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

Final Report for GS00-003

Project Type: Graduate Student
Funds awarded in 2000: $6,900.00
Projected End Date: 12/31/2003
Region: Southern
State: Kentucky
Graduate Student:
Major Professor:
Mark Coyne
University of Kentucky
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Project Information

Summary:

Organic farm management requires soil building practices such as cover cropping and does not allow chemical fertilizers. Enhanced soil biological activity is a goal of many organic farmers. The ability of three organically managed soils to resist change in or to recover biological functions after perturbation (soil resilience) was related to properties affected by management. Managing nitrogen (N) in organic farm systems is challenging. Partitioning of N among various forms was quantified. Because organic farmers’ soil management practices are directed by knowledge and beliefs about soil, Kentucky organic farmers were interviewed.

Undisturbed soils on two organic farms had higher soil quality than cropped soils on those farms, as measured by organic matter content, light fraction organic matter, and selected physical properties. Cropped and undisturbed soils were similar in resilience to perturbation for soil enzymes measured. Organically cropped soils on a research farm did not differ in soil quality characteristics or resilience from undisturbed soil on that farm.
Cropped soils had more N in mineral forms at all times than undisturbed soils, but undisturbed soil had more potentially mineralizable N. Added N was not fully assimilated into organic forms over a brief incubation in any soil. Gross N-mineralization rates were similar for cropped and undisturbed soils over a 6-d incubation. If N-retention is a goal, management attention should be paid to light fraction organic matter, a strong sink for N.
Organic farmers are likely to concentrate on building soil organic matter rather than on managing N. They do not consider N-loss a problem. Organic matter is difficult to increase because most organic management practices are land- and labor-intensive. Attention to N-management as well as organic matter could benefit organic farms. The land grant university could best help organic farmers by encouraging the creation of markets and infrastructure appropriate to organic farming.

Introduction

This project investigated soils under certified organic management. Organic management practices such as cover cropping, compost or manure usage, and intensive mechanical weed control can affect some soil characteristics (e.g. cation exchange capacity, water-holding capacity, microbial diversity and activity, and pH) that influence soil biological behavior.

Soil resilience, the ability of the soils to resist change or to recover biological functioning after environmental disturbance, was related to properties affected by management. Soil resilience is an aspect of soil quality, in that soils of “higher” quality are expected to display more resilient behavior and to be less susceptible to permanent degradation. Long-term organic management may increase soil resilience. The measurement of soil resilience is challenging; a widely agreed-upon dataset or protocol for describing soil resilience does not yet exist.

Partitioning of N among various forms in soil was quantified and related to properties affected by management. The relationship of organic farm practices to N is a particular concern because of the necessity of N for crop production, its instability and easy loss from soil, and its expense and possible off-site environmental impacts. The main difficulty in managing N organically is to match temporal N availability and crop demand. The complementary processes of N mineralization and immobilization are key soil functions; adding active organic matter, as occurs in organic farming, may change mineralization and immobilization rates.

The project focused on describing soils in certified organic management systems, rather than comparing organically managed soils to conventionally managed soils. Because undisturbed natural soils, such as those under forest or grassland, generally have higher SOM and microbial activity than cultivated soils, comparisons were drawn between cultivated organic farm soils and undisturbed soils on the same farms.

The lack of basic research in certified organic agriculture means that perceptions about organic N-management practices may or may not be supported by data. Farm decisions are often made on the basis of personal observation, personal belief, and consultation with other farmers. Kentucky organic farmers’ typical N-management practices, the information on which they base their decisions, and the reasons why they believe their practices are effective and beneficial were investigated. The object was to identify areas for research and extension helpful for organic farmers.

Project Objectives:

Describe organic practices used on two organic Kentucky farms, plus the Kentucky State University (KSU) research farm.

Investigate N-transformations in these soils.

Quantify the ability of the soils to recover N-transforming functions after disturbance, relating results to fundamental soil properties.

Describe common N-management practices used by Kentucky certified organic farmers and describe the beliefs and knowledge supporting management choices.

Identify research and extension topics that would benefit organic farms.

Cooperators

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  • Victoria Bhavsar

Research

Materials and methods:

This project used a combination of field work, microcosm studies, and personal interviews. Cropped and undisturbed soils from three Kentucky organic farms were used for all field work and microcosm studies. Laboratory analyses were performed by standard soil science methods.

Field work
The soils of the organic farms in this project were sampled periodically for pH, mineral content, total N, and total organic matter (OM). Winter mineral N accumulation at 30-cm depth was measured with buried ion exchange resins, to estimate potential nitrate loss. Selected soil physical properties (texture, aggregate stability, hydraulic conductivity, and water retention) were described.

Microcosm studies
Soil resilience
This experiment investigated the resilience of organically managed soils to short-term saturation, a common occurrence in late fall to early spring in Kentucky. Soils were saturated for 48 hr and allowed to recover for 6, 12, or 17 days. Before saturation and after recovery, the levels of key biological processes in N and carbon (C) cycling were measured (substrate-induced respiration, ß-glucosidase activity, denitrification enzyme activity, nitrification, and arginine ammonification).

A further objective was to investigate the effect of added organic matter on biological activity and resilience in these soils. Although organic farm management emphasizes organic matter addition, these soils may still be C-limited as regards maximum biological functioning. Therefore, the experiment was repeated with soils amended with a high rate of mature compost (equivalent of 45 Mg compost per hectare, or 20 tons per acre).

Nitrogen mineralization and immobilization
The objective of this experiment was to quantify the capacity of organically managed soils to convert added mineral N to other forms over a brief incubation. Although mineral N is not used in organic farming, it was used for this experiment so that the amount of mineral N added could be controlled.

Soils were incubated with 0, 67, or 134 kg N per hectare for 24 hr, 6 days, or 27 days. Nitrogen was added as 99 at.% 15N NH4Cl. Before and after incubation, soil mineral N and microbial biomass N were measured to estimate net N mineralization and immobilization rates. For selected samples, dilution of 15N was measured to estimate gross mineralization rates. During incubation, CO2 and N2O evolution were periodically measured by gas chromatography.

Interviews
The farmers of the two privately owned organic farms were interviewed in depth to describe the history of their farms, their farming practices, and their goals for their farms. Nineteen other Kentucky organic farmers were identified who had at least 1 acre in grain, horticultural products, or tobacco. Farmers were chosen from across the state’s five physiographic regions. These interviews focused on N management and research and extension needs.

Research results and discussion:

The objectives of this work, listed earlier in this report, were designed to investigate each of the three aspects of sustainability – agronomic, economic, and social. These conclusions address each objective in turn.

Organic soil management practices used on two organic Kentucky farms and the Kentucky State University (KSU) research farm.
Soil management practices on the two private farms included sporadic cover cropping, compost use, mulching, various biological stimulants, and mineral amendments. Organic management had not increased the quality of cropped soil, as indicated by soil organic matter, soil biological activity, and soil physical properties, to be comparable to that of undisturbed soils at the private farms in this dissertation.
At KSU, cover cropping was more consistent. No compost was used. Organic management has prevented soil disturbance from degrading compared to undisturbed soil. However, unless constant attention is paid to soil management, cropped soils on the research farm may degrade over time.
Mineral fertility was increased in all soils to be comparable or greater than mineral fertility in undisturbed soils.

N-transformations in organic farm soils.
Nitrogen loss over winter at two farms indicates that legume-based systems can lose as much N as systems with moderate amounts of N fertilizer. Lack of winter cover cropping may have led to higher N-loss in cropped soils compared to undisturbed soils on the second farm.

Undisturbed soils retained N in non-mineral forms to a greater extent than cropped soils although net mineralization was greater in undisturbed soils, indicating a higher level of potentially mineralizable N. This is probably due to the higher light fraction organic matter content in undisturbed soils. However, organically managed soils compared to conventionally managed soils did not retain a higher percentage of mineral N in the non-polluting NH4+ form. Adding N to soils stimulated CO2 release, especially in undisturbed soils high in labile organic matter. Adding N without addition of C may deplete soils of C and therefore microbial activity over the long term. In the short term, though, N stimulated microbial activity.

Because there were few differences in N-behavior or CO2 release in undisturbed vs N-fertilized soils at KSU, the addition of N-fertilizer at reasonable, recommended rates along with other good management practices such as cover cropping does not evidently harm soil life. However, N-fertilizers are not absolutely necessary if other means are used to supply readily mineralizable N. If N-retention is a goal for the cropped soils, special management attention should be paid to building organic matter, especially light fraction, which is a heavy sink for N. Light fraction organic matter responds readily to management.

Quantify the ability of the soils to recover N-transforming functions after disturbance.
Environmental perturbation that soils would normally experience did not affect the ability of soils to partition N. Excess N did cause changes in N-partitioning, as expected, but the changes were similar for organically and conventionally managed soils. These results together imply that soils of similar type in an area, provided that their management is not so different as to impact potential soil forming factors, are adapted to the stresses of their environment. However, soils from different environments, including management spanning more than a decade, may respond differently to perturbation.
Related to this conclusion, measuring soil resistance and resilience to perturbation remains challenging. Inherent soil characteristics such as texture and organic matter content were related to the soils’ responses; i.e., the soil with the heaviest texture and lowest organic matter was the least resistant and resilient overall. These characteristics are easier to measure than resilience at this time.
The ideal soil process for measuring soil resilience would differentiate among soils according to some parameter of interest such as soil management. Several different soil processes must be investigated to describe soil resistance and resilience since each soil process itself is different. Processes that respond to perturbation in similar ways but to different degrees depending upon the interesting parameters would be best. Soil functions with high inherent variability may not be good choices because they cannot differentiate among perturbation treatments. In this dissertation, L-arginine deaminase and β-glucosidase were the most consistent in differentiating among soils.
Soil quality and soil resilience are not yet well-defined concepts, despite the large amount of research focused on the areas. So many different possibilities exist for what to measure, under what circumstances, and for what purpose in which soils, that it is difficult to decide how best to describe either soil quality or soil resilience.

Common N-management practices used by Kentucky certified organic and beliefs and knowledge supporting management choices.
Organic farmers have a variety of strategies to build soil fertility and manage N on their farms. Major strategies such as cover cropping and composting are difficult to implement due to labor and land constraints.
Organic farmers rely more on observation and intuition than quantitative data when making management decisions. Farmers prefer to concentrate on the basic management practice of maintaining high organic matter, which often includes preventing soil loss, rather than investigate subtleties of soil biology and chemistry.
According to Kentucky organic farmers, resources for organic farming in Kentucky include Kentucky’s fertile soil, moderate climate, large local market for organic food, and Kentucky’s central location for interstate sales. Some of the challenges are agronomic challenges common to organic farming anywhere and changing life situations common to organic farmers anywhere. For the University of Kentucky to serve organic farmers, UK needs to investigate seriously and systematically the viability of organic farming and local food systems for Kentucky’s economic development, and if the results are positive, to support farmers and consumers in creating opportunities.

Identify research and extension topics that would benefit organic farms.
Useful research in soil science would concentrate on finding ways around constraints such as the expense of inputs or lack of available inputs, the difficulty of using cover crops in slow-drying soil, and dealing with loss of land area to cover crops. Research that further quantifies the behavior of N in organically managed soils could help farmers to make better decisions.
All research should be specific to organic farm systems. Research must be practical, accounting for constraints that farmers face. One management system that needs investigation is organic no-till. Organic farmers realize that the reliance on tillage for weed control is detrimental to soil quality. Another critical issue in no-till is N – mineralization is often stimulated by tillage. Without tillage, timing mineral N availability to match crop demands is even more difficult than usual. Trade-offs will be necessary; research could identify potential compromises or solve some of the problems.

Participation Summary

Project Outcomes

Recommendations:

Areas needing additional study

SUMMARY

This report covers an Enhancement Project (#iEG032-001) with an end date of 3/31/2005. Soil samples from a perturbation experiment comparing conventionally managed and organically managed soils were examined to determine whether there were significant differences in the effects of perturbation (drought, flooding, excess fertilizer N) on the mineralization of 15N-labeled vetch.

Soil samples were collected from vetch and vetch-rye cover crop treatments at Kentucky State University in Frankfort, KY and the Rodale FST and CUT studies at Kutztown, PA that involved conventional vs. organic management and compost utilization comparisons with dairy manure and N fertilizer. The soil samples were homogenized by management and perturbed by one-of-four treatments: none, drought for 28 days, flooding for 12 days, amendment with 800 kg N/ha. After the perturbation the samples were brought to approximately 20% gravimetric water content and incubated at 24 to 28 °C. At 1, 4, and 7 weeks a subset of samples from each management treatment was amended with vetch (3.655% total N) with an atom % 15-N enrichment of 10%. The total N added was 67 mg/kg. The amended samples were incubated one week to permit mineralization and the mineralized N in each treatment was extracted by KCl. The KCl extracts were diffused onto glass fiber disks and analyzed by a stable isotope mass spectrometer to determine the 15-N enrichment of the mineralized N.

The data indicated significant differences between treatments in terms of atom % 15-N of the mineralized extracts. The average atom % 15-N of the samples ranged from 1.0110 to 4.1711. The magnitude of atom % 15-N followed the trend: Flood > Control > Drought > N-fertilized, and this was relatively consistent regardless of week of sampling. The atom % 15-N in soil samples typically decreased with time as a result of mineralization of other N sources in the soil besides the labeled vetch.

The results confirm the initial comparisons performed on these soils in which total N mineralizing capacity was examined. The Rodale soils were better able to withstand perturbations than the Kentucky State soils, probably because of better structure and organic matter content, although individual management practices were relatively the same in terms of vetch mineralization.

OBJECTIVES/PERFORMANCE TARGETS

This study as designed to specifically compare the mineralization of 15-N labeled vetch in organically and conventionally managed soils at two sites:

Kentucky State University – Vetch Cover Crop
Kentucky State University – Vetch/Rye Cover Crop
Rodale Farming Systems Trial (FST) – Conventional
Rodale Farming Systems Trial – Legume-based Organic
Rodale Compost Utilization Trial (CUT) – Nitrogen Fertilized
Rodale Compost Utilization Trial – Uncomposted Dairy Manure
Rodale Compost Utilization Trial – Composted Dairy Manure

The soils were perturbed by flooding, drought, or excessive N fertilization and after 1, 4, and 7 weeks the one-week mineralization of added 15-N labeled vetch was examined. Our initial hypothesis was that organically managed treatments would be more resistant to the effects of perturbation, and therefore show higher rates of vetch mineralization than conventionally managed soils. The vetch was added to distinguish mineralization of the added organic N source from the mineralization of native organic N.

ACCOMPLISHMENTS/MILESTONES

Previous studies on these soils suggested that after perturbation net mineralization and nitrification occurred. However, in terms of the total inorganic N flux there was little difference in perturbation treatment. The Rodale soils were less affected by the perturbation than were the Kentucky State soils. There were no differences among farm management.

For the vetch mineralization study, 483 samples of diffused soil extracts were examined by stable isotope mass spectrometry at the Environmental Research and Training Lab at the University of Kentucky. The 15-N enrichment of all samples was greater than at the start of incubation, which was a clear indication that the added vetch was being mineralized in these samples. At the Kentucky State site the average 15-N enrichment of mineral N was routinely greater in the vetch treatment than the vetch/rye treatment, although on a week-by-week basis the differences between the two treatments were inconsistent. Likewise, the increase or decrease in 15-N enrichment was not consistent from week to week, although the magnitude of enrichment was.

In the Rodale FST samples the 15-N enrichment of the conventional system was routinely higher than the organic system on a week-by-week basis. Overall, the average 15-N enrichment of the conventional system was also higher than the organic system. The average 15-N enrichment of the FST trial exceeded that of the CUT trials as well.

In the Rodale CUT samples no treatment had consistently higher 15-N enrichment than any other. With recovery time there was also no consistent increase or decrease in atom % 15-N enrichment except for the N-fertilized perturbation in which the enrichment of 15-N uniformly increased with time. The flood perturbation consistently led to the highest 15-N enrichment.

IMPACTS AND CONTRIBUTIONS/OUTCOMES

The purpose of this enhancement grant was to determine the pool from which inorganic N was being generated in the soil samples recovering from perturbation. One of the conclusions from the previously funded study was that vetch was not mineralized after perturbation. The evidence from the 15 N study indicated that it was, because the 15-N enrichment of all soils amended with vetch increased after one week of vetch mineralization. This was most apparent in the N-fertilized perturbation, which was originally amended with 15-N to give an atom % 15-N enrichment of 0.60% (background enrichment was 0.37 atom % 15-N). Based on evidence from a flooded soil not amended with vetch, the atom % 15-N enrichment did not change during the course of incubation. Therefore, any increases in atom % 15-N came from the added vetch. That this increased with time in the N-fertilized treatment suggested that the initial hypothesis about vetch contributing to a priming effect was probably correct.

However, the fact that there was so little difference in the various treatments with respect to changes in the atom % 15-N indicated that the management systems in question were relatively unaffected by the type of perturbation imposed. This implies that all soils of similar type in an area, provided that there management is not so different as to impact soil forming factors, are adapted to the stresses of their environment in approximately equal degree.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.