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«RELATIONSHIP BETWEEN THE SOIL SEEDBANK AND WEED POPULATION AS INFLUENCED BY LAND USE INTENSITY IN SOUTHERN GUINEA SAVANNA OF NIGERIA Takim1*, F. O., ...»

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Nigerian Journal of Agriculture, Food and Environment. 9(3):36-41

Takim et al., 2013

Published September, 2013

RELATIONSHIP BETWEEN THE SOIL SEEDBANK AND WEED

POPULATION AS INFLUENCED BY LAND USE INTENSITY IN

SOUTHERN GUINEA SAVANNA OF NIGERIA

Takim1*, F. O., Fadayomi1, O., Ekeleme2, F. and Amosun3, J. O.

ABSTRACT

1Department of Agronomy, University of Ilorin, P.M.B. 1515, Ilorin. *email: felixtakim@yahoo.co.uk Department of Plant Health Management, Micheal Okpara University of Agriculture, Umudike, P.M.B 7267, Umuahia Abia State 3Institute of Agricultural Research and Training, Moor Plantation, OAU, Ile Ife, Nigeria.

This 2 year study investigated the relationship between the number of weed seeds in the soil seedbank and the emerged population of weed seedlings in 4 land use intensities in a southern Guinea savanna of Nigeria. Soil samples were collected soon after harrowing to a depth of 15cm and the weed seeds therein were enumerated. The emerged weed seedlings in the field sampling areas were counted over the following 12 or 15 weeks. The overall average proportion of the active weed seedbank emerging as seedlings at these fields range from 15.8 to 33.6 % of the total weed seedbank enumerated and found to be slightly differed across the cropping systems, weed control practices and land use intensities. The results showed a significant (P≤0.05) linear relationship between the weed seed numbers in the soil and the weed seedling numbers on the arable fields.

The result will be valuable in aiding the prediction of likely weed infestations in arable crops and provide a valuable input in timing of weed control.

Keywords: Soil, seedbank, land use, weed species, weed emergence, cowpea, maize

INTRODUCTION

Weeds are unique group of plant species because of their ability to infest and thrive in intensively disturbed habitats, despite extensive efforts to eliminate them. Weeds are successful because they are generally plastic plants that adapt to and survive changes in the environment. At maturity, weeds shed their seeds on agricultural land and thus add to the population of weed seeds in or on the soil. Yenish et al., (1992) described weed seedbank as the reservoir of viable weed seeds in the soil. For Clements et al.(1996) this reservoir corresponds to the seeds not germinated but, potentially capable of replacing the annual adult plants, which had disappeared by natural death, and perennial plants that are susceptible to plant diseases, disturbance and animal consumption, including man. All the viable seeds present in the soil or mixed with soil debris constitute the soil seedbank and it reflects the cumulative effects of many years of crop and soil management (Ndarubu and Fadayomi, 2006).

Weed seedbank gives an insight of the history of weed management successes or failures of a cropping system.

Seedbank studies help to increase the efficiency and efficacy of management decisions (Clements et al.,1996). As weed seedbank is an indicative of a field’s cropping systems history, it would be useful to know if weed seedbank and the aboveground community are closely related. If this relationship were predictive, seedbank data could be used in the design of predictive weed management. Although a number of studies have evaluated the relationship between the weed seedbank and the floristic compositions, results have not been consistent. While some studies have reported strong relationships between the weed seedbank and aboveground communities (Dessaint et al., 1997; Rahman et al. 2001 and 2006; Tuesca et al. 2004; Ndarubu and Fadayomi, 2006), others have found that correlations were generally low and very variable (Wilson et al. 1985; Forcella, 1993; Cardina and Sparrow, 1996; Webster et al. 2003). Despite the importance of weed seedbank as a propagule source for agricultural weeds (Cavers and Benoit, 1989), only few studies on the relationship between weed seedbank and aboveground weed community composition have been conducted in southern Guinea savanna (SGS) of Nigeria. This study was designed to compare the volume of weed seedbank with floristic composition under the different landuse intensities in SGS of Nigeria.

MATERIALS AND METHODS

This investigation was conducted at 4 study sites with known cropping history between 2009 and 2010 in Ilorin, southern Guinea savanna zone of Nigeria. Site I, had been under cultivation with different crop(s) per season continuously for 8 years. Site II was continuously cultivated with sole maize field between 2002 and 2008, site III which was adjacent to site II, had been under continuously sole cowpea cultivation for 5 years and site IV had been under natural weed fallow between 1997 and 2008. The field trial was designed as a randomized complete block with a split-plot arrangement and three replicates. At site I, the main plots consisted of four cropping systems, made up of maize and cowpea intercrop (MZCP), sole crop of maize (SMZ), sole crop of cowpea

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(SCP) and no- cropping (NCRP) treatment. Site II had sole maize and no cropping while site III had sole cowpea and no cropping as the main plots. The sub plots in each site consisted of three weed control methods, which included: 1. Chemical weed control (CWC); 2. Hoe weeding (HWC) at 3 and 6 weeks after planting and 3. No weed control (NWC). Site IV was divided into equal halves. Each portion was either cultivated to sole maize (FSMZ) or sole cowpea (FSCP), had the same treatments and established on the same date using the same experimental procedures as in site II and site III, respectively. The experiments were conducted on the same experimental sites and plots in 2009 and 2010 cropping seasons.





Data collection At site I, weed seedling emergence was monitored in the same fixed quadrats at 3, 6, 8, 10,12 and 15 WAP while on the others sites weed seedling emergence was monitored at 3, 6, 9 and 12 WAP using similar number and size of quadrant. In all the sites seedling emergence was assessed in two fixed 0.5 m2 quadrats per sub plot.

Soil sampling After harrowing, but before ridging, the experimental field, on each site, was divided into nine (9) cardinal points.

Two quadrants (1.0 m2) were randomly located at each of the cardinal points. Nine core soil samples were collected from each quadrant using a precision auger (7.4 cm in diameter) to a depth of 15 cm. Eight core samples from each of the two quadrants at each cardinal point were combined to form a composite sample for that cardinal point (Composite A). The remaining core sample from each of the two quadrants at each of the cardinal points were similarly be combined to form another cardinal point composite sample (composite B). These latter composite samples for each of the cardinal points were further combined to form an overall composite sample for the entire field. Thus, a total of ten composite samples (1 for each of the 9 cardinal points and one for the overall field) were analysed for weed seedbank. The composite samples were air-dried and passed through a 2 mm sieve.

The sieved samples were used for the estimation of the soil weed seedbank using the direct germination method.

Nine core soil samples were also taken from each sub-plot after the crops had been harvested in 2009 and 2010 cropping seasons. Samples from similar treatment combinations from each replicate were combined to form a composite sample.

Soil seedbank determination Nine hundred grams of the sieved composite soil samples were used to fill three plastic bowls (13 cm in diameter and 6 cm in depth) which were arranged in the screen house. Each of the bowls had four perforations at the base to facilitate drainage of excess water in the soil samples. The soil samples were watered to field capacity at the commencement of the experiment and on alternate days thereafter; then monitored for weed seed germination/seedling emergence at three weekly intervals. Germinating weed seedlings were enumerated either as broadleaves, grasses and sedges; identified to species level, counted and then pulled out. Identification of weed seedlings was carried out with the aid of the weed identification manual of Akobundu and Agyakwa (1998). Soil samples were stirred after each assessment to stimulate germination by bringing to the surface other weeds seeds that might have been deeply buried in them. The experiment was terminated at three months after its commencement.

Weed seedbank estimation The number (size) of weed seeds in the seedbank (Y) per land area (m2) was estimated by multiplying the number of seeds in soil sample (G) by the inverse ratio of the volume of soil in the auger sample to the volume of soil in 1 m2 area sampled to the depth of the auger (15 cm).

The ratio was computed as in Ndarubu and Fadayomi (2006):

Volume of soil from the auger sample (V1) V1 = π r2h, where π = 22/7, r = radius of the auger and h= depth of sampling V1 = 22/7 x (3.7 cm)2 x 15 cm = 645.2097 cm3 ; or 6.45 x 10-4 m3 Volume of soil from 1 m2 area sampled (V2) V2 = L x B x H, where L = length, B = breadth and H = depth of sampling.

V2 = 100 cm x 100 cm x 15 cm = 1.5 x 10 -1 m3 Y = V2/V1 x G, where Y = estimated density of weeds per m2 to the depth of 15 cm.

G = number of emerged weed seedling per soil sample.

The calculated inverse ratio of the volume of soil from an auger sample to the volume of soil per m2 was 232.56.

The data of weed density per soil samples were then extrapolated to weed density per m 2 by multiplying with 232.56.

Data analysis Data so obtained were subjected to analysis of variance (ANOVA) using Gen Stat statistical package (Discovery Edition 3) and the following comparisons were made: pre cultivation seedbank data among the different field sites; post-harvest seedbank data within individual fields and between the different field types; pre-cultivation seedbank data within a field type with floristic data in the same field for years I & II, respectively; post- harvest seedbank data in year I with floristic data in year II. The seedbank data were regressed against the floristic data with individual fields and between the different field types.

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RESULT Effect of previous land use on pre-cultivation weed seedbank The density of weed seeds obtained from the pre-cultivation seedbank estimation was significantly (P≤ 0.05) affected by the previous land use intensity in each experimental site (Table 1). Broadleaved weed seeds were significantly higher in the continuously cultivated maize fields followed by field with alternate cropping system and continuously cultivated cowpea fields while significantly lower density of broadleaved weed seeds were obtained from the natural fallow fields. Grass seedlings were significantly lower in natural fallow fields and those fields had similar density of grass weed seeds with continuously cultivated cowpea fields. Site I and continuously cultivated maize fields had similar grass seeds that were significantly higher than those obtained from other fields.

Sedge weed seeds were significantly highest in continuously cultivated maize fields followed by alternate cropping systems field while natural fallow fields had statistically lowest sedge seedlings. The continuously cultivated cowpea fields had similar sedge weed seedlings as in Site I and natural fallow fields. The total emerged weed seedling from the pre-cultivation seedbank was significantly lower under the natural fallow fields while the continuously cultivated maize fields had significantly higher weed seeds. Site I had similar volume of weed seeds with continuously cultivated maize fields while continuously cultivated cowpea fields had similar volume of weed seeds as in site I and natural fallow fields.

Effect of land use intensity on density of post harvest weed seedbank The total weed seedbank estimated was significantly affected by land use intensity in 2009 but not in 2010 (Table 2). In 2009, the post-harvest weed seedbank followed a fairly similar trend with the pre-cultivation weed seedbank except that continuously cultivated cowpea field had significantly higher density of weed seeds. The post-harvest weed seedbank in 2010 was not significantly affected by land use intensity. The progressive increase in the density of weed seedbank in each land use was significantly affected by year of cultivation and/ or estimation except in continuously cultivated cowpea fields. Site I had a significant increase in density of weed seedbank. The 2009 weed seedbank was 32% higher than the pre-cultivation density while 2010 post-harvest weed seedbank was 36% significantly higher than 2009 post-harvest weed seedbank. In the natural fallow fields, the density of post-harvest weed seedbank in 2009 was 5313 seeds/m2 in FSMZ and 3088 seeds/m2 in FSCP both were similar to the pre-cultivation weed seedbank density of 1679 seeds/m2 while the density of weed seedbank in the 2010 post-harvest weed seedbank in FSMZ field was significantly higher than the density obtained in the precultivation weed seedbank in the same field.

Effect of cropping system and weed management practice on post harvest weed seedbank At site I, total post- harvest weed seedbank estimated was not significantly affected by cropping system in both years of the study, while weed control practice significantly influenced total weed seedbank in 2009, but not in 2010 (Table 3). In 2009, total weed seeds were significantly higher in the unweeded control plots than in the other two plots. The herbicide treated plots had similar density of weed seeds as in the hand weeded plots. In the 2010 growing season, weed control treatment did not significant affect the density of weed seedbank.



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