6.3.2: Soils and the Water Cycle

Soils provide important ecosystem services through their control on the water cycle. These services include provisioning services of food and water security, regulating services associated with moderation, and purification of water flows, and they contribute to the cultural services of landscapes/water bodies that meet recreation and aesthetic values (Table \(\PageIndex{1}\); Dymond, 2014). At the pedon to hill slope scale, water stored in soil is used for evapotranspiration and plant growth that supplies food, stabilizes the land surface to prevent erosion, and regulates nutrient and contaminant flow. At a catchment and basin scale, the capacity of the soil to infiltrate water attenuates stream and river flows and can prevent flooding, while water that percolates through soil can replenish groundwater that can maintain water supplies and sustain surface water ecosystems while promoting a continued flow during periods of reduced precipitation (Guswa et al., 2014).

Table \(\PageIndex{1}\): Soil functions related to the water cycle and ecosystem services

Soil function	Mechanism	Consequence	Ecosystem service
Stores (storage)	Water held in soil pores supports plant and microbial communities	Biomass production Surface protection	Food Aesthetics Erosion control
Accepts (sorptivity)	Incident water infiltrates into soil with excess lost as runoff	Storm runoff reduction	Erosion control Flood protection
Transmits (hydraulic conductivity)	Water entering the soil is redistributed and excess is lost as deep percolation	Percolation to ground- water	Groundwater recharge Stream flow maintenance
Cleans (filtering)	Water passing through the soil matrix interacts with soil particles and biota	Contaminants removed by biological degradation/retention on sorption sites	Water quality

The soil functions of accepting, storing, transmitting and cleaning of water shown in Table \(\PageIndex{1}\) are inter-related. Soil water storage depends on the rate of infiltration into the soil relative to the rate of precipitation. Soil hydraulic conductivity redistributes water within and through the soil profile. The infiltration rate and hydraulic conductivity both depend on the water stored in the soil. The initially high rate of infiltration into dry soil declines as the soil water content increases and water replaces air in the pore space. Conversely, hydraulic conductivity increases with soil moisture content as a greater proportion of the pores are transmitting water. Water content and transmission times are also important to the filtering function of soil because contact with soil surfaces and residence time in soil are important controls on contaminant supply and removal (McDowell and Srinivasan, 2009).

The quantity of water which a soil can store depends on the thickness of the soil layer, its porosity, and soil matrixwater physical interactions. The latter are expressed as a water retention curve, the relationship between the soil water content and the forces holding it in place. The porosity and water retention curve are in turn influenced primarily by the particle size distribution and the soil bulk density, but also by the amount of SOM and the macropores created by biotic activity (Kirkham, 2014).

Optimum growth of most plants occurs when roots can access both oxygen and water in the soil. The soil must therefore infiltrate water, drain quickly from saturation to allow air to reach plant roots, and retain and redistribute water for plant use. An ideal soil for plant production depends on the climatic conditions. Soil structural stability and porosity are also important for the infiltration of water into soil. In addition to soil texture, organic matter improves soil aggregate stability (Das et al., 2014). While plant growth and surface mulches can help protect the soil surface, a stable, well aggregated soil structure that resists surface sealing and continues to infiltrate water during intense rainfall events will decrease the potential for downstream flooding resulting from rapid overland flow. Porosity (especially macropores of a diameter \(\geq 75 \mu \mathrm{m}\) ) controls transmission of water through the soil. In addition to total porosity, the continuity and structure of the pore network are as important to these functions as they are in filtering out contaminants in flow. Furthermore, the soil must support biota that will degrade the compounds of interest or have sorption sites available to retain the chemical species. Soil organic matter is important for these roles and, together with mineral soil (especially the clay fraction), provides sorption sites (Bolan et al., 2011). Flow through macropores, which bypass the soil matrix, where biota and sorption sites are generally located, can quickly transmit water and contaminants through the soil to groundwater or artificial drains, but for filtering purposes, a more tortuous route through the soil matrix is more effective (McDowell et al., 2008). There are multiple other links between soil biota and soil water, with water potential in particular having a pivotal role in the structure, growth, and activity of the soil microbial community (Parr et al., 1981).

Table \(\PageIndex{2}\):

Management action or other driver of change	Provisioning service impact	Regulating service impact	Supporting service impact	Cultural service impact
Land use change (increase change of agricultural to urban)	Decreased biomass; decreased availability of water for agricultural use	Increased impervious surface; decreased infiltration, storage, soil-mediated water regulation	Decreased genetic diversity; reduction of rainfall recycling, e.g. in the tropics	Decreased natural environment
Land use change (increase change of arable to intensive grassland)	Increased yield of animal over vegetable protein	Increased C sequestration; greater requirement of water; stress on ecosystem health of downstream waterways	Increased genetic diversity associated with mixed pastures	Change from traditional values and aesthetic value
Irrigation (increase)	Increased biomass over dryland agriculture; decreased availability of water for urban use	Increased C sequestration, but decreased filtration potential	Improved habitat for plant species	Infrastructure alters landscape decreasing spiritual connection with catchment
Drainage (increasing in marginal land)	Decreased soil saturation; increased biomass; removal of wetlands	Decreased C sequestration, denitrification, and flood attenuation	Better habitat for productive grassland plants, but loss of genetic diversity	Decreased recreational potential (e.g. eco- tourism)

Management of soil alters the ecosystem services provided by water (Table \(\PageIndex{2}\)). Soil conservation and sustainable management practices to combat desertification help to retain soil organic matter, structural stability, infiltration, and profile water holding capacity. The promotion of soil as a C sink to offset greenhouse gas emissions generally helps to maintain or improve soil hydrological functions as well. Deforestation, overgrazing, and excessive tillage of fragile lands, however, will lead to soil structural deterioration and a loss of infiltration, water retention, and surface water quality (Table \(\PageIndex{2}\); Steinfeld et al., 2006). Anthropogenic modifications to the water cycle can aid soil function. In dry regimes, inadequate soil moisture can be mitigated through supplementary irrigation, and where waterlogging occurs it can be relieved by land drainage. However, irrigation and drainage can have consequences for water regulation services. Irrigation that enables a shift to intensive land use can increase the contaminant load of runoff and drainage (Table \(\PageIndex{2}\); McDowell et al., 2011). Furthermore, drainage of wetland soils has been shown to reduce water and contaminant storage capacity in the landscape and can increase the potential for downstream flooding, as well as increasing the potential for GHG emissions due to the rapid decomposition of SOC in soil and dissolved organic C in drainage water (IPCC, 2013). The removal of surface or groundwater for irrigation disrupts the natural water cycle and may stress downstream ecosystems and communities. Irrigation of agricultural lands accounts for about \(70 \%\) of ground and surface water withdrawals, and in some regions competition for water resources is forcing irrigators to tap unsustainable sources. Irrigation with wastewater may conserve fresh water resources, but the fate of waterborne contaminants in soil and crops is a potential concern (Sato et al., 2013).

References:

Bolan, N. S., Adriano, D. C., Kunhikrishnan, A., James, T., McDowell, R. W., and Senesi, N.: Dissolved organic matter: biogeochemistry, dynamics and environmental significance in soils, Adv. Agron., 110, 1–75, 2011.

Das, B., Chakraborty, D., Singh, V. K., Aggarwal, P., Singh, R., Dwivedi, B. S., and Mishra, R. P.: Effect of integrated nutrient management practice on soil aggregate properties, its stability and aggregate-associated carbon content in an intensive ricewheat system, Soil Till. Res., 136, 9–18, 2014

Dymond, J.: Ecosystem services in New Zealand, Manaaki Whenua Press, Lincoln, New Zealand, 540 p., 2014.

Guswa, A. J., Brauman, K. A., Brown, C., Hamel, P., Keeler, B. L., and Sayre, S. S.: Ecosystem services: Challenges and opportunities for hydrologic modelling to support decision making, Wat. Resour. Res., 50, 4535–4544, 2014

IPCC: Supplement to the 2006 Guidelines for National Greenhouse Gas Inventories: Wetlands, Cambridge University Press, Cambridge, UK, 2013.

Kirkham, M. B.: Principles of soil and plant water relations, Academic Press, San Diego, CA, 2014.

McDowell, R. W. and Srinivasan, M. S.: Identifying critical source areas for water quality: 2. Validating the approach for phosphorus and sediment losses in grazed headwater catchments, J. Hydrol., 379, 68–80, 2009.

McDowell, R. W., Houlbrooke, D. J., Muirhead, R. W., Müller, K., Shepherd, M., and Cuttle, S. P.: Grazed Pastures and Surface Water Quality, Nova Science Publishers, New York, NY, 2008.

McDowell, R. W., van der Weerden, T. J., and Campbell, J.: Nutrient losses associated with irrigation, intensification and management of land use: a study of large scale irrigation in North Otago, New Zealand, Agric. Water Manage., 98, 877–885, 2011.

Parr, J. F., Gardner, W. R., and Elliot, L. F.: Water potential relations in soil microbiology: proceedings of a symposium, SSSA Special Publication number 9, Soil Science Society of America, Madison, WI, 151 pp., 1981.

Sato, T., Qadir, M., Yamamoto, S., Endo, T., and Zahoor, A.: Global, regional, and country level need for data on wastewater generation, treatment, and use, Agric. Water Manage., 130, 1–13, 2013.

Steinfeld, H., Gerber, P., Wassenaar, C. V., Rosales, M., and de Haan, C.: Livestock’s long shadow, Food and Agriculture Organization of the United Nations, Rome, Italy, available at: http://www.fao.org/docrep/010/a0701e/a0701e00.htm (last access: August 2014), 2006.

Excepted from:

Smith, P., Cotrufo, M. F., Rumpel, C., Paustian, K., Kuikman, P. J., Elliott, J. A., McDowell, R., Griffiths, R. I., Asakawa, S., Bustamante, M., House, J. I., Sobocká, J., Harper, R., Pan, G., West, P. C., Gerber, J. S., Clark, J. M., Adhya, T., Scholes, R. J., and Scholes, M. C.: Biogeochemical cycles and biodiversity as key drivers of ecosystem services provided by soils, SOIL, 1, 665–685, https://doi.org/10.5194/soil-1-665-2015, 2015. https://soil.copernicus.org/articles/1/665/2015/