319
Plant and Soil 188: 319–327, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.
Root and inorganic nitrogen distributions in sesbania fallow, natural fallow and maize fields Kindu Mekonnen, Roland J. Buresh1 and Bashir Jama International Centre for Research in Agroforestry (ICRAF), P.O. Box 30677, Nairobi, Kenya. 1 Corresponding author Received 5 March 1996. Accepted in revised form 23 August 1996
Key words: agroforestry, fallow, maize, nitrate, root distribution, root length density, root to shoot ratio, Sesbania sesban Abstract One hypothesis for a benefit of integrating trees with crops is that trees with deep root systems can capture and “pump up” nutrients from below the rooting zone of annual crops. Few studies have compared both root and nutrient distribution for planted trees, crops and grassland vegetation. A field study was conducted on a Kandiudalfic Eutrudox in the highlands of western Kenya to measure rooting characteristics and distribution of inorganic N and water in three land-use systems (LUS): (i) Sesbania sesban (L.) Merr. fallow, (ii) uncultivated natural weed fallow and (iii) unfertilized maize (Zea mays L.) monoculture. The maximum rooting depth was 1.2 m in the maize LUS, 2.25 m in a 13-month-old natural fallow, and > 4 m in a 15-month-old sesbania fallow. Total root length was 1.26 km m 2 for the maize LUS, 5.98 km m 2 for the natural fallow, and 4.56 km m 2 to 4 m for the sesbania fallow. Root length to 1.2 m was greater (p < 0.01) for natural fallow than for maize and sesbania fallow. A considerable portion of the sesbania root length to 4 m was in the subsoil; 47% was at 1.2 to 4 m and 31% was at 2.25 to 4 m. Deep rooting of sesbania coincided with lower soil water below 2 m in the sesbania fallow than the natural fallow. Nitrate-N, but not ammonium-N, to 4 m was affected by LUS. Total nitrate to 4 m was 199 kg N ha 1 for the maize LUS, 42 kg N ha 1 for the natural fallow and 51 kg N ha 1 for the sesbania fallow. Soil nitrate in the maize LUS was highest at 0.3 to 1.5-m depth on this Oxisol with anion sorption capacity. No such accumulation of subsoil nitrate was present under sesbania and natural fallow. Introduction A frequently cited benefit of agroforestry is the capture and “pumping up” of nutrients from below the rooting depth of annual crops by deep-rooted trees. Van Noordwijk (1989) also hypothesized that deep tree roots can act as a “safety net” below annual crops and intercept leaching nutrients. Nutrients captured by trees from outside the rooting zone of crops can potentially be transferred to surface soil in the form of leaf litter, roots, and prunings of tree leaves and branches (Schroth, 1995). Although spatial distribution and temporal patterns of root growth vary among tree species (Dhyani et al., 1990; Jonsson et al., 1988; Ruhigwa et al., 1992) there
is no doubt that tree roots can extend beyond the rooting depth of annual crops (Stone and Kalisz, 1991). The extent to which tree roots in deep soil layers contribute to the overall uptake of nutrients is, however, less clear (Van Rees and Comerford, 1986). De Willigen and van Noordwijk (1989) predicted higher N uptake in a deeprooted than a shallow-rooted system, using a model of N uptake with different root systems of a given total root length. Seyfried and Rao (1991) measured lower leaching losses of nutrients during 242 days from a mixed tree system than maize monoculture. Recovery of radioactive phosphorus injected at varying soil depths and distances from coffee in Kenya and Colombia, banana in Uganda, cacao in Ghana and oil palm in Cˆote D’Ivoire revealed that very little of the total P uptake by these perennials occurred from below 1-m
FAX No: +2542521001. E-mail:
[email protected]
ICPC: PIPS No.: 120544 BIO2KAP plso6158.tex; 24/04/1997; 11:19; v.6; p.1
320 depth (IAEA, 1975). In a review, Buresh (1995) concluded that the potential of trees to capture and pump up subsoil nutrients is greatest when trees have deep rooting systems and high demand for nutrients, water and/or nutrient stress occurs in the surface soil, and considerable reserves of plant-available nutrients or weatherable minerals occur in the subsoil. Short-duration planted tree fallows, especially with Sesbania sesban (L.) Merr., are a promising agroforestry alternative to traditional fallows for increasing the fertility of nutrient-depleted soils. Kwesiga and Coe (1994) showed that planted sesbania fallows of 1 to 3 year duration can increase yield of subsequent maize crops on a N-responsive soil. Hartemink et al. (1996) found lower subsoil nitrate and water in a sesbania fallow than unfertilized maize monoculture and suggested that fast-growing trees, such as sesbania, grown in rotation with cultivated annual crops can capture and recycle subsoil nitrate otherwise unavailable to shallow-rooted crops. However, they did not measure rooting; therefore they could not explain if differences in subsoil nitrate and water between sesbania fallows and other land-use systems were related to depth and distribution of roots. The objective of our study was to measure rooting depth, root distribution and distribution of inorganic N and water in the soil profile for sesbania and conventional weed fallows, as compared to maize monoculture.
Materials and methods A field experiment was carried out in western Kenya (0 060 N, 34 340 E) at an altitude of 1420 m on a very fine, kaolinitic, isohyperthermic Kandiudalfic Eutrudox. Rainfall is distributed in two crop growing seasons with an annual mean of 1800 mm. The growing season during the long rains extends from March to August, and the growing season during the short rains extends from September to January. Air-dried soil had the following characteristics in the top 15 cm: pH (1:2.5 soil/water suspension)=5.1, organic carbon = 15 g kg 1 , bicarbonate-EDTA extractable P = 2 mg kg 1 , KCl extractable Ca = 3.4 cmolc kg 1 , clay = 46%, and sand = 26%. Aluminum saturation in the top 2 m was less than 10%. Additional soil properties in the top 2 m are reported by Hartemink et al. (1996). The experiment was established in March 1993 as a randomized complete block with four replications. Treatments consisted of three land-use systems (LUS):
(i) a S. sesban fallow, (ii) an uncultivated natural weed fallow, and (iii) unfertilized maize monoculture. Plot size was 10 m by 10 m. In the first season (March to August 1993), maize was grown without fertilization in all plots. Sesbania (Kisii provenance) was direct seeded between maize rows in April 1993 to give a between-row spacing of 2.25 m and an in-row spacing of 0.4 m (11,100 plants ha 1 ). Maize was harvested on 15 August 1993. Thereafter, the sesbania plots were not cropped with maize, and natural regrowth of vegetation was allowed with no management in the natural fallow. Maize (hybrid 511 or 512) was grown at 53,330 plants ha 1 (0.75 by 0.25 m spacing) in the maize LUS for three subsequent seasons (1 September 1993 to 17 January 1994, 14 March to 4 August 1994, and 25 August 1994 to 7 January 1995). Root sampling Root sampling was conducted on a profile wall exposed from a pit dug inside each plot. Maize plots were sampled at maize maturity, between 30 July and 9 August 1994. In maize plots, the profile wall was perpendicular to two maize rows and extended from 0.37 m on the side of one row to 0.37 m on the opposite side of the adjacent row (1.5 m wide). Natural fallow plots were sampled from 1.5-m-wide profile walls-between 17 and 27 September, about 13 months after the start of the fallow. Sesbania plots were sampled between 26 October and 23 November, about 15 months after the start of the fallow. In sesbania plots, the profile wall was 10 cm from the base of a tree and perpendicular to sesbania rows; the profile wall extended from the midpoint between two sesbania rows to the center of a sesbania row (1.2 m wide). Immediately after digging the pit, plants adjacent to the profile wall were sampled. In maize plots, one maize plant from each of the two rows along the profile wall and all weeds from a 0.1 by 1.5 m area adjacent to the profile wall were harvested. In natural fallow plots, all plants from a 0.2 by 1.5 m area adjacent to the profile wall were harvested. In sesbania plots, the tree adjacent to the profile wall was harvested; and all weeds from a 0.2 by 1.2 m area adjacent to the profile wall were harvested. Harvested plants were oven dried at 70 C and weighed. Blocks of soil were collected with a metal sampler in 15-cm-deep layers to the lower extend of rooting in maize and natural fallow plots and to 4-m depth in sesbania plots. Each 15-cm-deep layer was sampled at ten adjacent 15 cm wide locations in maize and natural
plso6158.tex; 24/04/1997; 11:19; v.6; p.2
321 fallow plots and eight adjacent 15 cm wide locations in sesbania plots. The metal sampler (15 cm width by 15 cm height by 10 cm length) was hammered horizontally into the profile wall. Each soil sample was carefully retrieved from the profile wall to ensure that 2250 cm3 of soil was obtained. Each soil sample was placed in a bucket and soaked overnight to ease separation of roots from the soil. On the following day, the soaked soil was stirred and poured over a 0.5-mm sieve. Water was added to assist passage of the entire soil sample through the sieve. Roots and organic debris remaining on the sieve were placed in plastic bags with 17% acetic acid solution and stored in the refrigerator at 5 C. Roots were separated from organic debris and then sorted by plant species and into fine ( 2 mm diameter) and coarse size (> 2 mm diameter) with the assistance of hand-held magnifying lenses. Sorted roots were stored at 5 C in plastic bag with 17% acetic acid solution. Immediately before scanning, roots were stained with methyl violet solution (0.1% in 10% ethanol) and spread on a glass tray. Root length for maize LUS was determined with a Delta-T type RLS root length measurement system, using the formula of Newman (1966) when the tray contained no overlapping roots and the formula of Harris and Campbell (1989) when the tray contained overlapping roots. Root length for natural and sesbania fallows was determined with a Delta-T scan system. An image of the roots was obtained with a Hewlett Packard scanner and Aldus photostyler image analysis software at a resolution of 240 dots per inch. Root length was calculated by the formula of Newman (1966) when the tray contained no overlapping roots and by the procedure of Kirchhof (1992) when the tray contained overlapping roots. Both scanners were calibrated with reference targets of known length and diameter. After scanning, the roots were dried at 70 C for 48 hours and then immediately weighed to determine dry weight. Root length and biomass for each 15-cm-deep layer in a plot were calculated as the mean for the ten sampling locations in each maize and natural fallow plot and the mean for the eight sampling locations in each sesbania plot. Soil sampling On 22-23 November 1994, about 13 weeks after planting maize and 15 months after the start of the natural and sesbania fallows, soil was sampled to 4 m with a 7cm-diameter Edelman auger. Soil was collected from ten depths: 0 to 0.15, 0.15 to 0.3, 0.3 to 0.5, 0.5 to 1.0,
1.0 to 1.5, 1.5 to 2.0, 2.0 to 2.5, 2.5 to 3.0, 3.0 to 3.5, and 3.5 to 4.0 m. In each maize and natural fallow plot, soil was collected and composited from eight locations for layers above 1 m and from four locations for layers below 1 m. In maize plots, half the sampling locations were between maize rows and half were within rows. In each sesbania plot, the perpendicular distance from a sesbania row to the midpoint between two rows (1.12 m) was divided into three equal strata (0 to 0.37 m, 0.37 to 0.75 m, and 0.75 to 1.12 m from the sesbania row). For each stratum, soil was collected and composited from six locations for layers above 1 m and from three locations for layers below 1 m. One subsample of each soil sample was immediately dried at 105 C for 48 h in order to determine gravimetric water content. Another subsample was stored field moist at 5 C, and then analyzed for ammonium and nitrate. Soil bulk density was determined with cores collected from each depth from within a pit. The bulk density was used to convert nitrate values from mg kg 1 to kg ha 1 , and to convert gravimetric water to m3 m 3 . Nitrate, ammonium and water data for each sesbania plot were calculated as the mean for the three strata.
Analyses For determination of ammonium and nitrate, about 20 g of field moist soil was extracted with 100 mL 2 M KCl by shaking for 1 hour at 150 reciprocation per minute (30-mm-reciprocal stroke) and subsequent gravity filtering with prewashed Whatman 5 paper. Soil water content was determined on the stored field-moist soil at the time of extraction in order to calculate the dry weight of extracted soil. Ammonium in the extract was determined by a colorimetric method (Anderson and Ingram, 1993); and nitrate plus nitrite was determined by cadmium reduction (Dorich and Nelson, 1984) with subsequent colorimetric determination of nitrite (Hilsheimer and Harwig, 1976). No effort was made to separate nitrate and nitrite. Because nitrite was likely small relative to nitrate, the values were reported as nitrate for sake of simplifcation. Data for root length, root dry weight, nitrate and ammonium were tested for normality, and the data were not normally distributed. As a remedy, square root transformations were used for root data and logarithm (log to base 10) transformations were used for ammonium and nitrate data (Gomez and Gomez, 1984). The transformed data were then analyzed by the gener-
plso6158.tex; 24/04/1997; 11:19; v.6; p.3
322 al linear model procedure of the SAS Program (SAS Institute, 1989). Root length (km m 2 ) and nitrate (kg N ha 1 ) were reported in the untransformed scale for comparison of treatment means.
Results Rooting The maximum rooting depth was 1.2 m in the maize LUS, 2.25 m in the natural fallow and > 4.0 m in sesbania fallow. Weeds in the maize LUS were controlled by manual removal in the two months following maize planting, but appreciable weed growth occurred in the month before maize maturity. At maize maturity, weed roots accounted for 56% of the total root length in the maize LUS (Table 1). The maize root length measured at maturity (Table 1) likely underestimated the maximum root length because maize root length reportedly declines after the reproductive phase (Mengel and Barber, 1974; Wielser and Horst, 1993). The natural fallow had greater total root length (5.43 km m 2 ) in the top 1.2 m than the maize LUS (1.26 km m 2 ) (Table 1). Fine roots ( 2 mm diameter) comprised 99% of the root length above 1.2 m in the natural fallow. Nearly all roots in the natural fallow were above 1.2 m depth (Figure 1); only 9% of
Figure 1. Root length density for a 13-month natural fallow, a 15month sesbania fallow and maize plus weeds at maize maturity in unfertilized maize monoculture.
the total root length was below 1.2 m. Only fine roots were present below 1.2 m (Table 1). Fine roots comprised 95% of the sesbania root length above 1.2 m. Weeds comprised 6% of the total root length in the sesbania fallow above 1.2 m. Ageratum conyzoides L., Dichondra repens J. R. and G. Forst., Digitaria abyssinica (A. Rich.) Stapf and Paspalum scrobiculatum L. were predominant weed species in the three LUS. Hibiscus sp. and Guizotia scabra (Vis) Chiov. were common in only the natural fallow. Considerable sesbania roots extended below 1.2 m (Figure 1); 47% of the sesbania root length in the top 4 m was at 1.2 to 4 m depth, and 31% of the sesbania root length in the top 4 m was at 2.25 to 4 m depth (Table 1). The root length of sesbania at 2.25 to 4 m (1.36 km m 2 ) was comparable to the total measured root length in the maize LUS (1.26 km m 2 ). Coarse roots (> 2 mm diameter) of sesbania extended to 4 m, but they comprised a decreasing proportion of the root length with increasing depth. Weed roots below 2.25 m in the sesbania fallow came from guava (Psidium guajava L.), which was present in only one of the four sesbania plots. Root biomass was 28% of the total biomass in natural fallow and 23% of the total biomass in sesbania fallow. Root to shoot ratio was comparable for weeds in the natural fallow (0.41) and for sesbania (0.36) (Table 2). Variability among sesbania plots, as measured by standard deviation, was greater for aboveground biomass than for root biomass. Large standard deviations for the natural fallow are partly related to differences in plant species among field replications. The total root length was 1.26 km m 2 to a depth of 1.2 m for the maize LUS, 5.98 km m 2 to 2.25 m for the natural fallow, and 4.56 km m 2 to 4.0 m for the sesbania fallow (Table 1). At 0 to 0.3 m, root length was much greater for the natural fallow (3.82 km m 2 ) than for the maize LUS (0.77 km m 2 ) and the sesbania fallow (1.54 km m 2 ) (Table 3). Root length at 0.3 to 0.6, 0.6 to 1.2 and 0 to 1.2 m was also higher (p0.01) in the natural fallow than in the maize LUS and sesbania fallow (Table 3). The abundance of fine roots in the natural fallow was due to two major grasses, Digitaria abyssinica and Paspalum scrobiculatum. Root length at 0 to 0.3, 0.3 to 0.6, and 0.6 to 1.2 m was similar for the maize LUS and the sesbania fallow, but at 0 to 1.2 m root length was greater (p 0.05) for sesbania fallow than for the maize LUS (Table 3). Root length at 1.2 to 2.25 m was high for the sesbania fallow
plso6158.tex; 24/04/1997; 11:19; v.6; p.4
323 Table 1. Root distribution in three land-use systems Land-use systema
Depth (m)
Root source
Root length (km m Mean SDb
Maize
0 to 1.2
Maize Weeds
0.56 0.70
0.08 0.12
Natural fallow
0 to 1.2
Weeds 2 mm Weeds > 2 mm
5.39 0.04
0.78 0.02
1.2 to 2.25
Weeds 2 mm Weeds > 2 mm
0.54 0
0.22 0
0 to 1.2
Sesbania 2 mm Sesbania > 2 mm Weeds
2.17 0.11 0.15
0.59 0.04 0.12
1.2 to 2.25
Sesbania 2mm Sesbania > 2 mm Weeds
0.69 0.01 0.04
0.29 0.01 0.06
2.25 to 4.0
Sesbania 2 mm Sesbania > 2mm Weeds
1.35 0.01 0.04
0.39 0.01 0.07
Sesbania fallow
2)
a The maize land-use system was sampled at maize maturity; the natural fallow was sampled 13 months after establishment; and the sesbania fallow was sampled 19 months after relay sowing of sesbania with maize and 15 months after harvest of the maize. b SD=Standard deviation. Table 2. Effect of land-use system on aboveground and root biomass Land-use system
Plant
Aboveground biomass (g m 2 ) Mean SDa
Root biomass (g m Mean SD
Natural fallow
Weeds
693
202
266
127
0.41
0.23
Sesbania fallow
Sesbania Weeds
3799 65
1908 27
1092 64
126 70
0.36 0.83
0.22 0.57
2)
Root to shoot ratio Mean SD
a SD=Standard deviation. Table 3. Effect of land-use system on root length to 1.2-m depth Soil depth (m)
Mean root length (km m 2 )
Differences in root length (km m 2 )a Sesbania vs. Natural fallow Sesbania vs. maize vs. maize natural fallow
0 to 0.3 0.3 to 0.6 0.6 to 1.2 0 to 1.2
2.04 0.48 0.52 3.04
0.77 0.09 0.31 1.17
3.05 0.47 0.66 4.18
2.28 0.38 0.35 3.01
, Significant at 0.05 and 0.01 probability levels, respectively. a Analysis of variance was conducted on square root transformed data.
plso6158.tex; 24/04/1997; 11:19; v.6; p.5
324 Table 4. Soil nitrate and water to 4-m depth in three land-use systems Soil depth (m)
0 to 0.5 0.5 to 1.0 1.0 to 1.5 1.5 to 2.0 2.0 to 3.0 3.0 to 4.0 0 to 4.0
Mean nitrate-N (kg ha 1 )
20 23 20 12 14 10 98
Differences in nitrate-N (kg ha 1 )a Sesbania Natural Sesbania vs. fallow vs. vs. natural maize maize fallow -24 -44 -36 -18 -16 -10 -148
-31 -47 -39 -18 -14 -8 -157
7.1 3.1 3.1 -0.2 -1.6 -2.6 8.9
Mean water (m3 m
3)
0.36 0.43 0.40 0.39 0.34 0.30 0.35
Differences in water (m3 m 3 ) Sesbania Natural Sesbania vs. fallow vs. vs. natural maize maize fallow 0.018 -0.008 0.000 0.020 -0.008 -0.040 -0.010
0.020 0.005 0.020 0.028 0.028 0.028 0.023
-0.003 -0.013 -0.020 -0.008 -0.035 -0.068 -0.033
, Significant at 0.05 and 0.01 probability levels, respectively. a Analysis of variance was conducted on log transformed data.
in the top 0.5 m was greater (p0.01) in the sesbania fallow than the natural fallow. Ammonium-N throughout the soil profile, in contrast to nitrate-N, was not affected by LUS (data not shown). Ammonium ranged between 1 and 3 mg N kg 1 for the three LUS in the ten soil layers to 4 m. Ammonium-N to 4 m was 111 kg ha 1 for the maize LUS, 104 kg ha 1 for natural fallow, and 96 kg ha 1 for sesbania fallow. Soil water in the top 1 m was similar for the three LUS (Table 4). At 1.5 to 2 m, water was higher for natural fallow than for maize. At 1 to 1.5 m and 2 to 4 m, soil water was lower for sesbania than natural fallow. The lower soil water below 2 m for sesbania coincided with deeper rooting for sesbania than for other LUS (Figure 1). Figure 2. Nitrate-N in 15-month natural and sesbania fallows and at 13 weeks after planting the third maize crop in unfertilized maize monoculture.
(0.74 km m 2 ), as compared to the natural fallow (0.54 km m 2 ) (Figure 1). Inorganic-N and soil water In the soil sampling to 4-m depth, nitrate-N in the soil profile was strongly affected by LUS (Figure 2). Nitrate to 4 m was 199 kg N ha 1 for the maize system, 42 kg N ha 1 for the natural fallow, and 51 kg N ha 1 for the sesbania fallow. Nitrate-N throughout the profile was significantly greater (p0.01) for the maize LUS than sesbania and natural fallows (Table 4). Nitrate-N
Discussion Rooting was much deeper for sesbania than a natural fallow on this deep Oxisol with no chemical and physical barriers to rooting. The natural fallow had high root length density in the topsoil due to the fine roots of grasses, whereas sesbania with coarser roots in the topsoil had more than one-third of its total root length below 2 m (Figure 1). Deeper rooting of trees than grasses and the predominance of fine grass roots in surface soil are consistent with results from other studies (Atkinson, 1980; Eastham and Rose, 1990; Stone and Kalisz, 1991). The deep-rooted nature of sesbania might result in root channels that can improve the rooting depth of the subsequent crops (Van Noordwijk et al., 1991).
plso6158.tex; 24/04/1997; 11:19; v.6; p.6
325 Our observed root to shoot ratio for 19-month-old sesbania (0.36) was at the lower end of the range (0.31 to 0.87) reported by Dhyani et al. (1990) for four tree species at six months after planting. The root to shoot ratio is within the range (0.10 to 0.41) found by Toky and Bisht (1992) for 12 species of 6-year-old trees. Root to shoot ratio tended to be higher for weeds (0.41 in natural fallow and 0.83 in sesbania fallow) than sesbania (0.36). Eastham and Rose (1990) reported a root to shoot ratio of 0.32 for Eucalyptus grandis and 2.0 for tropical grasses under the eucalyptus. Because the natural fallow in our study included broad-leafed plants, like Hibiscus sp., it is not surprising that the root to shoot ratio for weeds was lower than reported for grasses. A bulge in nitrate occurred at 0.3 to 1.5-m depth in the maize LUS at the sampling (Figure 1), which coincided with 13 weeks after maize planting. An accumulation of nitrate was possible in the maize LUS because N mineralization exceeded the export of N by maize. Measurements of changes in ammonium and nitrate on this soil in the absence of plants suggest that the annual rate of N mineralization in the top 2 m is 175 kg N ha 1 (Buresh, unpubl. data). Yields of unfertilized maize in the maize LUS in the three seasons from September 1993 to January 1995 were only 0.4, 1.7, and 0.4 Mg ha 1 (Buresh, unpubl. data). Poor growth of maize partly resulted from low available soil P (bicarbonate-EDTA extractable P = 2 mg kg 1 ). Total annual export of N through removal of maize grain and biomass was about 80 kg N ha 1 , much less than the estimated annual N mineralization. A bulge in nitrate at 0.3 to 1.5 m could be further attributed to sorption of nitrate on positively charged soil surfaces. Hartemink et al. (1996) reported that about 60% of the nitrate at 1 to 2-m depth was sorbed on soil surfaces. Sorption of nitrate is known to delay its downward movement (Wong et al., 1987) and result in nitrate accumulation in the subsoil (Jones, 1976; Wild, 1972). In September 1993, at the start of the fallows, the total nitrate-N to 2-m depth was 118 kg N ha 1 in the maize LUS, 131 kg N ha 1 in the natural fallow and 101 kg N ha 1 in the sesbania fallow (Hartemink et al., 1996; Buresh, unpubl. data). The change in total nitrate to 2 m between September 1993 and our sampling in November 1994 (Figure 2) was +44 kg N ha 1 in the maize LUS, 107 kg N ha 1 in the natural fallow and 64 kg N ha 1 in the sesbania fallow. This reduction in nitrate under natural and sesbania fallows and the low subsoil nitrate under the fallows (Figure 2)
suggest that weeds in the natural fallow and sesbania utilized subsoil nitrate, presumably because of their high demand for N and deep rooting. The effect of the fallows on utilization of soil nitrate is likely even more dramatic than suggested by changes in nitrate between September 1993 and November 1994 because N mineralization was significantly higher (p0.05) in the natural and sesbania fallows than in maize monoculture (Maroko et al., unpubl. data). Hartemink et al. (1996) found lower (p0.05) soil water at 1.5 to 2 m for sesbania than natural fallows at 5 months after the start of the fallows in this experiment, but they did not sample below 2 m. The lower soil water at 2 to 4 m under sesbania than natural fallow (Table 4) and the deeper rooting of sesbania (Table 1) suggest that sesbania would be more effective than weeds in reducing the leaching of nitrate. In our study, the nitrate bulge occurred within the rooting zone of weeds (0.3 to 1.5 m). A natural weed fallow would likely be less effective than a sesbania fallow in capturing subsoil nitrate when appreciable nitrate accumulates below 2 m during annual crops before the fallow. Low available soil P probably limited early growth of sesbania and weeds, as well as maize. The limitation of low soil P on total plant biomass production and rooting after 13 to 15 months, however, would likely be less for perennials than an annual crop monoculture with periodic removal of weeds. Weeds and sesbania in the fallows unlike weeds and maize in the maize LUS, grew continuously from September 1993 to November 1994, enabling their roots to eventually explore a large soil volume and continuously take up nitrate and water. The amount and duration of weed growth in the maize LUS, unlike the natural fallow, were presumably insufficient for a marked influence on subsoil nitrate. Weed biomass, root length and rooting depth were much less in the maize LUS than the natural fallow because of periodic hand removal. Whereas weed biomass at root sampling was 693 g m 2 in the natural fallow (Table 2), it was only 47 g m 2 in the maize LUS (data not presented). Weed roots in the maize LUS did not extend below 1.2 m, but root length of weeds below 1.2 m in the natural fallow (54 km m 2 ) was comparable to the total root length of weeds in the maize LUS (0.70 km m 2 ) (Table 1). Nitrate-N in the topsoil was greater in the sesbania fallow than the natural fallow (Table 4). Higher topsoil nitrate in sesbania may result from faster mineralization under a N-fixing tree than under grasses (Mazzarino et al., 1991). The low and similar amounts of ammonium-N among LUS could be due to rapid
plso6158.tex; 24/04/1997; 11:19; v.6; p.7
326 conversion of ammonium to nitrate-N (nitrification) in all LUS. Although researchers have identified through modeling that capture of leaching nutrients and subsoil nutrients can be an important contribution of trees integrated with crops (Robertson, 1994; Van Noordwijk, 1989), little data have been collected to compare subsoil rooting and nutrients under trees and crops. Our one-time measurement of nitrate, water and roots confirms recent findings (Hartemink et al., 1996) of lower subsoil nitrate and water under sesbania and natural fallows than unfertilized maize monoculture. We further show that the lower subsoil nitrate and water in the fallows coincides with deeper rooting of sesbania and weeds in the natural fallow than maize.
Acknowledgements We thank R Coe for assistance on statistics, R Mutura for assistance in data handling and J Kinyangi and staff at the Maseno Agroforestry Research Centre for assistance in root sampling and analysis. Financial support for K Mekonnen was provided by a fellowship from the African Network for Agroforestry Education (ANAFE). The Swedish International Development Authority (SIDA) and the Overseas Development Organization (ODA), UK provided financial support to ICRAF that made this work possible.
References Anderson J M and Ingram J S I 1993 Tropical Soil Biology and Fertility. A Handbook of Methods. Second edition. CAB International, Wallingford, Oxon. UK. 221 p. Atkinson D 1980 The distribution and effectiveness of the roots of tree crops. In Horticultural Reviews. Ed. J Janick. pp 424–490. AVI Publishing Company, Westport, Connecticut. Buresh R J 1995 Nutrient cycling and nutrient supply in agroforestry systems. In Integrated Plant Nutrition Systems. Eds. R Dudal and R N Roy. pp 155–164. FAO fertilizer and plant nutrition bulletin no 12, Rome. De Willigen P and van Noordwijk M 1989 Rooting depth, synchronization, synlocalization and N-use effciency under humid tropical conditions. In Nutrient Management for Food Crop Production in Tropical Farming Systems. Ed. J van der Heide. pp 145–156. Institute for Soil Fertility, Haren. Dhyani S K, Narain P and Singh R K 1990 Studies on root distribution of five multipurpose tree species in Doon Valley, India. Agrofor. Syst. 12, 149–161. Dorich R A and Nelson D W 1984 Evaluation of manual cadmium reduction methods for determination of nitrate in potassium chloride extract of soil. Soil Sci. Soc, Am. J. 48, 72–75.
Eastham J and Rose C W 1990 Tree/pasture interactions at a range of tree densities in an agroforestry experiment. I. Rooting patterns. Aust. J. Agric. Res. 41, 683–695. Gomez K A and Gomez A A 1984 Statistical Procedures for Agricultural Research. Second edition. John Wiley and Sons, New York. 680 p. Harris G A and Campbell G S 1989 Automated quantification of roots using a simple image analyzer. Agron. J. 81, 935–938. Hartemink A E, Buresh R J, Bashir Jama and Janssen B H 1996 Soil nitrate and water dynamics in sesbania fallow, weed fallows, and maize. Soil Sci. Soc. Am. J. 60, 568–574. Hilsheimer R and Harwig J 1976 Colorimetric determination of nitrate from meat and other foods: An alternative colour reagent for the carcinogenic 1-naphtylamine and an improved extraction method. Can. Inst. Food Sci. Technol. J. 9, 225–227. IAEA 1975 Root Activity Patterns of Some Tree Crops. Technical Report Series No. 170. Joint FAO/IAEA Division of Atomic Energy in Food and Agriculture. International Atomic Energy Agency, Vienna. Jones M J 1976 Water movement and nitrate leaching in a Nigerian savanna soil. Exp. Agric. 12, 69–79. Jonsson K, Fidjeland L, Maghembe J A and Hogberg P 1988 The vertical distribution of fine roots of five tree species and maize in Morogoro, Tanzania. Agrofor. Syst. 6, 63–69. Kirchhof G 1992 Measurement of root length and thickness using a hand-held computer scanner. Field Crops Res. 29, 79–88. Kwesiga F and Coe R 1994 The effect of short rotation Sesbania sesban planted fallows on maize yield. For. Ecol. Manage. 64, 199–208. Mazzarino M J, Oliva L, Abril A and Acosta M 1991 Factors affecting nitrogen dynamics in a semiarid woodland (Dry Chaco, Argentina). Plant Soil 138, 85–98. Mengel D B and Barber S A 1974 Development and distribution of the corn root system under field conditions. Agron. J. 66, 341– 345. Newman E I 1966 A method for estimating the total length of root in a sample. J. Appl. Ecol. 3, 139–145. Robertson W H 1994 Modelling soil nitrogen levels under acacia/sorghum rotations. Agrofor. Syst. 27, 283–292. Ruhigwa B A, Gichuru M P, Mambani B and Tariah N M 1992 Root distribution of Acioa barteri, Alchornea cordifolia, Cassia siamea and Gmelina arborea in an acid Ultisol. Agrofor. Syst. 19, 67–78. SAS Institute 1989 SAS/STAT User Guide, Version 6, Volume 2, Fourth edition. SAS Institute Inc., Cary, NC, USA. 846 p. Schroth G 1995 Tree root characteristics as criteria for species selection and systems design in agroforestry. Agrofor. Syst. 30, 125– 143. Seyfried M S and Rao P S C 1991 Nutrient leaching loss from two contrasting cropping systems in the humid tropics. Trop. Agric. Trinidad 68, 9–18. Stone E L and Kalisz P J 1991 On the maximum extent or tree roots. For. Ecol. Manage. 46, 59–102. Toky O P and Bisht R P 1992 Observations on rooting patterns of some agroforestry trees in an arid region of north-western India. Agrofor. Syst. 18, 245–263. Van Noordwijk M 1989 Rooting depth in cropping systems in the humid tropics in relation to nutrient use efficiency. In Nutrient Management for Food Crop Production in Tropical Farming Systems. Ed. J van der Heide. pp 129–144. Institute for Soil Fertility, Haren, The Netherlands. Van Noordwijk M, Widianto, Heinen M and Hairiah Kurniatun 1991 Old tree root channels in acid soils in the humid tropics: Important
plso6158.tex; 24/04/1997; 11:19; v.6; p.8
327 for crop root penetration, water infiltration and nitrogen management. Plant Soil 134, 37–44. Van Rees K C J and Comerford N B 1986 Vertical root distribution and strontium uptake of a slash pine stand on a Florida spodosol. Soil Sci. Soc. Am. J. 50, 1042–1046. Wiesler F and Horst W J 1993 Differences among maize cultivars in the utilization of soil nitrate and the related losses of nitrate through leaching. Plant Soil 151, 193–203.
Wild A 1972 Nitrate leaching under bare fallow at a site in northern Nigeria. J. Soil Sci. 23, 315–324. Wong M T F, Wild A and Juo A S R 1987 Retarded leaching of nitrate measured in monolith lysimeters in south-east Nigeria. J. Soil Sci. 38, 511–518. Section editor: R F Huettl
plso6158.tex; 24/04/1997; 11:19; v.6; p.9
