N2-fixation and weed competition: breaking the connection between crops and weeds

N2-fixation and weed competition: breaking the connection between crops and weeds

Summary

In 2005, tissue sample processing from the 2004 field experiment continued throughout the year. Due to multiple grindings and microsamplings required for 15N analysis, complete analysis for a season’s worth of samples requires several hundred hours of processing. Greenhouse experiments were conducted in the off season to verify herbicides that could be utilized without affecting mycorrhizal colonization within weeds and soybean cultivars. These herbicides aided in reducing the confounding effects of weed populations in the field. These experiments also confirmed that prickly sida was effectively colonized by mycorrhizae. Prickly sida was used at two research stations as one of our test weed species because sicklepod was not permitted at one research station.

The soybeans and weeds were planted in a timely manner and irrigated as necessary throughout the season when possible. Some weed seedlings were lost because of drought and were replaced by transplants. Pigweed species continued to be problematic with their tendency to flower in mid to late June. Experiments were harvested earlier than originally planned to avoid a large contribution of volunteer spiny amaranth to the soil seed bank. By avoiding this seed rain we sacrificed final yield data in order to remain in good standing with the research farm.

All samples were finished and sent off for isotopic ratio spectrometry analysis by late December. The statistical analysis and sample repeats continue. The variation within these data is high, which is always anticipated with 15N natural abundance analysis, but preliminary data analysis suggests that nitrogen transfer is occurring from soybeans to prickly sida but not to pigweed species (which is consistent with our original hypothesis).
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Objectives/Performance Targets

1. Critically evaluate the role of N transfer in the development of weed patches.
a. Determine N transfer distances and penetration into weed patches, and the impact of transfer on weed population dynamics.
b. Determine the influence of N transfer on weed competitiveness and reproduction.
c. Determine the impact of N transfer on the N nutritional status of weed seeds and the competitiveness of offspring.
2. Compare weed growth, competitiveness, and seed production in traditional organic production systems with the new, modified system that uses a “low N transfer” soybean variety in rotation with a sweetpotato variety that has a lower N requirement.

In 2005 identified management practices that could be used in support of our goal to simulate a low-input/ limited resource farmer situation, weed species that could be substituted and adjusted our approach with lessons learned from the previous season. Speed of plant tissue sampling/processing improved through better equipment, more hourly assistants and fine tuning the experimental design addressing our objectives. This allowed us to obtain information from the previous season before the beginning of the next.

Accomplishments/Milestones

• Identified herbicides to assist in minimizing the confounding effects of volunteer weed populations.

  Identified and used weed substitutes (prickly sida substitutes for sicklepod) that could be utilized on all research locations.

  Planted, maintained, harvested, processed and submitted samples for 15N analysis for all experiments addressing our objectives.

Impacts and Contributions/Outcomes

Identified methods that allow for more efficiency at all levels of data harvesting and processing.

Collaborators: