Forest soils and carbon sequestration

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  • Forest soils and carbon sequestration

    R. Lal *

    Carbon Management and Sequestration Center, OARDC/FAES, The Ohio State University, Columbus, OH 43210, USA

    Abstract

    Soils in equilibrium with a natural forest ecosystem have high carbon (C) density. The ratio of soil:vegetation C density

    increases with latitude. Land use change, particularly conversion to agricultural ecosystems, depletes the soil C stock. Thus,

    degraded agricultural soils have lower soil organic carbon (SOC) stock than their potential capacity. Consequently, afforestation

    of agricultural soils and management of forest plantations can enhance SOC stock through C sequestration. The rate of SOC

    sequestration, and the magnitude and quality of soil C stock depend on the complex interaction between climate, soils, tree

    species and management, and chemical composition of the litter as determined by the dominant tree species. Increasing

    production of forest biomass per se may not necessarily increase the SOC stocks. Fire, natural or managed, is an important

    perturbation that can affect soil C stock for a long period after the event. The soil C stock can be greatly enhanced by a careful site

    preparation, adequate soil drainage, growing species with a high NPP, applying N and micronutrients (Fe) as fertilizers or

    biosolids, and conserving soil and water resources. Climate change may also stimulate forest growth by enhancing availability of

    mineral N and through the CO2 fertilization effect, which may partly compensate release of soil C in response to warming. There

    are significant advances in measurement of soil C stock and fluxes, and scaling of C stock from pedon/plot scale to regional and

    national scales. Soil C sequestration in boreal and temperate forests may be an important strategy to ameliorate changes in

    atmospheric chemistry.

    # 2005 Elsevier B.V. All rights reserved.

    Keywords: Soil organic carbon; Sequestration; Carbon cycle; Climate change; Forest ecosystems

    www.elsevier.com/locate/foreco

    Forest Ecology and Management 220 (2005) 2422581. Introduction

    Carbon (C) storage in forest ecosystems involves

    numerous components including biomass C and soil C

    (Fig. 1). The total ecosystem C stock is large and in

    dynamic equilibrium with its environment. Because of

    the large areas involved at regional/global scale, forest

    soils play an important role in the global C cycle* Tel.: +1 614 292 9069; fax: +1 614 292 7432.

    E-mail address: lal.1@osu.edu.

    0378-1127/$ see front matter # 2005 Elsevier B.V. All rights reserveddoi:10.1016/j.foreco.2005.08.015(Detwiler and Hall, 1988; Bouwman and Leemans,

    1995; Richter et al., 1995; Sedjo, 1992; Jabaggy and

    Jackson, 2000). Land use change causes perturbation

    of the ecosystem and can influence the C stocks and

    fluxes. In particular, conversion of forest to agricul-

    tural ecosystems affects several soil properties but

    especially soil organic carbon (SOC) concentration

    and stock. The conversion to an agricultural land use

    invariably results in the depletion of SOC stock by

    2050% (Schlesinger, 1985; Post and Mann, 1990;

    Davidson and Ackerman, 1993). The depletion of.

  • R. Lal / Forest Ecology and Management 220 (2005) 242258 243

    Fig. 1. Components of the terrestrial carbon stock.

    Table 1

    Carbon stock in selected biomes of the world (recalculated from

    Adams et al., 1990; Eswaran et al., 2000; Dixon et al., 1994; Malhi

    et al., 1999)

    Biome Area

    (Mha)

    C density (Mg/ha) C stock (Pg)

    Vegetation Soil Vegetation Soil

    Tundra 927 9 105 8 97

    Boreal/Taiga 1372 64 343 88 471

    Temperate 1038 57 96 59 100

    Tropical 1755 121 123 212 216

    Wetlands 280 20 723 6 202

    Total 5672 Mean 54 Mean 189 373 1086SOC stock is attributed to numerous factors including:

    decrease in the amount of biomass (above- and below-

    ground) returned to the soil, change in soil moisture

    and temperature regimes which accentuate the rate of

    decomposition of organic matter, high decomposa-

    bility of crop residues due to differences in C:N ratio

    and lignin content, tillage-induced perturbations,

    decrease in soil aggregation and reduction in physical

    protection of the soil organic matter, and increase in

    soil erosion. Thus, agricultural soils and especially

    eroded agricultural soils usually contain lower SOC

    stock than their potential capacity. Afforestation of

    agricultural land can reverse some of the degradation

    processes and cause enhancement or sequestration of

    SOC stock (Ross et al., 2002; Silver et al., 2000).

    Interest in the ability of forest soils to sequester

    atmospheric CO2 derived from fossil fuel combustion

    has increased because of the threat of projected

    climate change. Thus, understanding the mechanisms

    and factors of SOC dynamics in forest soils is

    important to identifying and enhancing natural sinks

    for C sequestration to mitigate the climate change.

    This manuscript synthesizes available information

    on forest soils as a sink for atmospheric CO2, and

    identifies the management options that may enhance

    the capacity for C capture/storage in forest soils. Themanuscript focuses on SOC stock in forest soils and

    factors affecting its dynamics, assesses the role of

    forest management on SOC sequestration, outlines

    soil sampling and analytical procedures and modeling

    options, describes the likely effects of projected

    climate change on SOC stock in forest soils, and

    outlines challenges of achieving the potential. Rather

    than being a comprehensive literature review, the

    focus is to highlight the principle issues and

    opportunities and build upon previous reviews on

    the topic (e.g. Bouwman, 1990; McFee and Kelly,

    1995; Lal, 2001; Kimble et al., 2003).

  • R. Lal / Forest Ecology and Management 220 (2005) 242258244

    Table 2

    World forest area and area change (adapted from FAO, 2003)

    Region Land area (Mha) Forest cover (Mha) Mangrove forest (Mha)

    Area in 2000 Plantations Change Area in 2000 Annual change (%/year)

    Africa 2978.4 649.9 8.0 0.526 3.35 0.3Asia 3084.7 547.8 115.9 0.036 5.83 1.2Europe 2260.0 1039.3 32.0 +0.88

    North and Central America 2137.0 549.3 17.5 0.57 1.97 1.4Oceania 849.1 197.6 2.8 0.37 1.53 1.0South America 1754.7 885.6 10.5 3.71 1.97 1.0

    World 13063.9 3869.5 186.7 9.39 14.65 1.02. World forests

    Forests ecosystems covering about 4.1 billion

    hectares globally (Dixon and Wisniewski, 1995) are

    a major reserve of terrestrial C stock. There are three

    principle forest biomes: boreal, temperate and tropical

    (Table 1). The boreal or taiga forest occupies a

    circumpolar belt. Temperate forests cover mid-

    latitudes between 25 and 508 north and south of theEquator, and comprise both evergreen and deciduous

    species. Tropical forests occur about 258 north andsouth of the Equator, and comprise both evergreen and

    deciduous species. Predominant types of tropical

    forests include lowland rainforest, montane forests

    and mangrove forests.

    The geographical distribution of the worlds forests

    indicates large areas in South America, Asia, Europe,

    and North and Central America and Africa (Table 2).

    Worldwide, forest cover is decreasing at the net rate of

    about 9.4 Mha/year mostly due to deforestation of the

    tropical rainforest (TRF) in Brazil, Sumatra and West

    and Central Africa. The deforestation and conversion

    of TRF into agricultural ecosystems results inTable 3

    Estimates of terrestrial carbon stock in worlds forest zones (Pre-

    ntice, 2001)

    Biome Area

    (Mha)

    Terrestrial carbon

    stock (Pg)

    Carbon

    density

    (Mg C/ha)

    Plants Soil Total Plants Soil

    Tropical forests 1.76 340 213 553 157 122

    Temperate forests 1.04 139 153 292 96 122

    Boreal forests 1.37 57 338 395 53 296

    Total 4.17 536 704 1240 emission of 1.61.7 Pg C/year into the atmosphere

    (IPCC, 2000).3. Carbon stock in forest ecosystems

    Forest vegetation and soils contain about 1240 Pg of

    C (Dixon et al., 1994), and the C stock varies widely

    among latitudes. Of the total terrestrial C stock in forest

    biomes, 37% is in low latitude forests, 14% in mid-

    latitudes and 49% in high latitudes. The above-ground

    plant C density increases with decreasing latitude from

    tundra to tropical rainforest (Fisher, 1995).Typical plant

    Cdensityrangesfrom40to60 Mg C/hainborealforests,

    60 to 130 Mg C/ha in temperate forests and 120 to

    194 Mg C/ha in tropical forests (Table 3), with the C

    density of an undisturbed TRF as high as 250 Mg C/ha.

    Nonetheless, asmuchas two-thirdsof the terrestrialC in

    forest ecosystems is contained in soils (Dixon et al.,

    1994).ThesoilCstockmaycompriseasmuchas85%of

    the terrestrial C stock in the boreal forest, 60% in

    temperate forests and 50% in TRF (Dixon et al., 1994).

    The ratioof soil:plantC stockmay range from3 to17 for

    high latitude, 1.2 to 3 for mid-latitude and 0.9 to 1.2 for

    lowlatitude(Table4).Alargepartofthetotalsoilorganic

    carbon stock occurs in soils of tundra, pre-tundra and

    taiga regions. Ping et al. (1997) reported that density of

    SOC in selected pedons of Alaska ranged from 162 to

    1292 Mg C/ha. The SOC stock was 692 Mg C/ha in an

    arctic coastal marsh pedon, 314599 Mg C/ha in an

    arctic tundra pedon, and nearly 1300 Mg C/ha in an

    organic soil. Boreal forests

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