Much of the environmental movement of the 1970s and 1980s focused on decreasing the degradation of the environment, be it developing better fishing equipment to decrease damage to ocean bottoms and the trapping of nontarget fish to educational campaigns concerning the destruction of the rainforests. In the 1990s, the scientific community turned from monitoring degrading practices to evaluating a holistic attribute, ecosystem health. This direction began with work on soil quality, much like air and water quality assessments of the 1980s, and evolved into such work as rangeland health and forest ecosystem health. The National Research Council suggested that rangeland health be defined as "the degree to which the integrity of the soil and ecological processes of rangeland ecosystems are sustained." Healthy would be the dictionary definition of: (1) functioning properly or normally in its vital functions, (2) free from malfunction of any kind, and (3) productive of good of any kind. These definitions also apply to forest ecosystems.
At the core of ecosystem health is soil quality, defined as "the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health." The major issues are:
1. productivity—the ability of soil to enhance plant and biological productivity;
2. environmental quality—the ability of soil to attenuate environmental contaminants, pathogens, and offsite damage;
3. animal health—the interrelationship between soil quality and plant, animal, and human health.
Within the context of the soil quality concept, it is evident that soil biology, specifically microorganisms and their processes, will play a major role in determining the health of the ecosystem. Thus, in situ parameters or management that enhance or degrade microbial habitat will shift the health of the system. Attributes that affect the functioning of the soil system are listed in Table 17.2 and, when measured in the context of the proper ecological question, can be used to evaluate ecosystem health. The proper ecological question means "what is the function of the ecosystem?" or "what is the ecosystem going to be used for?" Thus, the soil quality attributes for an urban area may be different from the soil quality attributes for range- and pasture land.
TABLE 17.2 Proposed Soil Physical, Chemical, and Biological Characteristics to Be Included as Basic Indicators of Soil Quality (Adapted from Doran and Parkin, 1996)
Depth of soil and rooting Soil bulk density and infiltration" Water-holding capacity" Water retention Water content" Soil temperature" Chemical
Total organic C and N
Electrical conductivity Mineral N (NH4 and NO3), P, and K Biological
Microbial biomass C and N
Potentially mineralizable N Soil respiration
Biomass C:total organic C ratio Respiration:biomass ratio
Hydrometer method Soil coring or excavation Field-determined using infiltration rings Field-determined after irrigation of rings Water content at 33 and 1500 kPa tension Gravimetric analysis: wt loss, 24 h at 105°C Dial thermometer or hand temperature probe
Wet or dry combustion, volumetric basisb Field- or lab-determined, pocket pH meter Field or lab, pocket conductivity meter Field or lab analysis, volumetric basis
Chloroform fumigation/incubation, volumetric basis Anaerobic incubation, volumetric basis Field-measured using covered infiltration rings, lab-measured in biomass essay Calculated from other measures Calculated from other measures
"Measurements taken simultaneously in field for varying management conditions, landscape locations, and time of year.
'Gravimetric results must be adjusted to volumetric basis using field-measured soil bulk density for meaningful interpretations.
A major attribute of healthy soil is the level of SOM, which controls many of the chemical and physical parameters of soil. For example, in Table 17.2 SOM can influence bulk density, water-holding capacity and retention, and soil temperature and buffer the soil pH and electrical conductivity as well as influencing biological activity. However, as we have seen SOM can be rapidly lost through oxidation and by wind and water erosion processes. Most of the 1965 million hectares (Mha) of degraded land worldwide suffers from low organic matter content. Thus, the single most overriding factor for increasing soil quality and ecosystem health is increasing the level of SOM.
Coupling the ecosystem health concept with increasing SOM levels has recently drawn attention due to global warming and greenhouse gas issues. Since the most abundant greenhouse gas is CO2 and CO2 is the raw material for SOM after it is fixed by plants, increasing SOM will remove CO2 from the atmosphere. Significant scientific activity has been aimed at estimating the potential for soils to sequester more SOM and thus be a sink for atmospheric CO2. An example would be to determine from the literature that soils of the Midwest have lost upward of 50% of their SOM due to cultivation and then calculate how much C could be sequestered by returning this land to the native state. In reality, this is not a trivial calculation; however, this analysis can be done with many types of land and management practices (see suggested reading). Rangelands and forests have been identified as the ecosystems likely to store significant amounts of additional C in soil and plant material; however, it will be the soil microflora and their processing of plant C that will govern the rate and amount of C that is stored.
The magnitude of rangeland and forest land in the United States and worldwide is staggering. Grazing land in the United States constitutes 336 Mha, 48% of which is technically rangeland (161 Mha), about the same as cropland; rangeland combined with grassland pasture totals 239 Mha. Grazing land is estimated to cover 55% of the total U.S. land area and about the same on a global basis. Forest ecosystems occupy 298 Mha in the United States and 3650 Mha globally, excluding boreal forests. Ecosystem scientists believe that through proper management, these ecotypes occupying vast areas can sequester large amounts of C in the soil, providing an increase in soil quality and ecosystem health and removing the greenhouse gas CO2 from the atmosphere.
There are a number of management practices and land use changes that could benefit increasing soil C in range- and grazing land. Much of the semiarid and arid grazing lands are highly erodible subject to both wind and water erosion and would benefit by conversion to managed grasslands with greater input intensity. Improving grazing management has been shown to increase soil C compared to nongrazed exclosures. However, the increases in total soil C and N with grazing may be soil texture dependent. The conversion of forest to pasture and marginal cropland to pasture can also substantially increase soil C as can better management of fertility and plant species composition. The estimations of the amount of soil C that could be stored in managed U.S. privately owned grazing lands range from 30 to 110 million metric tons (MMT) C year-1 or 142 to 519 kg-C ha-1 year-1. The latter value can be compared to the CRP land that converts cropland to grassland and increases the sequestration of soil C by 400 to 1000 kg-C ha-1 year-1.
Forest management practices also contribute to the increase in soil quality and ecosystem health by increasing soil C. The conversion of marginal land and highly erodible land to forest will increase soil C and also provide an aboveground component of sequestered C to the ecosystem. Increasing intensively managed timberland and canopy cover in urban areas has a large potential effect on soil C and maximum C sequestration. Even with the negative aspects of wood removal and burning, growing short-rotation woody crops for energy has a substantial potential to sequester soil C. The U.S. estimates for these practices on forest and marginal land range from 276 to 529 MMT-C year-1; this can be compared to an estimate of 75 to 208 MMT-C year-1 from more intensive management of U.S. cropland.
The potential amount of increased C sequestration in soils of grazing lands and croplands of the United States amounts to about 13% of the 1600 MMT-CE year-1 (CE represents C equivalents) greenhouse gas emissions from the United
States. With the increase in C sequestered by forests in soil and above ground this percentage more than doubles. Thus, soil C sequestration is a viable mechanism of reducing greenhouse emissions from the United States. Considering the global magnitude of the land area in rangeland and forest, if these lands could also be managed, soil C could be a significant strategy in reducing global greenhouse gases.
Thus when speaking about the health of an ecosystem, we must consider the whole system, including feedbacks. The system just described shows the benefit of increasing SOM to forest ecosystem health in addition to the benefit of reducing greenhouse gases. However, since increasing SOM is a balance between primary plant production and microbial decomposition it is the maintenance of suitable microbial habitat that will control the ecosystem health.
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