP at TV and 8% at BD. -3 -4 0 2 4 6 8 10 12 14 16 -6 C -4 Soil ero Measuredvarbon profile Time since evacuation (Kyr) sion So Model hollow C profile il Flood Plain Fig.12 Model estimated SOC storage Ero Flood Plain sion Soi during hollow infilling l prod be uctio drock n from Contour: 1m interval Contour: 1m interval 0 20 40 60 80 Result 3:Long-term Trends in Soil Carbon Storage at Tennessee Valley 0 20 40 60 80 meter meter Distance (x, y) 2. C erosion flux at Black Diamond Tennessee Valley Black Diamond The empirically obtained relationship between soil C storage and Fig.1 Cross section view of a convex hillslope soil C mass balance 3 14 SOC Storage 14 slope curvature was used to create the continuous soil C storage ∇ Slope transect In Watershed 13kyr BP On concave slopes (- 2z is negative), soils continue to thicken due to sediment input until they ) Convex slope 2 1 Shoulder 12 - ) Hollow Convergent slope 1 12 ? map, which allowed the estimations of soil C storage within convex r - ) are eventually evacuated through landslides, and the cycle then repeats. Here, we have r 1 y - y r 2 2 summit - y - 10 110 t C 2 slope and convergent slope. Here we took an extreme case that simplified hollow geometry (Fig. 2) and analytically modeled how hollow soil thickness varies 10 - m 1 m m C C Shoulder g C with time, following the approach of Dietrich et al., [1986]. In this approach, the soil thickness ( g entire convergent hollow was excavated at 13 kyrs BP. Erosion 8 g 8 ( ( x Present u x x β l f along a hollow axis is described as a function of diffusivity (K), side slope angle ( ), 0 u u l l n Deposition 6 F 6 o F i γ convergence angle ( ), and time (t): n s n o The watershed has been accumulating C, and its accumulation rate i o r o -1 4 s i 4 e o − Backslope 1 s 1/ 2 1/2 (3) r = γ β − C has been nonlinearly decreasing. E H [2Ktcos tan ( 1) ] o O 2 2 r 2 β γ C S Convex Slope sin cos E Footslope 2 32 t C -2 O Erosion S C 0 During the soil thickening, the C content at depth z is modeled as the balance of plant C inputs O 0 S Hollow Evacuation Hollow Evacuation and SOC decomposition: -3 Deposition 0 50 100 150 200 250 -2 0 10 20 30 40 50 60 70 -2 -2 -1 0 50 100 150 200 250 300 350 Time d( zCz ) = Iz − (4) ANPP 2001-2002 (gCm yr ) ¡ k C dt z z z z Horizontal Distance from Summit (m) ANPP 2003 (gCm-2yr-1) Fig.7 Comparison of soil C erosion flux to measured Fig.8 Calculated soil C erosion flux Fig.13 The total SOC storage within the TV watershed between hollow evacuations The depth integration of this C profile yields soil C storage. Above ground Net Primary Productivity (ANPP) along hillslope transect Fig.9 Comparison of soil C erosion flux to ANPP Hollow Slope Angle θθ Side Slope Angle β β Table 2 A summary of the C fluxes for different components of the two watersheds. Acknowledgements It would have not been possible to excavate all the soil pits without Aaron Miller. The Eroding Slope Depositional Slope Entire Hillslope generous help from Matt Cover was essential in numerous field and laboratory All rates are averaged over the convex slope All rates are averaged over the convergent slope All rates are averaged over the entire slope activities.

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