The Root of the Matter – Are Changes in Corn Root Morphology Responsible for Improved Yield and Higher Nitrogen Use Efficiency in Diversified Cropping Systems?

Date: 
Aug 2019

Issue

There is an important need to develop sustainable alternatives to the corn-soybean cropping system that dominates the upper Midwest, as evidenced by recurring problems with water quality, susceptibility to weather extremes and low profitability. Many of the shortcomings of the existing corn-soybean system are due to its sole reliance on short-season annual crops and the exclusion of deep-rooted perennial crops like alfalfa that provide long periods of living cover. The Iowa Nutrient Reduction Strategy noted that "extended rotations" can reduce both applications of nitrogen (N) inputs and undesired N discharges to the environment.

Objective

Researchers will investigate mechanisms whereby extending a corn-soybean rotation with small grain and legume forage crops can simultaneously increase corn yield and N-use efficiency. We expect that soil resistance to corn root penetration will be lower, corn root systems will be larger and deeper, and the rate of potential N mineralization from soil organic matter will be greater for corn following alfalfa than following soybean. Empirical data from these research plots will be used within the Agricultural Production Systems Simulator model (APSIM), which can assess the impact of various soil, climate and crop management factors on crop productivity, nitrate leaching and nitrous oxide emissions to the atmosphere. Insights from this project will inform strategies promoting higher agronomic productivity, greater N use efficiency and lower potential for N discharges to water and air. This project will use findings to parameterize a widely used crop growth model, the Agricultural Production Systems sIMulator (APSIM).

Approach

We will measure soil physical properties, corn root architecture, soil N dynamics, and corn performance in a long-term experiment in Boone County, Iowa, that includes a two-year corn-soybean rotation and a four-year corn-soybean-oat/alfalfa-alfalfa rotation. Data will be analyzed with linear mixed-effects models and non-linear regression techniques to evaluate the effects of rotation system and time since planting on corn shoot mass, N uptake and root mass and depth of soil penetration. Root front velocity, maximum root depth, root mass and specific root length (root length per unit of root mass) will be used in the APSIM model to evaluate corn growth and yield, and N discharges from soil, in the contrasting rotation systems.

We will produce scientific publications and conduct outreach activities to share information about the effects of extended rotations on corn root growth, yield and water quality.

Project Updates

December 2019

Measurements to quantify corn root distribution and mass were made in 2019 in two contrasting cropping systems, a 2-year corn-soybean rotation and a 4-year corn-soybean-oat/alfalfa-alfalfa rotation that periodically received cattle manure. Plots of the two cropping systems were present in four replicate blocks within a 9-hectare experiment established in 2001 at the Iowa State University Marsden Farm in Boone County. Corn planting was delayed by wet weather until June 3, 2019. Maximum corn root depth was determined seven times during the growing season, from 23 to 105 days after planting, based on four cores per plot drawn with a 19-mm.-diameter soil probe. Averaged over all sampling dates, corn root depth was 18.5% greater in the 4-year rotation (68.5 cm) than in the 2-year rotation (57.8 cm). The greatest corn root depth was observed at 77 days after planting, at which time roots were 14.8% deeper in the 4-year rotation (109.5 cm) than in the 2-year rotation (95.4 cm). These observations corroborated our hypothesis that maximum corn root depth would be greater for corn following alfalfa in the 4-year rotation than following soybean in the 2-year rotation.

Corn roots were sampled on five dates, from 3 to 105 days after planting, in 15 cm. increments to a depth of 60 cm. using a 32-mm-diameter soil probe. Four cores were drawn from each plot at a location 10 cm. to the side of corn rows. Soil from each depth increment within a plot was composited and roots were recovered from soil using a sequence of elutriation, flotation and air column separation. Roots are being removed from organic debris using tweezers and a stereo microscope. When that step is complete, roots will be dried and weighed, and comparisons  made by rotation system and depth increment. At the same dates that root samples were collected, corn shoot mass was measured by harvesting, drying and weighing eight plants per plot. Shoot mass data are being analyzed.

Samples with which to determine total aboveground corn N uptake were taken at reproductive maturity and are being analyzed. Corn grain yield was determined on Nov. 6, using a combine harvester. Grain yield did not differ significantly (p=0.74) between the 4-year rotation (12.7 Mg ha-1) and the 2-year rotation (12.8 Mg ha-1), though the application rate of nitrogen fertilizer was 58% lower in the 4-year rotation (81 kg N ha-1) than the 2-year rotation (193 kg N ha-1).

Root front velocity, maximum root depth and root mass data will be used in the APSIM model to evaluate corn growth and yield and N discharges from soil in the contrasting rotation systems. The modeling work will be part of a Ph.D. dissertation.

Publications during this period: