Biosphere & Pedosphere Dynamics: Carbon Cycling and Soil-Climate Feedback

Biosphere. The biosphere describes the plants, animals, fungi, bacteria, algae, and other living organisms that inhabit Earth. Of particular interest in the atmospheric sciences are terrestrial ecosystems. Much of the carbon in biologically active pools is stored in vegetation and soil (Ciais et al. 2013). It is estimated that plant biomass contains 450-650 Pg (1 Pg = 10¹⁵ g) of carbon. Soils hold more than three times as much carbon (1500-2400 Pg), with an additional ~1700 Pg C or more locked in permafrost. For comparison, the atmosphere contains about 800 Pg of carbon. The uptake of carbon during photosynthesis and the release of carbon during respiration by plants and soil microorganisms are important parts of the global carbon cycle. In addition, terrestrial ecosystems regulate other biogeochemical cycles that are important to climate (e.g., CH₄, N₂O), the emission of mineral aerosols into the atmosphere, and the emission of chemically reactive gases that affect air quality (e.g., biogenic volatile organic compounds). Plants, by regulating evapotranspiration, infiltration, and runoff, are a key component of the hydrologic cycle.

The natural vegetation of Earth has a distinct geographic pattern that corresponds to climate zones (Figure 2.4). Forests grow in tropical rainforest, humid subtropical, marine, humid continental, and subarctic climates. In these regions, precipitation is abundant year-round. Trees cannot survive in the bitter cold of tundra climates. Instead, small shrubs, herbaceous plants, and mosses grow in the short summers. Extensive grasslands occur in the semiarid and savanna climates of central North America, northern and central South America, central and southern Africa, central Asia, and Australia. Here climate is hot and dry. Short, dense woody bushes form chaparral (also known as Mediterranean) vegetation in the Mediterranean climate where summers are hot and dry and winters are mild and moist. Deserts, with sparse or widely spaced scrubby plants, establish in arid climates. The close correspondence between climate zones and vegetation zones is readily apparent. Climate zones such as tropical savanna, tropical rainforest, Mediterranean, and tundra are named after vegetation.

Fig. 2.4. Biogeography of natural vegetation prior to human land use. Data from ramankutty and foley (1999). See color plate section

Pedosphere. The lithosphere is the solid outer layer of Earth including the crust and upper mantle. Its outermost layer is called the pedosphere, or soil. Soil is the interface in the cycling of energy and materials between the atmosphere and land. It is the matrix through which energy, water, biomass, and nutrients flow. It is the location of large transformations of energy as radiation absorbed by the surface becomes sensible heat, latent heat, or is stored in the ground. Soil is the source of water and nutrients for plant growth. The recycling of nitrogen and other nutrients from vegetation back to the soil is crucial to both soil development and plant growth. Soils store vast amounts of carbon that is slowly released to the atmosphere during decomposition. The transformation of this material to decomposed humus releases nutrients that support plant growth. Numerous microflora and microfauna facilitate the cycling of carbon and nutrients among soil, living biomass, and air.

Soils develop over time in relation to climate, vegetation, and the underlying geologic parent material. There are twelve broad classes of soil, known as soil orders, that differ in color, texture, structure, and chemical and miner- alogical properties. Many of these twelve soil orders closely relate to climate and vegetation (Figure 2.5). Entisols and aridisols are soils with little organic matter or soil development and are common in arid climates in association with deserts. Gelisols are cold soils underlain with permafrost and develop throughout the Arctic in association with tundra. Histosols are organic soils with thick peat layers that develop in wet conditions associated with marshes, swamps, and bogs.

Fig. 2.5. Geographic distribution of the twelve soil orders. Image provided courtesy of the U.S. Department of Agriculture natural resources Conservation Service, Soil Survey Division, World Soil resources, Washington D.C. See color plate section

They are most prevalent in cold climates. Spodosols develop in boreal and cool temperate needleleaf evergreen forests, where the acidic litter enhances leaching. Mollisols are the thick soils with high organic matter content found in association with prairie vegetation. Ultisols are highly weathered clay soils that develop in warm to tropical climates. Oxisols are the most highly weathered soils, with high clay content, and develop where the climate is hot and wet throughout the year in association with tropical rainforests.