Microplot Design and Plant and Soil Sample Preparation for ¹⁵Nitrogen Analysis

Summary

A microplot design for ¹⁵N tracer research is described to accommodate multiple in-season plant and soil sampling events. Soil and plant sample collection and processing procedures, including grinding and weighing protocols, for ¹⁵N analysis are put forth.

Abstract

Many nitrogen fertilizer studies evaluate the overall effect of a treatment on end-of-season measurements such as grain yield or cumulative N losses. A stable isotope approach is necessary to follow and quantify the fate of fertilizer derived N (FDN) through the soil-crop system. The purpose of this paper is to describe a small-plot research design utilizing non-confined ¹⁵N enriched microplots for multiple soil and plant sampling events over two growing seasons and provide sample collection, handling, and processing protocols for total ¹⁵N analysis. The methods were demonstrated using a replicated study from south-central Minnesota planted to corn (Zea mays L.). Each treatment consisted of six corn rows (76 cm row-spacing) 15.2 m long with a microplot (2.4 m by 3.8 m) embedded at one end. Fertilizer-grade urea was applied at 135 kg N∙ha^-1 at planting, while the microplot received urea enriched to 5 atom % ¹⁵N. Soil and plant samples were taken several times throughout the growing season, taking care to minimize cross-contamination by using separate tools and physically separating unenriched and enriched samples during all procedures. Soil and plant samples were dried, ground to pass through a 2 mm screen, and then ground to a flour-like consistency using a roller jar mill. Tracer studies require additional planning, sample processing time and manual labor, and incur higher costs for ¹⁵N enriched materials and sample analysis than traditional N studies. However, using the mass balance approach, tracer studies with multiple in-season sampling events allow the researcher to estimate FDN distribution through the soil-crop system and estimate unaccounted-for FDN from the system.

Introduction

Fertilizer nitrogen (N) use is essential in agriculture to meet the food, fiber, feed, and fuel demands of a growing global population, but N losses from agricultural fields can negatively impact environmental quality. Because N undergoes many transformations in the soil-crop system, a better understanding of N cycling, crop utilization, and the overall fate of fertilizer N are necessary to improve management practices that promote N use efficiency and minimize environmental losses. Traditional N fertilizer studies primarily focus on the effect of a treatment on end-of-season measurements such as crop yield, crop N uptake relative to the N rate applied (apparent fertilizer use efficiency), and residual soil N. While these studies quantify the overall system N inputs, outputs, and efficiencies, they cannot identify nor quantify N in the soil-crop system derived from fertilizer sources or the soil. A different approach using stable isotopes must be used to track and quantify the fate of fertilizer derived N (FDN) in the soil-crop system.

Nitrogen has two stable isotopes, ¹⁴N and ¹⁵N, that occur in nature at a relatively constant ratio of 272:1 for ¹⁴N/¹⁵N¹ (concentration of 0.366 atom % ¹⁵N or 3600 ppm ¹⁵N^2,3). The addition of ¹⁵N enriched fertilizer increases the total ¹⁵N content of the soil system. As ¹⁵N enriched fertilizer mixes with unenriched soil N, the measured change of ¹⁴N/¹⁵N ratio allows researchers to trace FDN in the soil profile and into the crop^3,4. A mass balance can be calculated by measuring the total amount of ¹⁵N tracer in the system and each of its parts². Because ¹⁵N enriched fertilizers are significantly more expensive than conventional fertilizers, ¹⁵N enriched microplots are often embedded within the treatment plots. The purpose of this methods paper is to describe a small-plot research design utilizing microplots for multiple in-season soil and plant sampling events for corn (Zea mays L.) and to present protocols for preparing plant and soil samples for total ¹⁵N analysis. These results can then be used to estimate N fertilizer use efficiency and create a partial N budget accounting for FDN in the bulk soil and the crop.

Protocol

1. Field site description

NOTE: When performing ¹⁵N tracer field trials, selected sites should minimize variation due to soil, topography, and physical features⁵. Cross-contamination may occur following lateral soil movement due to slope, wind or water translocation, or tillage while the vertical distribution of soil N may be impacted by subsurface water flow and tile-drainage⁶.

1. Describe the experimental field site including past management (e.g., previous crops and tillage), latitude and longitude, soil physical and chemical properties (e.g., soil textural analysis, initial fertility conditions, pH, and soil bulk density).
2. Record GPS coordinates for the research site and the field corners.
3. Describe growing season management including pest and disease management (herbicide, insecticide, or fungicide use), soil fertility management (including rate, source, placement, and application timing), tillage, irrigation events and amounts, and residue management.
4. As crop growth and microbe mediated N transformations are affected by soil moisture, soil temperature, and air temperature, record climate information including daily high and low temperatures, daily precipitation, and soil moisture and temperatures at several depths that reflect the soil sampling depths.

2. Plot design

1. Plant six corn rows (~86,000 plants ha^-1) on 76 cm spacing with a final plot dimension of 15.2 m by 4.6 m.
   1. Establish border areas 1.5 m from each end of the lengthwise dimension (0-1.5 m, 13.7-15.2 m) and an additional border area 1.5 m long (9.8-11.3 m) adjoining the sampling and harvest areas (Figure 1).
   2. Designate rows 2 and 3 as the in-season plant and soil sampling area (1.5-9.8 m) and rows 4 and 5 as the harvest area (1.5-9.8 m) for corn grain yield.
   3. Establish a microplot area (11.3-13.7 m) with dimensions of 2.4 m by 3.8 m centered on the width dimension. Collect all ¹⁵N enriched plant and soil samples from this area, leaving 0.38 m of unsampled border on the length and width dimensions to minimize edge effects (Figure 2).
2. Delineate the treatment plot and microplot corners with different colored flags.

3. Soil and plant sample precautions

1. Use dedicated equipment and processing areas for unenriched and enriched materials. Contamination of unenriched materials (fertilizer, soil, or plant) by enriched materials and vice versa can drastically affect results.
2. Collect and process ¹⁵N enriched soil and plant samples in order of lowest to highest ¹⁵N expected enrichment to minimize cross-contamination. Ensure that work surfaces, gloves, utensils, and machinery are thoroughly cleaned between each sample to minimize cross-contamination from sample carryover.
3. Minimize foot traffic in microplots to prevent contamination of unenriched sampling areas. Wear protective shoe coverings when accessing microplots and remove them when exiting the microplot area.

4. ¹⁵N enriched fertilizer preparation and application

1. Following guidelines put forth by Ref. 2 for fertilizer ¹⁵N use efficiency (F¹⁵NUE) studies, dilute 10 atom % ¹⁵N enriched urea to 5 atom % ¹⁵N enriched urea and dissolve in 2 L of deionized water to ensure uniform enrichment of urea fertilizer.
   NOTE: The required concentration of ¹⁵N enriched fertilizer is dependent on the goals of the agronomic study. If the concentration of stock ¹⁵N enriched fertilizer exceeds the researcher's requirements, the stock fertilizer concentration may be diluted with similar conventional fertilizer using the following formula³.
   X2 = [(C1/C2) - 1] × X1
   X2 is the mass of the conventional unenriched fertilizer, X1 is the mass of the tracer fertilizer, C1 is the isotopic concentration [expressed as atom % excess (measured atom % enrichment minus the natural background concentration assumed to be 0.3663 atom %)] of the original tracer fertilizer, and C2 is the isotopic concentration of the final mixture. As an example, given 100 g of 10 atom % enriched urea, 92.7 g of conventional unenriched fertilizer would be required for a final isotopic concentration of 5 atom %;
   X2 = {[(10-0.3663)/5] - 1} × 100.
2. Analyze the solution for ¹⁵N concentration to verify enrichment. The authors utilized the analytical services provided by UC Davis Stable Isotope Facility.
   NOTE: Reactions of the soil-plant-microbe regime to fertilizer additions may be affected by the physical form of fertilizer. Depending on the goals of the study, the urea solution may be applied as a liquid or dehydrated to reform crystals. The crystals may be compacted into a cake using a Carver press at 10,000 psi, followed by crushing the cake and screening the particles to the desired size³.
3. Evenly apply the ¹⁵N enriched urea solutions to the microplots using a calibrated backpack CO₂ sprayer (Figure 3A). If multiple N rates or enrichment levels are used, consider using designated CO₂ sprayers for each enrichment level or use a single sprayer and apply solutions from the lowest to the highest enrichment to minimize treatment cross-contamination.
4. Incorporate urea-containing fertilizers with light tillage, hand rakes, or at least 0.6 cm of irrigation within 24 h of application to minimize volatilization loss potential.
5. No additional ¹⁵N enriched urea fertilizer is applied to the microplot during the second growing season. Apply conventional unenriched urea to the entire treatment to avoid a differential response in corn growth due to nitrogen.

5. Field sample processing: aboveground corn biomass

1. At each sampling stage, collect a six-aboveground corn plant composite sample from within the sampling area (¹⁵N unenriched) and a six-aboveground corn plant composite sample from the ¹⁵N enriched microplot. At least two plants should separate each sampled plant to avoid significantly altering plant growth dynamics. The authors collected plant samples at the V8 and R1 corn physiological development stages¹¹ and at physiological maturity (Figure 2).
2. Following the principles described in steps 3.1 and 3.2, chop V8 and R1 aboveground biomass (≤5 cm by ≤5 cm); a yard waste chipper is a satisfactory option. Place chopped biomass in labeled fabric or paper bags and dry in a forced-air oven at 60 °C until constant mass. Record the biomass dry weight (Figure 3B).
3. Partition physiologically mature corn plants into stover (all vegetative tissues including leaves, husks, and stalks), grain, and cob fractions. Chop and dry in a forced-air oven at 60 °C until constant mass. Record the biomass dry weight.
4. Within the microplot, cut all corn stalks at the soil surface, tie into a bundle, label according to plot, and remove from the field (Figure 3C). Adjust microplot corner flags to be nearly flush with the soil surface to minimize the risk of removal by the combine during harvest or tillage post-harvest.
5. Harvest grain from the harvest area and report yield at 15.5% moisture content¹². Harvest remaining research areas with a plot combine.
6. Rake unenriched biomass from off the microplot area. Chop and reapply microplot aboveground biomass to the correct plot (Figure 3D).
7. Incorporate residue into the soil surface with tillage taking care to minimize soil and corn residue transport into or out of the microplot area. Replace any microplot corner flags removed due to tillage.
8. Plant second-year corn on the same rows as the first-year corn.
9. Collect second-year aboveground corn biomass only at physiological maturity and process like first-year corn samples as described in step 5.3. Collect microplot samples from the center of the microplot area (1.52 m by 0.76 m) to avoid any potential signal dilution following tillage (Figure 2). Harvest grain from the harvest area and report yield at 15.5% moisture content.
10. Following the principles of steps 3.1 and 3.2, thoroughly mix and grind 100 to 200 g of dried plant material to pass through a 2 mm sieve. Thoroughly mix the ground material and store a subsample in a labeled coin envelope for further processing.
    NOTE: A Thomas Wiley mill is a satisfactory option for plant tissue grinding while a Perten Laboratory Mill 3610 is a satisfactory option for grinding grain.
    CAUTION: People grinding plant samples should wear ear protection and be protected from inhaling dust by wearing a National Institute for Occupational Safety and Health approved N95 Particulate Filtering Facepiece Respirator.

6. Field sample processing: soil

1. Collect first-year soil samples 8 days after ¹⁵N enriched fertilizer application, V8, R1, and post-harvest before tillage. Collect second-year soil samples at pre-plant and post-harvest. Due to logistical sampling constraints, the authors collected in-season soil samples at 0- to 15-, 15- to 30-, and 30- to 60-cm depths, post-harvest soil samples at 0- to 15-, 15- to 30-, 30- to 60-, and 60- to 90-cm depths, and second-year pre-plant soil samples at 0- to 30-, 30- to 60-, 60- to 120- cm depths.
   NOTE: If a soil probe is unable to collect a soil core to the deepest desired depth as a single core, collect deeper depth cores from the same boreholes as the upper depths discarding the top 1-cm of soil to avoid contamination from soil falling from upper depths.
   1. Collect a four-core (1.8-cm diameter) composite soil sample from the unenriched sampling area at V8 and R1 using a hand probe. Collect one core in the corn row and three cores between the corn rows.
   2. Collect a two-core (5-cm diameter) composite soil sample from the unenriched sampling area at pre-plant and post-harvest using a hydraulic probe.
   3. Collect a 15-core (1.8-cm diameter) composite soil sample from the microplot area 8 days after ¹⁵N enriched fertilizer application, V8, and R1 using a hand probe. Collect three to four cores in the corn row and 11 to 12 cores between the corn rows.
      NOTE: Soils are extremely heterogeneous. The greater number of cores collected from within the enriched microplot provides a better estimate of the true ¹⁵N enrichment of soil N¹³.
   4. Collect a three-core (5-cm diameter) composite soil sample from the microplot area at pre-plant and post-harvest using a hydraulic probe.
   5. Homogenize each composite soil sample in a bucket and place it in a pre-labeled paper bag.
2. Dry soil samples at 35 °C in a forced-air oven until constant mass. Grind each sample to pass through a 2 mm sieve. A mechanical soil grinder is satisfactory if it can be thoroughly cleaned between each sample.
   NOTE: Soil samples may be air-dried by spreading samples on trays in a thin layer. Trays should be in an area free from contamination by outside N sources. Unenriched and enriched samples should be physically separated to prevent cross-contamination.
   CAUTION: People grinding soil samples should wear ear protection and be protected from inhaling dust by wearing a National Institute for Occupational Safety and Health approved N95 Particulate Filtering Facepiece Respirator.

7. Lab sample processing: grind soil and plant samples

1. Dry ground plant samples (2 mm) overnight in an oven at 60 °C.
2. Following the principles described in step 3, grind dried plant samples or soil material to a fine, flour-like consistency. A roller jar mill is a satisfactory option.
   NOTE: The authors' jar mill is a custom-built conveyor belt system that can process 54 roller jars at a time.
   1. Fill each roller jar (250 mL borosilicate glass jar with a screw-top lid) with 10 to 20 g of ground plant or soil sample and seven stainless steel rods (8.5 cm long, 0.7 cm diameter).
   2. Roll roller jars at 0.4 x g for 6-24 h or until samples have a fine, flour-like consistency.
   3. Transfer the finely ground material into a clean, labeled 20 mL scintillation vial.
   4. Between each sample, wash roller jars, stainless steel rods, and lids with soap and water to remove any residue.
      1. Immerse roller jars and lids in a 5% HCl acid bath (prepared from 36-38% concentrated stock) overnight¹⁴.
         CAUTION: Hydrochloric acid is corrosive. It can cause severe skin burns, eye damage, and is harmful if inhaled. Always wear protective clothing, gloves, and eye and face protection. Flush contacted tissue thoroughly with water. Always use a secondary container when transporting acids. Always add acid to water as this reaction is exothermic. Immediately neutralize acid spills with baking soda.
         NOTE: A large acid bath may be prepared as 100 L of 5% HCl in a 208 L plastic container. Prepare several smaller volumes in a fume hood and then transfer the solutions to the plastic container. Replace the solution every three months.
      2. Triple rinse roller jars and lids with deionized water and air dry.
      3. Immerse stainless steel rods in a 0.05 M NaOH bath (prepared by dissolving 2 g of NaOH in 1 L of deionized water) overnight¹⁴. Prepare a new 0.05 M NaOH bath each day.
         CAUTION: Sodium hydroxide can cause severe skin burns and eye damage. Always wear protective clothing and eye protection. Immediately remove contaminated clothing and rinse skin or eyes with water for several minutes.
      4. Rinse the rods under running hot tap water for 5 minutes. Decant and triple rinse the rods with deionized water. Allow the rods to air dry on a paper towel-lined tray.

8. Weigh ground plant and soil samples for total N and ¹⁵N analysis

1. Analyze a few representative plant and soil samples for total N content (e.g., combustion analysis¹⁵). Calculate the sample mass that provides adequate N content for ¹⁵N analysis according to the analyzer specifications.
   NOTE: The authors utilized the analytical services provided by UC Davis Stable Isotope Facility. Enriched sample weights were optimized for 20 µg of N with a maximum of 100 µg of N.
2. Organize like-samples from lowest to highest expected ¹⁵N enrichment. Duplicate every eighth to twelfth sample in each run to check sample precision. Include at least one check sample per run¹⁶.
3. Label a clean 96-well plate and fitted lid with individual well evaporation rings. Cut a clean index card to fit just inside the lid to prevent sample movement between wells during transport.
4. Wearing nitrile gloves, clean the microscale, work surfaces, spatula, and forceps with laboratory wipes and ethanol. Place cleaned utensils on a Kimwipe on the lab bench.
   NOTE: Unenriched and enriched samples should be processed using separate scales and utensils to prevent cross-contamination.
5. Use forceps to place a pre-formed 5 mm x 9 mm tin capsule on a clean work surface, such as a stainless steel block with 5 mm x 8 mm well. Gently tap the capsule into the well to reform the cylindrical shape and flatten the bottom of the capsule if needed.
   NOTE: Because sample masses will be very small, the risk of sample contamination is high. Never touch the capsules with gloves. Discard the capsule if it touches any surface other than the forceps, clean work surface, scale weigh pan, or 96-well plate.
6. Use forceps to gently flare out the top 1 mm of the capsule to facilitate manipulation. To avoid scale damage when taring the weight of the capsule, hover and release the capsule 1 to 2 mm above the microscale weigh pan. Tare the capsule. Use forceps to return the capsule to the clean work surface.
7. Use a spatula to carefully add the required mass of finely ground sample material to the capsule. Avoid spilling sample material on the outside surface of the capsule or the work surfaces.
8. Using forceps, slowly crimp the top third of the capsule and fold over to seal. Using forceps, continue to fold and compress the capsule into a spherical shape taking care not to puncture or tear the tin.
   NOTE: Samples with low N content may require sample volumes that exceed the capacity of the 5x9 mm capsule. Larger capsules (e.g., 9 mm x 10 mm) may be used in these instances.
9. Use forceps to drop the wrapped capsule several times from a height of 1 cm onto a clean, dark surface or mirror to check for leaks. If no dust appears, weigh the sample using the same technique as described in step 8.6. Record the sample weight. Place the capsule in a 96-well plate and record the well placement.
   1. If dust appears on the dark surface, record the sample weight. Wrap the sample in a second tin capsule, recheck for leaks, and place it in a clean 96-well plate.
      NOTE: If the wrapped capsule is too large to fit in a 96-well plate, use a 24- or 48-well plate.
10. Between samples, clean each of the utensils and surfaces with ethanol and laboratory wipes paying especial attention to the spatula and forceps edges.
11. Secure the lid to the 96-well plate using tape and store in a desiccator.

9. Calculations

1. Calculate the mass of N (kg∙ha^-1) contained in the plant or soil samples using the following equations.
   
   
2. Calculate the fertilizer N fraction (N_f), fertilizer derived N (FDN), and soil derived N (SDN) for plant and soil samples¹⁷.
   
   where A is the atom % ¹⁵N enrichment.
3. 
   
4. Calculate fertilizer ¹⁵N use efficiency¹⁷.
   

Representative Results

The results presented in this paper come from a field site established in 2015 at the University of Minnesota Southern Outreach and Research Center located near Waseca, MN. The site was managed as a corn-soybean [Glycine max (L.) Merr] rotation prior to 2015 but was managed as a corn-corn rotation during the 2015 and 2016 growing seasons. The soil was a Nicollet clay loam (fine-loamy, mixed, superactive, mesic Aquic Hapludolls)-Webster clay loam (fine-loamy, mixed, superactive, mesic Typic Endoaquolls) complex. Soil fertility was managed according to university guidelines except for N¹⁸. Several N fertilizer treatments were arranged in a randomized complete block design with four replications but only the 135 kg N∙ha^-1 rate applied as urea at planting is presented in this paper. Soil bulk density was measured at the center of 0- to 15-, 15- to 30-, 30- to 60-, 60- to 90-, and 60- to 120-cm depth layers from two 5-cm deep samples per replication using the intact core method¹⁹. Bulk density was averaged within depth across replications and assumed to be constant across the field. Plot setup and plant and soil samples were collected and processed as described in the protocol section.

Total (FDN + SDN) aboveground biomass N increased with each successive sampling event over the first growing season (Figure 4). Fertilizer derived N concentration was greatest earlier in the growing season accounting for 44 ± 4% (mean ± standard error) of the total aboveground biomass N at V8 and decreased with each successive sampling period (Figure 4A). However, SDN consistently was the greatest fraction of aboveground biomass N illustrating the importance of soil N supply for optimal corn growth. At physiological maturity in the first year, 27 ± 1% of aboveground biomass N was from FDN with similar proportions in grain, stover, and cob fractions (Figure 4B). At physiological maturity in the second year, only 2 ± 0.1% of first-year FDN was recovered in the aboveground biomass with 1.6 ± 0.2 kg of first-year FDN ha^-1 exported in the grain (Figure 4A).

The soil-crop FDN budget is useful for quantifying FDN cycling within the system over time. Within 8 d of fertilizer application, the majority of FDN was in the top 15 cm of the soil profile, as expected (Figure 5). However, 22.2 ± 4.4 kg N ha^-1 had already moved into the deeper depths while 4 ± 10% of the FDN was unaccounted for. Unaccounted-for FDN is likely primarily driven by N loss mechanisms including leaching, denitrification, and volatilization that either move FDN below the soil sampling depths or remove the FDN from the system entirely. At V8 and R1, unaccounted-for FDN increased to 60.4 ± 4.7 kg N ha^-1 on average while soil N (0-15 cm) was 31.6 ± 6.8 kg N ha^-1 on average. Corn's rapid growth and high N demand from V8 to R1 resulted in an increase of 19.0 ± 4.4 kg FDN ha^-1 in aboveground plant biomass mirroring the 17.7 ± 5.2 kg FDN ha^-1 reduction from the 15- to 60-cm soil depths. Soil temperature and moisture conditions between these corn development stages tend to favor microbial growth resulting in rapid turnover of organic residues and re-utilization of mineralized N. These results suggest that corn roots mined inorganic FDN from the 15- to 60-cm depths while FDN in the 0- to 15-cm depth was primarily cycled between soil organic matter and microbial fractions. Additional isotopic analysis of soil inorganic and organic N pools is necessary to validate this hypothesis and provide greater detail and insight into FDN cycling dynamics¹⁰. By post-harvest year 1, 59 ± 2% of the original FDN was unaccounted for while 18.1 ± 3.9 kg FDN ha^-1 was in the top 30 cm of the soil (Figure 5) and 22.1 ± 2.3 kg FDN ha^-1 was exported in the grain (Figure 4B). Fertilizer ¹⁵N use efficiency was 24% (Equation 7) and is at the low end of commonly reported F¹⁵NUE measures (25-45%) reported by other studies²⁰. Although equipment was thoroughly cleaned between each sample, the lower F¹⁵NUE measures of the study could be an artifact of enriched sample dilution by processing enriched samples in order of lowest to highest expected enrichment. The amount of FDN in the top 30 cm doubled (36.0 ± 5.2 kg FDN ha^-1) from post-harvest year 1 to pre-plant year 2 due to partial residue breakdown since the previous fall but by post-harvest year 2 only 17.3 ± 3.3 kg FDN ha^-1 was still found within the soil-corn system (Figure 5). This study indicates that by the end of the first and second years, only 41 and 29%, respectively of first-year FDN was accounted for within the soil-corn system (including FDN exported in the grain) while the remainder was either lost to the environment or leached below the 90 cm soil sampling depth.

Spurious results may be obtained when samples are cross-contaminated affecting calculations of N_f, FDN, and SDN. For example, suppose a ¹⁵N enriched plant sample with an actual enrichment of 3.000 atom % ¹⁵N is contaminated with unenriched material diluting the ¹⁵N concentration to 2.500 atom % ¹⁵N. Further, assume Total N_Plant is 100 kg N ha^-1, the atom % ¹⁵N enrichment of the fertilizer was 5.000, and the atom % ¹⁵N enrichment of the unenriched plant sample was 0.366. The ¹⁵N enriched plant sample N_f would be reduced from 0.568 (actual) to 0.461 (contaminated sample) underestimating the true FDN by 10.7 kg N ha^-1. Overestimations of FDN may occur when samples with low ¹⁵N enrichment are contaminated with additional ¹⁵N. Thus, extreme care should be taken in all steps of sample collection and processing to minimize sample contamination, but most especially when sample masses are reduced (e.g., grinding and weighing procedures).

Figure 1: Plot design for the treatment plot and microplot. The figure illustrates the dimensions and relative placements of the border areas, unenriched sampling area, harvest area, and microplot area within the treatment plot. Please click here to view a larger version of this figure.

Figure 2: Microplot plant and soil sampling diagram. The figure illustrates the relative plant and soil sampling positions at each sampling stage that avoids altering corn N uptake patterns of later sampled corn plants. Sampling events occurred 8 days following the ¹⁵N enriched fertilizer application, at the V8 and R1 corn physiological development stages, at physiological maturity in the year of ¹⁵N enriched fertilizer application (PMY1) and the following year (PMY2), and prior to planting the second year (PPY2). Please click here to view a larger version of this figure.

Figure 3: Chronological depiction of microplot management. (A) Dissolve ¹⁵N enriched urea into 2 L of deionized water and spray onto the microplot at planting. (B) Collect and chop a six-aboveground corn plant composite sample from within the sampling area (¹⁵N unenriched) and a six-aboveground corn plant composite sample from the ¹⁵N enriched microplot at the pre-determined sampling times. (C) Following sample collection at physiological maturity, remove all remaining aboveground biomass from within the microplot. (D) Post-harvest, rake unenriched aboveground corn biomass from the microplot area. Chip and reapply the microplot corn aboveground biomass to the microplot area. Please click here to view a larger version of this figure.

Figure 4: Example of aboveground biomass N partitioned into fertilizer derived N (FDN) and soil derived N (SDN) fractions. Total aboveground biomass N was separated into its individual sources of FDN (solid color) and SDN (hashed color) in (A) and (B). Error bars represent the standard error of the mean. (A) Aboveground biomass N was measured at the V8 and R1 corn physiological development stages and at physiological maturity in the year of ¹⁵N fertilizer application (PMY1) and the year following ¹⁵N fertilizer application (PMY2). The value above each column represents the percentage of the total N that was FDN. (B) Aboveground biomass N measured at PMY1 and PMY2 is shown in its individual parts of cob (only Year 1), stover (stalk and leaves; includes cob for PMY2), and grain for FDN and SDN. Please click here to view a larger version of this figure.

Figure 5: Example of the soil-corn fertilizer derived N (FDN) budget. The mass of FDN recovered in aboveground (Abvgd) corn biomass and at various soil sampling depths is reported for six sampling events over two growing seasons. Sampling events occurred 8 days following the ¹⁵N enriched fertilizer application (PA), at the V8 and R1 corn physiological development stages, at physiological maturity in the year of ¹⁵N enriched fertilizer application (PMY1) and the following year (PMY2), and prior to planting the second year (PPY2). The difference between the applied fertilizer rate (135 kg N ha^-1) and the mass of FDN recovered in the soil-corn portions is the unaccounted for FDN fraction. The total mass of FDN for PPY2 and PMY2 was 113 kg FDN ha^-1 because 22 kg FDN ha^-1 was exported out of the soil-corn system as first-year grain. Error bars represent the standard error of the mean. Please click here to view a larger version of this figure.

Discussion

Stable isotope research is a useful tool for tracking and quantifying FDN through the soil-crop system. However, there are three main assumptions associated with N tracer studies that if violated may invalidate conclusions drawn from using this methodology. They are 1) the tracer is uniformly distributed throughout the system, 2) processes under the study occur at the same rates, and 3) N leaving the ¹⁵N enriched pool does not return³. Because this study is interested in the distribution of total FDN throughout the soil-crop system, assumptions 2 and 3 are of minimal concern²¹.

The high cost of ¹⁵N enriched material generally limits the size of ¹⁵N tracer studies. Therefore, prior to initiating a N tracer study, the researcher should carefully plan the research project's objectives considering: the number of sampling events, the length of the study (days to years), the N fertilizer application rate, and the ¹⁵N enrichment concentration required to measure differences from natural abundance (0.366 atom %) following ¹⁵N enriched fertilizer dilution by bulk soil². Commonly used ¹⁵N enrichment levels and application rates are reported for different types of agronomic research in Ref. 2. After determining the study objectives, the microplot must be sufficiently large to accommodate soil and plant sampling and avoid edge effects. The plot design described in this protocol uses a non-confined plot requiring that non-sampled border areas be employed⁶. The ¹⁵N concentration in border areas is diluted by mass flow across the microplot boundary and N uptake from outside the microplot by lateral corn roots growing in rows 1 and 6. Confined plots, where physical barriers are driven into the soil, do not require border areas but do require additional work during microplot establishment and may limit routine field operations⁶. References 3, 6, 22-25 provide additional guidance on selecting microplot sizes, border widths, and when confined or non-confined plots may be most appropriate.

The plant and soil sampling scheme of this study is designed to allow for multiple sampling events over two consecutive growing seasons. Early season plant and soil samples are taken near the outside edges of the microplot. Each successive sampling event moves closer to the center of the microplot to avoid sampling previously sampled areas. At least two corn plants separate each sampled plant to minimize changes in corn physiological development. One challenge with this study's soil sampling technique is that the soil core sampling method may not accurately intercept the heterogeneous distribution of ¹⁵N in the soil profile³. Spatial variability of soil total N is high with an estimated coefficient of variation of 15%³. Complete microplot excavation would improve ¹⁵N quantification accuracy but requires processing significant volumes of soil and limits sampling to a single event³ that is not in line with the objectives of this study. Subdividing the microplot into smaller sampling units allows for multiple excavation events but may increase the required microplot size to ensure non-sampled units are unaffected by modifications to the crop canopy and soil water dynamics. Despite the potential reduction in accuracy, many studies use the soil core technique for microplots ≥1 m^{2 9,22,26,27,28}. Sample precision may be increased by increasing the number of soil cores collected and composited per microplot using the following formula¹³:

n = (Z²)(CV²)/(d²)

where n is the number of soil cores, Z is the standardized normal variate for the corresponding alpha level (1.96 for 0.05 and 1.65 for 0.10), CV is the coefficient of variation, and d is the margin of error in the plot mean (as a decimal). Based on this formula, the authors expect that 15 cores per microplot would estimate total N to ±7.6% on 95% of the plots (n = 15; Z = 1.96; CV = 15%; d = 0.076). Reference 25 used a similar number of cores but subdivided the microplot into 32 sampling units collecting plant and soil samples from four units at each sampling event.

Others have shown that the microplot data can be extrapolated to the entire plot²⁹. However, for this assumption to be valid, the treatment plot and microplot must be similarly managed. If possible, fertilizer N should be applied in the same chemical and physical forms (e.g., urea dissolved in water) as these properties impact fertilizer-soil dynamics including N loss mechanisms, immobilization, and availability to soil microbes and plants³.

The roller jar grinding method described in this protocol is capable of pulverizing large volumes of plant and soil samples, ideal for ensuring a representative, homogenized sample. However, the technique requires significant manual labor and time to load, unload, roll, and clean the roller jars. Sample processing is limited by the available number of roller jars, the capacity of the conveyor belt unit, and the size of the acid bath. Commercial grinding vials may be an alternative to roller jars but may limit the volume of plant and soil samples processed. Lab-made, single-use grinding vials may be constructed that potentially serve as both the grinding and sample storage vessel. The main consideration of any of these grinding methods is to minimize cross-contamination between samples.

Finally, because ¹⁵N enriched fertilizer material is expensive, ¹⁵N enriched aboveground biomass and soil samples may be retained and homogenized for use in future studies. These products may be especially useful when investigating residue decomposition, mineralization potential, or other nutrient cycling processes²¹.

Materials

Name	Company	Catalog Number	Comments
20 mL scintillation vial	ANY; Fisher Scientific is one example	0334172C
250 mL borosilicate glass bottle	QORPAK	264047
48-well plate	EA Consumables	E2063
96-well plate	EA Consumables	E2079
Cloth parts bag (30x50 cm)	ANY	NA	For corn ears
CO2 Backpack Sprayer	ANY; Bellspray Inc is one example	Model T
Coin envelop (6.4x10.8 cm)	ANY; ULINE is one example	S-6285	For 2-mm ground plant samples
Corn chipper	ANY; DR Chipper Shredder is one example	SKU:CS23030BMN0	For chipping corn biomass
Corn seed	ANY	NA	Hybrid appropriate to the region
Disposable shoe cover	ANY; Boardwalk is one example	BWK00031L
Ethanol 200 Proof	ANY; Decon Laboratories Inc. is one example	2701TP
Fabric bags with drawstring (90x60 cm)	ANY	NA	For plant sample collection
Fertilizer Urea (46-0-0)	ANY	NA	~0.366 atom % 15N
Hand rake	ANY; Fastenal Company is one example	5098-63-107
Hand sickle	ANY; Home Depot is one example	NJP150	For plant sample collection
Hand-held soil probe	ANY; AMS is one example	401.01
Hydraulic soil probe	ANY; Giddings is one example	GSPS
Hydrochloric acid, 12N	Ricca Chemical	R37800001A
Jar mill	ANY; Cole-Parmer is one example	SI-04172-50
Laboratory Mill	Perten	3610	For grinding grain
Microbalance accurate to four decimal places	ANY; Mettler Toledo is one example	XPR2
N95 Particulate Filtering Facepiece Respirator	ANY, ULINE is one example	S-9632
Neoprene or butyl rubber gloves	ANY	NA	For working in HCl acid bath
Paper hardware bags (13.3x8.7x27.8 cm)	ANY; ULINE is one example	S-8530	For soil samples and corn grain
Plant grinder	ANY; Thomas Wiley Model 4 Mill is one example	1188Y47-TS	For grinding chipped corn biomass to 2-mm particles
Plastic tags	ULINE	S-5544Y-PW	For labeling fabric bags and microplot stalk bundles
Sodium hydroxide pellets, ACS	Spectrum Chemical	SPCM-S1295-07
Soil grinder	ANY; AGVISE stainless steel grinder with motor is one example	NA	For grinding soil to pass through a 2-mm sieve
Tin capsule 5x9 mm	Costech Analytical Technologies Inc.	041061
Tin capsule 9x10 mm	Costech Analytical Technologies Inc.	041073
Urea (46-0-0)	MilliporeSigma	490970	10 atom % 15N