The Impacts of Native Plant Diversity on Native Bee Development and Soil Health

Progress report for GS19-207

Project Type: Graduate Student
Funds awarded in 2019: $13,101.00
Projected End Date: 08/31/2022
Grant Recipient: University of Arkansas
Region: Southern
State: Arkansas
Graduate Student:
Major Professor:
Dr. Neelendra Joshi
University of Arkansas
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Project Information

Summary:

Current agricultural conventions tend to favor unbroken landscapes of monoculture crops. Though these fields can be efficient for farm equipment, there is evidence that they are inadequate for soil, pollinator, and ecosystem health. With this study, we aim to investigate whether the addition of native plant mixes and a higher diversity of floral resources in an area can improve the nutrient quality of the soil and the healthy development of native bees. We will divide 12 plots of land into three treatment groups: a buckwheat monoculture control and two different seed mixes of native and naturalized North American flowers. To investigate soil health, we will perform an analysis of the soil microbial community using 16S ribosomal RNA sequencing, as well as measuring pH and the levels of nitrogen, potassium, and phosphorus. To assess native bee health, we have constructed wood and mesh cages over each plot to house blue orchard bees (Osmia lignaria). After being exposed to either the buckwheat crop or one of the wildflower mixes, the bees will be measured on several endpoints to determine fitness, including number of offspring produced, larval body weight and development time, and adult emergence from pupae. We will then perform a comparative analysis to determine whether the native flower mixes had a positive effect on soil quality and bee development when compared to the monoculture control.

Project Objectives:
  • Determine whether native seed mixes, when compared to monoculture plantings, have a positive impact on native bee development health.
  • Evaluate the soil quality of plots separately planted with native seed mixes and nonnative monocultures.
  • Recommend more sustainable agriculture systems that support soil health and robust pollinator populations.

Research

Materials and methods:

This is a semi-field study on the effects of different wildflower or monoculture plantings on bee and soil health. The field plots are located at the University of Arkansas Agricultural Research Farm in Fayetteville, AR.

In the spring of 2020, the field plots were planted with either a buckwheat monoculture or with wildflowers. Buckwheat (Fagopyrum esculentum) was chosen due to its fast development time (~4-6 weeks from planting to bloom period) and its long period of bloom. It is also a fairly heat tolerant crop and can grow well in the rising temperatures of late spring in Arkansas. The other eight field plots were planted with wildflowers, four with a “honey bee flower mix” and four with a “eastern pollinator flower mix,” both purchased from the Sustainable Seed Company. The honey bee flower mix notably contains lacy phacelia (Phacelia tanacetifolia), a wildflower that is preferred by Osmia lignaria, as well as several other mason bee species, including Osmia bicornis. Additionally, several wildflower species have already been established at the research site, including dandelion (Taraxacum officinale), red clover (Trifolium pratense), and butterfly weed (Asclepias tuberosa). These wildflowers were removed from the buckwheat plots, but allowed to grow in the wildflower plots. Soil samples were collected from each plot before planting and 3 months following planting. An amount of soil (~56.7 g) was set aside from each plot and stored in a -80°C freezer for the soil microbiome analysis. The bacterial 16S rRNA gene from these samples will be amplified by PCR and then sent to Eurofins Genomics for sequencing, in order to measure the diversity of the bacterial communities. The rest of the samples were dried for a week and then sent to the Marianna Soil Test Laboratory in Marianna, AR for analysis of the pH and nutrient content (levels of nitrogen, phosphorus, and potassium).

Wooden cage frames were built over each plot in 2020, but due to working restrictions related to the Covid-19 pandemic, the cages were not able to be completed in time to release the blue orchard bees in 2020.

In the spring of 2021, we have collected the pre-planting soil samples and planted the buckwheat and wildflowers at the research site. The cage construction has been completed and the cages are currently being screened in with aluminum mesh screen to prevent the bees from escaping. These cages will allow us to control the bees’ diet and deny them access to alternative pollen and nectar sources. Each cage will also include a nesting box for the bees, a clean water source, and a small muddy pit for the bees to collect nesting materials.

 The blue orchard bees (Osmia lignaria) have been ordered from a well-established commercial supplier and will be released in April, to align with their natural emergence time. The bees have arrived in cocoons and are currently being stored until their emergence and release into the field cages. Blue orchard bees were chosen for several reasons. They are a native species, found widespread across the United States. They are efficient pollinators of several fruit, vegetable, and nut crops, and because of this are one of the few solitary bee species to be commercially managed. Finally, they are a generalist species and able to feed on a variety of flower species. Generalist feeders can be more flexible to changes in their diet, but in many cases can benefit from having a variety of floral resources available. Blue orchard bees naturally emerge from their cocoons in mid-Spring, though the exact time can vary depending on the region and weather conditions. We plan to release the bees into the cages in mid-April. Following emergence, the adult bees mate and then the females spend around 20 days collecting pollen and nectar to create a provision for their offspring. During this time, the female bees need adequate access to floral resources, both for their own health and development and to create this food supply for the offspring. While the adult bees are foraging, we will collect data on any feeding preference they may show toward certain wildflower species in the different mixes.

Once the eggs have been laid and the adult bees have died off for the year, we will collect the nest boxes from each plot in order to monitor the weight and development of the offspring. Blue orchard bees prefer to nest in small tubes and tunnels, creating a linear series of cells, with each cell containing an egg and a provision of pollen and nectar. The nest boxes are made of wood and filled with tubes of diameter 0.25″ and length 7″. In each tube we will insert a plastic straw of the same dimensions, which will allow us to safely remove the offspring from the nesting boxes and observe their development. We will observe the time spent in each larval instar and the overall development time from egg to pupa. We will weigh the offspring at their fifth and final larval instar and at their pupal stage. The following spring, we will count the number of offspring that successfully emerge from their cocoons.

Research results and discussion:

Results

Soil samples were collected in March and September of 2020, in order to analyze the nutrient content and microbial communities in the soil. The preparation and analysis of the soil microbiome samples have not yet been completed. The soil nutrient samples were sent to the Marianna Soil Test Laboratory in Marianna, AR. The analysis included the amounts of phosphorus, potassium, zinc, sulfate, nitrate, magnesium, and iron in the soil, as well as the pH and the estimated cation exchange capacity (CEC). The results of the pre-planting and post-planting soil nutrients were compared, as nutrients can fluctuate throughout the year. The post-planting results of the buckwheat, honey bee flower mix, and eastern flower mix groups were then compared to samples taken from nearby grassy fields at the research site. Post-planting samples were analyzed using a one-way ANOVA, by treatment group. In the case of the soil pH and nitrate levels, a nonparametric Kruskal-Wallis H test was performed instead.

Both pH and phosphorus levels were significantly affected by time of year, across all treatment groups (P = 0.0015 and P = 0.0122 respectively). There was a general increase in pH toward neutral and a decrease in phosphorus levels. Potassium and nitrate levels showed no significant change based on time of year (Fig. 1).

There was no significant effect of flower planting type on post-planting soil pH. In all plots, pH was neutral or mildly acidic, ranging from 6.5-7.4. Post-planting nitrate levels were also not significantly affected by flower planting type. The eastern flower planting and honey bee flower planting groups had higher nitrate level averages than both the buckwheat and grassy field groups, though this difference was not significant.

There was a significant effect of flower planting on post-planting potassium levels, though this was only seen between the grassy field and buckwheat groups (Fig. 2). The grassy field has significantly higher potassium levels. Both the eastern flower mix and the honey bee flower mix had average potassium levels higher than the buckwheat and lower than the grassy fields, but these differences were not significant.

Both of the wildflower planting groups, the eastern mix and the honey bee mix, had significantly higher post-planting phosphorus levels than the grassy field samples (Fig. 3). There was a general across all treatment groups of phosphorus levels decreasing from the March pre-planting samples to the September post-planting ones. The wildflower groups retained higher phosphorus levels later in the year than the buckwheat and grassy field groups.

Discussion

With modern agriculture, there has been a rise in more intensive growing systems, including monoculture plantings, increased use of pesticides and fertilizers, and frequent tilling. There has also been an increase in the amount of land used either as rangeland for livestock or fields for crop plantings. These changes have brought with them a great deal of investigation into how these intensive agricultural practices may effect ecosystem health and plant growth. Monoculture plantings without crop rotation, for example, have been shown to cause decreasing yields over time. This is likely due to the depletion of soil nutrients taken up by the crop plants, which over time can interfere with plant growth. Crop producers often deal with these problems either through the implementation of a crop rotation system or through the heavy use of fertilizers, though the use of fertilizers can cause further issues, such as algal blooms in local water supplies. As such, it is important to investigate how different agricultural methods can affect soil health and ways to ameliorate the negative effects of intensive agriculture in order to develop more sustainable agricultural practices.

Soil pH can vary based on time of year, region, and habitat type, and can impact nutrient absorption by plants and species richness of soil microbial communities. The highest diversity of microbial communities are generally found in soils near neutral pH, from ~6.5-7.5. Though some plants prefer highly acidic soils, below pH 5.5, these conditions can reduce  microbe diversity and interfere with calcium and magnesium absorption. In this study, pH was consistent across planting groups and close to neutral in all. Soil was slightly more acidic in March and became closer to neutral in September.

Potassium is an essential nutrient for plant growth, involved in enzyme regulation, gas exchange, and ATP production.  Plants with sufficient potassium tend to be hardier in drought condition and more resistant to certain diseases. Potassium levels were highest in the grassy field group. The fields were mowed regularly, but otherwise left undisturbed, without any tilling or planting. After one year of planting, buckwheat had the lowest potassium levels and levels that are considered below the ideal of 131-175 ppm for this region. It will be interesting to see whether this trend continues after the second year of planting.

Phosphorus is vital for plants for their energy storage, in the form of ATP. Insufficient phosphorus can slow or even stunt plant development. Uptake of phosphorus by plants depends on both the soil pH, with a pH between 6.5 and 7.0 being the optimum, and organic matter content of the soil, which can be affected by the variety of plants grown in the area. Phosphorus was higher in the pre-planting samples than the post-planting ones, and decreased later in the year, though both time of year averages were within the range for adequate phosphorus levels. In the post-planting analysis, the phosphorus levels in the grassy field samples were significantly lower than those in either of the wildflower planting groups. Buckwheat levels were on average lower than the wildflowers and higher than the grassy field, though not significantly. Wildflower plantings could have the potential to maintain higher phosphorus levels when compared to grassy fields and help prevent any phosphorus deficiencies in nearby crop plants, though the range of these effects are still unknown.

Nitrates are a form of usable nitrogen that can be taken up by plant roots and used for plant growth and leaf development. They are also an essential nutrient and insufficient nitrate levels can interfere with growth. Plants can differ in their nitrate requirements. Some vegetables, such as tomatoes, squash, and sweet corn take up higher levels of usable nitrogen, and over time can deplete the levels in the soil. Others, particularly legumes (Family: Fabaceae), can increase levels of available nitrogen through their association with nitrifying Rhizobium bacteria. Levels of nitrates at the research site were low across all treatment groups. They were on average higher in the two wildflower treatments, though these differences were not significant. Both wildflower mixes included members of the Fabaceae family, which could increase the nitrate content of the soil. There were, however, over 20 species of wildflower in each mix, and of those only 1 species in the honey bee mix and 3 in the eastern mix were legumes, so the nitrifying effect could be minor.

The soil nutrient analysis will continue in 2021 and the blue orchard bee analysis will be performed. Further results and discussion will be added as they are collected and analyzed.

 

Participation Summary
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.