Author: Hansen K1
Reviewer: Ribbons R2
Measurement unit: kg ha-1 y-1; Measurement scale: plot; Equipment costs: €€; Running costs: €€; Installation effort: medium; Maintenance effort: low; Knowledge need: medium; Measurement mode: manual
Soil leaching is the downward movement of dissolved mobile plant nutrients in the soil profile following percolating water such as rain or irrigation water. Leaching occurs when the soil pores become filled with water and water moves downward in the soil. EcologyDictionary.org defines leaching as ‘The process by which soluble constituents are dissolved and filtered through the soil by a percolating fluid’. Nutrients such as nitrate, phosphorus, and base cations, as well as other constituents such as dissolved organic carbon and contaminants, are lost from the soil via leaching below the rooting zone of the vegetation. Nutrients therefore become out of the reach of plants which leads to reduced soil fertility and plant yield (Cameron et al., 2013). Leaching may further create environmental concerns when the constituents move into ground- and surface waters and oceans and cause eutrophication resulting in unwanted growth of weeds and algae. Ecosystems normally have an intra-system cycle with a closed loop recycling essential nutrients. In such a system the demand for nutrients for growth is often higher or equal to the supply of the system. However, if the supply of nutrients is higher than the demand, leaching may occur, which is often an indication of saturation and a broken nutrient cycle. Climate change will affect the distribution of precipitation and extreme events such as droughts, and thus influence the amount of nutrients lost from a system and may lead to changes in the trends of leaching. Other global change drivers, such as land-use, land-use change, or pollution can also impact soil leaching. We are still learning more about the importance and effects of such phenomena (e.g. Reichenau et al., 2016). Accurate sampling of soil solutions followed by chemical composition analysis in the laboratory provides opportunities for effectively evaluating the availability and mobility of nutrients and pollutants as an early warning of potential groundwater contamination.
2.2.9.1. What and how to measure?
Knowledge on nutrients being lost from the soil, such as through nutrient leaching below the root zone (Addiscott, 1990), is required to understand nutrient cycling. Most commonly, soil leaching is determined by two factors: i) the soil solution nutrient concentration below the root zone combined with ii) the use of hydrological models to estimate the flow of water through the soil profile. Applying these two together we may estimate the amount of nutrients leached from the system.
Soil solution sampling aims to specify the quality of soil pore water. Generally, sampling can be performed using either non-destructive or destructive methods. We recommend the use of non-destructive methods which involve the installation of an in situ soil solution collector. A range of non-destructive soil solution sampling techniques are available and advantages and disadvantages are compared by Litaor (1988) and later by Weihermüller et al. (2007), Fares et al. (2009), and Curley et al. (2011). The non-destructive samplers (porous cups, porous plates, capillary wicks, pan lysimeters, resin boxes, lysimeters) vary in shape, size, and chemical and physical properties.
The samplers collect water either with applied tension (suction methods) or without applied tension (zero tension). Zero-tension lysimeters are constructed from pans or PVC pipes and collect gravitational water. They generally have significantly larger collection areas than tension lysimeters and are more difficult to install causing relatively larger disturbance to the plot, especially at greater depths. Tension lysimeters, on the other hand, are normally smaller and comparatively easy to install, and they have been produced using different materials such as ceramic, glass, acrylic, porous PTFE (Teflon), and other materials which are chemically inert. During recent years Teflon lysimeters have been more frequently used. Lysimeters collect water from soil pores at specific spots and partially filter the water that enters the sampler. A system with continuous suction is preferred if a low sampling frequency is applied (monthly is recommended). The basic principle is that a constant vacuum is applied to a suction cup, which allows water to pass through. The applied suction should preferably be equivalent to the suction of the soil at field moisture capacity.
When using tension lysimeters, large soil pits do not need to be dug because lysimeters can be installed using soil cores. Tension lysimeters thus cause only negligible disturbance of the soil, especially when installed at an angle, and are cost-effective. There is no consensus as to the best techniques for soil solution collection. Tension lysimeters have been used more extensively than zero-tension lysimeters, and are currently the most universally used technique for extracting soil water.
Tension lysimeters can be installed with medium training, but the installation is rather time-consuming. The number of samplers should be based on achieving an efficient cover of the spatial variation of the specific soil. This is often compromised by the need to reduce soil disturbance that installation of the samplers causes. If possible, the number of samplers should be more than three to capture the variation between samplers.
In climate-change plot experiments it is normally impossible to install lysimeters horizontally into the soil layer of interest because disturbance would be too great. It is therefore recommended to install the lysimeters at an angle of approximately 45 degrees below the root zone depth. The lysimeters are inserted into holes of appropriate depth made by a soil auger where a slurry of non-toxic silica flour (SiO2) has been applied. The slurry should be mixed at a ratio of approximately 5 kg silica flour to 1 L of deionized water. The mixed slurry is poured into the hole to a depth of at least 15 cm and the lysimeter unit is then placed in the auger hole pressing out the excess silica slurry. This procedure guarantees the porous surface of the lysimeter will be in close contact with the capillaries of the soil column.
Samples are evacuated using a portable pump that requires power. When the soil solution suction is less than the applied vacuum, the soil solution is drawn across the porous wall into the lysimeter by the induced pressure gradient. The soil solution sample is led from the cup by PTFE tubing to a storage collection bottle which is installed in an insulated box in order to protect the samples from temperature extremes and changes. The installation should preferably follow the manufacturer’s manual. Several installation protocols are available online: a thorough guide to installing Prenart lysimeters (http://www.prenart.dk/) is found at https://lter.kbs.msu.edu/protocols/42 but it may well be used for other lysimeter types. The maintenance of the system is low. Usually, when the system is operated continuously, there are few problems. However, plugging of the sampler pores does sometimes occur and it will then be necessary to remove the samplers and flush them through. The lysimeters can be reinstalled at new sites after rinsing.
The volume of water sampled by each sampler should be recorded so that the functioning of the lysimeter may be checked. Ideally, soil solution should be sampled throughout the year but some sites will be drier than others and in dry periods water will be hard to extract and the volume of water will decrease. A large enough volume of soil water for all included measurements should be stored in plastic bottles and transported to the laboratory as fast, dark and cold as possible. The soil samples are stored in the refrigerator until they can be processed and concentrations are determined. Fast measurements are recommended to avoid any changes in the samples. To interpret the soil leaching from measurements of soil concentration, a calibrated hydrological model is used to determine the hydrological flow of water in the soil profile. Subsequently, the soil concentration is coupled to the water movement in the soil profile. Different models exist to determine the hydrological flow of water (see reviews by Pechlivanidis et al., 2011; Devia et al., 2015; Sood & Smakhtin, 2015). Such a hydrologic model is often calibrated with soil moisture data derived from using a standard method such as time domain reflectometry (TDR) or time domain transmission (TDT) as well as climate data from a nearby climate station. Also see protocol 3.1 Soil moisture.
Where to start
Addiscott (1990), Cameron et al. (2013), Fares et al. (2009), Sood & Smakhtin (2015)
2.2.9.2. Special cases, emerging issues, and challenges
Ceramic lysimeters have been criticised (Raulund-Rasmussen, 1989) because the chemical composition of the soil solution can be modified by contact with the ceramic material.
Destructive methods involve soil sampling and subsequent extraction of soil solution in the laboratory by centrifugation. Laboratory extraction of soil water is generally less time-consuming and cheaper than lysimeter installation. However, for obvious reasons, such methods involve considerable destructive disturbance to the site that often make them unsuitable for most experiments.
Plant Root Simulator (PRS®) probes are non-destructive ion exchange resins that are easily inserted into the soil. Both anion (e.g. NO3–, HPO42-, SO42-) and cation (e.g. NH4+, K+, Ca2+, Mg2+) probes exist. These measure ion supply in situ in the upper soil profile and cannot be used to estimate soil leaching.
A range of studies have compared the chemical composition of soil solutions collected by zero-tension v. tension lysimeters (Haines et al., 1982; Nyberg & Fahey, 1988; Swistock et al., 1990; Hendershot & Courchesne, 1991; Marques et al., 1996). Estimates of soil solution composition and water flow differed according to lysimeter type. Zero-tension lysimeters collected more water at the upper soil depths but less water at greater depths. In general, there are no obvious patterns of divergences between lysimeter types across the various studies.
2.8.3. References
Theory, significance, and large datasets
Addiscott (1990), Cameron et al. (2013)
More on methods and existing protocols
Curley et al. (2011), Fares et al. (2009), Pechlivanidis et al. (2011), Sood & Smakhtin (2015)
All references
Addiscott, T. M. (1990). Measurement of nitrate leaching: a review of methods. In R. Calvert (Ed.), Nitrates, Agriculture, Water (pp. 157-168). Paris: INRA.
Cameron, K. C., Di, H. J., & Moir, J. L. (2013). Nitrogen losses from the soil/plant system: a review. Annals of Applied Biology, 162(2), 145-173.
Curley, E. M., O’Flynn, M. G. & McDonnell, K. P. (2011). The use of porous ceramic cups for sampling soil pore water from the unsaturated zone. International Journal of Soil Science, 6(1), 1-11.
Devia, G. K., Ganasri, B. P., & Dwarakish, G. S. (2015). A review on hydrological models. Aquatic Procedia, 4, 1001-1007.
Fares, A., Deb, S. K., & Fares, S. (2009). Review of vadose zone soil solution sampling techniques. Environmental Reviews, 17, 215-234.
Haines, B. L., Waide, J. B., & Todd, R. L. (1982). Soil solution nutrient concentrations sampled with tension and zero-tension lysimeters: report of discrepancies. Soil Science Society of America Journal, 46, 658-661.
Hendershot, W. H., & Courchesne, F. (1991). Comparison of soil solution chemistry in zero tension and ceramic-cup tension lysimeters. Journal of Soil Science, 42, 577-583.
Litaor, I. (1988). Review of soil solution samplers. Water Resources Research, 24(5), 727-733.
Marques, R., Ranger, J., Gelhaye, D., Pollier, B., Ponette, Q., & Goedert, 0. (1996). Comparison of chemical composition of soil solutions collected by zero-tension plate lysimeters with those from ceramic-cup lysimeters in a forest soil. European Journal of Soil Science, 47, 407-417.
Nyberg, R. C., & Fahey, T. J. (1988). Soil hydrology in lodgepole pine ecosystems in south-eastern Wyoming. Soil Science Society of America Journal, 52, 844-849.
Pechlivanidis, I. G., Jackson, B. M., Mcintyre, N. R., & Wheater, H. S. (2011). Catchment scale hydrological modelling: a review of model types, calibration approaches and uncertainty analysis methods in the context of recent developments in technology and applications. Global NEST Journal, 13(3), 193-214.
Raulund-Rasmussen, K. (1989). Aluminium contamination and other changes of acid soil solution by means of porcelain suction cups. Journal of Soil Science, 40, 95-101.
Reichenau, T. G., Klar, C. W., & Schneider, K. (2016). Effects of climate change on nitrate leaching. In W. Mauser & M. Prasch (Eds.), Regional Assessment of Global Change Impacts. (pp 623-629). Springer, Cham.
Sood, A., & Smakhtin, V. (2015). Global hydrological models: a review. Hydrological Sciences Journal, 60(4), 549-565.
Swistock, B. R., Yamona, J. J., DeWalle, D. R., & Sharpe, W. E. (1990). Comparison of soil water chemistry and sample size requirements for pan vs. tension lysimeters. Water, Air, and Soil Pollution, 50, 387-396.
Weihermüller, L., Siemens, J., Deurer, M., Knoblauch, S., Rupp, H., Göttlein, A., & Pütz, T. (2007). In situ soil water extraction: a review. Journal of Environmental Quality, 36(6), 1735-48.
Author: Hansen K1
Reviewer: Ribbons R2
Affiliations
1 Swedish Environmental Protection Agency, Stockholm, Sweden
2 Biology and Geology Departments, Lawrence University, Appleton, USA