Agronomy Journal 92:1081-1085 (2000)
© 2000 American Society of Agronomy
BANANA
Effects of Mulching on Biomass, Nutrients, and Soil Water in Banana Inoculated with Nematodes
Beverly D. McIntyrea,
Paul R. Speijera,
Susan J. Rihab and
Fred Kizitoc
a International Institute for Tropical Agriculture, East and Southern Africa Center, P.O. Box 7878, Kampala, Uganda (now deceased)
b Dep. of Soil, Crop and Atmospheric Sciences, Cornell Univ., Ithaca, NY 14853 USA
c Banana Program, Kawanda Agricultural Research Institute, P.O. Box 7065, Kampala, Uganda
sjr4{at}cornell.edu
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ABSTRACT
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The objective of this study was to determine whether mulching mitigated the impact of nematodes on banana (Musa AAA) inoculated with Radopholus similis (Cobb) Thorne and Helicotylenchus multicinctus (Cobb) Golden. The study was conducted at International Institute of Tropical Agriculture's (IITA's) Sendusu station, Uganda. Treatments included mulched and bare soil with or without nematode inoculation. Mulched treatments produced over three times more biomass than bare soil treatments. This increase in biomass was likely due to improved fertility as a result of mulching, since mulched treatments had higher concentrations of soil organic C, P, and exchangeable K and Mg, and foliar K. Mulched banana took up more water from both the 0- to 0.3-m and 0.3- to 0.5-m depths than banana grown without mulch and soil water recharged more quickly in the mulched treatments as a result of increased porosity from 0- to 0.3-m depth. Nematode inoculation had little effect on biomass or foliar nutrient concentrations of P, K, Ca, and Mg in the bare soil treatments. In contrast, in the mulched treatments nematode inoculation reduced banana biomass and yield. In both cases, more root necrosis was observed in the inoculated treatments. It appears that growth was so restricted by low fertility in the bare soil treatment that nematode damage was not a limiting factor. As mulching increased soil fertility, nematode damage did appear to restrict the growth potential of banana. Our study suggests that mulching may mitigate the impact of nematodes on bananas when applied to low fertility systems.
Abbreviations: IITA, International Institute of Tropical Agriculture
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INTRODUCTION
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BANANA yield loss associated with the nematodes Radopholus similis (Cobb) Thorne and Helicotylenchus multicinctus (Cobb) Golden can exceed 50% in Uganda (Speijer et al., 1994). Yield loss is assumed to be the result of decreased functional root biomass (Gowen and Quénéhervé, 1990). Some cultural practices, such as mulching (Sikora et al., 1989) and the use of clean planting material (Kashaija et al., 1999) are reputed to mitigate the impact of nematode infestation. Obiefuna (1991) found that a mulch of crop residues was effective at decreasing nematode populations and increasing yield in plantain (Musa AAB).
Factors that could be involved in mitigating the impact of nematodes in mulched soil are (i) increased nutrient availability, (ii) increased porosity/ improved infiltration, (iii) increased surface rooting. Alternatively, increased surface rooting could have a negative impact if nematodes reside primarily in the upper layers of the soil profile, that is, a larger percentage of total root biomass could potentially be exposed to nematode activity in the event of preferential surface rooting. Our objectives in this study were to determine the effect of mulching on above- and below-ground biomass, soil and foliar nutrient levels, and soil water uptake in banana inoculated with R. similis and H. multicinctus.
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Materials and methods
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Experimental Site, Design, and Cultural Practices
The research was conducted at Sendusu field station (0.53 N, 32.58 E; 1150 m asl), IITA, Uganda. The soil at the experimental site is classified as a isohyperthermic Rhodic Kandiudalf (USDA taxonomy) with slope averaging 4% and pH ranging from 5.4 to 6.4 in the surface 0.2 m. Rainfall at the site averages 1200 mm annually and is bimodally distributed, with the first season lasting from March to June and the second season beginning in September or October and ending in late December or early January.
Treatments included two cultural practices (mulched and bare soil) with or without banana nematode (R. similis and H. multicinctus) inoculation. Banana (Musa AAA, cv. Mbwazirume) was planted in June 1993. The planting material was gathered from farms near the experimental site and selected for absence of weevil damage, pared, and disinfected for 20 min in hot water (53–55°C) (Colbran, 1967). At the time of planting, inoculation was performed by exposing the planting material to root segments treated with nematode inoculum. Inoculation was repeated again 1 yr after planting by exposing corms to material treated with nematode inoculum. Complete details of inoculum preparation and application can be found in Speijer et al. (1999). Mulch was applied to a depth of 0.1 m at the time of planting. The mulch, comprised of a mixture of chopped maize (Zea mays L.) stalks and grass (Paspalum spp.) grown outside the plots, was maintained throughout the trial. After each harvest, banana pseudostems and leaves were chopped and left on the plot in both the mulched and bare soil treatments. Mulched plots were hand-weeded, while the bare soil plots were weeded with hoes. Six-meters wide clipped-grass alleyways separated experimental blocks of the four replications. Treatment plot size was 18 by 18 m with banana mats (three successive generations grown as a production unit) spaced at 3 by 3 m. Analysis of variances (ANOVA) were performed with Minitab Release 10 (Minitab Inc., Lebanon, PA, 1996). Mention of statistical significance refers to P 
0.05.
Measurements
Air temperature, wind speed, relative humidity, and incoming solar radiation were sampled at 3 m above the ground every 5 min and hourly averages were recorded with a CR10 datalogger (Campbell Scientific, Pullman, WA) at a site situated on clipped grass at the northeastern end of the trial. Rainfall was recorded every 15 min. During the period when soil water was measured (19 Dec. 1996–19 Jan. 1998), rainfall totaled 1620 mm and potential evapotranspiration (ETo) totaled 1686 mm. Average daily maximum and minimum air temperatures were 27.3 and 17.4°C, respectively, and solar radiation averaged 16.5 MJ m-2 d-1. Reference evapotranspiration (ETo, potential ET for a well-watered grass) was calculated on a daily basis with the Penman equation. Average daily wind speed was used to calculate the aerodynamic resistance to heat and vapor transfer and the bulk surface resistance to vapor diffusion was assumed to be zero.
Profile soil water was measured in all plots from 19 Dec. 1996 through 19 Jan. 1998 with a neutron probe (Institute of Hydrology, Oxfordshire, UK). For graphic presentation, days are numbered consecutively from the beginning until the end of water measurements (Day 0–396). Two aluminum access tubes were installed 3-m apart in the middle of the plot midway between banana mats in each of the plots. Soil moisture was measured in the surface 0.3 m of the profile and every 0.2 m thereafter to a depth of 1.7 m. Measurements were made beginning at 830 h on each measurement day. Prior to and after each field measurement session, a water standard, Rw, was determined by averaging three 64-s measurements made in a water-filled drum at a depth experimentally determined to result in the highest number of counts. To give a count rate relative to the water standard, daily soil water measurements, Rs, were divided by the average Rw for the study period. For calibration, cores for gravimetric and soil bulk density determination were taken within 0.15-m radius under conditions of varying soil water status. Regressions of neutron probe relative count rates against gravimetric measurements were significant:
for the surface 0.3 m of the mulched treatments and
for all depths in the bare soil treatments and depths >0.3 m in the mulched treatments.
Canopy interception of photosynthetically active radiation (PAR) was measured in October 1997 and January 1998 with a linear PAR ceptometer (Decagon Devices Inc., Pullman, WA).1
The ceptometer is an 0.8-m hand-held probe with PAR sensors located at 1-cm intervals. Measurements were made above and below the canopy (a measurement pair) between 1300 and 1400 h. This measurement pair was used to estimate fractional light interception by the canopy
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Aboveground biomass was determined in each plot of three replications (one replication was preserved for later use) by destructively sampling four banana mats contiguous to the neutron probe access tubes. The mats selected for sampling were similar in phenological stage: the oldest plant in a mat within ±4 wk of anthesis. Total fresh weights of leaves, midribs, bunches, and pseudostems were recorded for each mat. Subsamples were weighed and oven-dried at 60°C to constant weight. Moisture content was used to calculate total dry weight. Individual mat weights were averaged to give a plot mean.
The below-ground biomass (roots and corm) of suckers and mother plants was determined in each plot of three replications for 0- to 0.3-m depth and in 0.2-m increments thereafter. A 1.5-m radius was scored in the soil around the base of the four banana mats selected for sampling. The soil was then excavated to the appropriate depth and the roots were separated from the soil by sieving. After sieving, the roots were washed with water. Free water was removed from roots by blotting with absorbent paper and fresh weights were recorded. Subsamples from each depth were removed for analysis of moisture content, root necrosis, and nematode density.
Root segments were randomly selected with a combined length of 2 m (30–40 g fresh weight) for each depth and scored for percentage root necrosis. Thus the analyzed roots were from suckers as well as mother plants. The segments were divided length-wise and the percentage necrotic cortex estimated (Speijer and de Waele, 1997). After scoring, the root segments were split into smaller pieces (0.005-m segments) and thoroughly mixed. A 5-g fresh weight subsample was used for nematode extraction, using a modified Bearmann funnel (Hooper, 1990). Nematodes were counted in three aliquots of 2 mL from a 25-mL sample. If nematode densities were high, samples were diluted to 50 or 75 mL. Nematodes were identified to species-level and the counts included all stages and sexes.
Soil and foliar nutrient status were assessed through samples taken in November 1996, May 1997, and December 1997 from all plots in each of the four replications. Soil samples from 0 to 0.2-m depth were collected from six random locations within a plot and bulked prior to subsampling for analysis. Soil samples were oven-dried at 40°C for 24 h and analyzed for organic C, exchangeable Ca, Mg, and K. Foliar samples were taken from 0.1 by 0.2 m strips on both sides of the midrib (lamina 3) on three flowering or recently flowered plants. After oven-drying at 40°C for 24 h, foliar samples were analyzed for P, K, Ca, and Mg.
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Results and discussion
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Nematode Populations and Root Necrosis
At the end of the trial (January 1999) there was no significant effect of inoculation on populations of R. similis or H. multicinctus. However, at an earlier sampling date (October 1996) treatments inoculated with nematodes did have significantly greater populations of R. similis and H. multicinctus than those treatments that were not inoculated (Speijer et al., 1998). There was no significant effect of mulching on nematode populations. Obiefuna (1991) found that mulching did suppress nematode populations in plantain (Musa AAB).
Mulching did not significantly affect banana root necrosis at any soil depth (Table 1)
. Root necrosis was significantly greater from 0- to 0.3-m depth in the inoculated treatments than in the uninoculated treatments, but root necrosis did not differ with inoculation below 0.3-m depth. However, in October 1996 necrosis was lower in the mulched treatments (Speijer et al., 1998). This earlier analysis compared necrosis in roots from suckers with necrosis in roots from flowered plants. The contrast between the analyses (aggregated root systems vs. disaggregated) suggests that treatment effects may be diluted when entire mats and consequently, root systems from a variety of plant stages are used. If, on the other hand, whole-root system analyses are a more accurate reflection of damage, this suggests that the initial prophylaxis associated with clean planting material wanes with time.
Above- and Below-Ground Biomass
There was a significant treatment effect on above-ground biomass due to mulching: the mulched treatments had almost four times as much standing biomass as the bare soil treatments (Table 2) . Pseudostems from peri-anthesis plants in the mulched treatments had significantly greater circumferences at the base of the mat (0.71 vs. 0.47 m) and at 1-m height (0.47 vs. 0.30 m). Plant height to first leaf axil on peri-anthesis plants was also significantly greater in mulched treatments (2.46 vs. 1.56 m). Mean PAR interception was 50 and 31%, respectively, in the mulched and bare soil treatments. There was no significant effect of nematode inoculation and there was no significant interaction between mulching and nematode inoculation on aboveground biomass.
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Table 2 Banana pseudostem and leaves (dry mass) at the conclusion of the trial and average banana fruit yield (fresh mass) during the trial (4.5 yr) in Sendusu, Uganda, as affected by mulching and nematode inoculation
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Rooting depth (mean = 1 m) was similar across treatments. Mulched treatments had more biomass from 0- to 0.3-m depth, but this difference was only significant for the mulched uninoculated treatment (Table 2). A similar situation was noted in the corms; the mulched treatments had more biomass, but the difference was only significant for the mulched uninoculated treatment. The percentage of total root biomass found in the surface of the profile (0–0.3-m depth) was similar for mulched (85%) and bare soil (90%) treatments. Total root biomass was strongly correlated with banana yield (r2 = 0.91). As with aboveground biomass, there was no significant effect of nematode inoculation and there was no significant interaction between mulching and nematode inoculation on below-ground biomass.
The ratio of aboveground to below-ground biomass among treatments (mean = 0.8:1) did not differ significantly, but the sum of above- and below-ground biomass was much greater (1.14 kg m-2) in the mulched treatments than in the bare soil treatments (0.34 kg m-2). These differences in biomass at the conclusion of the trial reflected differences in the banana yield over the 4.5 yr of the trial.
Banana fresh fruit biomass during the 4.5 yr of the trial averaged 14.1 and 9.7 Mg ha-1 yr-1, respectively, in the mulched uninoculated and inoculated plots, while yield averaged only 5.2 and 4.0 Mg ha-1 yr-1 in the bare uninoculated and inoculated plots (Table 2). There was a significant interaction between mulch and nematode inoculation. Yield in the mulched treatment was most likely suppressed by the presence of nematodes as indicated by the difference in root necrosis between the mulched inoculated and mulched uninoculated treatments (Table 1).
Soil and Foliar Nutrient Status
At the conclusion of the study, soil organic C, P, and exchangeable K and Mg were significantly higher in the mulched treatments than in the bare soil treatments (Table 3)
. Inoculation resulted in no significant effects on soil nutrient status. This trend was evident on all three sampling dates (Table 3).
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Table 3 Soil nutrient status in banana fields at Sendusu, Uganda, as affected by mulching and nematode inoculation
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Potassium is critical to banana production; it is taken up in greater quantities than all other nutrients combined (Turner et al., 1989). Landon (1991) cited values ranging from 0.32 to 0.74 cmolc kg-1 as adequate for exchangeable soil K. Walmsley et al. (1971) cited 0.4 cmolc kg-1 as the critical level for soil K. By either criterion, soil K was deficient in the bare soil treatments (Table 3). Foliar K was significantly higher in the mulched treatments but according to diagnostic norms established by Angeles et al. (1993), 44.9 g K kg-1 is optimal for banana production. According to this index, K was deficient in the mulched as well as the bare soil treatments (Table 4)
. If soil K was inadequate in the mulched treatments (1.13 cmolc kg-1), then the range of adequacy (0.32–0.74 cmolc kg-1) reported in Landon (1991) is inappropriate for the study soil. Potassium deficiencies in banana have been previously reported on soils with similar values (1.0–1.2 cmolc kg-1) of exchangeable K (Turner et al., 1989).
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Table 4 Banana foliar nutrient concentrations at Sendusu, Uganda, as affected by mulching and nematode inoculation
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With K fertilization, foliar K increases and foliar Ca and Mg decrease (Messing, 1974; García et al., 1980). A negative relationship was also noted in the present trial; mulch increased soil K (Table 3) and subsequently foliar K, while foliar Ca and Mg declined (Table 4). Although no significant difference among treatments was noted in soil Ca, foliar Ca was significantly lower in the mulched treatments.
A ratio of approximately 0.3 K:1 Mg in the soil is considered by some researchers to be more important nutritionally to banana than the actual soil Mg level (Delvaux, 1995). The ratio of soil K:Mg was significantly higher in the mulched treatments, but according to this criterion, none of the treatments exhibited optimal proportions (Table 3). The diagnostic norm for foliar Mg (4.1 g kg-1) established by Angeles et al. (1993) indicates that all treatments were deficient in Mg.
In summary, foliar analyses indicated that all treatments were deficient in both K and Mg. In relative terms, however, the mulched treatments were better nourished than the bare soil treatments primarily as a result of a higher soil K status. Nematode inoculation did not significantly influence the ratio of nutrients in the leaves, suggesting that nematode pressure does not create an imbalance in nutrient uptake. Rather it appears that root systems damaged by nematodes take up a reduced quantity of nutrients.
Soil Water Uptake
Water uptake patterns were similar to other crops, with the rate of decrease in soil water most rapid in the surface horizon and uptake at lower depths occurring at a slower rate and subsequent to the drying of the upper soil horizons (Fig. 1)
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Fig. 1 (a) daily rainfall and soil volumetric water content from (b) 0–0.3-m (c) 0.3–0.5-m, and (d) 0.5 to 1.5-m depths in mulched uninoculated and bare soil uninoculated treatments
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Mulching resulted in an increase in radiation interception from 31 to 50%, due to a higher leaf area index. Consequently, potential transpiration was considerably higher in the mulched vs. the bare soil treatments. When potential transpiration is low, the low demand for water results in little, if any, water uptake at lower depths. More water was removed from both the 0 to 0.3-m depth and the 0.3- to 0.5-m depth in the mulched treatment compared to the bare soil treatment (Fig. 1b,c). The increase in aboveground biomass resulted in greater demand for soil water, which was reflected in the generally lower soil water status of the mulched treatments (Fig. 1b,c).
Soil bulk density (
b) was significantly lower in the mulched treatments (1.30 Mg m-3) than in the bare soil treatments (1.47 Mg m-3) in the 0- to 0.3-m depth. At the 0.3- to 0.5-m depth, b was again lower in the mulched compared to the bare soil plots (1.34 Mg m-3 vs. 1.49 Mg m-3). Below 0.5-m depth there were no significant differences between the mulched and bare soil treatments.
The decrease in b in the mulched treatment is indicative of an increase in total porosity and could result in higher rates of water infiltration at the surface (Hillel, 1998), particularly during intense rainfall events, and consequently more rapid recharge at depth. The recharge of the mulched soil below 0.5 m before the bare soil indicates that mulching enhanced infiltration (Fig. 1d).
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Conclusions
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The mulched treatments were more productive in terms of both above- and below-ground biomass than the bare soil treatments. Though mulching improved plant nutrition, none of the treatments were optimally nourished, as evidenced by foliar Mg and K status. However, the higher soil K status and the subsequently higher foliar K status of the mulched plots appeared to be critical to biomass accumulation and improved yield.
Lower soil bulk density and faster water recharge at depth in mulched treatments indicated that mulching increased soil porosity and improved infiltration. Since plant nutrition enhanced aboveground biomass production, water uptake was greater in mulched than in bare-soil treatments.
Nematode inoculation reduced root biomass from 0- to 0.3-m depth in both the bare soil and the mulched treatments. However, the inoculated plots produced significantly more yield when mulched because the enhanced soil fertility resulted in greater growth. Additional research is needed to determine if additions of inorganic nutrients would render banana grown on bare soil more successful at coping with high populations of nematodes. More important in terms of long-term maintenance of productivity will be work addressing optimal combinations of mulch (amelioration of soil physical structure) and fertilizer (adequate nutrition) for banana.
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ACKNOWLEDGMENTS
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We are grateful to the Rockefeller Foundation, USA, for the financial support provided for this research.
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NOTES
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1 Mention of a specific trade or product name does not necessarily mean the endorsement or exclusion of other products by Cornell University. 
Received for publication January 22, 1999.
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REFERENCES
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ABSTRACT
INTRODUCTION
