The quick answer is that there islittle difference that can be safely ascertained and that there is no reason toalter practice of the account of it. More detailed analysis may tell us something different but I suspectthat any such information would indicate additional effort also.
At least someone asked thequestion and it looks as something that is safe to ignore as an issue.
Organic and Conventional Farming Methods Compete to Eliminate WeedSeeds in Soil
Released: 4/21/2011 9:00 AM EDT
Newswise — Weeds are hard to kill; they seem to come back no matterwhat steps people take to eradicate them. One reason is because of thepersistence of weed seeds in the soil. Organic farming and conventional farmingsystems both have their methods of taking on weed seeds, but does one showbetter results than the other?
The authors of a study reported in the current issue of thejournal Weed Science conducted tests that compared conventionalfarming with organic systems. This research determined weed seed viabilityunder both systems over a two-year period in two separate locations.
To compare these systems, researchers buried seeds of two types ofweeds, smooth pigweed and common lambsquarters, in mesh bags. Tests wereconducted at agricultural research locations in Maryland and Pennsylvania
Depth of seeds in the soil, environmental conditions, and soilmanagement are among the factors that affect seed persistence. Underconventional soil management, tillage is an important practice that manipulatesthe depth of seeds and environmental conditions that can influence weed seedpersistence. 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 microbialbiomass than the soils of the conventional farming tests. This was measured byphospholipid fatty acid content. But the results of the tests did not leadresearchers to conclude that this microbial biomass has a dominating role inseed mortality.
Pigweed seeds showed a shorter life span under the organic system intwo of four experiments. Organic system lambsquarters seeds had a shorter lifespan in just one of the four experiments, while the conventional methods hadthe shorter life span in two of four experiments. These results leave anambiguous answer to the question of which farming system can better eliminateseeds deep in the soil to control weeds from their source.
Full text of the article, “Weed Seed Persistence and Microbial Abundance in Long-TermOrganic 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 MicrobialAbundance in Long-Term Organic and Conventional Cropping Systems
Silke D. Ullrich, Jeffrey S. Buyer,Michel A. Cavigelli, Rita Seidel, and John R. Teasdale*
Weedseed persistence in soil can be influenced by many factors, including cropmanagement. This research was conducted to determine whether organic managementsystems with higher organic amendments and soil microbial biomass could reduceweed seed persistence compared with conventional management systems. Seeds ofsmooth pigweed and common lambsquarters were buried in mesh bags in organic andconventional systems of two long-term experiments, the Farming Systems Projectat the Beltsville Agricultural Research Center, Maryland, and the FarmingSystems Trial at the Rodale Institute, Pennsylvania
Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; smooth pigweed, Amaranthushybridus L. AMACH
Received: September 29, 2010; Accepted: January 10, 2011
*First, second,third, and fifth authors: Research Associate, Chemist, Soil Scientist, andPlant Physiologist, U.S. Department of Agriculture-Agricultural ResearchService Sustainable Agricultural Systems Lab, Beltsville, MD 20705; fourthauthor: Agroecologist, Rodale Institute, Kutztown, PA 19530. Correspondingauthor's E-mail:
Persistence of weed seeds in soil is critical to the survival of annualweeds in agroecosystems. Seed mortality is a demographic process that can havea major impact on presence and dynamics of weed populations (Cousens and Mortimer 1995; Mohler 2001). Many factors have been shown to affect weed seedpersistence 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 onseed persistence is particularly important because it offers producers theopportunity to manage weed seed populations through cultural practices. Tillageis an important operation that can allow manipulation of seed depth andassociated environmental conditions that influence seed persistence (Lutman et al. 2002; Steckel et al. 2007). Amending the soil with organic inputs inreduced input or organic farming systems also can influence seed persistenceand 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 insoil, ranging from methods that maintain seed in a natural soil environmentwith less precision of recovery to methods that create an artificial soilenvironment but increase precision of recovery. An example of the formerapproach is to begin an experiment with a high natural population (Schweizer and Zimdahl 1984; Steckel et al. 2007) or to pulse with a defined initialdensity of seeds that may be subject to experimental management practices (Buhler and Hartzler 2001; Lutman et al. 2002) followed by subsequent soil sampling. Moreprecise recovery of the initial seed lot can be achieved by installation ofseeds in unenclosed soil cylinders that can be subsequently recovered in theirentirety (Schutte et al. 2008; Teo-Sherrell et al. 1996). Mesh bags or other porous seedenclosures provide an artificial barrier to the surrounding soil, and permithigh recovery precision of the initial seed lot (Conn et al. 2006; Davis et al. 2005; Gallandt et al. 2004). Schutte et al. (2008) foundthe mesh bag and core methods resulted in similar seed persistence andmortality, althoughVan Mourik et al. (2005) warnthat high seed-to-soil ratios in mesh bags can lead to excessive fungus-inducedmortality.
Gallandt et al. (1999) suggestedthat enhanced soil biological activity resulting from improved soil qualitycould increase the mortality of weed seeds in soil. Kennedy (1999)proposed that no-tillage and biologically basedsystems that enhance soil carbon, moisture, and microbial activity maypredispose these systems to greater seed decomposition. Soil managed for highorganic matter and reduced tillage had soil quality indicators, enzymeactivity, and water stable aggregates that were associated withweed-suppressive bacterial isolates (Kremer and Li 2003). However, the major causes of seedmortality include not only microbe-induced decay, but also fatal germination,physiological aging, and predation (Gallandt et al. 2004). Losses from predation can bedistinguished from the other mortality factors by confining seed in containersthat exclude the predator in question. It is often difficult to distinguishamong microbial decay, aging, and fatal germination in container experiments.Fatal germination can occur when seed germination is induced under conditionsthat do not permit successful emergence (Benvenuti et al. 2001; Gallandt et al. 2004). Research oriented to crop seedpreservation has shown that glassification of seed sugars can protect seed fromdestruction by various physiological aging mechanisms involving free-radicaland enzymatic destruction of essential cell molecular components (Bernal-Lugo andLeopold 1998). Thus, microbial destruction of the seed coat couldlead not only to microbial decay of seeds but also to moisture conditions thatwould induce a phase shift in glassification and acceleration of physiologicalaging processes.
Long-term systems experiments with a range of cropping systems thatdiffer in soil organic inputs offer an opportunity to investigate the potentialimpact 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 SystemsTrial (FST) near Kutztown , PA ,was initiated in 1981 and is the longest running comparison of conventional andorganic cropping systems in the United States
The objective of this research was to compare the persistence of smoothpigweed and common lambsquarters in the FSP and FST organic and conventionalcropping systems within mesh bags that eliminated mortality effects ofpredators. Our hypothesis was that increased soil microbial biomass in organic systemsare associated with shorter seed persistence. Since soil depth can influencethe environmental conditions and microbial activity that seeds encounter, asecond objective was to compare seed persistence at two seed depths, 5 and15 cm.
Materials and Methods |
Experimental Site Description.
Seed persistence experiments were conducted at two long-termexperimental sites: FSP, begun in 1996 and located at the BeltsvilleAgricultural 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 croppingsystems, a conventional chisel-tillage system and an organic system. Thesesystems 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. Detaileddescription of the systems and crop performance during the first 10 yr ofthis cropping systems experiment are outlined in Cavigelli et al. (2008). In brief, the conventional system wasfarmed using locally recommended fertilizer and herbicide programs. The organicsystem 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 retainingnutrients, and on rotary hoeing and sweep cultivation for weed control. Boththe conventional and the organic systems included plowing and disking forseedbed preparation.
At Rodale, experiments were conducted in two of three FST croppingsystems, a conventional and an organic system. A corn–corn–soybean–corn–soybeanrotation was followed in the conventional system with recommended fertilizerand herbicide inputs. The legume-based organic system at FST was chosen becauseit followed a corn–soybean–wheat rotation similar to that used in the FSP 3-yrorganic system. This organic system relied on various legume cover crops forproviding nitrogen to grain crops, on a rye cover crop for retaining nutrientsfollowing corn, and on rotary hoeing and sweep cultivation for weed control.Both the conventional and the organic systems included plowing and disking forseedbed preparation. One exception to cropping patterns occurred in 2003, whenoats (Avena sativa L.) were planted as a uniformity crop over the entire trial. Further detailson 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 collectedat BARC were placed in flat, square nylon bags with 6.75 cm interiorlength per side and a mesh size of 0.5 mm. Seed viability was initiallytested 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 andsystem and dry sieved to eliminate resident weed seed. Bags were installed in ahorizontal 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 croppingsystem per location.
In order to eliminate crop effects, bags were only put in soil duringthe soybean phase of each rotation. Bags were installed in plots in the fallafter harvest of the previous corn crop and planting of a rye cover crop (lateOctober to early November). Four sets of bags were installed, with each set tobe retrieved at half-year intervals over a 2-yr period. Bags were retrieved inthe spring before field preparation for soybean planting (late April to earlyMay). The set designated for removal in the first spring was taken to the labfor testing while the remaining sets were temporarily placed in soil near theedge of adjacent plots within the same cropping system while the experimentalplots were prepared for planting. After soybean planting, sets designated forlater retrieval were replaced into their designated plots at their designateddepths beneath the soybean row to avoid damage by subsequent between-rowcultivations. Bags were again retrieved in the fall after soybean harvest. Theset designated for removal in the first fall was taken to the lab and theremaining sets were moved to plots planted to rye and destined for soybeans inthe next season. A similar process was repeated to exhume the sets designatedfor spring and fall removal during the second season. The first experimentconsisted of four sets of bags that were initially installed in the fall of2001 and removal was completed in the fall of 2003. A second experimentconsisted of an additional four sets of bags that were initially installed inthe 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 blotteddry. Bags were dipped in 70% ethanol for 30 s, rinsed with deionizedwater, dipped in 10% Clorox1 for 5 min, rinsed again with deionized water, and blotted dry witha paper towel. When bags were dry, seeds were transferred to petri dishescontaining Whatman no. 3 filter paper and 7.5 ml deionized water.Dishes were sealed with parafilm and placed in a growth chamber at constant30 C for smooth pigweed or 20 C for common lambsquarters. Germinatedseeds were counted and removed from the dish after 1 wk. Plates weremoistened as needed, resealed, and counted after another week. Remainingungerminated seeds were squeezed with a forceps and identified as viable if theendosperm was white and firm (Sawma and Mohler2002).
A composite of 10 soil samples was taken to 15-cm depth near eachburial site in each plot for soil microbial biomass and community analysis. Sampleswere taken at three times during the season, in early April before springtillage, in June after soybean planting, and in early October before soybeanharvest. Soil was placed in a sealed plastic bag and transported in a cooler tothe 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 aspreviously 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 usedas a biomarker for eubacteria (Frostegård and Bååth, 1996). The sum of polyunsaturated fattyacids was used as a biomarker for eukaryotes, and the total phospholipid fattyacid (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 usedas a biomarker for eubacteria (Frostegård and Bååth, 1996). The sum of polyunsaturated fattyacids was used as a biomarker for eukaryotes, and the total phospholipid fattyacid (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 computedby dividing the measured number of germinated plus viable ungerminated seeds bythe initial number of viable seeds in each packet ( = 200). For each locationand experiment, survival values from each blockof each system were regressed onto time (unit = yr) using a log-logistic model withasymptotes at 100 and 0 (Schabenberger et al. 1999) where b is a shape parameter and LT50 is anestimate of the lethal time for 50% seed mortality. Thus, eight data pointsrepresenting four times of exhumation and two replications at each time wereused to determine LT50 valuesfor each of four blocks of each system. A four-way ANOVA was conducted toanalyze LT50 values relative to location, system, experiment, and depth using amixed model procedure (PROC MIXED, SAS version 9.1.32). Since the organic systems and conventional systems weresimilar at the two locations, these were coded as common systems (organic orconventional) across locations. Only threespecies–location–system–experiment–depth blocks from this factorial set oftreatments 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 generallinear model (PROC GLM). A one-way model was employed in which the main effectconsisted of the four cropping systems across both locations with year andsampling time treated as replications. For MANOVA, biomarkers were firsttransformed as the square root of the proportional biomarker (Hellingertransformation; Legendre and Gallagher 2001) and then standardized to unitvariance. A canonical variates analysis, generated by the MANOVA, was used toidentify the linear combination of variables that best separated the croppingsystems (Buyer et al. 1999; Seber 1984).Pearson coefficients were determined for the correlation between PLFAbiomarkers and seed survival half-lives.
Results and Discussion |
Seed Persistence.
The majority of seed buried in these experiments did not persist for 2yr. Generally, most seed persisted through the first winter (mean survival atthe first spring exhumation date was 87 and 83% for common lambsquarters andsmooth pigweed, respectively), but then exhibited higher mortality in the firstsummer (mean survival at the first fall exhumation date was 49 and 61% forcommon lambsquarters and smooth pigweed, respectively). By the end of2 yr, mean survival was 20 and 29% for common lambsquarters and smoothpigweed, respectively. Because this pattern resulted in a sigmoid-shapedresponse for most data sets, data were fit to a log-logistic function ratherthan to an exponential decay function, which is often employed for fitting seedpersistence data (Conn et al. 2006;Lutman et al. 2002).
Analysis of half-life values demonstrated a location by experiment bysystem interaction for smooth pigweed (P = 0.0945) and common lambsquarters(P = 0.0048). Smooth pigweed seed half-life was shorter in organic than inconventional systems in the second experiment at both locations (Table 1). Common lambsquarters seed half-life also was shorterin organic than in conventional system in the second Rodale experiment, but waslonger in organic than in conventional systems in both BARC experiments. Smoothpigweed and common lambsquarters survival was high in both systems at the firstspring exhumation date, but survival differentials between systems usuallybecame most pronounced at the first fall exhumation date. This survival patterncan be seen in the second BARC experiment in which system differences wereparticularly prominent for both species (Figure 1).
The inconsistent survival responses to system, location, and experimentcan not be explained by weather. Seasonal air temperatures were relativelysimilar between years and locations, although temperatures at Rodale tended tobe slightly, but consistently lower than those at BARC (Table 2).Rainfall also followed a similar pattern at the two locations, with rainfall atRodale higher than that at BARC in all but one of the 6-mo periods. Rainfalldifferentials between Rodale and BARC were most pronounced during the summersof 2002 and 2004. This rainfall difference may have contributed to the lowersmooth pigweed half-life at Rodale than BARC (1.0 vs. 1.6), but had no effecton the half-life of common lambsquarters (1.1 and 1.0 at Rodale and BARC,respectively). The difference between higher temperatures during summer burialperiod (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 bothlocations (Figure 1) was probably related to this temperaturedifferential. Mortality mechanisms dependent on biochemical or metabolicreactions would be enhanced by higher temperatures.
Despite the differences among system half-lives observed, thesedifferences were relatively small (months), and half-lives in both systems wereshort (less than 2 yr) in all location-experiments (Table 1).The seed persistence half-lives of approximately 1 to 2 yr reported hereare similar to those reported by other researchers using a range ofmethodologies. 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 yrin 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 of2.5 yr was observed under relatively colder Alaskan conditions (Conn et al. 2006),which undoubtedly would have delayed decay or aging processes compared to thosein warmer climates. A longer common lambsquarters half-life of 4.5 yr wasobserved at one location in a United Kingdom experiment (Lutman et al. 2002), but this data set was limited by anunusually low initial recovery value following pulse seeding that probablyupwardly biased this half-life estimate. Common lambsquarters seeminglypersisted for many years when buried in cylinders in Nebraska
It is also clear that a minority of smooth pigweed and commonlambsquarters seed can persist for many years. In all of the studies reportedabove, 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 bothshort-lived and long-lived seeds. The production of heterogeneous seed typeswith respect to dormancy, dispersion, and persistence is a strategy thatensures survival under potentially variable edaphic and environmentalconditions (Matilla et al. 2005). Common lambsquarters is known to produceheteromorphic brown and black seeds that vary in seed coat thickness, dormancy,and stress tolerance (Yao et al. 2010). Future research on seed persistence probablyrequires a more thorough characterization of both the initial viability of seedlots 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 wasshorter 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 atKutztown (1.0 vs. 0.9 yr, respectively). There was a location byexperiment by system by depth interaction for common lambsquarters half-life,but this involved a significant difference in depth at only one of eightlocation–experiment–system levels while there were no significant differencesat the other seven levels (data not shown).
Other research using mesh bag methodology has reported no effectamongst 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 seedenclosures, seed on the soil surface would be more prone to predation thanthose buried in the soil (Menalled et al. 2007; Mohler and Galford 1997; Navntoft et al. 2009); however, losses to mechanisms otherthan predation appear to have a more subtle relationship to depth. Fatalgermination is a potential loss mechanism that could account fordepth-dependent seed mortality. Common lambsquarters and redroot pigweed havebeen 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 thanthese could be fatal. However, redroot pigweed and common lambsquarters alongwith 18 other species were shown to undergo depth-induced dormancy, wherebyapproximately 85% of seed (90% of pigweed and lambsquarters) did not germinate(Benvenuti et al. 2001), a condition thought to be mediated byreduced oxygen levels with increased depth. This suggests that most seed at the5- and 15-cm depths in our study would have been unlikely to undergo fatalgermination.
Bags buried to either depth were temporarily exhumed during tillageoperations in the spring and fall, thus potentially exposing seed near thesurfaces 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 beensensitized by overwinter field burial or laboratory stratification (Bouwmeester and Karssen 1993; Gallagher and Cardina 1998a,b). Consequently, there was potential for fatal germination ofsome seed as a result of light exposure during bag transfer operations.However, the mechanisms of light induction can be complex because light isknown to interact with many other factors such as temperature (both magnitudeand diurnal amplitude), soil moisture, nitrates, and seed age for fullexpression to be achieved (Botto et al.2000; Bouwmeester and Karssen 1993; Gallagher and Cardina 1998a,b; Henson 1970). In addition, reburial would have a potentialcounter effect of inducing dormancy by placement at depths with reduced oxygenas described above (Benvenuti et al. 2001). Thus, fatal germination from seed thathad lost dormancy in these experiments is unknown but is a potential lossmechanism that may have been operative. Since the overall half-lives that weobserved were similar to those reported by others, this mechanism waspresumably not more operative in our experiments than in those reported byothers. Since seeds in both systems were handled similarly, potential fatalgermination probably does not explain the observed differences between systems.
PLFA.
Total soil microbial biomass, as measured by phospholipid fatty acidcontent, was highest in the organic systems at both locations, although thedifference between the organic and conventional systems was only significant atthe Rodale location (Table 3). Differences in total PLFA were driven primarily byhigher levels of gram-negative PLFA in the organic system at both locations andhigher levels of gram-positive PLFA in the organic system at Rodale. HigherPFLA levels of actinomycetes in the Rodale organic system and of eubacteria inthe BARC organic system also contributed to total PLFA differences; whereas,eukaryote, fungi, and protozoa contributed negligibly to total PLFA levels.
Multivariate analysis ofHellinger transformed PLFA data (based on biomarker proportion) showed that thefirst canonical variate explained 95% of variation with a nonsignificantcontribution by the second canonical variate (Figure 2).The BARC organic system data had the most positive values along the firstcanonical 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 Rodaledata were clustered to the negative side of the BARC data, and there was nosignificant difference between the Rodale organic and conventional systemprojected means (−0.87 and −1.15 for the Rodale organic and conventionalsystems, respectively). The gram-negative bacteria vector was associated withBARC organic system data at the positive end of the first canonical variate;whereas, actinomycetes and gram-positive bacteria vectors were associated withthe conventional BARC and the Rodale data at the negative end of the axis.
Soil organic matter andmicrobial 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 theorganic versus conventional systems at the initiation of these experiments(18.6 vs. 16.6 g kg−1 at BARCand 26.9 vs. 21.9 g kg−1 atRodale, respectively), it is not surprising that total microbial PLFA biomasswas higher in organic than conventional systems (Table 3).However, multivariate analysis of proportional PLFA data showed a divergence oforganic systems at BARC and Rodale, with the BARC organic system most highlyassociated with the gram-negative bacteria vector (Figure 2).Buyer et al.(2010) have shown that gram-negative PLFAs tend to associate with systems withmore utilizable carbon; whereas, gram-positive PFLAs are associated with morecarbon-limited systems. Peacock et al. (2001) showedthat a system with manure plus cover crops had greater microbial PLFA and agreater proportion of gram-negative bacteria than a system with only covercrops. In the systems reported here, both manure- and cover crop–based organicamendments were used in the BARC organic system, while the Rodale organicsystem was solely legume-based. Thus, the organic amendments used in the BARCorganic system may have provided more available carbon for supporting a higherproportion of gram-negative bacteria than the Rodale organic system.
Correlations between Seed Persistence andMicrobial PLFA.
There were very few significant correlations (P < 0.05)between PLFA biomarkers and seed persistencehalf-lives for Experiment 1 at both locations or for BARC Experiment 2 (datanot shown). However, there were many significant negative correlations betweenPLFA biomarkers and seed persistence for Rodale Experiment 2 (Table 4).The highest correlations between biomarkers and persistence in RodaleExperiment 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 similarfor total PLFA, eubacteria, gram-positive and gram-negative bacteria, andactinomycetes for both species at this depth (Table 4).Correlations of these biomarkers with smooth pigweed persistence at 15 cmwere lower than those at 5 cm, while there were no correlations for commonlambsquarters at the 15-cm depth. The soil used for PFLA analysis, that camefrom a 0- to 15-cm sampling depth, was probably more representative of soilsurrounding the seed at the 5-cm depth than at the 15-cm depth, therebyproviding a possible explanation for higher correlations between microbialbiomass and seed persistence at the 5-cm depth.
The absence of correlations between microbial PLFAs and seedpersistence half-lives in Experiment 1 probably resulted from the relativelysmall range of differences between seed half-lives in organic and conventionalsystems with few significant differences at both locations (Table 1).In contrast, there was a greater range of differences between seed half-livesin organic and conventional systems in Experiment 2, but only at the Rodalelocation were these half-life differences correlated with microbial PLFA. AtBARC, the opposite responses of smooth pigweed and common lambsquartershalf-lives to system (Table 1) suggests that factors not associated with overallelevated levels of organic carbon and microbial biomass in organic systems weredriving results. Given these unknown, but potentially complex set of factorsdriving results at this location, it is not surprising that there were nosignificant simple correlations between microbial PLFA and seed persistence atthis location. The presence of correlations in the Rodale second experimentsuggests that microbial activity could have been involved in seed mortality inthis location-experiment. Higher microbial total PLFA (Table 3)and lower seed persistence of both species in organic vs. conventional systemsin the Rodale second experiment (Table 1)provide plausibility that, in one of the four location-experiments reportedhere, the enhanced microbial hypothesis for weed seed mortality may have beenoperative.
The ambiguity of our results reflects a similar inconsistency in otherresearch that has investigated relationships between soil organic matter,associated microbial populations, and weed seedbank persistence. Soil densityof common lambsquarters seed was lower in a system relying on organicamendments than inorganic fertilizer (Gallandt et al. 1998); however, this was more likely due tolower weed biomass and seed production in this system than to higher seedmortality. Higher soil microbial biomass in California
At best, correlations between general metrics of microbial abundance orcommunity structure and seed persistence can reveal potential associations, butthey do not indicate causality. More importantly, they do not target thespecific microbial biochemical activities that may be the primary mechanismsfor seed mortality. Overall microbial biomass or community structure may notreflect the populations responsible for production of seed coat degradingenzymes, toxin production, or other activities detrimental to seed persistence.The seed coat is highly important for maintaining seed viability as shown by therapid mortality of seeds with experimentally damaged seed coats and by thecorrelation of persistence with seed coat thickness (Davis et al.2008). Sugars in seeds can form a viscous glassy state thatphysically stabilizes seed, suppresses deteriorating free-radical and enzymaticreactions, and provides for relatively high seed survivability. However, whenthis glass is weakened by increased water content and temperature (that couldresult from breaching the seed coat), a phase separation of sugars takes place,leading to initiation of rapid aging and seed mortality (Bernal-Lugo andLeopold 1998). Controlled aging treatments in the lab correlated tofield persistence of seeds of 27 species suggesting that inherent biochemicalresistance to moisture and temperature stress underlies aging and fieldpersistence (Long et al. 2008). Thus, to understand the role of microbialagents in this complex process of seed protection against both aging anddecaying influences, more precise and physiologically based research will beneeded. Cropping system management may provide overall conditions thatencourage enhanced microbial abundance and activity, but a more preciseunderstanding of the mechanisms of seed mortality will be required in order tomanage microbial populations for the targeted function of reducing weed seedviability.
Acknowledgments |
This research was funded, in part, by a USDA-ARS Headquarters ResearchAssociate Award. The authors are grateful for the technical oversight providedby Ruth Mangum and Stanley Tesch and to Jon Clark, Gloria Darlington, JonathanMelzer, and Elizabeth Reed for the hours of patient work that this projectrequired. We thank Bryan Vinyard, USDA-ARS Biometrics Consulting Service, foranalysis of the seed persistence data.
Sources of Materials |
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