Soil Management and Greenhouse Effect

John M. Kimble, Elissa R. Levine, B.A. Stewart, B.A. Stewart

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351415842
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Landwirtschaft

Soil Management and Greenhouse Effect focuses on proper management of soils and its effects on global change, specifically, the greenhouse effect. It contains up-to-date information on a broad range of important soil management topics, emphasizing the critical role of soil for carbon storage. Sequestration and emission of carbon and other gases are examined in various ecosystems, in both natural and managed environments, to provide a comprehensive overview. This useful reference includes chapters that address policy issues, as well as research and development priorities. The material in this volume is valuable not only to soil scientists but to the entire environmental science community.

CHAPTER 1

World Soils as a Source or Sink for Radiatively-Active Gases

R. Lal, J. Kimble, and B.A. Stewart

I. Introduction

Terrestrial ecosystems, comprising world biota and soils, play a major role in regulating atmospheric concentrations of radiatively-active gases. Oceans comprise the largest reservoir for carbon and may play a major role in buffering the atmospheric concentration of CO₂. The geological reservoir has become active only within the last two centuries since humans began mining fossil fuel.

Distribution of carbon pool among various reservoirs, based on estimates by Post et al. (1990) and Sundquist (1993), are shown in Figure 1. In addition to being a major carbon pool, world soils influence atmospheric concentration of CO₂ and other radiatively-active gases through their effects on biota. Estimates of annual fluxes of carbon among various pools shown in Table 1 indicate a net imbalance of about 1.8 ± 1.4 gigaton C yr⁻¹. This imbalance is attributed to the terrestrial ecosystems among which world soils are the major component. Land use and soil management can influence both eflux and influx of C between soil and the atmosphere, and of several other radiatively active gases e.g., N₂O, NO_x, CH₄, and CFCs.

Figure 1. 1988-Estimates of carbon pool among various reservoirs.

Table 1. Carbon exchange among various reservoirs

Flux		Reservoir	Rate (gigaton C yr^−1)
Efflux to the atmosphere	(i)	Fossil fuel burning	5.3
	(ii)	Land use	0.6–2.6
	(iii)	Plant respiration	40–60
	(iv)	Residue decay	50–60
		Sub-total	95.9–127.9
Influx from the atmosphere	(i)	Photosynthesis	100–120
	(ii)	Ocean uptake	1.6–2.4
		Sub-total	101.6–122.4
			1.8 ± 1.4
Imbalance (eflux-influx)

(Calculated from Post et al., 1990; Sundquist, 1993.)

I. Soil Degradation and Greenhouse Gas Emissions

Soil is a principal medium for plant growth, and about 99% of the human food supply comes from soil and related ecosystems. Soil degradation, reduction in productive capacity due to misuse and mismanagement aggravated by harsh environmental conditions is rapidly increasing because of increasing demand on the finite extent, and fragile nature of soil resources. Worldwide, about 2 billion ha of soil have been degraded to some extent (ISRIC, 1991–92; WRI, 1992–93), and there is a widespread land hunger especially in densely-populated Asia (Pimentel et al., 1994). Land hunger leads to cultivation of unsuitable and marginal lands for food crop production or grazing that further exacerbates soil degradation and feeds the vicious cycle. In addition to decreasing food production, soil degradation accentuates environmental problems. In developed economies, soil degradation leads to increased reliance on agricultural chemicals (e.g., fertilizers, pesticides) and irrigation to off-set adverse effects on productivity. These chemicals, especially fertilizers, may accentuate greenhouse gas emissions from soil-related processes. In developing economies, soil degradation leads to deforestation and expansion of agricultural activities to marginal lands in ecologically-sensitive ecoregions (Myers, 1989). Deforestation and cultivation of grasslands are among the major causes of greenhouse gas emissions (Houghton, 1994).

Depending on the interactive effects among biophysical and socioeconomic factors, there are three principal types of soil degradation i.e., physical, chemical, and biological. Soil physical degradation is set-in-motion by decline in structural attributes that lead to crusting, compaction, low infiltration rate, high runoff rate, and accelerated soil erosion (Figure 2). Worldwide 83 × 10⁶ ha or 4% of degraded soils have been subjected to some degree of physical degradation (Oldeman et al., 1990). Accelerated soil erosion is the most severe form of physical degradation, and about 80% of the world’s agricultural land presumably suffers moderate to severe erosion and 10% slight to moderate erosion (Speth, 1994). Worldwide 1094 × 10⁶ ha or 56% of degraded soils are prone to erosion by water and 548 × 10⁶ ha or 28% of degraded soils are prone to wind erosion (Oldeman et al., 1990). Chemical degradation leads to loss of bases from the soil solum and to acidification. Nutrient imbalance, deficiency of some nutrients and toxicity of others, is a major problem of low productivity in vast areas of the tropics and sub-tropics. Worldwide 240 × 10⁶ ha or 12% of degraded soils have been subjected to some degree of chemical degradation (Oldeman et al., 1990). Soil biological degradation, reduction in soil organic carbon content and in biomass carbon with concomitant effects on activity and species diversity of soil fauna and flora, has a major effect on greenhouse gas emissions from soil. These soil degradative processes are driven by socio-economic and political factors and in turn affect flux of radiatively-active gases from soils (Figure 2). Soil degradative processes, and especially the biological degradation may drastically influence eflux of CO₂, N₂O, NO, and CFCs and decrease influx of CH₄ (Figure 2).

Figure 2. Interactive effects of socio-economic and biophysical factors on soil degradation and greenhouse gas emissions.

III. Soil Management and Carbon Dynamics

The magnitude and type of greenhouse gas emissions from soil degradative processes depend on land use, cropping systems, and soil management. Soil management affects carbon dynamics and gaseous emissions through its influence on soil properties and processes (Figure 3). Principal soil properties affected by soil management are soil moisture and temperature regimes, aeration, and aggregation. Micro-aggregation can be grossly affected by soil management with strong effects on release of carbon hitherto inaccessible even to microbes.

Figure 3. Soil management effects on greenhouse gas emissions from soil-related processes.

Mineralization of carbon and nitrogen in soil follow a similar pattern. A simple model to predict the rate of change of C (and N) in soil is shown in equation 1 (Stevenson, 1982):

$\frac{dC}{dt}=a-kC$

(1)

where k is the decomposition constant, C (or N) is the carbon content of a given mass of soil at time t, and a is the accretion constant reflecting the amount of C added to the soil through agricultural operations e.g., crop residue return, application of compost and farmyard manure, dung from grazing cattle, etc. It is the magnitude of the term (a – kC) that determines whether soil is a sink (sequesteration) or source (eflux) of carbon. Magnitude of the decomposition constant k depends on land use, cropping systems, soil and crop management (Lai, 1989; Figure 4). All other factors remaining the same, the magnitude of decomposition constant k is generally more for tropical than temperate environment. Furthermore, the magnitude is low for natural ecosystems, and for agricultural practices based on science-based systems of soil and crop management involving judicious off-farm inputs and conservation tillage involving crop residue return. The magnitude of k is also low for soil restorative measures that enhance soil fertility. In contrast, the magnitude of k is high for practices that involve deforestation, expansion of agricultural activities to ecologically-sensitive ecoregions, biomass burning, cultivation of marginal lands, resource-based and low-input agriculture, and plow-based methods of seedbed preparation that accentuate extremes of soil moisture and temperature regimes, and systems that exacerbate soil degradative processes (Figure 4).

Figure 4. Factors affecting decomposition constant k of soil organic carbon.

Soil tillage is an important tool to regulate decomposition constant k. While plow-based tillage may increase k, conservation tillage systems decrease k. Kern and Johnson (1991) estimated that the organic carbon content of soils in the contiguous United States to 30 cm depth is 5,304 to 8,654 Tg C with 1,710 to 2,831 Tg C at 0 to 8 cm depth, and 1,383 to 2,240 Tg C at 8 to 15 cm depth. Maintaining the 1990 levels of conventional tillage (73% of arable land) until 2020 would result in loss of 46 to 78 Tg C. Increase in conservation tillage area from 27% in 1990 to 57% by 2020 would result in loss of 27 to 45 Tg C. However, increase in conservation tillage to 76% of the land by 2020 would result in loss of 14 to 24 Tg C. Widespread adoption of soil conserving systems, especially in the tropics and sub-tropics, can lead to substantial reduction in loss of soil organic carbon and emission of gr...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1. World Soils as a Source or Sink for Radiatively-Active Gases
• A. Land Use and Carbon Sequestration
• B. Agricultural and Grassland Ecosystems
• C. Forest Ecosystems
• D. Cold Ecosystems
• E. Wetlands and Rice Paddies
• F. Arid Lands
• G. Tropical Ecosystems
• H. Policy Issues
• I. Managing Soils for Mitigating Greenhouse Effect
• J. Research and Development Priorities