OVERVIEW
In this study, we quantified Cd and Pb content in soils and leafy
greens from five community gardens and compared our results with
regional patterns of soil Cd and Pb across the Phoenix area. A
previous city-wide analysis of soil metals was conducted at 200
locations in the 2000 Ecological Survey of Central Arizona (ESCA), as
a part of the Central Arizona–Phoenix Long-term Ecological Research
Project (Zhuo and Shock 2010, Zhuo et al. 2012). This survey showed an
uneven distribution of soil Pb and Cd across the Phoenix metro area,
with high concentrations in areas of former agricultural and urban use
(Zhuo 2010, Zhuo et al. 2012). We expected soil metal content of
community gardens to follow this larger, city-wide pattern. We also
expected that soil from elevated, raised beds would contain less Pb
and Cd than the non-raised beds that are located in the ground (McLean
and Bledsoe 1992, Clark et al. 2008, CDPH 2014). Finally, we expected
there would be no difference in metal content between shallow (0-15
cm) and deeper soils (15-30 cm) within raised beds. Growing plants in
raised beds can limit root contact with potentially contaminated
pre-existing soil. In addition to examining soil contamination, we
investigated the metal content of leafy greens in our five study
gardens to determine if it was related to the metal content of the
soil in which the plants were grown. Because soil metal content in the
ESCA city-wide survey was below the recommended EPA guidelines for
health concerns, and because metal content in plants can reflect how
much metal is found in the soil (Toth et al. 2016), we expected that
the heavy metal content for leafy greens grown in community gardens
would not exceed the existing guidelines for ingestion (from the
European Union; 0.3 mg Pb/kg fresh weight and 0.2 mg Cd/kg fresh
weight). EPA guidelines for ingestion of heavy metals in foods do not
currently exist.
SITE DESCRIPTION
In September 2014, we contacted gardens in the Phoenix metropolitan
area that aim to combat food insecurity and economic marginalization
issues in their communities. Among these, five different community
gardens agreed to participate in this study. The gardens vary in
location across the metro area, in age, size, and structure; and
community members utilize a variety of growing techniques, such as
importing soils, building raised beds, and using compost and
irrigation to enhance productivity. For privacy, garden names are not
disclosed.
EXPERIMENTAL DESIGN
In each garden, we sampled from planting bed types that were used
specifically to grow leafy greens. In some gardens, these sampling
locations were raised beds, in which soil was elevated and contained
by a wall such as wood or recycled car tires (hereafter, called
‘Raised’ beds). In other locations, leafy greens were grown using
in-ground beds, where soil was not bounded or raised (hereafter called
‘In-ground’ beds; Fig. 1). In many gardens, only one bed per category
(i.e. Raised or In-ground) grew leafy greens. In the case where there
were multiple raised or in-ground beds growing leafy greens within a
garden, we randomly chose beds and sampled soils and plants in each
(i.e. some gardens have two or three replicate soil samples of plants
or soil in Raised or In-ground beds).
SOIL AND PLANT SAMPLE COLLECTION
For each soil sample, we collected two separate soil cores using a
slide hammer core from 0 to 15 cm depth and another two from 15 to 30
cm depth, with cores located at least 50 cm apart from one another.
The two cores from each sampling location and depth were then combined
into a single plastic bag and homogenized to compose one soil sample
to be analyzed. From each of our five gardens, we collected and
analyzed at least one homogenized soil sample (composed of two soil
cores each) from each bed type that was present at that location. In
sum, we analyzed 2-6 soil samples from each of five 14community
gardens. Out of the 28 soil samples analyzed, 17 were collected from
0-15 cm depth, and 11 were collected from 15-30 cm depth. In addition
to soil samples, we took three samples of leafy greens that were
growing in each garden and bed type, where possible. These plant
samples came from Cavalo nero (kale) or Spinacia oleracea (spinach),
both of which were present in most gardens. At each site, one leaf
from each of two-three individual plants was randomly chosen for
sampling. Inner leaves were chosen by gently pulling back an outer
leaf and then, using scissors, cutting the next available leaf
approximately 2 cm from the stem of the plant.
To compare metal concentrations between our sampled leafy green plants
and leafy green plants commonly sold in grocery stores, we purchased
one bunch of conventional spinach from each of three separate grocery
stores – Sprouts, Safeway, and Food City – in the Phoenix metropolitan
area in March of 2015. We sampled and analyzed 3 leaves each from 3
conventional spinach bunches. Leaves were clipped above the stem, and
the 3 leaves of each bunch combined in a bag prior to processing.
SOIL AND PLANT SAMPLE PREPARATION AND METAL ANALYSIS
We sieved soils to 2 mm prior to analyses. All plant leaves were
rinsed thoroughly using tap water to replicate average consumer
habits. We then placed both soils and plant samples in a 105°F oven to
dry overnight. Once dried, we 15pulverized each sample into fine
powder using a ball mill at the Goldwater Environmental Laboratory at
ASU.
We measured soil Pb and Cd content on plant and soil samples using
standard EPA methods for soil trace metals (EPA 1996). We first
digested about 0.25 g each of dried soil and plants in a solution of
HNO 3 , HF, and HBrO 3 to dissolve the soil and plant material prior
to analyses. We then diluted the samples and used inductively coupled
plasma optical emission spectrometry (ICP-OES) to determine metal
concentrations. The detection limit for our analyses was 0.001- 0.01
mg/kg of soil or plant material for Pb and <0.0001 mg/kg for Cd. We
did not complete a full spectral analysis of other metals in the soil
samples, although this method would have controlled for interactions
that other metals may have with metals of interest (Cd and Pb).
ANALYSIS OF SOIL CHARACTERISTICS
In addition to metal analysis, on each soil sample we measured a suite
of soil properties that can affect metal solubility, including texture
(particle size analysis), pH (a measure of acidity or alkalinity), and
organic matter content. Soil texture analysis. Soil particle size, or
texture, influences soil porosity, water holding capacity, and how
metals move through soil. We determined soil texture by using a
modified hydrometer method (Bouyoucos 1962), which estimates the soil
content (in %) of sand (2.0-0.05 mm diameter particles), silt
(0.05-0.002 mm), and clay (<0.002 mm) (Gee and Bauder 1986). We
shook a solution of 40 g of oven-dried soil with 100 mL of a sodium
hexametaphosphate solution to prepare the samples for analyses. After
shaking, we put each sample in a 1-L suspension cylinder and filled
the cylinder to a 1-L mark with deionized water. We used a mixing rod
to mix the sample until it was homogenized within the cylinder, then
we placed the hydrometer into the sample. After 40 additional seconds
of no mixing, we recorded the hydrometer reading. We took hydrometer
readings on each sample at 40 seconds to determine the combined
percent silt and clay content and 7 hours to determine the percent
clay content. We subtracted the clay content from the clay plus silt
content to determine percent silt, and we determined percent sand
content by subtracting the percent silt plus clay from 100%.
Soil pH and organic matter analysis. Soil pH was assessed using a
modified EPA method (Ghose and Pettygrove 2014). We shook
approximately 15 g of soil in 30 ml of deionized (DI) water for 30
minutes, and then used a calibrated pH meter to read the pH of each
sample. Percent soil organic matter was determined using a modified
loss on ignition (LOI) method (Schulte and Hopkins 1996). We placed 20
g of oven dried soil samples in a 550°C furnace for 6 hours and
measured the loss of mass to determine organic matter percentage.
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