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Published online 6 February 2007
Published in Agron J 99:390-398 (2007)
DOI: 10.2134/agronj2005.0330
© 2007 American Society of Agronomy
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Crop Rotations

Cotton Roots, Earthworms, and Infiltration Characteristics in Sod–Peanut–Cotton Cropping Systems

Tawainga W. Katsvairoa,*, David L. Wrighta, James J. Maroisa, Dallas L. Hartzogb, Kris B. Balkcomb, Pawel P. Wiatrakc and Jimmy R. Richa

a Univ. of Florida, NFREC, 155 Research Rd, Quincy, FL 32351
b Auburn Univ., Wiregrass Reg. Res. & Ext. Center, P.O. Box 217, Headland, AL 36345
c Univ. of Clemson, Clemson, SC 29634

* Corresponding author (katsvair{at}ufl.edu)

Received for publication December 8, 2005.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Diverse cropping systems offer many advantages to farmers. We evaluated root growth, soil water infiltration, and earthworm population densities in a conventional peanut (Arachis hypogaea L.)/cotton (Gossypium hirsutum L.) rotation using conservation tillage (CT), and a peanut/cotton/bahiagrass (Paspalum notatum Fluegge) farming system. The rotations were initiated in 2000 in Quincy, FL, and in 2001 in Headland, AL, in both cases on a Dothan sandy loam (fine, loamy siliceous, thermic Plinthic Kandiudults). In 2003, a year with more uniform rainfall, cotton in the sod-based rotation had larger average crown root diameter per plant (22.6 vs. 16.3 mm), root area (87.2 vs. 57.4 cm2), root length (640 vs. 460 cm), and root biomass (18.59 vs. 10.45 g) as compared with cotton in the peanut/cotton rotation. Water infiltration rates were higher in both cotton and peanut after bahiagrass compared with the conventional peanut/cotton rotation in 2003. Earthworm population densities were greater in the sod rotation compared with the traditional peanut/cotton cropping system. Water infiltration was positively correlated to earthworm population densities. Despite the improvements in soil quality, cotton yield in the sod rotation was the same as the traditional cropping systems. Cotton developed excessive vegetative growth in the bahiagrass system at the expense of lint yield. Further research is needed to determine the N rate for the sod-based rotation in comparison with the conventional cotton/peanut rotation.

Abbreviations: CT, conservation tillage • SE, southeastern USA


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOILS IN THE SOUTHEASTERN USA (SE) pose unique challenges to peanut and cotton growers. They are generally infertile and sandy, hence prone to soil erosion and fertility and moisture deficits. Cotton provides few plant residues and does not increase organic matter nor protect from soil erosion (Reeves, 1997; Reddy et al., 2004). Cotton growers in the SE understand these problems and have responded by practicing more CT than all the other regions of the USA combined (National Crop Residue Management Survey, 2002). Conservation tillage increases plant residues on the surface (Rhoton, 2000; Havlin et al., 1990) and decreases soil erosion.

A natural compaction zone exists 15 to 20 cm deep in most SE coastal region soils (Kashirad et al., 1967; Campbell et al., 1974), creating another challenge for growers. This compaction layer restricts root growth and limits the soil volume in which roots can explore for available moisture and nutrients. Consequently, the resulting shallow-rooted crops are prone to water deficiencies even during minor dry periods. Further exacerbating the issue, the SE is known for high temperatures which increase evapotranspiration. Compaction zones may be broken through deep tillage and in-row subsoiling, which enable roots to reach lower soil profiles. However, fuel and machinery costs make this practice expensive.

We recommend diversifying the traditional peanut/cotton rotation to include bahiagrass, a common perennial pasture grass in the SE, because it contributes numerous benefits towards soil health and can alleviate some of the previously mentioned problems. Bahiagrass has a large and extensive root system (Blue and Graetz, 1977; Impithuksa and Blue, 1978) which can penetrate through the compaction zone (Elkins et al., 1977). The roots of the subsequent crops can exploit the channels created by the bahiagrass sod as low resistance pathways to explore larger soil volume and reach deeper depths (Long and Elkins, 1983). Up to 40 times higher root numbers have been reported in cotton following bahiagrass compared with continuous cotton (Long and Elkins, 1983). Cotton roots in the sod rotation extended up to 60 cm in depth but not in the continuous cotton rotation. The authors further reported increased nutrient uptake for N, P, and K at deeper depths for cotton in the sod rotation.

Perennial grasses can increase the amount of plant residue on the soil surface which in turn can reduce evapotranspiration losses, increase soil moisture (Wilhelm et al., 2004; Unger and McCalla, 1980; Weil et al., 1993), and reduce soil erosion (Gantzer et al., 1990). Presence of more plant residues can also result in increased biological activity of both micro- and mesofauna, including earthworms (Weil and Magdoff, 2004; Katsvairo et al., 2006). Traditionally, earthworms have been regarded as good indicators of soil health. As with the roots of perennial crops, earthworm burrows also provide preferential pathways for root elongation (Logsdon and Linden, 1992; Hirth et al., 1997; Kladivko and Timmenga, 1990). In fact, Wang et al. (1986) and Logsdon and Linden (1992) suggested that earthworm channels can sometimes be the only means for roots to reach lower horizons in soils with compacted layers. Edwards and Lofty (1978, 1980) also reported a close relationship between the distribution of roots and regions of earthworm activity. Earthworms facilitate conversion of plant residues to organic matter (Lavelle, 1988; Lee, 1985). They also impart numerous other benefits to soil health, and a number of other reviews and books outline those benefits (Katsvairo et al., 2006; Edwards, 2004; Linden et al., 1994).

The use of perennial grasses results in increased earthworm population densities which, in turn, can result in increased soil water infiltration. Water can flow at higher rates through both root channels (Rasse and Smucker, 1998) and earthworm burrows (Katsvairo et al., 2002). Meek et al. (1990) reported greater infiltration rates in cotton after alfalfa (Medicago sativa subsp. sativa). Numerous articles report positive correlations between earthworms and soil water infiltration (Teotia et al., 1950; Trojan and Linden, 1992; Bowman, 1993; Katsvairo et al., 2002). Furthermore, CT results in more plant residues for earthworms to consume (Berry and Karlen, 1993) and conserves root channels (Edwards et al., 1998) and earthworm burrows which can result in increased soil water infiltration rates (Cassel et al., 1995; Edwards et al., 1998; Gaskin et al., 2002).

Depressed commodity prices and environmental concerns have prompted the reintroduction of sod and livestock into row crop production systems (North Florida Research and Education Center, 2006). The integrated system provides improved crop health, higher crop yield, greater profitability, reduced risk, and better environmental stewardship (Katsvairo et al., 2006; Clark, 2004; Allen et al., 2005; Krall and Schuman, 1996). However, few studies have been conducted to examine the effects of incorporating bahiagrass into the cotton cropping systems under CT on soil health. From this review, it is feasible that the use of perennial grasses affects equally the tripart complex of root development, infiltration, and earthworm proliferation. The objectives of this study were to compare cotton root development, earthworm population densities and activities, and soil water infiltration rates in a peanut/cotton rotation with a bahiagrass/peanut/cotton rotation 2 to 5 yr after implementation.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This research was initiated in Florida in 2000 and Alabama in 2001 as part of a long-term multidisciplinary study to examine the influence of 2 yr of bahiagrass rotation on root growth and soil physical and biological properties on subsequent peanut and cotton crops in the rotation. The soils at both locations are Dothan sandy loam. The work in Florida was conducted at the University of Florida's North Florida Research and Education Center in Quincy, FL (84°33' W, 30°36' N). The 1.75-ha experimental site was planted to cotton in the summer of 1999 and fallowed in the following fall and winter. The large-scale study in Alabama was conducted at the Wiregrass Research Station in Headland, AL. Prior to the experiment, a large portion of the 6.0-ha Alabama site had been in a peanut/cotton cropping system and a smaller portion had been under switchgrass (Panicum virgatum L.) for 12 yr. Rotation treatments were distributed among the old system, and the small portion which was under switchgrass was allocated to the bahiagrass rotation. Hence, this portion of the field had been in perennial grasses for an extended time period, and in this article is referred to as bahiagrass/switchgrass rotation.

The experimental design was a strip plot design (also called split-block) with three replications (Little and Hills, 1978). In our study, the use of a lateral move irrigation unit imposed the use of the strip-plot treatment layout. Three 128-m-long by 45.7-m-wide strips consisted of alternating irrigated and nonirrigated treatments. The irrigation unit stayed in the same area all 5 yr, and irrigation was based upon extension recommendations for each crop in the irrigated treatment (Smajstrla et al., 2006). Crop rotation subplots were 45.7 m long by 18.3 m wide (20 rows) and were aligned perpendicular to the irrigated and nonirrigated strips. Crop rotations studied were a peanut–cotton–cotton rotation, which is the conventional rotation used by growers in the region, and a bahiagrass–bahiagrass–peanut–cotton rotation. We had all phases of the rotations in all years (Table 1). In the conventional rotation, the first and second-year cotton was also compared. A 2-yr bahiagrass sod was used for this study because it was difficult to get a good stand of bahiagrass after 1 yr under our environmental conditions. Oat (Avena sativa L.) was planted as a winter cover crop prior to planting the row crops (cotton and peanut) in the summer of each following year.


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  		Table 1. Crop sequence for bahiagrass, peanut, cotton, and oat in Florida, 2001 to 2004.

 
The same rotation treatments used in Florida were used in Alabama but with fewer replications. With rotation plot sizes at Alabama being at least 1.5 ha, several samples (depending on parameter being evaluated) per plot were taken and considered as replicates. Sampling points were identified in each plot and georeferenced. All soil measurements were then taken from the same locations throughout the 2 yr of data collection to reduce experimental errors due to soil spatial variation and to enable us to run correlations on the measurements with more accuracy. Center pivot irrigation was used at the Alabama site, and irrigation was applied across all plots as needed. Ryegrass (Lolium multiflorum) was grown in winter as a cover crop at the Alabama site. Stocker cattle either grazed on the ryegrass in winter, or the ryegrass was used for hay in the stocker operation. A herd of 68 calves was added in the fall of 2002 and allowed to graze throughout winter on the ryegrass which was planted after peanut and cotton harvest. Cattle were then removed from the field in time for peanut and cotton planting.

Bahiagrass
Bahiagrass stands were established in the spring of 2000 in Florida and 2001 in Alabama at a rate of 44 kg ha–1 seed, using a no-till drill. In subsequent years, the bahiagrass was also seeded in the spring and killed in the fall of the second year with glyphosate [N-(phosphonomethyl)glycine] (1.1 kg a.i. ha–1). Killing the bahiagrass in the fall reduced chances of regrowth; however, another application of glyphosate was usually required in the spring. Fertilizer (5–10–15, N–P–K) was broadcast at planting at a rate of 560 kg ha–1. Plots were fertilized with 34–0–0 at a rate of 112 kg N ha–1 approximately 30 and 60 d after planting in both years. Two hay cuttings were made at the Florida site in both years, but the bahiagrass was grazed at the Alabama site. No insecticides or herbicides were applied to the bahiagrass.

Cotton
Plot rows were strip tilled using a Brown Ro-Till implement (Brown Manufacturing Co., Ozark, AL) in April of each year, immediately prior to cotton planting at both sites. At the Florida site, the cotton variety Paymaster PM 1500 BG/RR was planted in 2000, while in 2001 to 2004, Deltapine DP 458 BR was planted. Deltapine 555 was planted at the Alabama site. All plantings were made between late April to early May with a Monosem planter at 144 000 seed ha–1. Thimet 20G [Phorate: O,O-diethyl S-(ethylthio) methyl phosphorodithioate] was applied in-furrow for insect control at a rate of 6.72 kg a.i. ha–1. As the season progressed, insecticide applications were made to control plant bugs, Dysdercus suturellus (Herrich-Schaeffer), and stink bugs, Nezara viridula L. Fertilizer (5–10–15) was applied in a band, near the row, at planting in all cotton plots at a rate of 28, 56, and 84 kg ha–1 of N, P2O5, and K2O, respectively. Cotton was sidedressed with 34–0–0 at a rate of 67 kg N ha–1 at squaring. Weed control was done using standard extension recommendations for each site (Ferrell et al., 2006). Plant densities were determined when cotton was about 10 cm tall by counting the number of plants along the entire length (15 m) of the two harvest rows in each subplot in Florida and by counting all the plants in a 10-m length in each of the four harvest rows per plot at the Alabama site.

Prior to harvest, cotton was defoliated with a tank mix of Finish [S,S,S-tributyl phosphorotrithioate (1.681 kg ha–1) + thidiazuron (N-phenyl-N'-1,2,3-thiadiazol-5-yl-urea) (0.056 kg ha–1); Ethephon (2-chloroethyl) phosphonic acid (0.84 kg ha–1); and Leafless [dimethipin (2,3-dihydro-5,6-dimethyl-1,4-dithiin 1,1,4,4-tetraoxide) + thidiazuron] in mid September to October each year at 2% v/v when 60 to 70% of cotton bolls were open. The plots were harvested with a spindle picker (International Harvester model 1822, Case Corp., Racine, WI) in October or November of each year. Lint and cotton seed were weighed for each sample.

Peanut
‘Georgia Green’ peanut was planted using a Monosem pneumatic planter (ATI Inc., Lenexa, KS) at 20 seed m–1 of row in mid to late May of each year. All plots were sprayed with fungicides to control foliar diseases using Alabama Extension Services Guidelines. The peanut crop was dug in October or November with a KMC (Kelley Mfg. Corporation, Tifton, GA) two-row digger and allowed to air-cure for 3 d prior to combining.

Ryegrass and Winter Oat
Winter oat and ryegrass were planted at a rate of 100 kg ha–1 using a Great Plains no-till drill at the Florida and the Alabama sites, respectively, in late October to early November of each year after cotton and peanut harvest. Before planting the ryegrass and oat, cotton stalks were shredded with a rotary mower. Nitrogen was applied at 30 kg ha–1 to the cover crop at the Florida site to stimulate growth, while the Alabama site was fertilized more frequently because the ryegrass was utilized for cattle grazing. Other cultural practices were based on the respective extension guidelines. At the Florida site, oat was grown primarily to increase the OM content of the soil and prevent erosion. As a result, oat was not allowed to reach maturity, but was instead killed with herbicides in April prior to planting cotton or peanut.

Cotton Roots
Cotton roots were harvested only at the Florida site, after cotton had reached physiological maturity in October. The roots were carefully dug from three 1-m-long by 0.5-m-wide sample areas in each rotation plot. Digging to the natural compaction zone was done using shovels and spades, then 10 to 15 cm further into the natural compaction zone. The difficulty of digging into the compaction zone limited sampling depth. The roots were carefully washed to remove all soil and organic debris. Diameter of the crown root was measured using calipers. The roots were scanned using an office paper scanner (hp scanjet 5530). The scanned images were imported into Assess, a digital image software program (Lamari, 2006). After calibration with objects of known dimensions, the software was used to determine total root area and length. The roots were then dried and weighed. Root image analysis is illustrated in Fig 1 . We report average diameter of root crowns, root area and length, and total biomass.


Figure 1
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  		Fig. 1. Illustration of root scanning using the computer program Assess.

 
Earthworms
Sampling for earthworms was done in mid January at both sites, when temperatures are relatively cool in the SE. Both soil temperature and moisture remain fairly constant during this time of the year. In summer, soil temperatures and moisture fluctuate significantly during the day (morning and afternoons), and this affects earthworm distribution and can potentially obscure treatment effects. The disadvantage of sampling in January is that earthworm numbers may not be as high as summer sampling. Shovels were used to dig to, then past the compaction zone, to a depth of 10 cm or more where possible. This method of sampling is adequate to sample for shallow-dwelling earthworms but not for deep-dwelling earthworms. The area sampled was 0.203 m2, and four samples were taken per plot in Florida while 10 samples were taken from each large plot (1.5 ha) from georeferenced locations in Alabama. The soil was hand-sifted, and earthworms were counted manually. At the time of sampling, the Florida site was in winter oat, and the Alabama site was under cattle grazing on ryegrass. In the discussion, comparisons are made between earthworms sampled in winter and summer crops of the previous year.

Infiltration
Water infiltration rates were determined at the surface and also in the compaction zone during the growing season at both the Florida and Alabama site during the latter half of the growing season in both years using PVC tubes. The PVC tubes were cut into rings of 15-cm diameter and 10-cm height. The rings had a sharpened edge, were inserted into the ground, and hammered to a depth of 2 to 5 cm to achieve a good seal on the soil surface and at the compaction zone. We then poured 500 mL of water into each ring and recorded the time for the water to disappear from the ring and percolate into the ground. This method to determine infiltration is rather crude because with 15-cm-diam. infiltration tubes, there is significant amount of lateral flow which could increase infiltration. However, the method works for comparative purposes, and was adopted as it is convenient and easily repeated by farmers. Infiltration was determined at four locations per plot in Florida and at 10 georeferenced locations per plot in Alabama. The depth to the compaction zone was also measured using a meter stick, and soil moisture was determined using the gravimetric water content method before the 500 mL of water was added into each ring. Water infiltration data were converted to natural logarithms to improve homogeneity of variance. The data was back-transformed into original units for presentation.

Statistical Analysis
All data were analyzed by ANOVA procedures using the SAS Statistical Software Package (SAS Institute, 2002). Although the rotation study started in 2000, we report data collected from 2003 to 2005. Combined analyses over years as well as separate analysis for each year are presented. Irrigation main effects are confounded with block effects and as a result, they cannot be discussed from a statistical significance standpoint, but are discussed using means. However, irrigation x rotation interactions are appropriate for discussion. Mean separation for main effects and interactions were obtained by Fisher's protected LSD, as described by Little and Hills (1978). Effects were considered significant in all statistical calculations if P ≤ 0.05.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Precipitation
Precipitation differed between the two growing seasons at both sites. We show the precipitation and irrigation applied in Florida as Table 2. We report only the precipitation in Florida because root studies were only conducted in Florida. Total irrigation amounts of 110 and 125 mm were applied to the cotton and bahiagrass in the irrigated treatment in 2003 and 2004, respectively. July of 2004 was dry and had rainfall below the 30-yr average. July and/or August are important months in cotton production in the SE because that is when cotton blooms and fruit set occurs. The year 2004 was also marked by three hurricanes which were mostly felt as strong winds in Quincy, FL, resulting in some lodged plants.


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  		Table 2. Monthly precipitation and rainfall in Florida in 2003 and 2004 growing seasons.

 
Cotton Roots
The labor-intensive nature of root studies limited this work to the Florida site. Compared with cotton in the conventional rotation, cotton in the bahiagrass rotation had larger root crown diameter (39%), total root area (52%), total root length (39%), and total root biomass (77%) in 2003 (Table 3). A larger root system enables crops to explore larger soil volume to extract more nutrients and moisture. The more extensive root growth in cotton after the sod can be attributed to recolonization of the sod root channels. Cresswell and Kirkegaard (1995) used the term biodrilling to describe cases where biopores left by cover crops can provide low resistance pathways for subsequent crops. Long and Elkins (1983) reported a 10-fold increase in cotton root numbers after sod compared with cotton in the conventional rotation. Additionally, the authors reported a greater N, P, and K uptake in the sod rotation compared with the conventional rotation. Improved root performance is reported in other crops following deep-rooted cover crops (Williams and Weil, 2004; Stirzaker and White, 1995; Rasse and Smucker, 1998). Other results from this study showed that the cotton in the sod rotation was taller and developed more total aboveground biomass and leaf area index (Marois et al., 2004). The larger amount of aboveground biomass shaded weeds more quickly (Katsvairo et al., 2004a). There were no rotation x irrigation interactions for either crown root diameter, root area, root length, or biomass in 2003.


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  		Table 3. Diameter of the cotton root crown, area, length, and biomass at maturity for cotton under conventional and bahiagrass rotation in Florida in 2003 and 2004.

 
No statistical differences were observed for either root area or length between the rotations in 2004. However, differences were found between the rotations in root biomass and root crown diameters (Table 3). Cotton roots in the sod rotation in 2003 had thicker diameter than the conventional systems not only in 2003, but the diameters were thicker than both the conventional and sod rotation in 2004 (Table 3). Thicker diameters enable plants to exert more pressure on the soil and develop a deeper root system. While the year 2004 was overall wetter than 2003, 2004 was much drier in May and July, which are important periods of root development (Table 2). The more uniform rainfall distribution in 2003 could have resulted in cotton achieving a big rooting system including more fibrous roots in the sod rotation. Roots in the sod rotation had thicker diameters compared with second-year conventional cotton, but not different from first-year conventional (Table 3). It should be pointed out that it was very difficult to dig up and wash roots thoroughly, such that complete recovery was impossible.

The difficulty in root recovery and cleaning was also reported by Box (1996) and Merrill et al. (2002). This situation is exacerbated if the finer roots lie in compacted soil layers. In the present study, the finer ends of the tap root were not recovered, and neither were the adventitious roots which grew from the end of the tap root and were embedded in the compaction layer. Because most of a plant's root length and area is in the smaller roots (Merrill et al., 2002), it is possible that the lack of differences in total area and length between the rotations in 2004 may have been because we lost significant adventitious roots from the sod rotation which were never recovered from the compaction zone. Visually, the sod rotation had more roots in the compacted zone than the conventional rotation. A deep root system is an adaptive advantage during periods of stress, and enables cotton to escape active zones of plant-parasitic nematodes, which are highest in numbers in the top 15 to 30 cm. Minimizing parasitic attack becomes more essential under conditions of plant stress. Deep-rooted plants are also able to extract moisture from deeper soil profiles, greatly reducing irrigation requirements. Calculations by Elkins et al. (1977) showed that it is possible to reduce irrigation frequency from once every 3 d if roots were 15 cm long to once a month if roots were 152 cm long. Elkins (1985) observed cotton roots growing to 180 cm in sod rotation, but only up to 28 cm in the conventional rotation. He attributed the improvement in root growth to an eightfold increase in large soil pores in the sod rotation. In the present study, irrigated cotton had greater root area (52.27 vs. 44.79 cm2) and root length (552.79 vs. 457.85 cm) due to more water and nutrients being available for growth. Although differences in precipitation were recorded between 2003 and 2004, overall, the total precipitation for both seasons was close to the long-term average (Table 2), again indicating that cotton can respond to irrigation if rainfall is not uniformly distributed. There was a significant rotation x irrigation interaction for crown diameters (data not shown). We did not conduct an analysis across years because the first-year cotton treatment in the conventional rotation was present in 2004 but not in 2003.

Zobel (1992) reported that while the potential maximum root growth of plants is governed by genetics, there is response to the environment, including soil conditions. This plasticity can be exploited in designing and management of diverse cropping systems.

Earthworms
Earthworms were sampled in January of 2004 and 2005. In this article, however, we discuss how earthworms were influenced by rotations of the previous seasons, 2003 and 2004. Greater earthworm densities were observed for cotton in the sod rotation compared with cotton in the conventional rotation at the Florida site at the 2004 sampling time, relating to the 2003 growing season (Table 4). At the Alabama site, earthworms averaged 6.3 m–2 in the section of the field which had been under bahiagrass/switchgrass for 12 years. No earthworms were observed in other sections of the field which had been under continuous peanut/cotton cropping. At the Alabama site, the section of the field formerly under switchgrass could be expected to have built up higher levels of organic matter over the years. Several articles report that plant residues and organic matter are among the major factors that influence earthworm population densities (Jordan et al., 1997; Hubbard et al., 1999). At the Florida site, the higher number of earthworms in the sod rotation could have been partially a result of greater food availability from the sod and greater soil moisture conservation. Improved soil conditions in the sod rotation also could have assisted in increasing moisture recharge and moisture retention in that rotation. Plant residue and soil moisture are regarded as the most important factors affecting earthworm densities, with higher moisture promoting more earthworms (Hendrix et al., 1992; Buckerfield et al., 1997; Berry and Karlen, 1993). Irrigation at the Florida site during the 2003 growing season did not increase earthworm densities, perhaps because the season had uniformly distributed rainfall.


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  		Table 4. Earthworm population densities under conventional and bahiagrass-rotated cotton in Florida in 2004 and 2005.

 
Earthworms showed a more complex distribution at the Florida site in the January 2005 sampling period compared with the 2004 sampling time (Table 4). Earthworm densities showed rotation and rotation x irrigation effects at the Florida site for the 2005 sampling. Greatest numbers of earthworms were found in the sod rotation, followed by second-year cotton, and were least in the first-year cotton in the 2004 growing season. The reasons for a twofold difference in earthworm population densities between first-year and second-year cotton are not clear. It is possible that there may be more plant residues remaining in the field after cotton than after peanut. However, peanut hay was not removed. Peanut digging disturbs the soil, and this could have reduced earthworm populations. As was the case in the 2003 growing season, greater soil moisture was present under the sod rotation compared with the conventional system (data not shown). Earthworm densities were more than threefold greater under irrigation (20.1 vs. 6.6 m2) compared with nonirrigated conditions at the Florida site for the 2005 sampling period. The previous season, 2004, had less uniformly distributed rainfall. Earthworms also showed a rotation x irrigation interaction for the 2004 compared with the 2003 growing season. Cotton in the sod rotation under irrigation had greater earthworm densities compared with cotton in both phases of the conventional cropping system. Soil moisture could have been the most overriding factor that influenced earthworm populations for the 2005 sampling period at the Florida site. At the Alabama site, earthworms showed a wider distribution across rotations for 2004 compared with the 2003 growing season. In addition to observing greater earthworm numbers in the section of the field formerly under switchgrass, we also observed greater densities in peanut after bahiagrass but not in peanut in the conventional rotation. Overall, the earthworm numbers were low at both sites. It is possible that the cool temperature in January could have resulted in the earthworms burrowing deeper into the ground. The difficulties of digging into the compaction layer could have prevented us from capturing them if they had burrowed deeper into the compaction layer.

Earthworm burrows in the soil improve gaseous exchange, aeration, soil water infiltration, and root growth (Jakobsen and Dexter, 1988; Linden et al., 1994; Logsdon and Linden, 1992; Katsvairo et al., 2006). Burrows were observed at both sites in the present study, and channels were also visible within the compaction zone. Cotton roots were found in the channels left behind by the earthworms. The presence of earthworms in the zone of natural compaction and burrows being recolonized by cotton roots are illustrated in Fig. 2 and 3 . Although the earthworm population densities reported in this study are low, we observed evidence of earthworm activity including middens and burrows. The observed evidence further indicated that our sampling may not have been deep enough to account for the deep burrowers nor to capture the main zone of worm activity at the time we sampled. This further explains the low numbers in our earthworm counts. Since both root channels and earthworm burrows were present, the questions arises—could earthworms have exploited the bahiagrass channels for their burrows or did bahiagrass exploit the earthworm burrows for their channels? It is possible that earthworm burrowing and root growth are mutually beneficial and can have synergistic effects (Linden et al., 1994). We also observed more earthworm burrows and casts in rotations and regions with more root growth. A similar pattern is also reported by Edwards and Lofty (1978).


Figure 2
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  		Fig. 2. A soil clod showing an earthworm, earthworm burrows, and roots growing through earthworm channels in the natural soil compaction zone in Florida in 2003. The soil clod was dug from a sod rotation plot.

 

Figure 3
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  		Fig. 3. Earthworms and cotton roots in the natural soil compaction zone in bahiagrass-rotated cotton in Florida in 2004.

 
Infiltration
Field infiltration was higher in the sod-rotated cotton at both the surface and the natural compaction zone compared with the conventional system at the Florida site in 2003 (Table 5). Infiltration was greater only at the surface and not at the natural compaction layer in the peanut after 12 yr of bahiagrass/switchgrass compared with cotton after peanut, first-year bahiagrass, and peanut after cotton in Alabama in 2003 (Table 6). Infiltration rates were on average ninefold greater in the sod-rotated cotton compared with the conventional rotation in the zone of natural compaction at the Florida site. Rates of infiltration were sevenfold greater in the peanut after bahiagrass/switchgrass compared with the other treatments at the Alabama site. The root channels resulting from perennial grass rotations could have contributed to greater infiltration rates at both sites. Increased soil water infiltration is often reported after perennial crops (Meek et al., 1990). Also, the overall improved soil structure after the sod could have resulted in the improved infiltration. Higher infiltration rates reduce runoff losses and ensure greater volume of stored water. This, in turn, would reduce irrigation requirements for subsequent crops.


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  		Table 5. Soil water infiltration rates under conventional and bahiagrass-rotated cotton in Florida in 2003 and 2004.

 

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  		Table 6. Soil water infiltration rates in livestock/peanut/cotton cropping system in Alabama in 2003 and 2004.

 
Lower infiltration rates were observed at the surface in the sod rotation, and there were no treatment differences between the rotations in the zone of compaction in either the Alabama or Florida sites in 2004 (Tables 5 and 6). At the time soil water infiltration was determined, both sites were generally wet. We attempted to determine infiltration rates at about the same stage of cotton development in both years; however, that stage of cotton development in 2004 occurred at a time when there were continuous rains and hurricanes in both Florida and Alabama. The lower infiltration rates are probably a result of greater moisture retention in the soil profile in the sod rotations compared with the conventional rotations. Other reports on the same study showed greater soil moisture for the sod rotation (Katsvairo et al., 2004b). At both sites, gravimetric water content was greater in the sod rotation compared with the conventional rotation (data not shown).

The sod rotation which had the greatest infiltration rates in FL in 2003 was also the rotation which had the greatest earthworm population densities at the Florida site in 2004. Regression analysis shows a positive relationship (P ≥ 0.0086) between earthworms and infiltration in the compaction zone, with R2 of 0.92 for Florida. It was not necessary to conduct regressions at the Alabama site in 2003 because earthworms were only found in one treatment, the peanut after 12 yr of switchgrass. Our findings are consistent with several other researchers who document positive relationships between infiltration rates and earthworm densities (Katsvairo et al., 2002; Trojan and Linden, 1992; Bowman, 1993). For the Florida site, the positive correlation between infiltration and earthworm density were most prevalent for the compaction layer and not at the surface. Traffic from equipment resulted in surface compaction, which could affect infiltration rates, while the soil environment would be expected to remain fairly stable at the deeper depths. In 2004, regression analysis showed a positive relation (P ≤ 0.0091 and R2 = 0.99) between infiltration and earthworm densities only under irrigated conditions at the Florida site. The irrigated treatment had more earthworms than the nonirrigated treatment, and may have resulted in the positive relationship between infiltration and earthworm densities.

Yield
The sod rotation maintained cotton yield at the same level as the conventional cropping system in 2003 (Table 7). In 2004, the sod rotation had greater yield than first-year cotton in the conventional rotation, and the same yield as second-year cotton in the conventional rotation. The cotton in the sod rotation developed much more biomass and root growth but did not influence yield in 2003. The standard N application rate recommended for the conventional cropping system was used in the sod rotation; however, the standard N application rate resulted in excessive vegetative growth for the sod-rotated cotton. Nitrogen rates will be reduced in subsequent years of this experiment. At the Alabama site, peanut after bahiagrass yielded 3950 kg ha–1, while the peanut after cotton yielded 4280 kg ha–1 in 2003. In 2004, a relatively drier year, peanut after bahiagrass yielded 4290 kg ha–1 compared with 3950 kg ha–1 for the peanut in the conventional rotation. The low yield in the peanut after bahiagrass in 2003 may have been a result of competition for resources from bahiagrass regrowth, when the bahiagrass was killed in spring. In 2004, the bahiagrass was killed in fall of the previous year, reducing regrowth. Another article from the same region reports lower peanut and cotton yields when bahiagrass is killed in spring compared with fall of the previous year (Wright et al., 2006).


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  		Table 7. Cotton yields under conventional and bahiagrass rotation in Florida in 2003 and 2004.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Incorporating bahiagrass into the traditional peanut/cotton cropping system resulted in improved cotton root development including larger total root area, length, and biomass. In one season cotton in the sod rotation developed a thicker crown root, indicating a deeper tap root system. A deep tap root system enables the cotton to extract nutrients and soil moisture from deeper in the soil profile. Greater earthworm densities were observed in cotton and peanut in perennial grass cropping systems. Higher levels of plant residues and improved soil moisture conditions after perennial grasses could have resulted in the higher earthworm population densities. In a drier year, irrigation also increased earthworm population densities. The sod rotation improved soil water infiltration, probably due to the increased presence of earthworm burrows and root channels. The sod rotation overall maintained cotton yields at the same level as the conventional system, but excessive vegetative growth was observed. We are currently working on developing management systems to reduce the vegetative growth and increase yield and bring the sod rotation to its full potential. We consider sod rotations in the SE as the next important step to enhance CT and promote sustainability.


   ACKNOWLEDGMENTS
 
This study was supported in part by Cotton Incorporated, Peanut Checkoff, USDA Special Research Grants, and Northwest Florida Water Management District. Special mention is made of Brian Kidd, Iwona Jarczykowska, and Cynthia Davis-Holloway for help with conducting the field and laboratory work.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Mention of a trademark, proprietary product, or vendor does not constitute a guarantee of warranty for the product, and does not imply its approval to the exclusion of other products or vendors that may be suitable.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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Home page
 Agron. J.Home page
T. W. Katsvairo, D. L. Wright, J. J. Marois, D. L. Hartzog, K. B. Balkcom, P. P. Wiatrak, and J. R. Rich
Performance of Peanut and Cotton in a Bahiagrass Cropping System
Agron. J., August 10, 2007; 99(5): 1245 - 1251.
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