The Vertical Distribution of Soil Organic Carbon and Its Relation to Climate and Vegetation

Abstract

Abstract. 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 are (1) to examine the association of SOC content with climate and soil texture at different soil depths; (2) to test the hypothesis that vegetation type, through patterns of allocation, is a dominant control on the vertical distribution of SOC; and (3) to estimate global SOC storage to 3 m, including an analysis of the potential effects of vegetation change on soil carbon storage. We based our analysis on Ͼ2700 soil profiles in three global databases supplemented with data for climate, vegetation, and land use. The analysis focused on mineral soil layers.Plant functional types significantly affected the vertical distribution of SOC. The percentage of 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 in the 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 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 with depth, climate dominating in shallow layers and clay content dominating in deeper layers, 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 ranges and compared distributions for vegetation types within each range. The percentage 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 Ͻ 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 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 1502 Pg estimated for the first meter (which is similar to the total SOC estimates of 1500-1600 Pg made by 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 to control SOC distri...

Storage and Turnover of Organic Matter in Soil

“…Historical global estimates for the top meter of soil ranged from 800 to 2,400 Pg C, converging on the range of 1,300-1,600 Pg C to 1 m. Batjes (1996) estimated that an additional 900 Pg C is stored between 1 and 2 m depth, and Jobbágy and Jackson (2000) revised that estimate to 500 Pg between 1 and 2 m and another 350 Pg between 2 and 3 m depth. Global organic C stocks to 3 m are currently estimated at 2,300 Pg, with an additional 1,000 Pg contained in permafrost and peatlands (Jobbagy and Jackson 2000;Zimov et al 2006). Soil C distribution with ecosystem type is shown in Figure 1.…”

Emerging land use practices rapidly increase soil organic matter

“…1). Agricultural cultivation has likely decreased soil C stocks by one-half to two-thirds, with a cumulative loss near 30-40 Mg C ha À 1 (ref.…”

Drying/rewetting cycles mobilize old C from deep soils from a California annual grassland

“…Values in parentheses are standard deviations. Analytical precision is AE3& for D 14 C and AE0.2& for d 13 C (Xu et al, 2007 There is a large pool of old, deep soil C (Jobbágy and Jackson, 2000). The mechanisms for its stability, however, have not been clear.…”

“…Deep soils (below 20 cm) are the one of the major reservoirs of organic C on earth, containing more C than is in either vegetation or the atmosphere (Jobbágy and Jackson, 2000). The nature of that C pool, however, remains something of a mystery, with questions remaining about its chemical nature, turnover time, and the processes that regulate it (Baisden and Parfitt, 2007).…”