Student Research: Broccoli - Living Mulch

Agroecologists are interested in food production systems that resemble natural systems in important ways. For example, natural systems are often species diverse – forests, meadows, lakes, and oceans can have many different species living within one community. Generally, our agricultural practices don’t look like these communities, favoring monocultures instead. Growing our food in diversified fields can be challenging, especially given that conventional monocropping has led to high yields, low weed pressure, and relatively easy harvesting.

Broccoli transplants in plastic-covered raised beds at Woodman Farm, University of New Hampshire. 

Despite the challenges, there are many benefits to diversification that are important for long term health and stability of our agricultural land. Rotating crops diversifies species over time and has been recognized as an important method of keeping soils productive and disease pressure down. Diversity can also occur in space, such as with intercropping (planting more than one species together at the same time). However, this method in particular has the potential to lower cash crop yields if the species used compete with each other for resources.

Our lab “diversified” a broccoli field by intercropping the broccoli with a cover crop. The cover crop in this case is called a living mulch, because it is grown between the broccoli rows and one of its functions is to keep weeds down, similar to other types of mulches. Past research has shown there is a tendency for living mulches to have a variable effect on yields, and understanding why, or even how to prevent yield loss, is an important step in making this a viable option for growers.

To see if we could optimize this diversified system, that is, to create a scenario in which competition does not reduce yields, we grew broccoli with or without a living mulch, and with different amounts of fertilizer. We expected that some amount of fertility would cancel out any competitive effects from the living mulch, and then we could decide if that much fertilizer was an acceptable investment.

Establishing the experiment: a living mulch (perennial ryegrass and white clover) was planted in between half of the raised beds, while  plots in the other half of the field were kept weed free. For each type of field management (living mulch and bare soil), sections of the bed row were given different fertilizer rates to see if this resource changed broccoli yields in the presence or absence of the living mulch. 

The results, in a nutshell, were mixed. In the first year we did the experiment, the broccoli yields were not different between living mulch plots and bare soil plots at the recommended fertilizer rates. In the second year, the yields in the living mulch plots were lower for all fertilizer rates. Why the difference?

When the broccoli was mature, the heads were weighed from living mulch or bare soil sections (seen above). Living mulch can provide many ecological services, such as nitrogen fixation, soil stabilization and erosion prevention, or increased soil organic matter. However, they can also compete with the cash crop for available nutrients during the growing season. 

Well, we don’t know for sure, but we have some ideas. We don’t think that competition for water is the problem because the broccoli was irrigated. We also don’t think that the living mulch competed with broccoli for light because it was occasionally mowed. It’s possible that despite high levels of fertilization, the living mulch could still compete with broccoli for nutrients. There are environmental circumstances where the roots of the living mulch can grow aggressively to find nutrients, and this was certainly a possibility, especially in the second year when yields were lower.

Interestingly, some plants react to the presence of neighbors by detecting and responding to the ratio of red to far-red light. A nearby plant (or living mulch), reflects a different light signature than say, the bare ground does. We may be observing the broccoli’s response to this light signal, which is ultimately expressed through lower yields.

The take-away is that efforts to diversify agricultural systems remain important, and we should continue to work on methods that balance short and long-term productivity. Living mulches have a lot of potential, and under some circumstances they have been successfully used without negative yield consequences. Understanding why and when they compete with crops will be important information in order to make them a more useful tool to growers.

The results of this study will be appearing soon in HortScience (in press).

Student Research: Cover Crops & Hayfields

The Agroecology lab, Master's student Myers Shaiyen, and Vicky Stafford of the Strafford County Conservation District are working on a project to establish cover crops in existing hayfields in order to improve soil health in those fields. We are working with a broad mixture of species and several approaches to seed establishment. There is a a nice overview of the work by Lori Wright, featured in an article for the NH Agricultural Experiment Station.

Disturbing the soil in Dover, NH, using an Aerway aerator. 

A short excerpt from the article:
“We are trying to figure out ways to practice agriculture sustainably. The conventional approach to increasing hayfield productivity would be to apply chemical fertilizers or manure, or to use tillage to establish the cover crops; however, these approaches would likely not improve soil health over the long term and could have unintended environmental impacts. So instead of relying on external inputs and tillage, we are testing strategies aimed at ‘ecological intensification,’ which involves using biotic interactions among species to drive improvement in fertility, productivity, and soil quality. Ideally, such an approach would not only improve productivity and soil health, but also other ecological services that underpin agricultural sustainability and human wellbeing,” -Rich Smith

Student Research: Pasture Intercropping System

Intercropping annual grains into perennial pasture

There are several reasons why two seemingly conflicting agricultural systems—annual grains and perennial pastures—might work well together.  The motives to explore this potential symbiosis relate to low input dairy production. Dairy farmers are faced with many challenges including the increasing price of grain. The purchase of organic feed grains represents a significant cost for organic dairy farmers in the Northeast and has led to increasing interest in on-farm production of grains. However, organic and low-input feed grain production often relies on intensive tillage for seed bed preparation and weed control, which can negatively impact soil quality. Intercropping annual feed grains with perennial pasture could reduce the need for intensive tillage, and may lessen the environmental impacts of dairy production and the need for purchased off-farm inputs.

Yet, despite the potential benefits of intercropping systems, little is known about the agronomic, environmental, and economic effects of intercropping annual feed grains into established pasture under organic and low-input management.

To evaluate these systems Jennifer Wilhelm and Rich Smith have established an annual grain rotation (corn in 2013) into alfalfa stands using a “gradient of tillage intensity.” Different tillage types fall on a spectrum of soil disturbance, from complete inversion of the soil (e.g. conventional tillage, or mold board plowing) to minimal soil disruption (e.g. strip tillage and no tillage). The traditional method of conventional tillage “resets” the entire soil and plant community, whereas strip tillage disturbs only the seeding row, leaving the inter-row plant community intact.

A Yeoman's Plow, one of several tillage methods used.

Different degrees of soil disturbance after the initial tillage is completed.

Some tillage methods leave the alfalfa pasture intact more than others.

Of course there are always tradeoffs: We might expect intense tillage methods to favor corn growth, but also to disturb the pasture beyond recovery, creating greater space for weed community establishment. Low-intensity tillage may favor the pasture and reduce weed establishment, but the pasture plant community can compete with the corn for resources, reducing crop yield.

The results of the first year of this study will be available soon. This project is funded by a graduate student grant from the Northeast SARE Program.

After tilling, all plots were planted with grain corn....

...

...Jennifer in front of the corn later in the season.

Student Research: Monitoring Agricultural Greenhouse Gas Emissions

Update on Agricultural Season Extension with High Tunnels: a Greenhouse Gas Source or Sink?

Tessa's research evaluating the differences between greenhouse gas emission in high tunnels and agricultural fields is continuing into the fall and winter seasons. The tomato crops have been pulled out of the ground and the soil has been plowed. Even without crops, it's still important to know what's happening below-ground by measuring the carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions that result.

Because the soil has been disturbed and the rows of crops removed, she's trying a new approach to measuring these trace gas emissions:

Sampling trace gas emissions using a "portable flux chamber" (an upside down cooking pot) as an alternative to semi-permanent plastic collars that were used during the growing season. This method may allow for an improved snapshot of greenhouse gasses emitted from agricultural soils, especially because it is more adaptable for sampling around typical agricultural management practices.  

The portable flux chamber in the traditional high tunnel. UNH's new moveable high tunnel and several low tunnels are in the background. 

Student Research: Using a Phytometer to Assess Cover Crop Performance

Ghosts of cover crops past: quantifying the effects of cover crops on subsequent crop-weed-soil interactions

Elisabeth’s research aims to assess the effects of cover crops on agroecological processes such as weed suppression, nutrient capture, and yield response in future crops. Replicated plots of different cover crops, some common and some novel, were established at the UNH Woodman Farm last summer to evaluate these effects over the course of an entire year. These cover crops were plowed under in the spring and a uniform test crop, sorghum-sudangrass (“sudax”), was seeded over the experimental site.

This photo was taken with an unmanned aerial vehicle and shows
how the test crop responded to the previous cover crop treatments.  

The area in blue outlines the location of the previous year’s cover crop experiment. A checkerboard pattern is visible in the sudax where the different cover crop treatments were located. The area around the square was left fallow during the cover crop experiment by periodically cultivating. Notice the difference in color of the sudax growing outside of the experiment compared to where the cover crops were growing! Aerial photographs, such as this one, are proving to be great visual aids when sharing findings of our studies with growers.

We found that sudax yield was higher where legume cover crops and forage radish had been grown previously. In 2013, the study was repeated at an adjacent site.

Sudax inflorescence

Elisabeth’s favorite cover crop among those she evaluated is forage radish (Raphanus sativus var. longipinnatus, trade name “Tillage Radish”). This cover crop is an annual in our region and is very effective at competing with weeds when sown in late summer and early fall.  In our trials, forage radish suppressed weeds in the fall better than the other cover crops, and this effect carried over into the next season. Forage radish produces very long taproots that are effective at capturing nitrogen stored deep in the soil profile.  As these roots decompose in the spring, these nutrients are released for the next crop to use.

Forage radish up close

In the next few weeks we will measure yield and weed populations growing in our current test crop to verify trends we observed last year between cover crop treatments. Growers in New Hampshire have expressed interest in trying forage radish on their farms after hearing about our research – and we’ll have more to add to the discussion shortly.

Student Research: Monitoring Agricultural Greenhouse Gas Emissions

Agricultural Season Extension with High Tunnels: a Greenhouse Gas Source or Sink?

New England is undergoing dramatic agricultural changes - we are expanding and intensifying production to meet increasing food demands. These changes have important consequences for our agricultural ecosystems, and Tessa's research aims to describe the potential atmospheric effects of agricultural intensification through high tunnels. 

A high tunnel planted with tomatoes at UNH's Woodman Farm.

Many farmers in the Northeast are growing crops under high tunnels as a means to extend their growing season and protect their crops from inclement weather (see NRCS's High Tunnel Initiative).  By growing crops under these plastic covered metal structures, farmers can grow high-value specialty crops, and protect their crops from frost, temperature fluctuations, excess precipitation, pests and disease. While this form of plasticulture is widely used, little is known about the environmental impacts of such practices. 

To address this question, Tessa is measuring carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions (trace gasses) from soils under high tunnel management and comparing these data to trace gas emissions from the same crop planted in an adjacent field. These data will provide insight into the effects of increased high tunnel production on climate change and strategies for mitigating these effects.

Gas samples are taken simultaneously from tomato interrows and rows in the field. 

Tessa, extracting and storing a gas sample. 

Research Assistant Matt Morris measuring soil moisture.
Soil moisture is an important driver of trace gas production and is sampled each week. 

Student Resarch: Precision Zonal Management

Rewiring the network? Structural and functional responses of soil faunal food webs to agricultural disturbances.

Lesley is currently working in collaboration with the Pennsylvania State University, and several other universities, on precision zonal management (PZM) in corn and soybean rotations. Corn and soy are the two biggest "major" crops grown in the US and occupy over 150 million acres of farmland (EPA). Their group is examining the effects of ridge tillage, a system that creates raised rows (for example by using a ridge-till planter), on a range of important crop, soil, and soil faunal (animal) responses. 

An example of the soil profile in a ridge till system. 

Here's a brief overview of Lesley's goals, objectives, and potential outcomes:

"Conventionally managed annual row crop systems often rely heavily on synthetic fertilizers, pesticides, crop monocultures, and tillage to produce consistent cash crop yields. Our research will determine how agricultural management practices affect soil food web structure and its capacity to function. The cumulative impacts of agricultural management may reduce the functional stability of soil fauna communities by constraining successional processes and rewiring consumer-prey relationships. My objectives are to:

A.    Determine the effects pesticide seed treatments have on the functional capacity and composition of the soil food web.

B.    Ascertain how zonal tillage affects the soil food web in terms of community diversity, structure, and functional stability.

C.    Determine how divergent the soil food webs become at the sub-rhizosphere scale in heterogeneously managed agroecosystems.

We will address these objectives through a combination of field experiments and modeling. This research will further our understanding of how agricultural disturbances affect soil food web structure and its capacity to function. More broadly, this research will provide farmers with practical information regarding the direct and indirect effects management practices have on soil food web communities and their capacity to perform agriculturally important ecosystem services."

A happy Lesley in PA, early in the season. 

Litter bags - food for the soil fauna.
Lesley measures how much  is consumed over the course of the growing season. 

Soil cores are used for a variety of measurements, including soil moisture,
to sample the nematode community, and to measure corn roots.