Understanding N Fixation by Legume Cover Crops in Organic Vegetable Systems

Understanding N Fixation by Legume Cover Crops in Organic Vegetable Systems

Summary

This project is part of our work to examine asynchronies in N availability and uptake in organic vegetable production on California’s Central Coast and to evaluate possible means of reducing the asynchronies. One large gap we have found in working with budgets and models is a lack of N fixation estimates by two of the region’s most important winter legume cover crops, bell beans (Vicia faba) and woollypod vetch (Vicia villosa ssp. dasycarpa). We are using naturally occurring isotopes to develop N fixation estimates for locally important species and local growing conditions to more accurately model these vegetable production systems.

Objectives/Performance Targets

Objective 1. Determine if N sources used by legumes and reference species are the same

To estimate N fixation, we have been using the 15N natural abundance method. This method uses the difference between the ratio of 15N and 14N (δ15N) in legumes and non-fixing “reference” plants grown in the same environment to determine the percent of plant N derived from the atmosphere (%Ndfa). The method assumes that pools of N accessed by the legume and reference are the same, but this assumption may be problematic. In earlier field research we saw that the number of years since compost application was negatively correlated with δ 15N of the reference species but had no relationship with δ 15N of the legumes. This indicates that the reference and legume species were accessing different spatial or temporal soil N pools, with the reference species taking up more N from the compost. We proposed two projects to better observe this difference in resource use:

1. Field experiment: We would insert metal sleeves 1 m deep in the soil to create two replicates of eight 0.25-m2 plots at five sites on a single farm that we used for our earlier work. Each set of plots would be planted with oats (non-fixing reference plant) and vetch (legume) grown in four fertility treatments (no, low, moderate and high additional N applied as liquid fish emulsion). Aboveground biomass would be sampled four times and dried, ground plant samples would be submitted to UC Davis for δ 15N analysis, along with soil samples.

2. Field observation: We would harvest aboveground biomass of 10 individuals of oats, bell beans and vetch at three farms every three weeks, then analyze the samples for total aboveground N.

Objective 2. Determine how much over- or underestimates of fixation affect the N balance of organic vegetable production systems in this region

We would construct an N input and output budget for the farm used in the experiments in Objective 1. We would also use three models to examine N dynamics in local organic vegetable systems. We would then compare budget inputs and outputs to estimate the N balance, as well as the sensitivity of the balance to a range of fixation estimates, based on our previous work. In addition, we would test the models for ability to predict NO3- concentrations in soil and soilwater, and see what changes in fixation estimates cause in the models’ output under average, wetter and drier climate conditions.

Objective 3. Educate growers, researchers and advisors on how much N legume cover crops add to organic vegetable productions in this region and its importance to the system

As part of a network of organic vegetable and strawberry producers, researchers, advisors and NGO and industry representatives from California’s Central Coast, we would share our research findings at our 2-3 meetings each year. We also would publish the results in a research brief through the Center for Agroecology and Sustainable Food Systems at UC Santa Cruz.

Accomplishments/Milestones

Objective 1. Determine if N sources used by legumes and reference species are the same

1. Field experiment. Update: We planted the field experiment in Dec. 2005, after switching from metal sleeves to plastic pots with the bottoms removed. After planting, we experienced high seed predation in several fields, probably due to birds and rodents, and many of the seedlings that germinated were killed by slug predation in mid to late Dec. A second round of seeds planted in early Jan. also was greatly damaged by slugs. At this point much of the fertilizer we had applied had probably been leached during rain events, so we decided to harvest the few plots that had good survivorship and to try the experiment again.

We modified and expanded the experiment for the greenhouse to avoid predation problems and to answer several questions: 1) does %Ndfa vary within and across farm sites (including ones with different compost histories) and with fertility level, 2) are oats an appropriate reference species for estimating vetch %Ndfa, and 3) if %Ndfa does vary with fertility, can oat biomass or oat total N uptake serve as a proxy for estimating relative soil fertility? In this way fixation estimates used in nutrient budgets or models could be adjusted depending on soil fertility levels.

Seven oat or vetch seeds were planted in late Oct. 2006 in fertility-amended field soil in 983-ml pots. Topsoil collected from six farm sites was used as the growing medium; the sites were four fields at the UC Santa Cruz organic research and teaching farm (Farm 1, including two fields in production ~ 25 years that received compost nine years ago and eight years ago, respectively, the farm orchard, which has received no compost, and an uncultivated area, which also has received no compost) and two organic farms in Watsonville, CA (Farms 2 and 3). Prior to planting, the soil in each pot was sieved and mixed with Phytamin organic fertilizer added at the rate of 0, 75, 125 or 200 kg N/ha, with 5 replicates per species x soil source x fertility treatment. After emergence, plants were thinned to 3 per pot.

Plants were destructively harvested 17 weeks after emergence. Plants from each pot were separated into above- and belowground biomass. Roots were washed, and all above- or belowground biomass from each pot was combined. The plants were oven-dried at 70 C for three days, weighed, and ground on a Thomas-Wiley mini-mill. Dried plant matter was analyzed on a continuous flow isotope ratio mass spectrometer at the UC Davis Stable Isotope Lab for δ15N. As of Jan. 2008 we have δ15N measurements for all treatments and are awaiting the analysis of duplicate samples to confirm the trends we’ve observed.

We calculated the mean oat and vetch biomass for each treatment, as well as the %Ndfa. Uncertainty was calculated using Monte Carlo simulations. A two-way ANOVA for oat biomass was calculated in Excel.

Based on our preliminary isotope data, no striking negative relationship between estimated %Ndfa and fertility was observed, except for one field at Farm 1 as well as Farm 3. Overall, the high %Ndfa estimates in our experiment are in the same range we have seen in previous field experiments, with most estimates across fertility treatments and soil sources falling around 80-100%. This is well above the 50% benchmark commonly used in estimating cover crop N fixation for nutrient budgets. Literature values for Vicia faba range widely. In a review, Unkovich and Pate (2000) found values from 19 to 97 %Ndfa. A six-year study of legumes in rotation with cotton in Australia (also winter planting, with plants sown in May and harvested in Sept.), had generally high %Ndfa for both Vicia villosa and Vicia faba, with estimates from 50-92 %Ndfa and 79-84 %Ndfa, respectively (Rochester and Peoples 2005).

Mean total above- and belowground oat biomass was negatively correlated with mean total vetch biomass for each site, although the strength of fit varied. Farm 2 had the strongest correlation (y = -0.51x + 21, R2 = 0.90), Farm 3 had a moderate correlation (y = -0.37x + 18, R2 = 0.39), and the Farm 1 field with nine years since compost application had the weakest relationship (y = -0.70x + 15, R2 = 0.25).

Again just looking at one field for Farm 1 and at Farms 2 and 3, oat biomass was not strongly correlated with site or fertility treatment, although the site x fertility treatment effect approached statistical significance at the 95% confidence level. The overall mean oat biomass per site was similar across sites (13.1 g at Farm 1, 11.8 g at Farm 2, 11.7 g at Farm 3) but varied widely within and across fertility treatments.

The weak correlation between oat and vetch biomass may indicate oats are not an appropriate reference species for this system. Likewise, the lack of a clear relationship between oat biomass and fertility treatment indicates that oat biomass alone is not a good proxy for soil fertility in these sites. Although oats are easily accessible and grow well during the period when cover crops are generally grown in this area, growth may not be limited by soil fertility at the fertility levels commonly found in vegetable fields in the region. An ideal reference species would be a non-nodulating isoline of the legume species under study, but such lines are not available for Vicia. In other work we are examining the potential of other species (including mustard and phacelia, both dicotyledonous) for use as reference species.

We are now finishing this study by incorporating the last of the isotope values as the samples are analyzed and comparing the results for the fields within Farm 1 as well as across all three farms.

2. Field observation. Update: We were able to take the samples as planned, although the sampling period at one of the sites was shorter than expected due to warm weather allowing for early cover crop incorporation. The plant samples were analyzed for total N content on a macro CN analyzer at UC Santa Cruz. We are finishing this part of the project by comparing seasonal N uptake among the three cover crops (data forthcoming).

Objective 2. Determine how much over- or underestimates of fixation affect the N balance of organic vegetable production systems in this region

Update: UC Santa Cruz researcher Joji Muramoto and I began building N budgets in Jan. 2007 and tested the budgets with two growers last spring.

The budget for Farm 1 shows the estimated N input, output and balance for a single vegetable production field over 7 years, with a positive balance of 170 kg N/ha. If we use a higher %Ndfa as seen in our greenhouse and field studies, the balance changes; at 90 %Ndfa there is a positive N balance of 518 kg N/ha. This triples the estimated loss for the field.

We will continue to modify the budget as we work with the results from our other two studies and will include these different N fixation scenarios as part of three models examining N dynamics in these systems.

Objective 3. Educate growers, researchers and advisors on how much N legume cover crops add to organic vegetable productions in this region and its importance to the system

Update: We have been talking and working with the growers and researchers in our network, including the two growers whose farms we have been studying, and shared some of our initial findings with them at a network meeting last year. In fall 2007 we presented our preliminary results at the Sustainable Ag Expo in Paso Robles, CA and at the National Conference on Agriculture and the Environment in Asilomar, CA; both conferences included many Central California growers, researchers and advisors. We will continue to share our results at network meetings and other conferences in 2008 and through a research brief.

Impacts and Contributions/Outcomes

As stated in the update for Objective 3, we are sharing our findings with the local community of growers, researchers and advisors through meetings and conferences, as well as a forthcoming research brief. Our aim is to increase awareness of how high %Ndfa by cover crops may be and to facilitate its use in budgets and models used by growers and researchers in planning fertility management.

Collaborators: