Non-Peer ReviewedThis poster includes background information about how PRS™-probes are used to measure soil nutrient supply rates and how factors of the soil environment influence nutrient supply rate measurements. The discussion contains research examples of the effects of soil moisture, soil temperature, and competing sinks as well as the effect of the duration of PRS™-probe burial. These effects are important to consider when interpreting supply rate data

Baron, V.S.

Greer, K.J.

Schoenau, J.J.

Sulewski, C.A.

English

University of Saskatchewan Research Archive

Factors Affecting Nutrient Supply Rate Measurements with PRS™-Probes.  C.A. Sulewski1, K.J. Greer1, J.J. Schoenau2, V.S. Baron3  1Western Ag Innovations Inc., 3 – 411 Downey Road, Saskatoon, SK, S7N 4L8 2Saskatchewan Centre for Soil Research, University of Saskatchewan, Saskatoon, SK, S7N 5A8 3Agriculture & Agri-Food Canada Lacombe Research Centre, Lacombe, AB, T4L 1W1  Key Words:  PRS™-probe, nutrient supply rate, nitrogen, phosphorus, potassium, sulfur, soil moisture, soil temperature, root competition  Abstract  This poster includes background information about how PRS™-pr bes are used to measure soil nutrient supply rates and how factors of the soil environment influence nutrient supply rate measurements.  The discussion contains research examples of the effects of soil moisture, soil temperature, and competing sinks as well as the effect of the duration of PRS™-p obe burial.  These effects are important to consider when interpreting supply rate data.  What is a PRS™-Probe?  Plant Root Simulator (PRS™) probes are comprised of an anion or cation exchange resin membrane encapsulated in a plastic holding device (Figure 1).  A high concentration of ion adsorption sites on the resin surface allows the PRS™ membrane to act as an ion sink when buried in soil, similar to a plant root surface (Schoenau et al., 1993).  The PRS™ assesses nutrient supply rates by continuously adsorbing charged ionic species from the soil during the burial period.       Figure 1.  Anion (on the left) and cation (on the right) PRS™-probes.    PRS™-probes are buried in situ under conditions similar to those in which plant roots grow and absorb nutrients.  This provides an integrated measure of supply as affected by all of the factors that affect nutrient supply to plant roots (free ion activities, buffer capacity, mineralization, immobilization, temperature, moisture, crop uptake, diffusion etc.).  What are PRS™ Ion Supply Rates?  Ion supply rates are reported as amount of ion adsorbed per unit surface area ion exchange membrane per entire tim  of PRS™-probe burial, conventionally “mg 10cm-2 duration of burial-1”.  PRS™ supply rates represent ion fluxes adsorbed from both the soil labile ion pool as well as ions replenished by more slowly supplying pools.  They are a function of soil ion pool sizes as well as soil physical, chemical and biological conditions, meaning all factors of the soil environment must be considered when interpreting PRS™ supply rate data.  PRS™ supply rates are well correlated with plant nutrient uptake (Table 1).  Table 1.  Relationship between PRS™ Supply Rates, Soil Ion Concentrations Determined by Conventional Chemical Extraction and Plant Ion Uptake. Correlation (R2) with Chemical Species PRS™ Type Conventional Extraction Method Plant Uptake References Nitrate Anion 0.69 0.86 Qian & Schoenau, 1995 Phosphate Anion 0.57 0.71 Schoenau et al., 1993 Sulfate Anion 0.73 0.98 Greer & Schoenau, 1994 Borate Anion 0.79 n/a Greer & Schoenau, 1994 Chloride Anion 0.81 n/a Greer & Schoenau, 1994 Potassium Cation 0.87 0.68 Qian et al., 1996 SAR Cation 0.95 n/a Greer & Schoenau, 1995 Sodium Cation 0.86 n/a * Calcium Cation 0.68 n/a * Magnesium Cation 0.68 n/a * Ammonium Cation n/a n/a * Chromium DTPA-Anion 0.98‡ 0.99‡ Tejowulan et al., 1994 Manganese DTPA-Anion 0.50 0.68 Tejowulan et al., 1994 Iron DTPA-Anion 0.61 0.71 Liang & Schoenau, 1995 Nickel DTPA-Anion 1.00‡ 1.00‡ Liang & Schoenau, 1995 Copper DTPA-Anion 0.78 0.75 Tejowulan et al., 1994 Zinc DTPA-Anion 0.83 0.74 Tejowulan et al., 1994 Cadmium DTPA-Anion 0.98‡ 0.98‡ Liang & Schoenau, 1995 Lead DTPA-Anion 0.97‡ 0.98‡ Liang & Schoenau, 1995 2,4-D amine Anion 0.98‡ n/a Szmigielska & Schoenau, 1994Metsulfuron Anion n/a 0.98 Szmigielska et al., 1998 Glucosinolates Anion 0.98 n/a Szmigielska et al., 2000 ‡ Experiments using soils spiked at increasing rates. * Unpublished data.  What Factors Influence PRS™ Supply Rates?  Duration of Burial in Soil  The longer a PRS™-probe is buried in the soil, the greater the opportunity to adsorb ions, the higher the ion supplies measured.  The effect of PRS™ burial time on nutrient supply rates measured in three soils is shown in Table 2.  Supply rates of all nutrients are visibly higher after 24-hour versus 1-hour burial.  Supply rates would be even higher following a 2-week PRS™ burial, particularly for ions such as N and S that are released through mineralization processes.  Table 2.  PRS™ Nutrient Supplies Determined during Two Burial Periods in Three Soils under Controlled Environmental Conditions. (Schoenau et al., 1993) NO3-N PO4-P K SO4-S Soil Burial Time mg cm-2 burial time-1 1 hr 20.9 0.66 35.5 3.6 1 24 hr 142.9 1.60 72.0 15.5 1 hr 4.7 0.07 17.6 10.1 2 24 hr 84.1 0.23 28.3 106.6 1 hr 5.9 0.14 20.9 2.3 3 24 hr 21.6 0.25 30.6 5.4  PRS™-probes should be buried for equivalent time periods among treatments to be compared.  Supply rates can not be divided into smaller time units, as ion adsorption may not be linear over time.  However, measurements made by repeated burial of PRS™-p obes in the same soil slot over time can be added together to achieve cumulative supplies (i.e. over the growing season as in Table 6).  “Snapshots” of soil nutrient supplying power are obtained by 1 - 24 hour burial of PRS™-probes, while 1 - 4 week burials are used to measure release of soil ions over time.  Soil Temperature  When burying PRSä-probes, particularly in the field, it is important to account for the effects of soil temperature on ion diffusion and mineralization. Microbial activity, and therefore mineralization/immobilizat on processes, will be reduced at lower soil temperatures, which will affect measurements of mineralizable nutrients such as N and S in particular.  Th s is demonstrated in Table 3 in which microbial respiration progressively increases as soil temperature increases.  At the same time, N supply rates measured by PRS™-probes also increase.  Alternatively, higher temperatures could result in reduced supply rates as increased microbial activity causes ion immobilization (see Competing Sinks section).  Table 3.  Microbial Activity and Nutrient Supplies in Triplicate Samples of a Loamy Sand Soil (O.M.= 1.8%) Incubated for One Week at Three Temperatures.  (P>F = 0.02) (NO3 + NH4)-N PO4-P Incubation Temperature (ºC) mg 10cm-2 week-1 Cumulative Respiration (mg CO2-C kg-1 O.D. soil) 5 62 2.2 37 a 23 90 2.1 189 b 32 300 2.4 337 c Although differences may be greater following long duration burials, there is also an effect of temperature on ion movement to the PRS™ during short-term burials (Table 4).  At reduced soil temperatures, ions diffusion decreases and thus, movement of ion diffusion limited nutrients such as PO4-P to the PRSä-probes will be reduced.  For these reasons, soil temperature data should be reported along with nutrient supply rate data where possible.  Table 4.  Nutrient Supply Rates Measured in Triplicate Soil Samples over Two Time Periods at Four Soil Temperatures.  (P<= 0.05) Nutrient Supply Rate (mg 10cm-2 time of burial-1) (NO3 + NH4)-N PO4-P K SO4-S Incubation Temperature (ºC) 1 hr 24 hr 1 hr 24 hr 1 hr 24 hr 1 hr 24 hr 4 63 b 244 b 2.3 b 7.8 b 217 c 221 b 17 c 61 b 10 74 ab 296 ab 2.7 ab 8.1 b 248 bc 195 ab 21 c 72 ab 20 86 a 290 ab 3.0 a 12.1 a 275 ab 175 a 25 a 82 a 30 90 a 352 a 3.0 a 10.3 ab 292 a 186 a 25 a 85 a   Soil Moisture  Soil moisture content has a large influence on physical movement, biological uptake and chemical reaction of ions in the soil, which in turn influence ion supplies to plant roots.  An example of how moisture affects PRS™ nutrient supply rates is provided in Table 5 in which the PRS™-probes were buried in soils incubated for 1 hour at 5 moisture contents (values represent means of 3 replicates).  Table 5.  Influence of Soil Moisture Content on PRS™ Soil Nutrient Supply Rates.  (Schoenau et al., 1993) PRS™ Soil Nutrient Supply Rate (mg 10cm-2 hour-1) Soil Moisture Content NO3-N + NH4-N PO4-P K SO4-S Saturated 282 4.5 218 50 100% F.C. 200 2.7 181 39 70% F.C. 196 1.4 155 37 45% F.C. 113 0.9 93 26 15% F.C. 24 0.3 48 12  At lower soil moisture contents, lower supply rates are measured as microbial activity (and therefore mineralization) is limited and physical ion movement is restricted by greater tortuousity in the path the ions must travel to reach the PRS™ membrane.  Microbial activity may also be limited in very wet soils due to anaerobic soil conditions and N supply rates may be reduced in saturated soils as a result of denitrification.  Increased microbial activity at higher soil moisture contents could also result in reduced supply rates due to microbial immobilization of ions (see Competing Sinks section).  As a result of limited ion movement in dry conditions, researchers often moisten the soil in the vicinity of the PRS™-probe during short-duration burials to obtain a “snapshot” of soil nutrient supply.  For long-duration, in-field burials, soil moisture levels should be continuously monitored and reported as interpretive data along with nutrient supply rates.  Competing Sinks  Any factor responsible for removing i ns from the available soil nutrient pool can compete with the PRS™-probes for ions and can be responsible for reduced nutrient supply rate measurements.  For example, if there is a large organic carbon source present in the soil with a wide C:N, C:P or C:S ratio, micro-organisms may compete with the PRS™ for ions.  This process, known as nutrient immobilization, is one process within the whole cycle of mineralization/immobilization transformations that can affect the supply rates measured.    Plant roots can also compete with the PRS™ for ions.  In studies extending over days or weeks, burying the PRS™ near plant roots results in a measure of the difference between net mineralization and plant uptake (nutrient surpluses rather than net mineralization).  Net mineralization without root competition can be assessed by inserting a “root exclusion cylinder” (Figure 2) into the ground, removing plants, and burying the PRS™ within the soil area isolated by the cylinder.  By burying PRS™ both inside and outside of the cylinders, a more complete picture of the soil NO3-N dynamics can be obtained.  The impact of plant root competition on soil nutrient supplies measured over a growing season is shown in Table 6.     Figure 2.  Root exclusion cylinder consisting of a PVC pipe inserted to a 10” depth from which plants were removed to isolate the PRS™-probe from plant root competition.  Table 6.  Impact of Plant Roots on NO3-N Ion Adsorption by PRS™-Probes Buried in Different Pasture Systems Biweekly (12 times) over a Growing Season at Lacombe, Alberta, 1999. (Dr. V.S. Baron, unpublished data) No Roots Roots N Uptake Pasture Type mg NO3-N 10cm-2 24 wk-1 Old Perennial 1713 237 140 Alfalfa 2686 1328 330 Mixed Brome/Alfalfa 2446 895 320 Annual Rye Grass 2955 2007 200 Low Input 1349 262 140 Significant F ** ** * SE LSMeans 185 165 40 Nitrate-N supplies were much lower in the presence of roots than where the PRS™-probes isolated from root competition.  The influence of N fixation is also apparent in that N uptake is greatest in the treatments containing alfalfa while PRS™ cumulative supply rates measured in the absence of roots indicate greatest N supply in the annual rye grass treatment.  When N-fixing plants are being studied, it is important to remember that N uptake is influenced by both N supply as measured within the root exclusion cylinder as well as N fixation by plants growing outside of the cylinder.  Conclusions  The sensitivity of PRS™-probes to soil environmental conditions makes them a powerful tool for measuring soil nutrient supplies under field conditions similar to those in which plants grow.  This sensitivity also means that care must be taken in planning experiments with the PRS™ and when interpreting PRS™ supply rate data.  If possible, it is recommended that soil moisture and temperature be measured during the duration of PRS™ burial and that competition from plant roots be accounted for.  References  Greer, K.J. and Schoenau, J.J.  1996.  A rapid method for assessing sodicity hazard using a cation exchange membrane.  Soil Technology.  8:  287-292.Greer, K.J. and Schoenau, J.J.  1994.  Salinity and salt contamination assessment using anion exchange membranes.  Proc. Soils and Crops Workshop 1994.  Extension Division, University of Saskatchewan, Saskatoon, SK.  Pp. 44-48. Liang, J. and Schoenau, J.J.  1995.  Development of resin membranes as a sensitive indicator of heavy metal toxicity in the soil environment.  Int. J. Envir. Anal. Chem.  59:  265-275. Qian, P. and Schoenau, J.J.  1995.  Assessing nitrogen mineralization from soil organic matter using anion exchange membranes.  Fert. Res.  40:  143-148. Qian, P., Schoenau, J.J., Greer, K.J., and Liu, Z.  1996.  Assessing plant available potassium in soil using cation exchange membrane burial.  Can. J. Soil Sci. 76:  191-194. Schoenau, J.J., Qian, P. and Huang, W.Z.  1993.  Ion Exchange Resin Membranes as Plant Root Simulators.  Proc. Soils and Crops Workshop 1993.  Extension Division, University of Saskatchewan, Saskatoon, SK.  Pp.  392-400. Szmigielska, A.M., Schoenau, J.J. and Levers, V.  2000.  Determination of glucosinolates in canola seeds using anion exchange membrane extraction combined with the high-pressure liquid chromatography detection.  J. Agric. Food Chem.  48:  4487-4491. Szmigielska, A.M., Schoenau, J.J. and Greer, K.  1998.  Comparison of chemical extraction and bioassay for measurement of metsulfuron in soil.  Weed Sci.  46:  487-493. Szmigielska, A.M. and Schoenau, J.J.  1994.  Determination of 2,4-D amine in soils using anion exchange membranes.  J. Agric. Food Chem.  43:  151-156. Tejowulan, R.S., Schoenau, J.J. and Bettany, J.R.  1994.  Use of ion exchange resins in soil and plant testing for micronutrient availability.  Proc. Soils and Crops Workshop 1994.  Extension Division, University of Saskatchewan, Saskatoon, SK.  Pp. 255-267. 

Factors affecting nutrient supply rate measurements with PRS™-probes

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Factors affecting nutrient supply rate measurements with PRS™-probes

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