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

Winter legume cover crops such as bell beans (Vicia faba) and woollypod vetch (Vicia dasycarpa) play an important role in fertility management in organic vegetable systems in California’s Central Coast. As a rule of thumb, growers estimate 12.5-42.0 kg N/ha are fixed annually, but more accurate estimates based on specific, regional data may help growers better track the relative input and output of N sources. In addition, a better understanding of how fixation values affect N cycling in these systems, including how much N is available and when for the following crop, can help growers manage N more efficiently and lose less N to the environment. Specifically, our research objectives are 1) to determine if N sources used by legumes and reference species (non-fixing legumes used to calculate N fixation) are the same and thus provide accurate regional N fixation estimates, and 2) to determine how much over- or underestimates of fixation affect the N balance of organic vegetable systems. Our educational objective is to educate growers, researchers and advisors on how much N winter legume cover crops add to organic vegetable production in this region and its importance to the balance of the system.

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

One common method of estimating fixation is the 15N natural abundance method. This method uses the difference between the ratio of two stable N isotopes (15N and 14N, expressed as δ15N) of the soil and the δ 15N of the atmosphere to determine the percent of plant N derived from the atmosphere (%Ndfa). %Ndfa is calculated by comparing the δ 15N of a non-fixing “reference” plant with the δ 15N of a legume grown under the same conditions; the reference plant is meant to provide an integration of the δ 15N of the soil N that is available for plant growth over the growing season. This use of reference species assumes that the pools of N accessed by the legume and reference are the same.

My research indicates that this assumption may be problematic. In a previous experiment, vetch and bell beans were grown in single-species plots, along with two reference species, oats and mustard. The δ 15N of the plants indicated that the reference and legume species were accessing different pools of soil N, with the reference species taking up more available N from compost than the legumes.

To better understand which N sources are used by each legume and reference species in this system, I will conduct two projects. First, I will use soil from five field sites within a single farm that have a range of compost histories as the medium for pots in a greenhouse study. I will establish two replicates of eight pots for each of the five soil types and two replicates of eight pots in soil from two other organic vegetable farms. Each set of pots will have oats and vetch grown individually in four fertility treatments: no, low, moderate and high additional N applied as liquid fish emulsion. I will harvest aboveground biomass from the plots four times during the growing season and submit dried, ground plant samples to UC Davis for δ 15N analysis. This will allow me to compare the sources of N each species is using at different points during the growing season. In addition, I will submit soil samples from the top 0-15 cm, 15-45 cm and 45-90 cm of the soil adjacent to where I took the soil for each set of pots to determine δ 15N.

Second, I harvested aboveground biomass of 10 individuals of oats, bell beans and vetch growing in standard fields at each of the three farms every three weeks. I will analyze the dried, ground plants for total aboveground N, allowing me to compare N uptake among the plants during the winter. These studies, along with the past two years of research, will help me develop regional %Ndfa estimates for bell beans and vetch in organic vegetable systems.

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

I will construct an N input and output budget for the organic farm used in the above experiments. In addition, I will use three models to examine N dynamics in local organic vegetable systems. The models are DNDC (DeNitrification and DeComposition), NDICEA (Nitrogen Dynamics In Crop rotations in Ecological Agriculture), and a new model developed by David Crohn at UC Riverside. For the budget and models I will use soil and soilwater nitrate concentration, soil moisture, plant N uptake and other data I have collected the past two years. Budget inputs and outputs will be compared to estimate N deficit or surplus, and the models will be tested for ability to predict NO3- concentrations in soil and soilwater (validation methods from Moreels et al. 2003).

My estimates of N fixation from 2003-04 varied by up to 29% for a single legume with a single reference species on a single farm, and varied more when compared across reference species. Based on this wide range that I’ve observed, I want to test what kind of effect higher and lower fixation rate have on budget and model outcomes. I will use a simplified uncertainty analysis for N fixation in the budget and each of the models and observe whether a 15% increase or decrease from the mean for both bell beans and vetch changes N deficits, surpluses or balances. I will also test the +/- 15% of the mean under different climate scenarios in the models to see the effect if winters are 30%, 50%, or 75% wetter or drier than average.