Grasslands

Unmanaged and lightly managed grasslands can be seen as having a low rank for ecosystem service for products directly consumed by human beings, but they often sustain populations of large herbivores that serve as food and other animal products used by humans (Table 2.2). Unmanaged grasslands are extensive in the tropics, and range along a broad gradient of precipitation that strongly influences both production and biological diversity, aboveground and belowground, merging into parkland and, ultimately, forest at the wet end. These grasslands are often the guardians of important watersheds. They have a disproportionately high aesthetic value, deriving from an inherently high biodiversity, the visibility of some wildlife, and a large recreational potential (McNaughton et al. 1983). Where grasslands are the product of low precipitation, management options are limited because animal production is ultimately a product of (inverse) stocking density (Ruess & McNaughton 1987). However, in mesic systems, where many managed grasslands are the product of historical forest clearance and/or intensive grazing, production can often be enhanced in proportion to fertilizer input and manipulation (through tillage and seeding) of plant species composition. Such management carries major implications for water quality (reduced by large rises in dissolved C and N compounds in runoff water) and for greenhouse gas emissions (Williams et al. 1998). There is also, arguably, a fall in aesthetic value and restrictions in the availability of land for recreational purposes.

A large functional diversity of soil organisms in grassland soils contributes to the sustainability of ecosystem goods and services. In unmanaged grasslands, there is a strong positive role of soil organisms, whereas intensified grassland management results in reducing the role of soil organisms in the decomposition of soil organic matter and the mineralization of nutrients due to the addition of mineral fertilizers or liquid manure (Bardgett & Cook 1998). Reduced activity of burrowing soil organisms decreases the contribution of managed grasslands to water storage, which results in enhanced runoff of rainwater and, consequently, flooding in downstream areas (Bardgett et al. 2001). In managed grasslands, nitrifying microorganisms may contribute to the leaching of mineral nitrogen to surface water or groundwater because the amount of available mineral nitrogen exceeds the uptake by plants (Smith et al. 2002). The predominance of bacterial-based soil food webs reduces the capacity of managed grasslands to act as sinks for carbon (Burke et al. 1989; Brye et al. 2002; Mader et al. 2002).

The possible management interventions are mowing, stocking density, fertilization, pesticide application, seeding, tillage, and enclosure. In reality, there is a gradient of management from light to intensive, and also a major difference between dryland grassland systems and mesic ones. The total number of land uses imposed on managed grasslands is very large. Grasslands may also be created on a temporary basis when tropical forest is felled, or permanently by invasive species if the soil is subsequently exhausted

Table 2.2. Provision of goods and services in a temperate grassland ecosystem.

This ecosystem is considered in its unmanaged and managed states. A rank from —3 (strong disservice) through 0 (neutral) to +3 (strong service) is given for each good or service, indicating its value to human societies. Under each category we have distinguished between biotic (mediated by living soil organisms) and abiotic (mediated by chemical, physical and geological, climatological or historical factors, over which living organisms have, essentially, no overwhelming control on the short and medium term) processes. The relative contributions of biotic and abiotic processes ( = ecosystem functions, including anthropogenic inputs and perturbations) to each good or service is given by asterisks * (small) to ""^large). Note that in Chapter 5, Tables 5.A1—5.A3 have been developed using the same principle, but there the relative contributions of abiotic and biotic processes have sometimes been slightly differently valued. This is due to the comparison between unmanaged and managed.

Unmanaged Grassland * Managed Grassland **

Goods or Services Rank Biotic Abiotic Rank Biotic Abiotic

Food production

Plant products 0 0

Animal products 2 *** decomposition, ** soil type, topography, 3 * nutrient transforma- *** chemical inputs, bioturbation, organic fire, climate tion, nitrification, etc. soil type, topography, matter transformation climate

+ nutrient cycling, resistance to pests & diseases

Water quality (Riparian context: quality to local streams and rivers)

retention of N in ** soil type and cover, biomass, physical stabilization, interception of runoff, soil organisms topography, fire, climate (esp. precipitation)

Phosphate liberation, DON, DOC, microbiological pollution of runoff, soil organisms

*** soil type and cover, topography, fire, climate (esp. precipitation), compaction

Water volume' (Flow to watersheds)

*** moisture retention by OM, évapotranspiration, soil organisms

** soil type and cover, topography, fire, climate (esp. precipitation), infiltration, runoff

—2 * moisture retention by organic matter, soil organisms

*** soil type and cover, topography, fire, climate (esp. precipitation), flood, run-off, infiltration, compaction

Table 2.2. (continued)

Unmanaged Grassland * Managed Grassland **

Goods or Services Rank Biotic Abiotic Rank Biotic Abiotic

Fiber 2 *** decomposition, *** soil type, topography fire, 1 * nutrient transforma- *** chemical inputs, soil

(e.g., wool, leather) bioturbation, organic climate tion, e.g., nitrification type, topography, matter transformation etc. climate

+ nutrient cycling, resistance to pests & diseases

Recreation

' wildlife

1 landscape

1 * wildlife, hiking, etc.. according to habitat

*** amenity value reduction, e.g., topography amendment, fencing, denial of access

C sequestration (storage of carbon in biomass or soil organic matter, mitigating global warming)

2* *** organic matter formation/accumulation, CaCO^ deposition

** complexing organic matter, texture, fire, CaCO^ deposition

** net C accumulation (despite bacterial based foodwebs enhancing C loss)

** complexing organic matter, texture, fire, CaCO^ deposition

*** nitrifiers, denitrifiersjoss of CH4 oxidation texture, climate, pH

Trace gases and atmospheric regulation 2 *** maintenance of C ** texture, climate, pH —3

(production of CH4 and and N balances

Noxides by microbes, also oxidation of CH4)

# includes lightly managed freerange grasslands with no fertilizer and pesticide inputs, excluding winter livestock feed. ## intensively managed animal and plant production systems with fertilizer and pesticide inputs and/or irrigation.

^ not including charging of groundwater.

* because of higher accumulation of surface and subsurface organic matter.

through subsistence agriculture. Unmanaged grasslands are mainly used for free-range animal production.

In conclusion, our example shows that in the type of grasslands considered, the largest impacts of management on ecosystem goods and services are on water quality and quantity and on trace gases. There may be a management trade-off between enhanced production and increased leaching and trace gas production, but in this trade-off the total area needed for food production as well as the price of land will undoubtedly play an important role.

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