Optimizing Biological Nitrogen Fixation in Organic Cropping Systems for Sustainable Nutrient Mangaement

Biological nitrogen fixation (BNF) is the major source of new nitrogen in organic agriculture, yet there has been almost no research devoted to understanding how organic management practices impact this process. Furthermore, few widely used, temperate green manures have been characterized in terms of their nitrogen fixing traits. The long-term goal of our proposed research is to understand the ecology of biological nitrogen fixation within organic cropping systems so that this process can be most effectively managed. Specifically, we seek to understand how organic management strategies and their resulting long-term soil legacies interact with plant and microbial species to regulate nitrogen fixation and assess the economic consequences of these interactions. Funded by the USDA CSREES Integrated Organic Program, I am using molecular techniques to 'fingerprint' rhizobia strains that have been extracted from nodules of soybean and red clover grown on organic farms that have been under organic management for varying lengths of time. We hope that our research will show us how specific management practices are impacting the ecology of rhizobia found in symbiosis with commonly grown legumes in organic systems.

The Role of Soil Organisms in Carbon Cycling in Anthropic Soils of the Brazilian Amazon

In this project we, in close collaboration with Brazilian scientists at the Centro de Energia na Agricultura, are looking at how the soil microbial community influences a fascinating tropical soil system known as "Terra Preta" (Anthropogenic Dark Earth). Terra Preta (TP) soils found in the Brazilian Amazon are known for their unusually high soil C contents. It is now widely accepted that these soils were created between 500 and 2,500 years before present by indigenous pre-Colombian Indians for their own agricultural use. Since the existence of Terra Preta soils is a strong piece of evidence supporting the hypothesis of high population densities in the Amazon prior to Spanish conquest, fertility characteristics of TP soils have been increasingly studied in recent years. However, their biological properties remain a mystery. I am interested in the way diverse microbial populations found in TP soils are influenced by and themselves influence C content and quality.

We think that the soil microorganisms found in TP soils can serve as a major contributor to C sequestration by helping to mineralize as well as stabilize certain fractions of soil organic matter. Because the carbon cycling in TP has been shown to be very different from other soils, we think that soil microbial life in TP soils will also be distinct, and hope to characterize this diversity through our research. We are using both traditional and novel molecular techniques to assess microbial abundance and diversity. We are using Polymerase Chain Reaction (PCR) to amplify target sequences of organism DNA, and Denaturing Gradient Gel Electrophoresis (DGGE).

Terra Preta soils are often darker
than the darkest Munsell color swatch!


Pottery is found in high concentrations
in Terra Preta soils

Nitrogen Fixation in Organic Coffee Agroecosystems in Chiapas, Mexico

Coffee throughout Latin America is cultivated under both shaded (a traditional practice for small landholders) and unshaded (full sun and high external inputs of agrochemicals, especially nitrogen fertilizer) conditions.In the early 1970's a steady increase in coffee prices in Latin America stimulated a transformation from diverse shaded coffee agroecosystems, where many productive tree species were intercropped with the coffee plants, into homogenous coffee systems containing no trees and coffee that needed to be fed with technological packages of agrochemicals such as N fertilizers. Many of the best-preserved traditionally shaded coffee farms are found in Chiapas, Mexico. This project looks at alternatives to fertilization with synthetic N-fertilizers in Chiapas. In particular I investigate a process called nitrogen fixation.

Coffee ecosystems are agricultural systems and therefore export most of the nutrients in the harvestable products such as berries and firewood. To sustain annual yields, these lost nutrients need to be subsidized either with inorganic purchased fertilizers, or bio-fertilizers such as leguminous tree leaf litter or compost. Full sun systems require the use of chemical inputs to make up for the loss of the shade tree leaf litter that they once had. Inputs often need to be purchased from large corporations, offsetting profits of increasing yields. Many small-scale poor farmers just cannot afford the fertilizers and thus their coffee goes without the N that would possibly increase their coffee yeilds.

Interest in the use of leguminous species for N addition in shaded systems has increased in recent years. Leguminous tree species, 'fix' N through a symbiotic association with soil bacteria and can therefore serve as a biological N addition to the system without needing purchased inputs. This is an important mechanism of bio-fertilization for farmers who are either certified organic, or are in the processing of becoming certified. Organic coffee producers are restricted from using agrochemicals, thus rely in part on nitrogen fixation to provide N to their crop.

My research addresses numerous facets of N-fixation on organic coffee farms. These include farmer understanding of soil fertility enhancement processes that they use as a basis for decision-making and experimentation, diversity of N-fixing organisms that are in symbiosis with the shade tree Inga, and the effectiveness of the Inga-rhizobia symbiosis as a source of N for organic coffee. It is hoped that this research will help farmers by investigating if N-fixation is indeed a feasible and reliable alternative to chemical N-fertilization.

Julie interviews a young coffee farmer

Coffee farm shaded by Inga tree species