The quick answer is that there is
little difference that can be safely ascertained and that there is no reason to
alter practice of the account of it.
More detailed analysis may tell us something different but I suspect
that any such information would indicate additional effort also.
At least someone asked the
question and it looks as something that is safe to ignore as an issue.
Organic and Conventional Farming Methods Compete to Eliminate Weed
Seeds in Soil
Released: 4/21/2011 9:00 AM EDT
Newswise — Weeds are hard to kill; they seem to come back no matter
what steps people take to eradicate them. One reason is because of the
persistence of weed seeds in the soil. Organic farming and conventional farming
systems both have their methods of taking on weed seeds, but does one show
better results than the other?
The authors of a study reported in the current issue of the
journal Weed Science conducted tests that compared conventional
farming with organic systems. This research determined weed seed viability
under both systems over a two-year period in two separate locations.
To compare these systems, researchers buried seeds of two types of
weeds, smooth pigweed and common lambsquarters, in mesh bags. Tests were
conducted at agricultural research locations in Maryland
and Pennsylvania
Depth of seeds in the soil, environmental conditions, and soil
management are among the factors that affect seed persistence. Under
conventional soil management, tillage is an important practice that manipulates
the depth of seeds and environmental conditions that can influence weed seed
persistence. Organic soils have enhanced biological activity, with more carbon,
moisture, and microbial activity that could lead to greater seed decomposition.
The organic soils in this study were higher in total soil microbial
biomass than the soils of the conventional farming tests. This was measured by
phospholipid fatty acid content. But the results of the tests did not lead
researchers to conclude that this microbial biomass has a dominating role in
seed mortality.
Pigweed seeds showed a shorter life span under the organic system in
two of four experiments. Organic system lambsquarters seeds had a shorter life
span in just one of the four experiments, while the conventional methods had
the shorter life span in two of four experiments. These results leave an
ambiguous answer to the question of which farming system can better eliminate
seeds deep in the soil to control weeds from their source.
Full text of the article, “Weed Seed Persistence and Microbial Abundance in Long-Term
Organic and Conventional Cropping Systems,” Weed Science, Vol. 59, No.
2, April-June 2011, is available at http://www.wssajournals.org/doi/full/10.1614/WS-D-10-00142.1.
About Weed Science
Weed Science is a journal of the Weed Science Society of
WEED BIOLOGY AND ECOLOGY
Weed Seed Persistence and Microbial
Abundance in Long-Term Organic and Conventional Cropping Systems
Silke D. Ullrich, Jeffrey S. Buyer,
Michel A. Cavigelli, Rita Seidel, and John R. Teasdale*
Weed
seed persistence in soil can be influenced by many factors, including crop
management. This research was conducted to determine whether organic management
systems with higher organic amendments and soil microbial biomass could reduce
weed seed persistence compared with conventional management systems. Seeds of
smooth pigweed and common lambsquarters were buried in mesh bags in organic and
conventional systems of two long-term experiments, the Farming Systems Project
at the Beltsville Agricultural Research Center, Maryland, and the Farming
Systems Trial at the Rodale Institute, Pennsylvania
Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; smooth pigweed, Amaranthus
hybridus L. AMACH
Received: September 29, 2010; Accepted: January 10, 2011
*First, second,
third, and fifth authors: Research Associate, Chemist, Soil Scientist, and
Plant Physiologist, U.S. Department of Agriculture-Agricultural Research
Service Sustainable Agricultural Systems Lab, Beltsville, MD 20705; fourth
author: Agroecologist, Rodale Institute, Kutztown, PA 19530. Corresponding
author's E-mail:
Persistence of weed seeds in soil is critical to the survival of annual
weeds in agroecosystems. Seed mortality is a demographic process that can have
a major impact on presence and dynamics of weed populations (Cousens and Mortimer 1995; Mohler 2001). Many factors have been shown to affect weed seed
persistence including species (Buhler and Hartzler 2001), maternal environment (Schutte et al. 2008), depth in soil (Conn et al. 2006),
environmental conditions (Davis et al.
2005), and soil management (Davis et al.
2006; Fennimore and Jackson 2003; Lutman et al. 2002; Steckel et al. 2007). Understanding soil management effects on
seed persistence is particularly important because it offers producers the
opportunity to manage weed seed populations through cultural practices. Tillage
is an important operation that can allow manipulation of seed depth and
associated environmental conditions that influence seed persistence (Lutman et al. 2002; Steckel et al. 2007). Amending the soil with organic inputs in
reduced input or organic farming systems also can influence seed persistence
and weed seedbank levels (Davis 2007; Davis et al. 2006; Fennimore and Jackson 2003; Gallandt et al. 1998).
Several approaches have been used to assess the persistence of seeds in
soil, ranging from methods that maintain seed in a natural soil environment
with less precision of recovery to methods that create an artificial soil
environment but increase precision of recovery. An example of the former
approach is to begin an experiment with a high natural population (Schweizer and Zimdahl 1984; Steckel et al. 2007) or to pulse with a defined initial
density of seeds that may be subject to experimental management practices (Buhler and Hartzler 2001; Lutman et al. 2002) followed by subsequent soil sampling. More
precise recovery of the initial seed lot can be achieved by installation of
seeds in unenclosed soil cylinders that can be subsequently recovered in their
entirety (Schutte et al. 2008; Teo-Sherrell et al. 1996). Mesh bags or other porous seed
enclosures provide an artificial barrier to the surrounding soil, and permit
high recovery precision of the initial seed lot (Conn et al. 2006; Davis et al. 2005; Gallandt et al. 2004). Schutte et al. (2008) found
the mesh bag and core methods resulted in similar seed persistence and
mortality, althoughVan Mourik et al. (2005) warn
that high seed-to-soil ratios in mesh bags can lead to excessive fungus-induced
mortality.
Gallandt et al. (1999) suggested
that enhanced soil biological activity resulting from improved soil quality
could increase the mortality of weed seeds in soil. Kennedy (1999)proposed that no-tillage and biologically based
systems that enhance soil carbon, moisture, and microbial activity may
predispose these systems to greater seed decomposition. Soil managed for high
organic matter and reduced tillage had soil quality indicators, enzyme
activity, and water stable aggregates that were associated with
weed-suppressive bacterial isolates (Kremer and Li 2003). However, the major causes of seed
mortality include not only microbe-induced decay, but also fatal germination,
physiological aging, and predation (Gallandt et al. 2004). Losses from predation can be
distinguished from the other mortality factors by confining seed in containers
that exclude the predator in question. It is often difficult to distinguish
among microbial decay, aging, and fatal germination in container experiments.
Fatal germination can occur when seed germination is induced under conditions
that do not permit successful emergence (Benvenuti et al. 2001; Gallandt et al. 2004). Research oriented to crop seed
preservation has shown that glassification of seed sugars can protect seed from
destruction by various physiological aging mechanisms involving free-radical
and enzymatic destruction of essential cell molecular components (Bernal-Lugo and
Leopold 1998). Thus, microbial destruction of the seed coat could
lead not only to microbial decay of seeds but also to moisture conditions that
would induce a phase shift in glassification and acceleration of physiological
aging processes.
Long-term systems experiments with a range of cropping systems that
differ in soil organic inputs offer an opportunity to investigate the potential
impact of enhanced soil microbial activity on weed seed persistence (Davis et al.
2006; Gallandt et al. 2004; Kremer and Li 2003). The Rodale Institute Farming Systems
Trial (FST) near Kutztown , PA ,
was initiated in 1981 and is the longest running comparison of conventional and
organic cropping systems in the United
States
The objective of this research was to compare the persistence of smooth
pigweed and common lambsquarters in the FSP and FST organic and conventional
cropping systems within mesh bags that eliminated mortality effects of
predators. Our hypothesis was that increased soil microbial biomass in organic systems
are associated with shorter seed persistence. Since soil depth can influence
the environmental conditions and microbial activity that seeds encounter, a
second objective was to compare seed persistence at two seed depths, 5 and
15 cm.
Materials and Methods
|
Experimental Site Description.
Seed persistence experiments were conducted at two long-term
experimental sites: FSP, begun in 1996 and located at the Beltsville
Agricultural Research Center (BARC), Maryland ,
and FST, begun in 1981 and located at the Rodale Institute near Kutztown , PA.
At BARC, experiments were conducted in two of the five FSP cropping
systems, a conventional chisel-tillage system and an organic system. These
systems were chosen because they followed a similar 3-yr corn (Zea mays L.)–soybean [Glycine max (L.)
Merr.]–wheat (Triticum aestivum L.)
rotation, so that confounding effects of rotation would be minimized. Detailed
description of the systems and crop performance during the first 10 yr of
this cropping systems experiment are outlined in Cavigelli et al. (2008). In brief, the conventional system was
farmed using locally recommended fertilizer and herbicide programs. The organic
system relied on cover crops (hairy vetch [Vicia villosa Roth] before corn and rye [Secale cereale L.] before soybean] plus poultry litter for providing and/or retaining
nutrients, and on rotary hoeing and sweep cultivation for weed control. Both
the conventional and the organic systems included plowing and disking for
seedbed preparation.
At Rodale, experiments were conducted in two of three FST cropping
systems, a conventional and an organic system. A corn–corn–soybean–corn–soybean
rotation was followed in the conventional system with recommended fertilizer
and herbicide inputs. The legume-based organic system at FST was chosen because
it followed a corn–soybean–wheat rotation similar to that used in the FSP 3-yr
organic system. This organic system relied on various legume cover crops for
providing nitrogen to grain crops, on a rye cover crop for retaining nutrients
following corn, and on rotary hoeing and sweep cultivation for weed control.
Both the conventional and the organic systems included plowing and disking for
seedbed preparation. One exception to cropping patterns occurred in 2003, when
oats (Avena sativa L.) were planted as a uniformity crop over the entire trial. Further details
on FST cropping systems management and system performance can be obtained in Pimentel et al. (2005).
Seed and Soil Handling.
Smooth pigweed and common lambsquarters seeds that had been collected
at BARC were placed in flat, square nylon bags with 6.75 cm interior
length per side and a mesh size of 0.5 mm. Seed viability was initially
tested by germination followed by a forceps squeeze test of ungerminated seeds.
Each bag was filled with 200 viable seeds of a species along with 72 cm3 of field soil, which was obtained from the targeted burial location and
system and dry sieved to eliminate resident weed seed. Bags were installed in a
horizontal position in plots at 5- or 15-cm depths, avoiding wheel track areas.
Bags were installed at two locations per plot and in four blocks per cropping
system per location.
In order to eliminate crop effects, bags were only put in soil during
the soybean phase of each rotation. Bags were installed in plots in the fall
after harvest of the previous corn crop and planting of a rye cover crop (late
October to early November). Four sets of bags were installed, with each set to
be retrieved at half-year intervals over a 2-yr period. Bags were retrieved in
the spring before field preparation for soybean planting (late April to early
May). The set designated for removal in the first spring was taken to the lab
for testing while the remaining sets were temporarily placed in soil near the
edge of adjacent plots within the same cropping system while the experimental
plots were prepared for planting. After soybean planting, sets designated for
later retrieval were replaced into their designated plots at their designated
depths beneath the soybean row to avoid damage by subsequent between-row
cultivations. Bags were again retrieved in the fall after soybean harvest. The
set designated for removal in the first fall was taken to the lab and the
remaining sets were moved to plots planted to rye and destined for soybeans in
the next season. A similar process was repeated to exhume the sets designated
for spring and fall removal during the second season. The first experiment
consisted of four sets of bags that were initially installed in the fall of
2001 and removal was completed in the fall of 2003. A second experiment
consisted of an additional four sets of bags that were initially installed in
the fall of 2002 and removal was completed in the fall of 2004.
In the lab, bags were washed clean of soil with tap water and blotted
dry. Bags were dipped in 70% ethanol for 30 s, rinsed with deionized
water, dipped in 10% Clorox1 for 5 min, rinsed again with deionized water, and blotted dry with
a paper towel. When bags were dry, seeds were transferred to petri dishes
containing Whatman no. 3 filter paper and 7.5 ml deionized water.
Dishes were sealed with parafilm and placed in a growth chamber at constant
30 C for smooth pigweed or 20 C for common lambsquarters. Germinated
seeds were counted and removed from the dish after 1 wk. Plates were
moistened as needed, resealed, and counted after another week. Remaining
ungerminated seeds were squeezed with a forceps and identified as viable if the
endosperm was white and firm (Sawma and Mohler
2002).
A composite of 10 soil samples was taken to 15-cm depth near each
burial site in each plot for soil microbial biomass and community analysis. Samples
were taken at three times during the season, in early April before spring
tillage, in June after soybean planting, and in early October before soybean
harvest. Soil was placed in a sealed plastic bag and transported in a cooler to
the lab. Soil was sieved through a 0.6-cm screen and stored at −20 C.
Phospholipid fatty acid analysis of soil microbial communities was conducted as
previously described (Buyer et al. 2010). Biomarkers for gram-positive bacteria,
gram-negative bacteria, actinomycetes, fungi, and protozoa were as described by Buyer et al. (2010). In addition, the sum of 15 
0, 15 1 iso and anteiso, 17 0 cyclo, 17 0 iso, and 17 1 iso and anteiso was used
as a biomarker for eubacteria (Frostegård and Bååth, 1996). The sum of polyunsaturated fatty
acids was used as a biomarker for eukaryotes, and the total phospholipid fatty
acid (PLFA) concentration was used as a biomarker for total microbial biomass (Zelles 1999).
0, 15 1 iso and anteiso, 17 0 cyclo, 17 0 iso, and 17 1 iso and anteiso was used
as a biomarker for eubacteria (Frostegård and Bååth, 1996). The sum of polyunsaturated fatty
acids was used as a biomarker for eukaryotes, and the total phospholipid fatty
acid (PLFA) concentration was used as a biomarker for total microbial biomass (Zelles 1999).
Data Analysis.
The percentage of seed surviving at each exhumation date was computed
by dividing the measured number of germinated plus viable ungerminated seeds by
the initial number of viable seeds in each packet ( = 200). For each location
and experiment, survival values from each block
of each system were regressed onto time (unit = yr) using a log-logistic model with
asymptotes at 100 and 0 (Schabenberger et al. 1999) where b is a shape parameter and LT50 is an
estimate of the lethal time for 50% seed mortality. Thus, eight data points
representing four times of exhumation and two replications at each time were
used to determine LT50 values
for each of four blocks of each system. A four-way ANOVA was conducted to
analyze LT50 values relative to location, system, experiment, and depth using a
mixed model procedure (PROC MIXED, SAS version 9.1.32). Since the organic systems and conventional systems were
similar at the two locations, these were coded as common systems (organic or
conventional) across locations. Only three
species–location–system–experiment–depth blocks from this factorial set of
treatments failed to give a reliable LT50 estimate; these were treated as missing values for the analysis.
PLFA biomarkers were analyzed using ANOVA and MANOVA with the general
linear model (PROC GLM). A one-way model was employed in which the main effect
consisted of the four cropping systems across both locations with year and
sampling time treated as replications. For MANOVA, biomarkers were first
transformed as the square root of the proportional biomarker (Hellinger
transformation; Legendre and Gallagher 2001) and then standardized to unit
variance. A canonical variates analysis, generated by the MANOVA, was used to
identify the linear combination of variables that best separated the cropping
systems (Buyer et al. 1999; Seber 1984).
Pearson coefficients were determined for the correlation between PLFA
biomarkers and seed survival half-lives.
Results and Discussion
|
Seed Persistence.
The majority of seed buried in these experiments did not persist for 2
yr. Generally, most seed persisted through the first winter (mean survival at
the first spring exhumation date was 87 and 83% for common lambsquarters and
smooth pigweed, respectively), but then exhibited higher mortality in the first
summer (mean survival at the first fall exhumation date was 49 and 61% for
common lambsquarters and smooth pigweed, respectively). By the end of
2 yr, mean survival was 20 and 29% for common lambsquarters and smooth
pigweed, respectively. Because this pattern resulted in a sigmoid-shaped
response for most data sets, data were fit to a log-logistic function rather
than to an exponential decay function, which is often employed for fitting seed
persistence data (Conn et al. 2006;Lutman et al. 2002).
Analysis of half-life values demonstrated a location by experiment by
system interaction for smooth pigweed (P = 0.0945) and common lambsquarters
(P = 0.0048). Smooth pigweed seed half-life was shorter in organic than in
conventional systems in the second experiment at both locations (Table 1). Common lambsquarters seed half-life also was shorter
in organic than in conventional system in the second Rodale experiment, but was
longer in organic than in conventional systems in both BARC experiments. Smooth
pigweed and common lambsquarters survival was high in both systems at the first
spring exhumation date, but survival differentials between systems usually
became most pronounced at the first fall exhumation date. This survival pattern
can be seen in the second BARC experiment in which system differences were
particularly prominent for both species (Figure 1).
The inconsistent survival responses to system, location, and experiment
can not be explained by weather. Seasonal air temperatures were relatively
similar between years and locations, although temperatures at Rodale tended to
be slightly, but consistently lower than those at BARC (Table 2).
Rainfall also followed a similar pattern at the two locations, with rainfall at
Rodale higher than that at BARC in all but one of the 6-mo periods. Rainfall
differentials between Rodale and BARC were most pronounced during the summers
of 2002 and 2004. This rainfall difference may have contributed to the lower
smooth pigweed half-life at Rodale than BARC (1.0 vs. 1.6), but had no effect
on the half-life of common lambsquarters (1.1 and 1.0 at Rodale and BARC,
respectively). The difference between higher temperatures during summer burial
period (18 to 21 C) and lower temperatures during the winter burial period
(1 to 8 C) was the most consistent weather trend observed (Table 2).
Higher mortality during the first summer than winter burial periods at both
locations (Figure 1) was probably related to this temperature
differential. Mortality mechanisms dependent on biochemical or metabolic
reactions would be enhanced by higher temperatures.
Despite the differences among system half-lives observed, these
differences were relatively small (months), and half-lives in both systems were
short (less than 2 yr) in all location-experiments (Table 1).
The seed persistence half-lives of approximately 1 to 2 yr reported here
are similar to those reported by other researchers using a range of
methodologies. Half-lives for Amaranthus species ranged from < 1 to 2 yr in experiments using natural seedbank decline (Schweizer and Zimdahl 1984), pulse seeding without containers
(Buhler and Hartzler 2001), and mesh bags (Davis et al.
2005). Common lambsquarters half-lives ranged from 1 to 2.5 yr
in experiments with natural seedbanks (Schweizer and Zimdahl 1984), pulse seeding (Lutman et al. 2002), and mesh bags (Conn et al. 2006; Davis et al. 2005). The relatively longer half-life of
2.5 yr was observed under relatively colder Alaskan conditions (Conn et al. 2006),
which undoubtedly would have delayed decay or aging processes compared to those
in warmer climates. A longer common lambsquarters half-life of 4.5 yr was
observed at one location in a United Kingdom experiment (Lutman et al. 2002), but this data set was limited by an
unusually low initial recovery value following pulse seeding that probably
upwardly biased this half-life estimate. Common lambsquarters seemingly
persisted for many years when buried in cylinders in Nebraska
It is also clear that a minority of smooth pigweed and common
lambsquarters seed can persist for many years. In all of the studies reported
above, some viable seed were always present at the conclusion of the experiment,
whether 4 to 6 yr (Buhler and Hartzler 2001;Schweizer and Zimdahl 1984; Steckel et al. 2007), 17 yr (Burnside et al. 1996), or 20 yr (Conn et al. 2006).
This suggests that seed cohorts of these species are made up of both
short-lived and long-lived seeds. The production of heterogeneous seed types
with respect to dormancy, dispersion, and persistence is a strategy that
ensures survival under potentially variable edaphic and environmental
conditions (Matilla et al. 2005). Common lambsquarters is known to produce
heteromorphic brown and black seeds that vary in seed coat thickness, dormancy,
and stress tolerance (Yao et al. 2010). Future research on seed persistence probably
requires a more thorough characterization of both the initial viability of seed
lots as well as the proportions of distinct heteromorphic seed types.
Seed Depth.
There were few significant effects of seed burial depth on half-life.
There was a location by depth interaction in which smooth pigweed half-life was
shorter when buried at the 5-cm than at the 15-cm depth at BARC (1.35 vs.
1.95 yr, respectively), but there was no significant difference at
Kutztown (1.0 vs. 0.9 yr, respectively). There was a location by
experiment by system by depth interaction for common lambsquarters half-life,
but this involved a significant difference in depth at only one of eight
location–experiment–system levels while there were no significant differences
at the other seven levels (data not shown).
Other research using mesh bag methodology has reported no effect
amongst 0- to 10-cm (Davis et al. 2005) or 2- to 15-cm (Conn et al. 2006)
depths on common lambsquarters persistence. Clearly, in the absence of seed
enclosures, seed on the soil surface would be more prone to predation than
those buried in the soil (Menalled et al. 2007; Mohler and Galford 1997; Navntoft et al. 2009); however, losses to mechanisms other
than predation appear to have a more subtle relationship to depth. Fatal
germination is a potential loss mechanism that could account for
depth-dependent seed mortality. Common lambsquarters and redroot pigweed have
been shown to readily emerge from depths to 2 cm in a field study (Grundy et al. 2003) or to 4 cm in a pot study (Benvenuti et al. 2001), so germination at depths greater than
these could be fatal. However, redroot pigweed and common lambsquarters along
with 18 other species were shown to undergo depth-induced dormancy, whereby
approximately 85% of seed (90% of pigweed and lambsquarters) did not germinate
(Benvenuti et al. 2001), a condition thought to be mediated by
reduced oxygen levels with increased depth. This suggests that most seed at the
5- and 15-cm depths in our study would have been unlikely to undergo fatal
germination.
Bags buried to either depth were temporarily exhumed during tillage
operations in the spring and fall, thus potentially exposing seed near the
surfaces of the bags to light. Light is a well-known stimulus to germination of Amaranthus and Chenopodium species and is particularly effective on seeds that have been
sensitized by overwinter field burial or laboratory stratification (Bouwmeester and Karssen 1993; Gallagher and Cardina 1998a,b). Consequently, there was potential for fatal germination of
some seed as a result of light exposure during bag transfer operations.
However, the mechanisms of light induction can be complex because light is
known to interact with many other factors such as temperature (both magnitude
and diurnal amplitude), soil moisture, nitrates, and seed age for full
expression to be achieved (Botto et al.
2000; Bouwmeester and Karssen 1993; Gallagher and Cardina 1998a,b; Henson 1970). In addition, reburial would have a potential
counter effect of inducing dormancy by placement at depths with reduced oxygen
as described above (Benvenuti et al. 2001). Thus, fatal germination from seed that
had lost dormancy in these experiments is unknown but is a potential loss
mechanism that may have been operative. Since the overall half-lives that we
observed were similar to those reported by others, this mechanism was
presumably not more operative in our experiments than in those reported by
others. Since seeds in both systems were handled similarly, potential fatal
germination probably does not explain the observed differences between systems.
PLFA.
Total soil microbial biomass, as measured by phospholipid fatty acid
content, was highest in the organic systems at both locations, although the
difference between the organic and conventional systems was only significant at
the Rodale location (Table 3). Differences in total PLFA were driven primarily by
higher levels of gram-negative PLFA in the organic system at both locations and
higher levels of gram-positive PLFA in the organic system at Rodale. Higher
PFLA levels of actinomycetes in the Rodale organic system and of eubacteria in
the BARC organic system also contributed to total PLFA differences; whereas,
eukaryote, fungi, and protozoa contributed negligibly to total PLFA levels.
Multivariate analysis of
Hellinger transformed PLFA data (based on biomarker proportion) showed that the
first canonical variate explained 95% of variation with a nonsignificant
contribution by the second canonical variate (Figure 2).
The BARC organic system data had the most positive values along the first
canonical variate axis and the mean projection on this axis was significantly
(P < 0.05)
higher than all other systems according to MANOVA
(1.77 for FSP organic vs. 0.002 for the BARC conventional system). The Rodale
data were clustered to the negative side of the BARC data, and there was no
significant difference between the Rodale organic and conventional system
projected means (−0.87 and −1.15 for the Rodale organic and conventional
systems, respectively). The gram-negative bacteria vector was associated with
BARC organic system data at the positive end of the first canonical variate;
whereas, actinomycetes and gram-positive bacteria vectors were associated with
the conventional BARC and the Rodale data at the negative end of the axis.
Soil organic matter and
microbial biomass and activity are often highly correlated in cropping systems
(Buyer et al. 2010; Cookson et al. 2008; Kremer and Li 2003; Peacock et al. 2001). Since soil carbon was higher in the
organic versus conventional systems at the initiation of these experiments
(18.6 vs. 16.6 g kg−1 at BARC
and 26.9 vs. 21.9 g kg−1 at
Rodale, respectively), it is not surprising that total microbial PLFA biomass
was higher in organic than conventional systems (Table 3).
However, multivariate analysis of proportional PLFA data showed a divergence of
organic systems at BARC and Rodale, with the BARC organic system most highly
associated with the gram-negative bacteria vector (Figure 2).Buyer et al.
(2010) have shown that gram-negative PLFAs tend to associate with systems with
more utilizable carbon; whereas, gram-positive PFLAs are associated with more
carbon-limited systems. Peacock et al. (2001) showed
that a system with manure plus cover crops had greater microbial PLFA and a
greater proportion of gram-negative bacteria than a system with only cover
crops. In the systems reported here, both manure- and cover crop–based organic
amendments were used in the BARC organic system, while the Rodale organic
system was solely legume-based. Thus, the organic amendments used in the BARC
organic system may have provided more available carbon for supporting a higher
proportion of gram-negative bacteria than the Rodale organic system.
Correlations between Seed Persistence and
Microbial PLFA.
There were very few significant correlations (P < 0.05)
between PLFA biomarkers and seed persistence
half-lives for Experiment 1 at both locations or for BARC Experiment 2 (data
not shown). However, there were many significant negative correlations between
PLFA biomarkers and seed persistence for Rodale Experiment 2 (Table 4).
The highest correlations between biomarkers and persistence in Rodale
Experiment 2 were found at the 5-cm depth for common lambsquarters (up to
−0.64) and smooth pigweed (up to −0.55). Correlations were relatively similar
for total PLFA, eubacteria, gram-positive and gram-negative bacteria, and
actinomycetes for both species at this depth (Table 4).
Correlations of these biomarkers with smooth pigweed persistence at 15 cm
were lower than those at 5 cm, while there were no correlations for common
lambsquarters at the 15-cm depth. The soil used for PFLA analysis, that came
from a 0- to 15-cm sampling depth, was probably more representative of soil
surrounding the seed at the 5-cm depth than at the 15-cm depth, thereby
providing a possible explanation for higher correlations between microbial
biomass and seed persistence at the 5-cm depth.
The absence of correlations between microbial PLFAs and seed
persistence half-lives in Experiment 1 probably resulted from the relatively
small range of differences between seed half-lives in organic and conventional
systems with few significant differences at both locations (Table 1).
In contrast, there was a greater range of differences between seed half-lives
in organic and conventional systems in Experiment 2, but only at the Rodale
location were these half-life differences correlated with microbial PLFA. At
BARC, the opposite responses of smooth pigweed and common lambsquarters
half-lives to system (Table 1) suggests that factors not associated with overall
elevated levels of organic carbon and microbial biomass in organic systems were
driving results. Given these unknown, but potentially complex set of factors
driving results at this location, it is not surprising that there were no
significant simple correlations between microbial PLFA and seed persistence at
this location. The presence of correlations in the Rodale second experiment
suggests that microbial activity could have been involved in seed mortality in
this location-experiment. Higher microbial total PLFA (Table 3)
and lower seed persistence of both species in organic vs. conventional systems
in the Rodale second experiment (Table 1)
provide plausibility that, in one of the four location-experiments reported
here, the enhanced microbial hypothesis for weed seed mortality may have been
operative.
The ambiguity of our results reflects a similar inconsistency in other
research that has investigated relationships between soil organic matter,
associated microbial populations, and weed seedbank persistence. Soil density
of common lambsquarters seed was lower in a system relying on organic
amendments than inorganic fertilizer (Gallandt et al. 1998); however, this was more likely due to
lower weed biomass and seed production in this system than to higher seed
mortality. Higher soil microbial biomass in California
At best, correlations between general metrics of microbial abundance or
community structure and seed persistence can reveal potential associations, but
they do not indicate causality. More importantly, they do not target the
specific microbial biochemical activities that may be the primary mechanisms
for seed mortality. Overall microbial biomass or community structure may not
reflect the populations responsible for production of seed coat degrading
enzymes, toxin production, or other activities detrimental to seed persistence.
The seed coat is highly important for maintaining seed viability as shown by the
rapid mortality of seeds with experimentally damaged seed coats and by the
correlation of persistence with seed coat thickness (Davis et al.
2008). Sugars in seeds can form a viscous glassy state that
physically stabilizes seed, suppresses deteriorating free-radical and enzymatic
reactions, and provides for relatively high seed survivability. However, when
this glass is weakened by increased water content and temperature (that could
result from breaching the seed coat), a phase separation of sugars takes place,
leading to initiation of rapid aging and seed mortality (Bernal-Lugo and
Leopold 1998). Controlled aging treatments in the lab correlated to
field persistence of seeds of 27 species suggesting that inherent biochemical
resistance to moisture and temperature stress underlies aging and field
persistence (Long et al. 2008). Thus, to understand the role of microbial
agents in this complex process of seed protection against both aging and
decaying influences, more precise and physiologically based research will be
needed. Cropping system management may provide overall conditions that
encourage enhanced microbial abundance and activity, but a more precise
understanding of the mechanisms of seed mortality will be required in order to
manage microbial populations for the targeted function of reducing weed seed
viability.
Acknowledgments
|
This research was funded, in part, by a USDA-ARS Headquarters Research
Associate Award. The authors are grateful for the technical oversight provided
by Ruth Mangum and Stanley Tesch and to Jon Clark, Gloria Darlington, Jonathan
Melzer, and Elizabeth Reed for the hours of patient work that this project
required. We thank Bryan Vinyard, USDA-ARS Biometrics Consulting Service, for
analysis of the seed persistence data.
Sources of Materials
|
Literature Cited
|
Benvenuti,
S., M. Macchia, and S. Miele. 2001. Quantitative analysis of emergence of seedlings from buried
seeds with increasing soil depth. Weed Sci.49:528–535. [Abstract]
|
|
Bernal-Lugo,
|
|
Botto, J.
F., A. L. Scopel, and R. A. Sánchez. 2000. Water constraints on the photoinduction of weed seed
germination during tillage. Aust.
J. Plant Physiol.27:463–471.
|
|
Bouwmeester,
H. J. and C. M. Karssen. 1993. Seasonal periodicity in germination of seeds of Chenopodium
album L. Ann.
Bot.72:463–473.
|
|
Buhler,
|
|
Burnside,
O. C., R. G. Wilson, S. Weisberg, and K. G. Hubbard. 1996. Seed longevity of 41 weed species
buried 17 years in eastern and western
|
|
Buyer, J.
S., D. P. Roberts, and E. Russek-Cohen. 1999. Microbial community structure and function in the spermosphere
as affected by soil and seed type.
|
|
Buyer, J.
S., J. R. Teasdale, D. P. Roberts, I. A. Zasada, and J. E. Maul. 2010. Factors affecting soil microbial
community structure in tomato cropping systems. Soil Biol. Biochem.42:831–841.
|
|
Cavigelli,
M. A., J. R. Teasdale, and A. E. Conklin. 2008. Long-term agronomic performance of organic and conventional
field crops in the mid-Atlantic region. Agron. J.100:785–794.
|
|
Cookson,
W. R., D. V. Murphy, and M. M. Roper. 2008. Characterizing the relationships between soil organic matter
components and microbial function and composition along a tillage disturbance
gradient. Soil
Biol. Biochem.40:763–777.
|
|
Cousens,
R. and M. Mortimer. 1995. Dynamics of Weed Populations.
|
|
Davis, A.
S. 2007. Nitrogen fertilizer and crop residue
effects on seed mortality and germination of eight annual weed species. Weed Sci.55:123–128. [Abstract]
|
|
Davis, A.
S., K. I. Anderson, S. G. Hallett, and K. A. Renner. 2006. Weed seed mortality in soils with
contrasting agricultural management histories. Weed Sci.54:291–297.[Abstract]
|
|
Davis, A.
S., J. Cardina, F. Forcella, G. A. Johnson, G. Kegode, J. L. Lindquist, E. C.
Luschei, K. A. Renner, C. L. Sprague, and M. M. Williams. 2005. Environmental factors affecting seed
persistence of annual weeds across the
|
|
Davis, A.
S., B. J. Schutte, J. Iannuzzi, and K. A. Renner. 2008. Chemical and physical defense of weed
seeds in relation to soil seedbank persistence. Weed Sci.56:676–684.[Abstract]
|
|
Fennimore,
S. A. and L. E. Jackson. 2003. Organic amendment and tillage effects on vegetable field weed
emergence and seedbanks. Weed
Technol.17:42–50. [Abstract]
|
|
Frostegård,
A. and E. Bååth. 1996. The
use of phospholipid fatty acid analysis to estimate bacterial and fungal
biomass in soil. Biol.
Fertil. Soils22:59–65.
|
|
Gallagher,
R. S. and J. Cardina. 1998a. Phytochrome-mediated
|
|
Gallagher,
R. S. and J. Cardina. 1998b. Phytochrome-mediated Amaranthus germination
II. development of very low fluence sensitivity. Weed Sci.46:53–58.
|
|
Gallandt,
E. R., E. P. Fuerst, and A. C. Kennedy. 2004. Effect of tillage, fungicide seed treatment, and soil fumigation
on seed bank dynamics of wild oat. Weed Sci.52:597–604.[Abstract]
|
|
Gallandt,
E. R., M. Liebman, S. Corson, G. A. Porter, and S. D. Ullrich. 1998. Effects of pest and soil management
systems on weed dynamics in potato. Weed Sci.46:238–248.
|
|
Gallandt,
E. R., M. Liebman, and D. R. Huggins. 1999. Improving soil quality: implications for weed management. J. Crop Prod.2:95–121.
|
|
Grundy, A.
C., A. Mead, and S. Burston. 2003. Modelling the emergence response of weed seeds to burial depth:
interactions with seed density, weight and shape. J. Appl. Ecol.40:757–770.
|
|
Henson,
|
|
Kennedy,
A. C. 1999. Soil
microorganisms for weed management. J. Crop Prod.2:123–138.
|
|
Kremer, R.
J. and J. Li. 2003. Developing
weed-suppressive soils through improved soil quality management. Soil Till. Res.72:193–202.
|
|
Legendre,
P. and E. D. Gallagher. 2001. Ecologically meaningful transformations for ordination of
species data. Oecologia129:271–280.
|
|
Long, R.
L., F. D. Panetta, K. J. Steadman, R. Probert, R. M. Bekker, S. Brooks, and
S. W. Adkins. 2008. Seed
persistence in the field may be predicted by laboratory-controlled aging. Weed Sci.56:523–528. [Abstract]
|
|
Lutman, P.
J. W., G. W. Cussans, K. J. Wright, B. J. Wilson, G. M. Wright, and H. M.
Lawson. 2002. The
persistence of seeds of 16 weed species over six years in two arable fields. Weed Res.42:231–241.
|
|
Matilla,
A., M. Gallardo, and M. I. Puga-Hermida. 2005. Structural, physiological and molecular aspects of heterogeneity
in seeds: a review. Seed
Sci. Res.15:63–76.
|
|
Menalled,
F. D., R. G. Smith, J. T. Dauer, and T. B. Fox. 2007. Impact of agricultural management on
carabid communities and weed seed predation. Agric. Ecosyst. Environ.118:49–54.
|
|
Mohler, C.
L. 2001. Weed life history: identifying
vulnerabilities. Pages 40–98 in M.
Liebman, C. L. Mohler, and C. P. Staver, eds. Ecological Management of
Agricultural Weeds.
|
|
Mohler, C.
L. and A. E. Galford. 1997. Weed seedling emergence and seed survival: separating the
effects of seed position and soil modification by tillage. Weed Res.37:147–155.
|
|
Navntoft,
S., S. D. Wratten, K. Kristensen, and P. Esbjerg. 2009. Weed seed predation in organic and
conventional fields. Biol.
Cont.49:11–16.
|
|
Peacock,
A. D., M. D. Mullen, D. B. Ringelberg, D. D. Tyler, D. B. Hedrick, P. M.
Gale, and D. C. White. 2001. Soil microbial community responses to dairy manure or ammonium
nitrate applications. Soil
Biol. Biochem.33:1011–1019.
|
|
Pimentel, D.,
P. Hepperly, J. Hanson, D. Douds, and R. Seidel. 2005. Environmental, energetic, and economic
comparisons of organic and conventional farming systems.BioScience55:573–582.
|
|
Sawma, J.
T. and C. L. Mohler. 2002. Evaluating seed viability by an unimbibed seed crush test in
comparison with the tetrazolium test. Weed Technol.16:781–786. [Abstract]
|
|
Schabenberger,
O., B. E. Tharp, J. J. Kells, and D. Penner. 1999. Statistical tests for hormesis and
effective dosages in herbicide dose response. Agron. J.91:713–721.
|
|
Schutte,
B. J., A. S. Davis, K. A. Renner, and J. Cardina. 2008. Maternal and burial environment
effects on seed mortality of velvetleaf and giant foxtail. Weed Sci.56:834–840. [Abstract]
|
|
Schweizer,
E. E. and R. L. Zimdahl. 1984. Weed seed decline in irrigated soil after six years of
continuous corn and herbicides. Weed
Sci.32:76–83.
|
|
Seber, G.
A. F. 1984. Multivariate
Observations.
|
|
Steckel,
L. E., C. L. Sprague, E. W. Stoller, L. M. Wax, and F. W. Simmons. 2007. Tillage, cropping system, and soil
depth effects on common waterhemp seed-bank persistence.Weed Sci.55:235–239. [Abstract]
|
|
Teasdale,
J. R., R. W. Mangum, J. Radhakrishnan, and M. A. Cavigelli. 2004. Weed seedbank dynamics in three
organic farming crop rotations. Agron.
J.96:1429–1435.
|
|
Teo-Sherrell,
C. P. A., D. A. Mortensen, and M. E. Keaton. 1996. Fates of weed seeds in soil: a seeded
core method of study. J.
Appl. Ecol.33:1107–1113.
|
|
Van
Mourik, T. A., T. J. Stomph, and A. J. Murdoch. 2005. Why high seed densities with buried
mesh bags may overestimate depletion rates of soil seed banks. J. Appl. Ecol.42:299–305.
|
|
Wagner, M.
and N. Mitschunas. 2008. Fungal effects on seed bank persistence and potential
applications in weed biocontrol: a review. Basic Appl. Ecol.9:191–203.
|
|
Zelles, L. 1999. Fatty acid patterns of phospholipids
and lipopolysaccharides in the characterisation of microbial communities in
soil: a review. Biol.
Fertil. Soils29:111–129.
|
