In the Midwestern United States, corn-soybean rotation is an essential agricultural practice to increase crop yield, often known as the rotation effect. Plant-parasitic nematodes, particularly soybean cyst nematode (SCN, Heterodera glycines) in soybean and Pratylenchus in corn, may contribute to the rotation effect and the nematode community is a sensitive indicator of changes in soil ecology. A long-term research site in Waseca, Minnesota was established in 1982 to study corn-soybean rotation. At the site, various corn-soybean crop sequences can be compared each year including corn and soybean in 1 to 5 years of monoculture, annual rotation of each crop, and continuous monoculture of each crop. Granular nematicides (terbufos or aldicarb) have been applied to half of each plot since 2010 to minimize nematode populations across crop sequences. If successful, the rotation yield effect could be compared with unaltered nematode populations and with minimal nematode populations to determine the role of nematodes in the rotation effect. For this thesis, crop yield and plant-parasitic nematode populations were measured from 2010-14 while the nematode community was assessed from 2013-14 at the long-term rotation research site. Specifically, the objectives of this research were to: (i) investigate the impact of crop sequences and nematicide application on plant-parasitic nematode populations and crop yield (Chapter 2), (ii) determine the role of plant-parasitic nematode populations in the corn-soybean rotation effect using nematicide application (Chapter 2), and (iii) study the impact of crop sequences and nematicide application on soil ecology based on the nematode community (Chapter 3). Research related to these objectives is reviewed in Chapter 1. SCN egg population densities significantly increased in soybean and decreased in corn—particularly in the initial 3 years of monoculture-- so populations were significantly greater in soybean than corn monoculture for most sequences (P ≤ 0.05, Fischer’s LSD). After two to three years of corn monoculture, SCN populations were below 200 eggs/100 cm3 soil, the proposed damage threshold guideline for SCN, in most seasons. Pratylenchus populations significantly decreased in soybean monoculture and increased in corn monoculture—particularly in the initial 3 years of monoculture-- leading to significantly greater Pratylenchus populations under corn than soybean monoculture for most sequences (P ≤ 0.05, Fischer’s LSD). Helicotylenchus population densities were similar among many crop sequences, but significantly greater in extended corn monoculture and smaller in extended soybean monoculture than most sequences (P ≤ 0.05, Fischer’s LSD). Xiphinema was present at the site, but population densities were small at an average of 8 nematodes/100 cm3 soil across plots and seasons. Corn yields decreased significantly in monoculture, particularly in the initial 3 years in monoculture (P ≤ 0.05, Fischer’s LSD). Soybean yields also decreased significantly in monoculture, often decreasing throughout the length of monoculture tested when comparing crop sequence treatments within single years (P ≤ 0.05, Fischer’s LSD). Aldicarb nematicide consistently significantly decreased Pratylenchus, Helicotylenchus, and Xiphinema populations but was inconsistent against SCN (P ≤ 0.05, ANOVA). Aldicarb nematicide consistently increased corn yield and decreased Pratylenchus and Helicotylenchus populations (P ≤ 0.05, ANOVA), suggesting these nematodes, particularly Pratylenchus, may have decreased corn yield. Aldicarb nematicide also significantly increased soybean yield in 2012 and 2013, but decreased SCN populations did not consistently correspond with increased soybean yield (P ≤ 0.05, ANOVA). While nematicide reduced nematode populations, there was still significant variation across crop sequences in most seasons (P ≤ 0.05, ANOVA), so nematicide application was not a successful method for determining the role of nematodes in the rotation effect in most seasons. However, the strong influence of crop sequences on SCN and Pratylenchus populations suggest nematodes have a role in the rotation effect. Nematicide application also impacted non-target nematodes and thus soil ecology with significantly decreased fungivore and bacterivore populations, diversity, and maturity; but significantly increased enrichment (P ≤ 0.05, ANOVA). The nematode community and soil ecology was significantly different in corn compared to soybean cropping systems and changed most during initial years of crop monoculture (P ≤ 0.05, Fischer’s LSD). Cropping systems in corn supported significantly greater fungivore populations, fungal decomposition pathways, more diversity, and a more mature ecosystem compared to soybean systems (P ≤ 0.05, Fischer’s LSD). Soybean systems supported significantly greater bacterivore populations and a more disturbed, enriched ecosystem (P ≤ 0.05, Fischer’s LSD). These differences between corn and soybean systems suggest nutrient mineralization by nematodes and other microorganisms may play a role in the benefits of crop rotation for plant growth.
University of Minnesota Ph.D. dissertation. May 2015. Major: Plant Pathology. Advisor: Senyu Chen. 1 computer file (PDF); vii, 115 pages.
Determining the role of plant-parasitic nematodes in the crop rotation yield effect, and the influence of crop rotation and nematicide application on the nematode community.
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