Matter

William Horwath

Introduction

Long-Term Carbon Cycle The Short-Term C Cycle Ecosystem C Cycling

Composition and Turnover of C Inputs to Soil Soil Organic Matter

Quantity and Distribution of Organic Matter in Soils Role of Methane in the C Cycle Future Considerations References and Suggested Reading introduction

Carbon (C) was deposited on earth from carbonaceous comets and asteroids (Anders, 1989) in both organic and inorganic forms. The extraterrestrial C contained complex compounds including hydrocarbons, organic acids, and amino compounds essential to the evolution of cellular life forms. The "carbon cycle" is the transfer of C among the atmosphere, oceans, land, and life. The C cycle is composed of both long-term and short-term cycles. The subject of this chapter is to understand the short-term C cycle, which includes the terrestrial components of plants, soil biota, and soil organic matter (SOM).

long-term carbon cycle

The thermal degradation of comets and asteroids entering the earth's atmosphere was presumed to be the initial source of atmospheric carbon dioxide (CO2). The accumulation of CO2 in the atmosphere resulted in the first major active component of the global C cycle: dissolved carbonates (CO3). The weathering of calcium (Ca)-magnesium (Mg)- and silicate (Si)-containing rocks dissolved by carbonic acid (HCO3) found in precipitation and other water sources consumed atmospheric CO2 (Fig. 12.1). The following describes this chemical reaction.

These chemical species, particularly Ca and carbonates, are transported in a dissolved state by rivers to the ocean, where the following reaction occurs:

The overall reaction consumes atmospheric CO2 and transfers it to the near-permanent geologic reservoirs through processes encompassing terrestrial weathering and marine carbonate sedimentation (Berner, 2004). Carbonates formed during pedogenesis, termed caliche, are small in comparison to marine carbonate sedimentation.

The weathering of Ca-Mg Si rocks and sedimentation of marine carbonates is a large sink of atmospheric CO2 (Box 12.1). The uplift of the Himalaya mountains is thought to have increased Ca-Mg Si rock weathering, consuming large amounts of atmospheric CO2. This resulted in the late Cenozoic cooling due to depletion of atmospheric CO2 (Raymo, 1991). The steep oregenic uplift associated with continental formation dynamics enhances primary mineral weathering by increasing

Volcanic

Volcanic

CaCO3 metamorphism and and digenesis

Organic C

FIGURE 12.1 The long-term C cycle (adapted from Berner, 2004).

CaCO3 metamorphism and and digenesis

Organic C

FIGURE 12.1 The long-term C cycle (adapted from Berner, 2004).

erosive events, constantly exposing Ca-Mg Si rocks to further weathering. Following the subduction of sedimentary rock into the earth's mantle, carbonates eventually undergo thermal weathering and are released as CO2 back to the atmosphere primarily through volcanic activity (Fig. 12.1). This process represented a critical step in controlling atmospheric CO2 content of the atmosphere early in the earth's history.

During the Devonian, the evolution of large vascular plants perturbed the long-term carbonate sedimentation cycle by accelerating the weathering of Si rocks and by removal of CO2 from the atmosphere through burial of organic matter in sediments. Accelerated weathering of silicate rocks occurred as a result of the production of organic acids during growth and decomposition of plant debris. The prolific growth of vascular plants led to the accumulation of large amounts of organic matter, eventually leading to the formation of large fossil fuel deposits so vital to today's society. This period also saw an increase in atmospheric oxygen (O2) from photosynthetic activity and absence of oxidation of buried C-rich organic matter. The evolution of microbial-resistant lignin, a component of the secondary cell wall of vascular plants, is surmised to have contributed to the large quantities of plant debris being deposited. It has been hypothesized that the initial burial of large amounts of lignaceous plant material occurred as a result of the absence of lignin-degrading organisms, which evolved later in the Paleozoic

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