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3 Soil Organic Carbon Assessment Methods
Charles W. Rice, Carlos B. Pires, James Lin, and Marcos V. M. Sarto
Summary
Globally, the amount of soil organic carbon (SOC) is more than twice that in the atmosphere or living vegetation. Soil organic carbon is an extremely important soil health indicator because it influences almost all soil biological, chemical, and physical properties and processes. Loss of SOC accelerates soil health problems such as soil erosion and decreases soil aggregation. This chapter explores those issues and discusses various SOC measurement methods.
Introduction
Soils contain several important C pools and play an essential role in the global C cycle. Total soil C consists of organic C and inorganic C, with organic C being part of soil organic matter (SOM). Estimated amounts of organic C stored in world soils range from 1100 to 1600 Pg, more than twice that in living vegetation (560 Pg) or the atmosphere (750 Pg) (Sundquist, 1993; Lehmann and Kleber, 2015). Most of the world’s productive soils are in cultivated agriculture, which through plow‐based management, has increased SOM loss, accelerated soil erosion, and decreased soil aggregation (Lal, 2015; Karlen and Rice 2015). Intensive tillage, low crop yield, lack of crop diversification, and excessive removal of crop residues have reduced C replenishment to the soil and thus negatively affected soil health (Rice, 2005; Bonini Pires et al., 2020). Those practices have decreased SOC by as much as 26% in the upper 30 cm and 16% in the top 100 cm of many soil profiles (Sanderman et al., 2017).
The Soil Science Society of America (SSSA) defines SOM as the organic fraction of soil exclusive of undecayed plant and animal residues (SSSA, 1997). Measurements of SOM include decayed plant residues, soil microorganisms, soil fauna, and byproducts of decomposition that lead to the production of humic substances in a process called humification (Horwath, 2007). SOM can be classified in two distinct pools, active and passive, based on their chemical composition, stage of decomposition, and turnover time (Cambardella and Elliot, 1994; Gougoulias et al., 2014). Active pools exhibit a turnover in months to years, while turnover in passive pools occurs in decades to millennium (Magdoff, 1996). Soil organic matter can also be divided into three major categories: particulate organic matter (POM), humus, and resistant organic matter (ROM) (Bell and Lawrence, 2009). Each SOM classification has a method of measurement and converts the data to SOM using conversion factors that vary. Therefore for consistency, we recommend reporting the values as organic C.
Origin and Factors Affecting SOC
Soil organic carbon is derived primarily from plant residues with transformations and storage of SOC being a function of biotic, chemical, and physical properties and processes interacting with plant residue quality or biochemistry as well as its accessibility to organisms. Plant residues are decomposed by soil microorganisms and most of the plant C is released to the atmosphere as CO2. Approximately 10 to 20% of the C in plant residue becomes SOM, sometimes referred to as “humus.” A portion of this C can persist in soils for hundreds to thousands of years. The theoretical potential for soil C storage is a function of climate and basic soil characteristics, while the amount of C residing in the soil is a function of plant and soil management.
Tillage and organic residue input are two primary drivers influencing organic carbon levels in soils. Soil disturbance (i.e., tillage) disrupts soil aggregates and decreases physical protection by exposing C within soil aggregates to microbial decomposition, which results in a conversion of organic carbon
