Krista Jacobsen

1. Introduction
On a national-level, community-based food systems are
experiencing a renaissance, with consumers increasingly
seeking access to locally-produced foods and to “know
their farmer.” Direct market relationships have allowed
farmers to capture more value from their crops, with the
public perception of locally-produced foods as healthier
and its production more sustainable.
This market expansion and diversification into
horticultural crop production are generally considered
positive from social, public health and economic
dimensions. However, we know little about the
processes underlying the environmental sustainability of
these systems. Horticultural production systems are
resource, input, labor and tillage intensive. Their
management practices have been shown to have
significant impacts on nutrient cycling in agroecosystems.
Further, much of our understanding of nitrogen and
carbon dynamics in agroecosystems are based on work
in agronomic cropping systems with limited diversity in
rotational practices and farming enterprises (e.g. West &
Post, 2002; Ma & Shaffer, 2001). Thus, the net result of
the social and economic incentives to diversify into
horticultural practices here and abroad are drivers in
these systems are land use trends that we have not
historically seen, nor have a strong understanding of.
The goal of this work is to improve our understanding
of the nitrogen and carbon inputs, outputs, and the key
pathways driving agroecosystem sustainability in
horticulture-based systems along a gradient of
intensification. Specifically, we seek to improve our
understanding of (1) nitrogen availability, efficiency, and
retention (2) soil carbon dynamics in labile carbon pools
in a diversity of crops and cropping seasons. In addition
to C and N dynamics, we are also seeking to
contextualize our soil- and plant-based flows within a the
broader farming system by using life cycle analytical
approaches.
Preliminary Results
Acknowledgments
Funding for this work was awarded from USDA Foundational
Programs Renewable Energy, Natural Resources, and Environment
(RENRA) Program (2013-67019-21403).
The PI’s wish to thank the cooperating farm managers taking
extensive data for this project, with special thanks to the Bell and
Stone families, Tiffany Thompson, and Dr. Mark Williams. Students
have provided significant project labor, and include Debendra
Shrestha, Matthew Deason, Garrett Steck, and Savannah McGuire.
Project support staff include Dr. Haichao Guo, Dr. Alexandra Williams,
Jason Riley, Brett Wolff, and Aaron Stancombe.
Materials and Methods
Future Steps
We will continue to take data in the field experiments for one to one-
and-a-half more years, and are beginning to analyze this data as
physical samples are processed. Trace gas data collection is
ongoing, and we are beginning to analyze this flux data for conference
presentations beginning Summer 2015. Nutrient cycling modeling
efforts will begin within the year based on these data. Energy
modeling activities are ongoing, and will begin to include the
conventional system and individual crops in a subset of the systems.
We anticipate these forthcoming results will inform future study
on ecological intensification in horticulture-based systems, as well as
additional interdisciplinary work examining how we quantify
“intensification” and “sustainability” in systems-oriented contexts such
as those explored in this work.
Krista Jacobsen1, Ole Wendroth2, & John Schramski3
1Department of Horticulture, University of Kentucky; 2Department of Plant and Soil Sciences, University of Kentucky; 3College of Engineering, University of Georgia
Toward Sustainable Nitrogen and Carbon Cycling on Diversified
Horticulture Farms Serving Community Food Systems
Experiment I
The goal of the first experiment is to characterize cycling of labile N and C pools, and estimate loss rates via trace gases and leaching from
a single rotation within three of the study systems for three years (Extensive Organic, Stationary Organic High Tunnel, and Conventional
systems, Table 1). Soil C pools are sampled semi-annually at the 0-15, 15-30, and 30-50cm depths and analyzed for total soil C measured
by LECO combustion and labile soil C measured by the permanganate oxidizable carbon (POX-C) method (Culman et al., 2013). Soil N
pools are sampled monthly at the 0-15, 15-30, and 30-50 cm depths for soil mineral N (NO3
- and NH4
+) and for resin-bound mineral N using
buried ion-exchange resin bags at the 7.5 and 22.5 cm depths. Mineral N samples are analyzed by colorimetric analysis on a microplate
reader (BioTek Instruments, Inc, Winooski, VT) after reduction of NO3
- samples via a cadmium reduction device (ParaTechs Co.,
Lexington, KY) (Crutchfield & Grove, 2011). Mineral N leaching losses are being evaluated using ion exchange resin lysimeters (after
Susfalk & Johnson, 2002) deployed at 50 cm and collected quarterly.
Trace gas fluxes of CO2, CH3, N2O, and NH3 are measured year-round on a weekly basis during the spring and early summer,
and a bi-weekly basis from mid-sum to early spring using a field-based FTIR gas analyzer (Gasmet Technologies, Helsinki, FI) (Figure 1).
Sites are instrumented with solid-state electrical resistance soil moisture sensors and tensiometers at the 15, 30, 45, and 60 cm depths.
Yield data are collected as harvested, and plant biomass samples collected at completion of each segment of the rotations (e.g.
after end of crop harvest, cover crop growth, etc.). Data from this study will also be used in modeling N dynamics and losses using the
LEANCH-M and RZWQM models (discussed in “Future Steps,” below).
Figure 1. Graduate student Debendra
Shrestha collecting trace gas data using
Gasmet DX4040 FTIR gas analyzer in
Fall 2014.
Experiment II
The goal of the second experiment is to characterize soil C and N processes in model crops in the spring, summer, and fall growing seasons in each of the five study systems.
Each model crop will be tracked for one year in each system, beginning with the crop and through succeeding crop and/or cover crops characteristic of the production system.
Life cycle analyses, using carbon as a surrogate for energy, will allow for comparison between input:output for each crop and system.
Project Overview
We have structured this work based on
five cropping systems that represent this
intensification gradient: 1) a low external
input organic system based on forages
and rotational grazing for subsequent
vegetable production, 2) an annual
cover-crop and organic fertilizer-based
system, 3) a year-round, moveable high
tunnel system that relies on rotation and
cover crops, as well as organic
fertilizers, 4) a year-round, stationary
high tunnel system that relies on organic
fertilizers, and 5) a conventional system
that relies on inorganic fertilizers. All
systems are tillage-intensive and rely on
varying degrees of pesticide and other
fertility inputs. We have parsed this
work into two field experiments based in
these systems, with complementing
nutrient cycling modeling and life cycle
analyses.
Energy Analysis
All labor, inputs, operations, and yield data are being collected for both Experiment I and II in order to model the ecological energetics of several of the systems in Experiment I
and the crops in several systems in Experiment II. Boundaries are set at the farming system or field boundary, respectively. Computational models are being developed in
Microsoft Excel, and incorporate as much of the “upstream” or “embodied” energy in each input as possible (after Schramski et al., 2013).
Table 1. Farming systems used in the project, as characterized by parameters characterizing the intensity of the production system.
*Seasonal indicates no or very little use of season extension technologies. Seasonal production for the study area is mid-March to mid-November.
Literature Cited
Culman, S. W. et al. 2013. Permanganate oxidizable carbon reflects a processed soil fraction that is sensitive to management. Soil Science Society of America Journal 76:
494-504.
Crutchfield, J. D., and J.H. Grove, 2011. A new cadmium reduction device for the microplate determination of nitrate in water, soil, plant tissue, and physiological fluids. Journal
of AOAC International 94: 1896-1905.
Ma, L., and M.J. Shaffer, 2001. A review of carbon and nitrogen processes in nine U.S. soil nitrogen dynamics models. In: Modeling Carbon and Nitrogen Dynamics for Soil
Management. CRC Press, Boca Raton, FL. p. 55-102
Susfalk, R. B., and D.W. Johnson, D. W., 2002. Ion exchange resin based soil solution lysimeters and snowmelt solution collectors. Communications in Soil Science and Plant
Analysis 33, 1261-1275.
Schramski, J. R., Jacobsen, K. L., Smith, T. W., Williams, M. A., and Thompson, T. M. (2013). Energy as a potential systems-level indicator of sustainability in organic
agriculture: Case study model of a diversified, organic vegetable production system. Ecological Modelling 267, 102-114.
West, T.O. and W.M. Post, 2002. Soil organic carbon sequestration rates by tillage and crop rotation: A global data analysis. Soil Science Society of America Journal 66(6):
1930-1946.
To date, preliminary results that are in shareable form are for the whole-
system energy analysis, based on data from January 2014 to January 2015
for the Extensive and Medium-Scale Organic systems. Cumulative inputs
and outputs for these systems are illustrated in Figures 2 and 3,
respectively. The cumulative ratio for the Extensive Organic system, which
is based on a five year pasture-three year vegetable rotation without
supplemental fertility, is approximately 10. The cumulative ratio for the
Medium Scale Organic system, which is based on cover crops and
imported compost and granular organic fertilizer, is approximately 48. It is
notable, however, that the greatest energy inputs into the Medium Scale
Organic system are natural gas inputs to heat the propagation greenhouse,
and electricity for refrigeration and packing operations. Outputs do not
begin until the first distribution of the Community Supported Agriculture
(CSA) program begins in late May, although inputs begin as soon as
organizing for the year begins in early January.
Figure 3. Energy input and output contributions for the Medium Scale Organic
system for 2014.
Figure 2. Energy input and output contributions for the Extensive Organic
system for 2014.

2. Extensive Organic Medium-Scale Organic
Movable Organic
High Tunnel
Conventional
Stationary Organic
High Tunnel
Production Seasonal* Seasonal* Year-round Seasonal* Year-round
Fallow
Periods
5 year forage-based fallow, with
rotational grazing
Annual cover crop once per year Annual cover crop twice per year Annual cover crop once per year None
Tillage
Frequency
None (fallow) -> Intensive semi-annual
primary and secondary (horticulture)
Annual primary tillage, bed shaping,
frequent shallow cultivation for weed
control
Semi-annual primary tillage, additional
semi-annual secondary tillage, frequent
cultivation for weed control
Semi-annual primary and secondary
tillage
Quarterly secondary tillage, frequent
cultivation for weed control
Nutrient
inputs
Fallow, cover crop, minimal compost
Cover crop, compost, granular manure-
based fertilizer
Cover crop, compost, granular manure
based fertilizer
Cover crop, synthetic fertilizer
Compost, granular manure-based
fertilizer
Intensification Low High