Resilience of Nitrogen Availability and Retention in Soils of Kentucky Certified Organic Farms
On one organic farm, cropped soils retained more mineral nitrogen (N) than a nearby undisturbed soil during a water extraction. Soils from a second organic farm behaved oppositely. The percent of soil organic matter (OM) present as labile OM may affect N-retention. Despite several years of OM addition, soils were carbon-limited for denitrification, an important N-transforming process. Soil resistance to perturbation and recovery from pertubation (“resilience”) appears more sensitive to soil type than management, based on Kentucky and Pennsylvania (Rodale Institute) soils. Rodale soil under organic vs. conventional management did have slightly different resilience characteristics but results are not conclusive.
1.) 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.
2.) 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 and 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 has been described by using ion exchange resins buried in these soils. Final data is not yet available.
Three microcosm experiments have been performed. One study investigated the extent of recovery of soils subjected to environmental perturbations of drought, flood, and heavy N-fertilization. Soils were taken from the Rodale Institute Long-Term Farming Systems Trial and the Compost Utilization Trial (Kutztown, PA), and from the KSU vegetable trial. Partitioning of N among organic, inorganic, and microbial forms was measured periodically. “More resilient” soils were considered those which after a recovery time most closely matched their original characteristics, or whose N-partitioning was not affected by the perturbation. Soil type seems more important than soil management in determining a soil’s potential for resilience. Within the same soil type at the Rodale Institute, organically managed soils did seem to recover more readily than conventionally managed soils, but results are not conclusive. Analysis of the heavy 15N isotope may give more definition to results.
A second study was performed by Dr. Ali Ebd El-galil, visiting scholar from South Valley University, El-Kothar, Sohag, Egypt. This study investigated the presence and activity of two groups of N-transforming bacteria, denitrifiers and NH4+ dissimilators, in soils from the private Kentucky organic farms and the KSU vegetable trial. All soils were carbon-limited for the complete transformation of nitrate (NO3-) to N2, meaning that at least some N lost from the soil in gaseous forms was released as nitrous oxide (N2O), a greenhouse gas. Organically managed cropped soils did not differ from undisturbed sods in this respect. Even after several years of deliberate management for building soil OM, carbon was still limiting for microbial processes in these soils.
A third study, not yet completed, investigates the effect of air-drying and re-wetting of soil samples on determination of denitrification potential. This study is mostly of interest to soil microbiologists as a refinement of methods.
Three microcosm experiments will be performed in 2002-2003, on cropped soils vs undisturbed soils from these farms. In all cases, experimental results will be related to organic matter characteristics that may be affected by organic management (i.e., quantity and quality).
One experiment will quantify the capacity of the soils to assimilate mineral nitrogen into organic forms. Using the heavy 15N isotope, this experiment will 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.
A second experiment will quantify the level of disturbance to N-transforming processes and their level of recovery after environmental perturbations to the soils. These mechanisms include respiration, denitrification, mineralization, immobilization, and nitrification. Perturbations will include flooding and N-overloading.
A third experiment will attempt to quantify the relationship between plant demand for mineral N and microbial release of mineral N in these soils. The microbial community may respond to shifts in the soil solution equilibrium, providing more or less mineral N depending upon the rate and quantity at which plants remove mineral N from soil solution.
Detailed case studies of the farms will be written. Financial data will be summarized. A survey of organic farmers in Kentucky will be performed to determine the goals of these farmers for their N-management programs.
Impacts and Contributions/Outcomes
Impact and Contributions
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.
Graduate Research Assistant
University of Kentucky
500 South Limestone St
Ag. North, N106R
Lexington, KY 40546-0091
Office Phone: 8592572103