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 ferti....

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, ini....

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, m.......

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 distributio.......

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