An artificial seed bank study was conducted at Pendleton, SC, USA, to investigate the persistence of Palmer amaranth seeds buried uniformly across a 10-cm depth in soil inside polyvinyl chloride (PVC) cylindrical pipes over 4 years. The experiment was conducted using a split-plot design, with year as the main plot factor and with or without soil disturbance (shallow tillage to a depth of 10 cm) as the subplot factor. Annual soil disturbance through tillage in the spring stimulated emergence during the first and second year after burial. A total of 0.5% to 0.8% of the seed bank emerged during the 4-yr burial period, and 99% or more of the 4-yr total emergence occurred during the first two years of burial. Seeds retrieved from 0 to 5 cm and 5 to 10 cm depths did not differ in viability. Soil disturbance influenced the decline of the artificial seed bank at least in the first year, with fewer viable seeds remaining in annually-disturbed plots. Regardless of soil disturbance, a small fraction of seeds (0.01% to 0.03% of original seed bank) remained viable in the soil after four years of burial. In conclusion, Palmer amaranth seeds buried across a 10 cm soil depth in the artificial seed bank had low persistence, which implies that burial may aid management of the weed seed bank.
Seed bank persistence is one of the major factors influencing weed infestations in agricultural production systems [
Physical and physiological dormancies are important mechanisms of weed seed persistence in the soil seed bank [
Tillage brings buried seeds to or near the soil surface (favoring emergence and seed bank depletion) or buries freshly shed seeds deeper into the soil (favoring seed dormancy and persistence) [
Seed bank persistence of a few Amaranthus species has been previously studied [
Palmer amaranth seeds used for the experiment were collected in late September of 2004 from an agricultural site near Pendleton, SC, USA. Seeds were cleaned and air-dried at room temperature until burial in the field. Percent viability of seeds just prior to burial was determined by placing four replicate samples of fifty seeds in a 9-cm-diam petri dish (Fisher Scientific, 3970 Johns Creek Court, Suwanee, GA 30024, USA) between two layers of filter paper (Whatman’s No. 1, Fisher Scientific, 3970 Johns Creek Court, Suwanee, GA 30024, USA) moistened with a 5 ml of 1% (v/v) fungicide (Captan 4-L fungicide, Drexel Chemical Company, P.O. Box 13327, Memphis, TN 38113-0327, USA) solution in deionized water. Preliminary experiments showed no adverse effect of 1% (v/v) fungicide on seed germination of Palmer amaranth. Petri dishes were wrapped with a transparent film (Fisher Scientific, 3970 Johns Creek Court, Suwanee, GA 30024, USA) to minimize moisture loss and were incubated for 14 days in the dark at 30˚C, which is an optimum temperature for germination of Palmer amaranth [
An artificial seed bank study was initiated in a field in 2004 at the Simpson Research Station near Pendleton, SC, USA (34.6506˚N, 82.7808˚W), to evaluate the persistence of Palmer amaranth in soil over 4 yrs (2005 to 2008), approximately 1 month after collecting mature seeds. The experimental site was maintained with tall fescue (Festuca arundinacea L.) turf for at least 15 yr prior to initiation of the experiment, ensuring no recent establishment and direct seed rain of Palmer amaranth in the experimental area. The soil was a Cecil sandy-loam (fine, kaolinitic, thermic Typic kanhapludults). In August 2004, the experimental site was sprayed with glyphosate at 870 g·ae·ha−1 to kill the existing vegetation, disked with two passes and then roto-tilled once to an approximate depth of 15 cm.
Artificial seed banks were established in late October 2004 using polyvinyl chloride (PVC) cylindrical pipes of 15 cm height and 50 cm diam with openings at both ends and were placed vertically to a depth of 10 cm into the soil by excavating the soil. The remaining 5 cm length of the pipe remained above the soil surface to prevent off-site movement of seeds. The excavated soil from a 0 to 10 cm depth was placed in a bucket. Twenty thousand viable seeds·m−2 were thoroughly mixed with soil in the bucket and the soil/seed mixture was returned to the pipe. Fine polypropylene microfilament mesh screens similar to those used by previous researchers [
Each November, soil from depths of 0 to 5 and 5 to 10 cm was excavated separately from each pipe, with a total of 32 soil samples from 16 plots [2 treatments (with and without soil disturbance) by 8 replications]. The excavated soil was stored at 4˚C until placed in flat trays in the greenhouse at 30˚C/24˚C (day/night) and 16 h photoperiod.
. Mean monthly soil temperatures at 10 cm soil depth and rainfall during the seed burial experiment at Pendleton, SC, USA
Soil temperature (˚C) | Rainfall (cm) | |||||||
---|---|---|---|---|---|---|---|---|
Month | 2005 | 2006 | 2007 | 2008 | 2005 | 2006 | 2007 | 2008 |
January | 9 | 9 | 10 | 7 | 5.5 | 10.9 | 14.6 | 6.2 |
February | 9 | 8 | 7 | 8 | 6.6 | 5.0 | 10.0 | 12.2 |
March | 10 | 12 | 12 | 11 | 7.1 | 4.7 | 9.2 | 11.8 |
April | 16 | 17 | 16 | 16 | 8.7 | 10.3 | 2.8 | 9.2 |
May | 21 | 21 | 22 | 21 | 10.5 | 2.8 | 3.8 | 5.8 |
June | 23 | 26 | 27 | 27 | 24.6 | 21.0 | 8.5 | 1.5 |
July | 28 | 28 | 27 | 28 | 23.0 | 5.5 | 7.7 | 3.6 |
August | 29 | 28 | 29 | 28 | 9.8 | 6.6 | 4.3 | 17.0 |
September | 27 | 25 | 27 | 26 | 1.7 | 13.0 | 5.7 | 1.2 |
October | 22 | 20 | 22 | 20 | 7.2 | 11.4 | 4.1 | 6.7 |
November | 15 | 14 | 14 | 12 | 9.5 | 7.4 | 3.2 | 4.9 |
December | 7 | 11 | 11 | 10 | 13.8 | 11.2 | 14.6 | 11.7 |
Multiple trays were used to accommodate the entire soil volume that was excavated for each burial depth. The soil was uniformly spread on the tray to a depth of 2.5 cm, which was chosen since Amaranthus species including Palmer amaranth can emerge only from shallow depths up to 2.5 cm [
All weed emergence and viability data were expressed as number per m2 and as a percentage of the original seed bank. Also, emergence each year as a percentage of the 4-yr total was calculated. Data were tested for homogeneity of variance and normality assumptions using Levene’s and Shapiro-Wilk’s tests, respectively. Data met both assumptions; hence, analyses were performed on non-transformed data. All data were subjected to ANOVA using PROC MIXED in SAS. Year, soil disturbance (with and without), and the interaction of year and soil disturbance were fixed effects, and replication and replication by year were random effects in the model used for determining the field emergence. Viability data were analyzed as a split-split plot design with year being the main plot, soil disturbance (with or without) being the subplot, and burial depths (0 to 5 and 5 to 10 cm) being the sub-subplot factor. Year, soil disturbance, and burial depth and their relevant interactions were fixed effects in the model, and replication, replication by year, and replication by year by soil disturbance were random effects. Means for the significant main effects and interactions were separated using Fisher’s Protected LSD test at P < 0.05.
The interaction of year by soil disturbance was significant for Palmer amaranth emergence from the seed bank (P-value = 0.0002). The effect of tillage-mediated annual soil disturbance on emergence was observed during the first two years following seed burial, with greater differences between the soil disturbance treatments in 2006 relative to 2005.
In 2006, 0.28% (56 seedlings·m−2) of initial seed bank (20,000 seeds·m−2) emerged in plots with annual soil disturbance compared with 0.07% (14 seedlings·m−2) of initial seed bank that emerged in plots disturbed once only during burial, a four-fold difference (
The stimulatory effect of annual soil disturbance through a shallow spring tillage on Palmer amaranth emergence observed during the first two years (2005 and 2006) after burial might be due to improved soil aeration, soil-seed contact, and light exposure that enhanced germination of buried Palmer amaranth seeds from the seed bank. Also reported in other small seeded weed species, shallow spring tillage (to a depth of 10 cm) stimulated seedling emergence due to increased exposure of seeds to a favorable microclimate for germination [
. Effect of annual soil disturbance (with and without) on Palmer amaranth field emergence from the artificial seed bank in Pendleton, SC, USA, over 4 years
Field emergencea | ||||||||
---|---|---|---|---|---|---|---|---|
Yearb | With disturbancec | Without disturbance | With disturbance | Without disturbance | ||||
% of initial seed bank | % of total emergence | |||||||
2005 | 0.51 | a A | 0.44 | b A | 64 | b A | 86 | a A |
2006 | 0.28 | a B | 0.07 | b B | 35 | a B | 14 | b B |
2007 | 0.01 | a C | 0.00 | a B | 1 | a C | 0 | a C |
2008 | 0.00 | a C | 0.00 | a B | 0 | a C | 0 | a C |
S.E. ± | 0.021 | 1.311 |
aThe initial seed bank was comprised of 20,000 seeds·m−2 buried in the top 10 cm of the soil in October 2004. Palmer amaranth emergence was monitored in the field every other week from the date of first observed seedling emergence until emergence ceased each year in September. bMeans within a year (row) followed by the same lowercase letter are not significantly different based on Fisher’s protected LSD test at P < 0.05. cMeans within a soil disturbance treatment (column) followed by the same uppercase letter are not significantly different based on Fisher’s protected LSD test at P < 0.05.
However, in the present study, the initial seed bank contained seeds uniformly distributed in the top 10 cm of soil and subsequently disturbed once a year or kept undisturbed for the remaining 4-yr burial period, methodology similar to that used in other natural or artificial weed seed bank studies [
Consistent with our results on Palmer amaranth, the effect of mechanical soil disturbance (to a depth of 10 cm) on redroot pigweed emergence was evident only in the first year during a 5-yr burial study [
Data shown in
In 2005, averaged over burial depths, 1.06% (212 viable seeds·m−2) of initial seed bank remained in the absence of annual soil disturbance compared with 0.63% (126 viable seeds·m−2) of initial seed bank that remained in annually disturbed tilled plots, a 1.7-fold reduction (
Similar to our findings on Palmer amaranth, previous researchers [
. Effect of annual soil disturbance on remaining viable seeds of Palmer amaranth in soil samples collected from the artificial seed bank in each of the 4 years, averaged across burial depths in Pendleton, SC, USA
Remaining viable seedsa | ||||
---|---|---|---|---|
Yearb | With disturbancec | Without disturbance | ||
% of initial seed bank | ||||
2005 | 0.63 | b A | 1.06 | a A |
2006 | 0.04 | a B | 0.09 | a B |
2007 | 0.03 | a B | 0.06 | a B |
2008 | 0.01 | a B | 0.03 | a B |
S.E. ± | 0.065 |
aThe initial seed bank was comprised of 20,000 seeds·m−2 buried in the top 10 cm of the soil in October 2004. Viability of seeds in soil samples collected from the field plots in November each year was assessed by monitoring the emergence in the greenhouse at 30/24 C (day/night) and 16 h photoperiod for 6 months, and any remaining seeds were recovered through flotation/centrifugation technique and tested for viability. Data shown in the table represent the total viable seeds that germinated or emerged in the greenhouse plus those that were tested positive for viability. bMeans within a year (row) followed by the same lowercase letter are not significantly different based on Fisher’s protected LSD test at P < 0.05. cMeans within a soil disturbance treatment (column) followed by the same uppercase letter are not significantly different based on Fisher’s protected LSD test at P < 0.05.
likely to occur between burial depths of 0 to 2 cm and below 2 cm as evident in common waterhemp, with seeds in the top 0 to 2 cm of soil, the zone of germination, being less persistent compared with seeds buried below 2 cm; however, difference in seed persistence was not evident between 2 to 6 and 6 to 12 cm of the soil profile [
In the present study on Palmer amaranth, accounting for total viable seeds in the 0 to 10 cm of the soil that included nondormant seeds that emerged in the field or germinated in the greenhouse bioassay plus dormant seeds recovered from the soil and were tested viable, approximately 98% of the artificial seed bank was lost during the 4-yr burial period. This confirms the previous findings that Amaranthus species have low persistence in soil seed banks [
The rapid decline of Amaranthus seed bank within a year of burial might be due to physiological death of seeds, fatal germination, seed herbivory, or microbial seed degradation in the soil, as reported in other natural or artificial seed bank studies [
Due to variability in methodology used by different researchers for seed burial, results from these experiments should be interpreted with caution. Weed seed survival in artificial seed banks may not mimic those in natural seed banks [
Deep burial of Palmer amaranth seeds would be a promising strategy for depleting the soil seed bank. Soil disturbance through shallow spring tillage reduced the Palmer amaranth seed bank through emergence losses, at least in the first year after seed burial. However, it should be recognized that Palmer amaranth is a prolific seed producer with a single plant producing almost 600,000 seeds [
The authors would like to thank US South Carolina Soybean Board for funding this research.