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

EG JOBBÁGY; RB JACKSON

ECOLOGICAL APPLICATIONS

Ecological Society of America

Lugar: Washington DC; Año: 2000 vol. 10 p. 423 - 436

1051-0761

As the largest pool of terrestrial organic carbon, soils interact strongly withatmospheric composition, climate, and land cover change. Our capacity to predict andameliorate the consequences of global change depends in part on a better understandingof the distributions and controls of soil organic carbon (SOC) and how vegetation changemay affect SOC distributions with depth. The goals of this paper are (1) to examine theassociation of SOC content with climate and soil texture at different soil depths; (2) to testthe hypothesis that vegetation type, through patterns of allocation, is a dominant controlon the vertical distribution of SOC; and (3) to estimate global SOC storage to 3 m, includingan analysis of the potential effects of vegetation change on soil carbon storage. We basedour analysis on .2700 soil profiles in three global databases supplemented with data forclimate, vegetation, and land use. The analysis focused on mineral soil layers.Plant functional types significantly affected the vertical distribution of SOC. The percentageof SOC in the top 20 cm (relative to the first meter) averaged 33%, 42%, and 50%for shrublands, grasslands, and forests, respectively. In shrublands, the amount of SOC inthe second and third meters was 77% of that in the first meter; in forests and grasslands,the totals were 56% and 43%, respectively. Globally, the relative distribution of SOC withdepth had a slightly stronger association with vegetation than with climate, but the oppositewas true for the absolute amount of SOC. Total SOC content increased with precipitationand clay content and decreased with temperature. The importance of these controls switchedwith depth, climate dominating in shallow layers and clay content dominating in deeperlayers, possibly due to increasing percentages of slowly cycling SOC fractions at depth.To control for the effects of climate on vegetation, we grouped soils within climatic rangesand compared distributions for vegetation types within each range. The percentage of SOCin 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, intermediatein grasslands, and shallowest in forests (P , 0.05 in all cases). The effect ofvegetation type was more important than the direct effect of precipitation in this analysis.These data suggest that shoot/root allocations combined with vertical root distributions,affect the distribution of SOC with depth.Global SOC storage in the top 3 m of soil was 2344 Pg C, or 56% more than the 1502Pg estimated for the first meter (which is similar to the total SOC estimates of 1500–1600Pg made by other researchers). Global totals for the second and third meters were 491 and351 Pg C, and the biomes with the most SOC at 1–3 m depth were tropical evergreenforests (158 Pg C) and tropical grasslands/savannas (146 Pg C).Our work suggests that plant functional types, through differences in allocation, helpto control SOC distributions with depth in the soil. Our analysis also highlights the potentialimportance of vegetation change and SOC pools for carbon sequestration strategies.