This study evaluated the effects of fumigation on soil nematode communities and soil food webs in pistachio orchards in California. Nematode taxa distributions were analyzed using structural, channel, and enrichment indices. Results showed that fumigation decreased the channel index, indicating a reduction in the fungal decomposition pathway. The enrichment index increased with fumigation, suggesting greater nutrient availability promoted colonizer nematode populations. While structure was initially low due to agricultural practices, there was a trend of higher structure index values under fumigation. Overall, the results demonstrate that fumigation disrupts the soil food web by reducing fungi and favoring bacterial decomposition.

1. Gavin John, Amanda Hodson, Ed Lewis
UC Davis Department of Entomology and Nematology
Acknowledgements
The authors thank David Doll for the use of his ongoing
support in almond and pistachio field experiments.
Conclusion
The findings of this experiment indicate that the
relative importance of fungal and bacterial mediated
decomposition channels is effect by fumigation
treatments. Additionally, the results suggest that
fumigation increases the availability of carbon, thus
allowing for the immobilization of nutrients.
Measurements of labile carbon between treatments
would be a useful measurement and should be
considered in future studies.
Aim and Hypothesis
The purpose of this study is to evaluate soil communities
by utilizing nematode taxa distributions as a means of
making inferences about the soil food web, specifically
after a major disturbance. The various indices developed
by Howard Ferris provide a useful tool for making
inference in regards to the soil food web, and this study
intends to elucidate the effects of fumigation on soil
communities in Pistachio orchards. We expect to see a
decrease in community structure in response to fumigation,
and also that fungal mediated decomposition pathways will
be more reduced compared to bacterial decomposition.
Introduction
A fundamental aspect of attributing soil ecological
function based on the diversity of nematodes is the
assignment of nematode taxa to a specific value (1-5)
on the colonizer-persister scale. Taxa that are
considered as persisters (3-5) occupy higher trophic
levels within the soil food web and are thus more
susceptible to disturbances. Taxa that are colonizers
(1-2) recolonize environments more rapidly compared
to persisting species and reproduce more quickly. The
relative abundance of colonizers to persister gives
insight into the soil community’s ability to reestablish
itself after a disturbance.
In this experiment we will examine are the
structural index (SI), channel index (CI), and
enrichment index (EI). Each index is based on the
relative abundance of each nematode taxa and their
position on the cp-spectrum. The enrichment index is
based on the expected response of opportunistic guilds
to recourse enrichment (Ferris). This index is thus
useful as an indicator of potential mineralization and
nutrient recycling. The channel index is determined by
the relative quantities of bacterial grazing and fungal
grazing nematodes, which gives insight into the
decomposition channels of the soil food web. The EI
measure the foodweb’s response to available resources,
while the CI indicates the dominant decomposition
pathways. Together, these two indices are intended to
give useful insight into nutrient availability, nutrient
leaching potential, and indicate how C and N input
should be managed to maximize productivity (Ferris
2001).
Method
Pistachio orchards were sampled for nematodes near
Livingston, CA as part of an ongoing experiment
comparing the susceptibility of pistachio rootstocks to
root feeding nematodes. For this experiment, soils were
analyzed from four blocks, each containing two rows,
where one was fumigated and one was not. Within each
block, 6 different rootstocks were planted, although we
only analyzed samples from two clonal lines of P.
antlantica x P. integerrima ‘UCB-1’ ( D110P and
D394W ) for this experiment. Prior to planting in early
spring 2013, one row in each block was fumigated with
Telone® II (1,3 dichloropropene) applied as a liquid
injection at row center at a rate of 33.3 gallons/acre, while
the remaining row in each block was not fumigated.
Nematodes were extracted using sugar centrifugation. Pest
nematodes were identified and the total amount of
nematodes per 200 mL soil was noted.After initial
counting, the extracted nematodes were centrifuged
again.Roughly 1 mL of extract was mounted on a slide
and the nematodes were identified to the genus level. The
total count was scaled up using the initial amount of
nematodes per 20 mL of soil, and indices were determined
using NINJA:Nematode INdicator Joint Analysis.
Results
The difference in blocks was not determined to be
significant, so the control and fumigant treatments were
considered irrespective of rootstock. Difference in indices
were compared by Kruskal Wallace tests (since assumptions
of normality could not be met for ANOVA) in R v.3.1.1. In
2015 there was a decrease in the average channel index after
fumigation (p=0.63, chi=.26)
The Enrichment Index increased in response to fumigation
(p=.01, chi=6.11)
There was a trend indicating that the SI increased in response
to fumigation. The average SI value for the control and
fumigation treatments were 20.4 and 27.4, respectively. The
standard error of measurements for were 3.7 and 6.2.
Discussion
The relatively low values obtained for the structure
index indicates that the initial structure is supressed due
to intensive agricultural practices. This could result in
low populations of predatory and omnivorous
nematode genre, thus providing a less valuable
comparison to post fumigation treatments.
Nevertheless, the decrease in the channel index after
fumigation indicates that the fungal mediated
decomposition channel is adversely affected by
fumigation. Additionally, the findings suggest that
fumigation results in an increase in the enrichment
index. These findings are in abidance with research
that suggests fumigation with chloroform results in the
immediate increase of nitrogen and labile carbon
(Jenkinson 1975). The rapid immobilization of
nitrogen via the oxidation of carbon substrate mediated
by microbes is reflected in the increase in opportunistic
nematode species. This increase in the EI could also
contribute to the higher SI value in the fumigated
treatment. Considering both treatments underwent
tilling in spring of 2013, it is likely significant structure
has yet to develop and the increase in EI allows for
colonizing nematodes to become established, paving
the way for persisting species.
Using nematode taxa distributions to assess soil food web
conditions after fumigation in pistachio orchards
C
A
B
D
Figure 1: Figure 1 shows characteristic representatives of each major
nematode feeding category consisting of 1A) a meloidogyne plant parasitic
nematode with a cp value of 2; 1B) a predatory Qudsianematinae with a cp
value of 4; 1C) an Aphelenchus fungal feeder with a cp value of 2 and 1D)
an Acrobeles bacterial feeder with a c value of 2
Figure 2: This figure shows the change in the channel index in response to
fumigation. The average value for the fumigated and control treatment
were 51.0 and 88.3, respectively. The respective standard errors were 12.1
and13.7
Figure 3: Figure 3 shows the increase in the EI in response to
fumigation. The average values for the fumigated and control
treatments were 28.0 and 10.1. The respective error of measurement
were 4.3 and 3.0.