The vertical distribution of soil organic carbon and its relation to climate and vegetation

Authors: Jobbágy EG, Jackson RB

.As the largest pool of terrestrial organic carbon, soils interact strongly with atmospheric composition, climate, and land cover change. Our capacity to predict and ameliorate the consequences of global change depends in part on a better understanding of the distributions and controls of soil organic carbon (SOC) and how vegetation change may affect SOC distributions with depth. The goals of this paper were to 1) examine the association of SOC content with climate and soil texture at different soil depths, 2) test the hypothesis that vegetation type, through patterns of allocation, is a dominant control on the vertical distribution of SOC, and 3) estimate global SOC storage to three meters, including an analysis of the potential effects of vegetation change on soil carbon storage. We based our analysis on > 2000 soil profiles in three global databases supplemented with data for climate, vegetation, and land use.

Plant functional types significantly affected the vertical distribution of SOC. The proportion of SOC in the top 20 cm (relative to the first meter) was 33%, 42%, and 50% on average for shrublands, grasslands, and forests, respectively. In shrublands, the amount of SOC in the second and third meters was 77% of that in the first meter, while in forests and grasslands the totals were 56% and 43%. Globally the relative distribution of SOC with depth had a slightly stronger association with vegetation than with climate, but the opposite was true for the absolute amount of SOC. Total SOC content increased with precipitation and clay content and decreased with temperature. The importance of these controls switched from climate in shallow layers to clay content in deeper layers, possibly due to increasing proportions of slower cycling SOC fractions at depth. To control for the effects of climate on vegetation, we grouped soils within climatic ranges and compared distributions for vegetation types within each range. The proportion of SOC in the top 20 cm relative to the first meter varied from 29% in cold arid shrublands to 57% in cold humid forests, and, for a given climate, was always deepest in shrublands, intermediate in grasslands, and shallowest in forests (P<0.05 in all cases). The effect of vegetation type was more important than the direct effect of precipitation in this analysis. These data suggest that shoot/root allocation combined with vertical root distributions alter the distribution of SOC with depth.

Global SOC storage in the top three meters of soil was 2344 Pg C, or 56% more than the 1502 Pg estimated for the first meter (and similar to 1500-1600 Pg estimates of other researchers). Global totals for the second and third meters were 491 and 351 Pg C, and the biomes with the most SOC at 1-3 m depth were tropical evergreen forests (158 Pg C) and tropical grasslands/savannas (146 Pg C).

Our work suggests that plant functional types, through differences in allocation, help control SOC distributions with depth in the soil; our analysis also highlights the potential importance of vegetation change and SOC pools for carbon sequestration strategies.


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