Soil organisms play a major role in the delivery of ecosystem goods and services that are crucial for supporting human societies and for the sustainability of natural and managed ecosystems. Soil organisms act on very small scales, but their effects may range from local (diseased plants, nutrient mineralization) to very large scales (plant succession, carbon sequestration, production of trace gases that contribute to global warming). The diversity of soil organisms may matter more for a process that is accounted for by only a few species than for a process that is accounted for by many species. However, empirical evidence on effects of species diversity on ecosystem processes is still relatively rare and does not yet allow generalizations. It is probable that the diversity of functions is more important for the sustainability of ecosystem goods and services than species diversity per se, but this area is still wide open for further studies.
The role of soil organisms is more prominent in grasslands (especially natural ones), forests, and low-input (no-till) arable land than in intensively managed grasslands and arable land. However, the relative importance of soil organisms for the performance of ecosystem processes (as compared with the importance of abiotic influences) differs along climatic gradients or between soil types. There are also differences between the relative contributions of different taxa of soil organisms to ecosystem processes along climate gradients or between soil or vegetation types. In cold areas, for example, soil microorganisms play a lesser role in the decomposition of organic matter, whereas soil fauna have a more dominant role. Earthworms are key species in mesic grasslands, but enchytraeids are crucial in coniferous forests and some arable land.
Human interventions, such as plowing, fertilization, and using pesticides, often lead to shifts in the major decomposition channels or to a by-passing of the role of soil organisms. In intensively fertilized tilled arable land, the decomposition pathway is bacteria-based and the role of symbiotic mutualists (mycorrhizal fungi and nitrogen-fixing microorganisms) is largely redundant. Stability of nutrient pools in these systems may be achieved by high-input measures, but this results in, for example, the leakage of nutrients to ground- and surface water. In these cases, human activity to enhance the delivery of ecosystem goods, such as food production, result in the loss of ecosystem services, such as water purification occurring in soils.
Other human-induced changes, such as land use change, deforestation, soil drainage, erosion, enhanced temperature, and increased CO2 concentrations may all affect soil ecosystem goods and services by affecting soil organisms either directly or indirectly. Erosion will affect soil communities through the direct loss of habitat, whereas rising temperature and CO2 concentration may lead to more incipient changes in soil communities and, therefore, of the functioning of soil systems and the sustainability of the delivery of ecosystem goods and services.
There are clearly management trade-offs for the role of soil organisms in the delivery of ecosystem goods and services. We do not have much evidence that these tradeoffs act through the loss of species diversity, but this is mainly due to our limited knowledge on the diversity of, for example, soil microbial communities and consequences for ecosystem processes. Management trade-offs clearly act through effects on the diversity of functions. Intensive tillage farming practice reduces the abundance of earthworms, which negatively affects both the water-holding capacity of soils as well as the population of mycorrhizal fungi. This, as well as changes in the soil due to deforestation, may enhance flooding incidence in lowlands due to increased peaks in run-off water.
The template that we have developed for the analysis of the contribution of soil organisms and abiotic soil factors to the organization of soil processes, and to the delivery of ecosystem goods and services, is applicable to a wide range of environmental contexts. The examples that we have presented, however, apply to temperate systems. The approach adopted here may well prove valuable for comparison with tropical systems, where the potential for soil biotic diversity may be higher.
The obvious weakness of the present approach is that the relative importance of the services and the relative contributions of biotic (essentially manageable) and abiotic (only partially manageable) processes are expressed only in comparative terms. To make absolute (monetary) valuations possible, some of the services (e.g., food and fuel production) could be costed for a given local economy, while other services could be assigned financial status from these by reference to the relative importance we have suggested. The difference between these biological valuations and other schemes of costing, which are the stock-in-trade of economists, is that in each defined ecosystem some processes are amenable to management, and others are not. The prices of services, which essentially reflect their availability for manipulation by humans, should be adjusted accordingly.
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