Understanding N Fixation by Legume Cover Crops in Organic Vegetable Systems

View the project final report

• 2008 annual report

Commodities

• Vegetables: broccoli

Practices

• Crop Production: cover crops, nutrient cycling, organic fertilizers
• Education and Training: networking, on-farm/ranch research
• Natural Resources/Environment: biodiversity
• Production Systems: agroecosystems
• Soil Management: nutrient mineralization, soil analysis

This project was part of our work to examine asynchronies in N availability and uptake in organic vegetable production on California’s Central Coast. While biological N fixation (BNF) is an important part of N budgets and models, there are no BNF estimates for two important winter legume cover crops, bell beans (Vicia faba) and woollypod vetch (Vicia villosa). We evaluated a key assumption underlying the natural abundance method of estimating BNF, developed regional BNF estimates for bell beans and woollypod vetch, and determined how a range of BNF estimates affects the balance of N budgets for organic vegetable systems.

Introduction

Legume or legume-mix (e.g., with grasses) cover crops can play a key role in the fertility management of agroecosystems, particularly by providing N through biological N fixation (BNF) with Rhizobium symbionts. Because of its importance, BNF is a necessary component of N budgets, tools used to compare agroecosystem N inputs and outputs and thus determine the potential for N loss via leaching or denitrification and any change in soil N storage. If N budgets show significant surpluses, growers and researchers can consider alternate fertility management practices to reduce potential N loss and thus reduce the negative impacts of N on the environment (Vitousek et al. 1997, Johnson et al. 2007) and human health (Wolfe and Patz 2002, Townsend et al. 2003). Accurate estimates of BNF in N budgets allow the separation of the contribution of atmospheric N from the contribution of soil N to plant growth. Significant underestimates of BNF would underestimate the N surplus of a system, while significant overestimates of BNF would lead to overestimates of N surplus.

In the past 20 years, the 15N natural abundance method (Shearer and Kohl 1986) has gained popularity as a means to estimate BNF. Like the traditional difference method, the natural abundance method compares a legume and a non-fixing “reference” plant grown under the same conditions. The difference method compares the total N content of the legume and reference plant and attributes any difference to BNF. However, the choice of reference species can have a strong influence on the estimate of fixation because it assumes the legume and reference plant have similar spatial and temporal soil N uptake patterns under the same growing conditions. This is dependent on growing conditions, and, as Unkovich and Pate (2000) show in a review of BNF estimation methods, is highly variable among years and can lead to “the unsatisfactory conclusion” that BNF is negative if the legume has lower biomass and total N than the reference plant. Unkovich and Pate (2000) conclude that available data show the difference method to be “unreliable” overall.

Alternately, the natural abundance method compares isotopic signatures (15N/14N, or δ15N) of the legume and reference plant. In N isotope studies, the atmospheric δ15N is set as 0. Biological processes discriminate against 15N, so nutrient pools that are high in biologically-transformed N are enriched (i.e., have a higher δ15N) compared to the atmosphere. In the natural abundance method, the reference plants grown in soil are assumed to have a δ15N signature that integrates the signature of the available soil N over the growing season (Boddey et al. 2000), while N-fixing legumes are assumed to have a signature that combines soil and atmospheric N signatures. Thus BNF is calculated as:

%Ndfa = 100(δ15Nref - δ15Nleg)/(δ15Nref - B)

where %Ndfa is the % of N derived from the atmosphere, δ15Nref is the signature of a non-legume cover crop in field plots, δ15Nleg is the signature of a legume cover crop in field plots, and B is the δ15N signature of a legume when all N is derived from fixation (Shearer and Kohl 1986).

One methodological challenge of the natural abundance method is that, as for the difference method, the choice of reference species can be important. For optimal accuracy of estimating BNF, the reference plant and legume ideally should have similar N uptake patterns, both spatially and temporally, as well as similar growth responses to a given climate and soil environment (Pate et al. 1994). A non-nodulating isoline may be the best candidate because of similarities in growth habit and phenology, however such isolines are not available for many legumes. Instead, a reference plant with N uptake patterns that correlate well with those of the legume may be suitable for use in N budgets. This use of reference species assumes that the pools of N accessed by the legume and reference are the same.

Biological N fixation by winter legume cover crops is an important component of fertility management in organic vegetable systems in California’s Central Coast, particularly as legume-cereal mixes on relatively small (<100 ha) organic farms (Brennan and Smith 2005). Bell beans (Vicia faba, same species as faba beans), woollypod vetch (Vicia villosa ssp. dasycarpa) and other legumes commonly are grown in a mix with cereals, particularly oats (Avena sativa). Growers in the region assume that 50% of cover crop N, or about 60-150 kg N/ha, is fixed annually (J. Leap, pers. comm.). This fixation estimate is a rule of thumb; despite widespread use of legume-oat mixes, there are few specific estimates of N fixation for bell beans and vetch in the Central Coast. As tighter N management is encouraged by regulatory agencies, more accurate estimates based on specific, regional data may help growers track the relative input and output of N sources and minimize potential N loss.

We have been working to estimate BNF for bell beans and vetch in the Central Coast. Our initial research indicated that the assumption that the legumes and their reference species used the same N pools may be problematic. We planted vetch, bell beans and two reference species (oats and mustard) in single-species plots in five sites with a range of 1-8 yrs since last compost application. All plots were on the same organic farm to minimize soil type and climatic differences. Aboveground biomass was analyzed for δ15N. The data showed that the number of years since compost application was negatively correlated with δ15N of the reference species but had no relationship with δ15N of the legumes. This indicated that the reference and legume species were accessing different pools of soil N, with the reference species taking up more available N from the compost than the legumes.

Following on this, we proposed two main objectives for this project. First, we wanted to determine if the N sources used by the legumes were the same as those used by the reference species. Second, we wanted to determine how different estimates of BNF would affect N budgets for vegetable production systems that use cover crops as part of fertility management.

Objective 1. Determine if N sources used by legumes and reference species are the same
To better understand which N sources are used by each species in this system, we proposed two projects:

1. Field experiment: We planned to 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. This would allow us to compare the sources of N each species is using at different points during the growing season.

2. Field observation: We planned to harvest aboveground biomass of 10 individuals of oats, bell beans and vetch at three farms every three weeks during the winter, then analyze the samples for total aboveground N. This would allow us to compare N uptake among the plants during the winter.

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