Resilience of Nitrogen Availability and Retention in Soils of Kentucky Certified Organic Farms

Resilience of Nitrogen Availability and Retention in Soils of Kentucky Certified Organic Farms

Summary

Soils from organically managed garden fields were not different from undisturbed soils on the same sites in terms of their resistance to perturbation and recovery from perturbation. Key N- and C-transforming processes were similarly affected by flooding in both soil management systems. These processes may not be as sensitive as necessary for testing soil resilience on a broader scale. Organic farmers were interviewed. The main needs identified that can be addressed by land grant institutions were public education, availability of seeds and materials for organic production, and local networking among organic farmers.

Objectives/Performance Targets

Quantify the ability of cropped vs undisturbed woodlot soils of two certified organic Kentucky farms to retain N in organic, microbial, and inorganic forms and to release N in mineralized forms to plants. Relate N-cycling in the soils to soil parameters that may respond to farming practices intended to build and retain soil organic matter. Repeat the objective for soils in a replicated vegetable production trial at the Kentucky State University research farm.
Quantify the ability of these soils to regain their N-retention and transformation functions after environmental perturbation. Relate soil resilience to soil parameters that may respond to farming practices intended to build and retain soil organic matter.

Accomplishments/Milestones

Total nitrogen (N) and carbon (C), organic N and C, and microbial biomass N and C have been described based on periodic soil sampling from cropped soils and undisturbed soils on two private organic farms and the Kentucky State University (KSU) Sustainable Vegetable Production Trial. Mineral N loss through leaching below the main rooting zone, 30 cm, in winter months was measured by using ion exchange resins buried in these soils from approximately December to March of each winter. The greatest total NO3- catch for any soil was approximately 19.54 mg NO3–N kg-1 soil, over a period of 98 d. As expected, areas under permanent cover lost less NO3- 2 out of 3 winters than cropped soils, and soils with cover crops lost less NO3- than uncovered soils. To minimize leaching losses on organic farms, cover crops must be a priority for winter soil management.

Three microcosm experiments have been performed. One study investigated the effect of air-drying and re-wetting of soil samples on determination of denitrification potential. This study is mostly of interest as a refinement of soil research methods, and the result is that denitrification potential should be performed on fresh soil samples. Drying and re-wetting of soils adds to the variability of denitrification enzyme assays so greatly that conclusive statements about soil management effects on denitrification are impossible to reach.

One experiment quantified the capacity of the soils to assimilate mineral nitrogen into organic forms. This experiment used the heavy 15N isotope to elucidate the extent of assimilation after a period of incubation, the major pathways of assimilation including biotic and abiotic, and the organic fraction (microbial vs humic) in which N finally resides. Final data is not yet available, as 15N analysis has not been performed.

The third experiment quantified the level of disturbance to N- and C-transforming processes and their level of recovery after flooding. Soils were flooded for 48 hr and water was removed. Soils recovered from flooding for 1, 2, or 3 weeks. These mechanisms included. At each time, substrate-induced respiration, ß-glucosidase activity, denitrification, N-mineralization, and nitrification were measured. Flooding reduced N-mineralization at all times but compost buffered the effect of flooding. Compost increased C-mineralization at all times, and flooding did not affect C-mineralization at any time. We found that organic farm soils were limited in active carbon content for maximum microbial activity, based on the response of the enzymatic processes measured to compost. Cropped soils and undisturbed soils were not different from one another. The processes we measured were probably not sensitive enough indicators of soil resilience to be useful in commenting upon soil quality, but they had not been tested previously for this purpose.

Interviews with 18 organic farmers in Kentucky, representing approximately 18% of Kentucky certified organic farmers, were performed. A “snowball” technique was used, in that interviews ceased when information became redundant. The purpose of the interviews was to determine needs for research and extension for organic farms in Kentucky, particularly regarding soil fertility issues. The most pressing needs for land-grant resources at this time is for public education about organic farming and to encourage availability of seeds and materials suitable for organic farming. Networking among local organic farmers is limited to the central region of Kentucky; more events in various places around Kentucky would be helpful. Information from the interviews was used to plan extension and education activities for grant proposals.

In the time remaining, detailed case studies of the farms will be written. Financial data will be summarized. Microcosm data will be summarized.

Impacts and Contributions/Outcomes

This research will benefit organic producers in the Southern Region by investigating the behavior of N in organically managed soils. If organically managed soils have tightly coupled N cycles, i.e., mineral N is not present unless demanded, their potential for N-pollution is minimized. The converse also holds.

Organic farmers are well aware of the need to supply organic matter to soil. Part of this research shows that readily available soil C is important to limit the contribution of soils to the buildup of N2O in the atmosphere. Although the contribution of this country’s organic farms to agriculturally generated N2O is probably very small (owing to the tiny organic acreage), most organic farmers likely would not wish to contribute to the problem at all. A more general conclusion is that even several years of attention to organic matter management is not enough to provide available soil C for all microbial processes to occur at optimum rates.

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