An important condition for erosion to constitute a C sink is that the decomposition of organic matter is slower in low lying depositional basins compared to that on the contributing, eroding slopes. We investigated if this criteria holds in an undisturbed, zero-order watershed in Northern California using in situ growing vegetation (Baccharis shrub and annual grasses) and common substrates (recalcitrant tongue depressors and labile filter paper) organic matter in eroding slope positions (summit, and back- and foot-slope positions) and two depositional settings (terrestrial depression or hollow and floodplain). We used a 30–day laboratory incubation and 18-months field, litterbag experiment to determine the effect of substrate quality, and local environmental conditions on organic matter decomposition. We found that there is no significant difference in rate of CO2 mineralization between eroding and depositional sites at comparable depths. During the 18months of our litterbag experiment, the in situ Baccharis decomposes slightly faster on the surface of the depositional sites (k=0.5) compared to the eroding locations (k=0.4). Grasses on the other hand on average decompose slower in the depositional sites (k=0.3) compared to the eroding slopes (k=0.5). Moreover, in our sites, both the labile (filter paper) and recalcitrant (tongue depressor) SOM decompose slower in the surface of the eroding slopes compared to the depositional basins. Therefore, we conclude that rate of SOM decomposition, in situ or ex situ do not follow simple, established principles of declining rates of SOM decomposition with depth in our sites since these sites experience complex vertical mixing and lateral redistribution of soil and associated SOM through the activity of pocket gophers and soil erosion, respectively.