Effects of different methods of soil tillage and biomass application on crop yields and soil properties in agroforestry with high tree competition

Agriculture Ecosystems & Enwronment ELSEVIER

Agriculture, Ecosystems and Environment 52 ( 1995 ) 129-140

Effects of different methods of soil tillage and biomass application on crop yields and soil properties in agroforestry with high tree competition G~3tz Schroth a'*, Ninkabou Poidy b, Thomas Morsh~iuser a, Wolfgang Zech a "Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany blnstitut National des Cultures Vivrikres, BP 2318, Lomd-Cacaveli, Togo Accepted 13 July 1994

Abstract

Root competition between trees and crops is a major problem of agroforestry associations, especially with fast-growing tree species. This has also been observed in an agroforestry system with rows of Senna siamea (syn. Cassia siamea) trees and annual crops on a shallow Ferric Acrisol in the subhumid savanna of central Togo. The possibility of reducing competition by different methods of soil tillage was studied with maize and cowpeas as associated crops. Treatments were traditional hoe-ridging or level ploughing with an ox-plough. Cajanus cajan pmnings were applied either as mulch or as green manure. Although the method of biomass application had no effect on crop yields, ploughing significantly increased maize yields compared with hoe-ridging. However, this effect was independent of the intensity of tree competition. Tree competition affected grain yields per maize cob, and tillage method influenced cob number per row. Ridging did not protect the crop root systems from tree root competition, which has thus to be alleviated by appropriate species selection and systems design. In the ploughed treatments, soil fertility declined rapidly during the experiment, probably because of increased soil erosion, surface runoff and mineralisation of soil organic matter, leading to losses of P and (non-significant) C, N and basic cations. Soil acidification was accelerated accordingly. This indicates a trade-offbetween short-term yield improvement and medium-term degradation of soil fertility by level ploughing compared with the traditional soil tillage method. Ridging seems better suited than level ploughing for these sites. For reasons of soil protection and labour economy, mulching is preferable to green manuring. Keywords: Competition; Green manure; Mineral nutrition; Mulch; Root distribution; Senna siamea; Tillage method

1. Introduction

In agroforestry, trees are often assumed to improve not only the economic output of a cropping system, but also its sustainability by exerting positive effects on soil fertility, hydrology and microclimate. However, simultaneous cultivation of woody perennials and annual crops on the same unit of land also implies * Corresponding author. 0167-8809/95 / $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIOI 6 7 - 8 8 0 9 ( 9 4 ) 0 0 5 4 5 - 1

automatically the danger of a unilateral or reciprocal impairment of the system components by competition (Anderson and Sinclair, 1993). Several authors have drawn the attention to this problem in agroforestry systems, which include associations with different woody species, with trees as well as with regularly pruned hedges, and in the dry as well as the wet tropics (Ssekabembe, 1985; Ghosh et al., 1987; Singh et al., 1989; Malik and Sharma, 1990; Lal, 1991; Szott et al., 1991 ). A solution to the problem of competition may thus

130

G. Schroth et al. / Agriculture, Ecosystems and Environment 52 (1995) 129-140

become essential to the future of permanent associations of trees and crops in agroforestry. Intensive competition between rows of Senna siamea (Lam.) Irwin & Barneby (syn. Cassia siamea Lam. ) trees and crops on adjacent fields has been observed in central Togo (Zech, 1988) and gave the incentive to this research. Competition effects between trees and crops include shoot competition for light and root competition for water and mineral nutrients. Shoot competition is relatively easy to control by pruning or thinning the trees. Root competition has been identified as a problem by many authors (Singh et al., 1989; Lal, 1991; Ong et al., 1991; Rao et al., 1991; Szott et al., 1991). Several studies on tree root distribution have been motivated by the possibility of root competition between trees and crops in agroforestry (Jonsson et al., 1988; Dhyani et al., 1990; Ruhigwa et al., 1992). Unfortunately, some fast-growing trees, which are demanded by farmers, possess particularly aggressive root systems, for example eucalypts (Ghosh et al., 1987; Malik and Sharma, 1990) and Senna siamea (Von Maydell, 1986). Root competition is most likely to occur on shallow soils, which cover vast areas in the West African savanna zone (Kessler and Breman, 1991 ). In border-line plantings with root-intensive tree species, the reduction of tree root competition may be attempted by digging trenches between the trees and the fields (Singh, 1987), but this involves a major amount of work and may reduce the acceptability of tree planting by farmers. Also, exclusion of tree roots from the fields negates their assumed positive effects like reduction of leaching losses and addition of organic matter to the soil through root turnover. Alternatively, intensive soil tillage can be used to destroy tree roots in the topsoil before sowing the crops. Pietrowicz (1983) reduced yield depressions around Grevillea trees from 50% to less than 10% by deep hoeing. Yield depressions in agroforestry with zerotillage have been interpreted as an effect of tree root competition (Ssekabembe, 1985). The present research was conducted in the savanna region of central Togo, where two tillage systems predominate: traditional ridging of the soil with a handhoe ('daba') and mechanised tillage with ox-plough and herse, which creates a fiat seedbed. Although the disturbance of the soil by ploughing is often slightly deeper than by hoe-ridging, the latter technique may have the advantage of creating a refuge for the crop

roots in the ridges, unless these are invaded by upwardgrowing tree roots. In this study, the two techniques were compared under agroforestry conditions with a root-intensive tree species (Senna siamea) in order to see which of these techniques better reduced root competition. In agroforestry, nutrient-rich biomass is often applied to the fields to improve soil fertility and substitute for some mineral fertiliser. This may be done either by distributing the biomass on the soil surface ( 'mulch' ), or by mixing it with the topsoil during the tillage ('green manure') (Wilson et al., 1986). The method of biomass application may influence the effects of tree roots on crops by affecting the efficiency of nutrient utilisation by the crops (Wilson et al., 1986; Kang and Mulongoy, 1992) and eventually the growth and distribution of roots in the soil, like the concentration of roots under a mulch layer (Schroth et al., 1992). This aspect was also studied. Soil tillage and biomass application may both affect organic matter mineralisation, soil erosion and crop growth. Hence, the development of soil properties and their effects on the nutrient supply to the crops were also measured during this experiment.

2. Study site and methods 2. I. Location and climate

The trial was conducted near Kazaboua in the subhumid savanna of Central Togo (l°5'E, 8°26'N, approximately 300 m above sea level). Mean annual rainfall between 1980 and 1991 was 1157 ram. The rainy season is unimodal and lasts from April to October. In the experimental years 1990 and 1991 the rainfall was 1136 mm and 1407 mm, respectively. The experiments were located on the western slope of a low hill with approximately 3% inclination. The soils are typically Ferric Acrisols according to FAO/ Unesco (1988). Table 1 gives the topsoil characteristics (0-15 cm) of the experimental plots. The sandyloamy topsoil of the site is moderately acid and very low in total C and N. The available P contents are low to medium, the K contents medium to high and the Mg and Ca contents are both high according to the classification for the southern United States (Jones et al., 1974; Cope and Evans, 1985). The Zn levels are well

G. Schroth et al. / Agriculture, Ecosystems and Environment 52 (1995) 129-140

131

Table 1 Fertility parameters of the topsoils (0-15 cm) at the beginning (1990) and at the end (1991) of the experiment with F-tests of the threefactorial ANOVA. Effects of biomass treaments and second-order interactions were non-significantin all cases. For interactions of main plot treatments see text Treatmenta Year Sand Silt Clay pH pH C N P K Mg Ca Zn (%) (%) (%) (H20) (KCI) (mgg -1) (mgg -I) (mgkg -1) (mgkg -I) (mgkg -l) (mgkg -l) (mgkg -l) P/M P/G D/M D/G Tillage Year T×Y

1990 1991 1990 1991 1990 1991 1990 1991

72.2 73.5 73.6 73.4 75.7 78.3 76.4 77.9 NS * NS

9.4 18.5 9.3 17.3 I1.1 15.3 11.2 15.4 12.0 12.3 12.0 9.8 10.6 13.0 9.8 12.3 NS NS NS NS NS NS

6.20 5.76 6.16 5.73 6.25 6.03 6.25 6.05 * ** *

5.19 4.96 5.10 4.95 5.38 5.39 5.36 5.34 ** NS NS

8.27 7.69 7.76 7.06 8.28 7.98 8.46 9.20 ** NS NS

0.569 0.558 0.540 0.509 0.602 0.604 0.617 0.679 ** NS NS

9.8 9.2 14.5 8.7 16.3 19.1 17.4 23.7 NS NS *

67.5 39.0 51.7 32.7 70.1 43.2 77.5 42.3 * ** NS

80.0 63.2 75.9 66.8 84.3 69.7 85.0 81.9 * ** NS

511 426 457 382 568 564 549 594 ** NS NS

0.43 0.33 0.44 0.37 0.61 0.58 0.55 0.60 ** NS NS

p, plough; D, daba; M, mulch; G, green manure. * P<0.05; **P<0.01; NS, not significant. a

below the critical value for maize in all treatments (0.8 mg kg - t ) (Council on Soil Testing and Plant Analysis, 1980). These classifications only approximate the real soil fertility, because calibrations of the extraction method with yield levels have not been made on this site. The effective cation exchange capacity is 3.7 cmol+c kg - j . A sandy-clayey, usually hardened subsoil with pH values (0.1 M KC1) mostly less than 5 and with only traces of available P restricts root growth at a depth of between 10 and 70 cm, with 20 to 40 cm being the most common. Considering the low organic matter and N contents of the site in combination with the shallowness o f the topsoils, the initial soil fertility has to be classified as low. The experimental plots were located on three field strips which were 24 m wide in the downslope ( E - W ) direction. The preliminary experiment was conducted on one field strip, and the main experiment on two adjacent field strips. The fields were framed on both sides by double-rows of trees which had been planted in 1984 on the contour lines to serve primarily as erosion barriers and for wood production ( Egger, 198 6). Very strong depressions in crop biomass and yield were observed on the field strips between the trees. The dominating tree species was Senna siamea ( L a m . ) Irwin & Barneby (Caesalpiniaceae). Within each double-row, the distance between the tree rows was 2 m, and the distance between the trees within a row was also 2 m. The double-rows of trees were framed on both sides by regularly pruned

hedges of Leucaena leucocephala (Lam.) de Wit. Fig. 1 gives the layout of one half of the main experiment. In September 1991, at the end of the trial, the average height of the Senna trees was 12.9 m, and the average circumference at breast height ( 1.3 m) was 61 cm. The radius of the crown projection of the trees was approximately 3 m. 2.2. Preliminary trial on tree root competition To obtain an estimate of the contribution of tree root competition to the observed competition effects, a preliminary trial was made in 1988 on the western border of one field, consisting of six plots of 7 m length in the direction of the tree rows and 5.6 m width perpendicularly to the tree rows. The western limit of the plots was formed by a Leucaena hedge, which was 3 m from the nearest Senna row. Thus, tree competition decreased from one side of the plots to the other. On 27 May, the plots were sown with alternate rows of maize and pigeon pea (Cajanus cajan (L.) Millsp.) at 80 cm distance between the rows and 40 and 30 cm, respectively, within the rows, following level ploughing with an ox-plough. Three weeks after sowing, the roots of the adjacent tree row were trenched in every second plot ( n = 3 ) by digging a U-shaped trench between the first crop row and the Leucaena hedge, with the hardened subsoil as the lower limit at about 50 cm depth. The trenches were closed to avoid distur-

G. Schroth et al. / Agriculture, Ecosystems and Environment 52 (1995) 129-140

132

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bance to soil hydrology. During the cropping season, the tree roots penetrating the trench were cut with a hoe every 2 weeks to a depth of 18 cm. The water tensions were measured from July to September at 10 cm soil depth with tensiometers in all plots at 4 and 5.6 m distance from the trees. Fertilisation was 103 kg h a of N P K ( 1 5 - 1 5 - 1 5 ) and 26 kg ha-~ of urea. On 20 September, the maize was harvested. The maize rows at 3.8, 5.4, 7.0 and 8.6 m from the Senna row were weighed separately and subsamples dried at 80°C to constant weight.

2.3. Main experiment: plots and treatments In 1990, four plots were placed along the trees on each side of two fields between the double-rows of trees, giving a total of 16 plots. Of each plot, one side was formed by the Leucaena hedge at 2.3 m average distance from the nearest tree row, and the opposite side was situated in the central part of the field (Fig. 1 ). Plot dimensions were 13 m parallel to the tree rows, of which the central 8 m were used for measurements, and 11 m perpendicularly to the tree rows, of which the first 10 m were used for measurements. The following treatments were applied at the beginning of both the 1990 and the 1991 cropping seasons. (1) Plough/mulch ( P / M ) : tillage with an oxplough 10-15 cm deep, parallel to the tree rows, followed by harrowing; the fiat soil surface was mulched

with 5.6 t ha -1 prunings of Cajanus cajan (fresh weight). (2) Plough/green manure ( P / G ) : tillage as in ( 1 ), but Cajanus prunings ploughed under. (3) Daba/mulch ( D / M ) : tillage by hand-hoe ( ' d a b a ' ) , forming ridges of 20 cm height, measured between the top of the ridge and the bottom of the furrow, parallel to the tree rows; Cajanus mulch applied on top of the ridges in the same quantity as above. (4) Daba/green manure ( D / G ) : tillage as in (3), but Cajanus prunings covered under the ridges. The prunings were obtained by cutting 1-year-old Cajanus cajan at 80 cm height. The 5.6 t ha-~ fresh weight corresponded to a dry weight (70°C) of 2.6 t h a - 1 in 1990 and 2.3 t h a - ~ in 1991, of which 0.8 t h a - l were leaves and 1.5 t h a - ~ were branches. The trial was set up as a latin square with four replications. In two further replications initially set up on the hilltop where topsoil depths were locally less than 10 cm, the trial was discontinued after the observation of an extreme within-plot heterogeneity of the stands, apparently because of very strong reactions of the crops to even small differences in soil depth.

2.4. Crops The crops were sown in rows parallel to the tree rows. The first row was 1 m from the Leucaena hedge and 3.3 m in average from the nearest tree row. To quantify the influence of the trees on the crops, the plots were divided in 1990 into two subplots of six rows each, and in 1991 into four subplots of three rows each. The average distances of the subplots in 1990 to the nearest tree row were 5.3 m (subplot I) and 10.1 m (subplot II). In 1991 these were 4.1 m (subplot Ia), 6.5 m (subplot Ib), 8.9 m (subplot IIa) and 11.3 m (subplot IIb). In 1990, cowpeas (Vigna unguiculata (L.) Walp.) were sown on 22 May at 80 cm X 25 cm after tillage and biomass application. The wide inter-row distance was used for reasons of comparability with the maize crop of 1991. Missing plants were replaced on 1 June. No fertiliser was applied. Insecticide treatments were applied on 10 and 25 July. The cowpeas were harvested in several passages during the first half of August. In 1991, maize (Zea mays L., variety Poza Rica) was sown at 80 cm × 40 cm, two plants per hill, on 27 May after tillage and biomass application. Missing

G. Schroth et al. / Agriculture, Ecosystems and Environment 52 (1995) 129-140

plants were replaced on 6 June. Fertilisation was 150 kg h a - l of NPK (15-15-15) on 14 June and 50 kg h a - 1 of urea on 17 July. The maize was harvested on 23 September. The fertile maize cobs were counted, and the grain dry weights were obtained by drying subsamples at 105°C to constant weight. Cowpea yields are given as obtained after sun-drying of the grains for several days, and maize yields at 14% water content. 2.5. Root studies

The distribution of Senna roots in the plots was investigated within 1 week after harvesting the maize. In every plot, profiles of approximately 1 m length parallel to the tree bands and at different distances to these were opened. The compacted subsoil formed the lower limit of the profiles at a depth of 20-40 cm. In the profile walls, root ends were carefully exposed and Senna roots counted. These were easily recognised in the profiles by their black colour. Most of the roots were less than 2 mm in diameter. With the applied method, the probability for a root to be counted in the profile depends on its direction of growth and its ability to resist the mechanical stress of profile preparation (Mackie-Dawson and Atkinson, 1991), thus the method gives relative rather than absolute information about root densities. To obtain more quantitative information on Senna root distribution, soil cores, 8 cm in diameter were taken from 0-10 cm depth in several distances from three single rows o f Senna trees on nearby fields. Three cores from the same distance and tree row were mixed after cutting long roots into pieces of a few cm length, and a subsample of 100--400 g was washed over a 0.5 mm sieve for root extraction, taking larger subsamples for greater distances from the trees. The method has been described by Schroth and Kolbe (1994). Senna roots were separated from other roots and organic debris under 10 X magnification without distinguishing between live and dead roots. Root length was measured after Tennant (1975), and root mass was obtained after drying at 70°C for 48 h. Carbon in the root samples was measured with a CNS-Elementar Analyser, and all root weights were converted to 45% C to correct for adhering soil particles. The soil water content was measured by drying subsamples at 105°C to constant weight, and total mass and volume of the soil sample and mass of the subsamole for root extraction were

133

used to convert root length into cm cm--3 of soil and root mass into kg ha-1 d m - i . The tree rows were characterised by measuring the height and circumference at breast height ( 1.3 m) of the nearest four to six trees. 2.6. Soil and plant analyses

Topsoil samples were taken from each main plot in May 1990 before the beginning of the tillage operations and in September 1991 after the maize harvest. In 1990, about 120 mm of rain had fallen between the onset of the rains in February and the soil sampling. For every sample, soil from nine points in a plot was mixed (three replicates from three distances to the tree bands). Sampling depth was 0-15 cm. Where the red subsoil was found at a lower depth, the sampling depth was reduced accordingly. In the ridged plots, the 1991 samples were taken from the middle between a ridge and a furrow as proposed by Moncriefet al. in James and Wells (1990), assuming that this corresponded to an average situation in the plots. After air-drying and sieving of the samples to pass 2 mm, the following analyses were conducted: texture by wet sieving and aerometer method after dispersion in a Na4P207 solution (Hartge and Horn, 1989); total carbon and nitrogen gas-chromatographically with a CNanalyser; pH with a glass-electrode in distilled water and in 1 M KC1 at a soil:solution ratio of 1:2.5; available fractions of phosphorus, potassium, calcium, magnesium and zinc by extraction with 0.05 N HCI and 0.025 N H2SO 4 (Mehlich I) at a soil : solution ratio of 1:4 and 5 min shaking time (Council on Soil Testing and Plant Analysis, 1980; Olsen and Sommers, 1982), followed by photometric measurement of P and measurement of cations by atomic absorption spectrometry. The extraction method is recommended for acid, highly weathered, relatively low cation exchange capacity soils (Cope and Evans, 1985). Maize leaf samples were collected at the silking stage in the last week of July 1991. Only subplots Ia and IIb were considered. From every subplot, the ear leaves of six plants were taken, washed in tap water, dried at 70°C and ground for analysis. N was measured gaschromatographically with a CN-analyser. For the other elements, subsamples were dry-ashed at 560°C for 14 h and the ash dissolved in 10% HCI. P and cations were measured as indicated above.

G. Schroth et al. /Agriculture, Ecosystems and Environment 52 (1995) 129-140

134

Table 2 Analysis of variance table for maize yield data Source of variation

d.f.

Subplots Main plots (MP) Rows Columns MP-treatments Tillage (T) Biomass (B) T×B Error a Distance (D) Error b D× T D× B DXTXB Error c

63 15 3 3 3 1 1 1 6 3 9 3 3 3 27

F

F (5%)

showed that the difference between the main plots had changed during the experiment. F (1%)

3. Results and discussion 3.1. Contribution of tree roots to total competition 6.265* 0.002 0.011

5.99 5.99 5.99

13.75 13.75 13.75

38.14"*

3.86

6.99

0.650 0.220 0.394

2.96 2.96 2.96

4.60 4.60 4.60

Fig. 2 shows an average increase of maize yield of 65% in the rows between 5.4 and 8.6 m from the nearest Senna row following tree root trenching in the preliminary trial. At 3.8 m from the tree row, many plants had been lost to seed predators hiding under the trees, and the maize had also been shaded by the Leucaena hedge during part of its development. Because of few repli-

1000

For the calculation of nutrient additions to and losses from the fields, nutrient concentrations were taken from Pieri (1989) for maize, Minist~re de la Coop6ration et du D6veloppement ( 1991) for cowpeas and Schroth et al. (1992) for Cajanus prunings. For the prunings, the leaf/branch ratio of 1 : 1.88 from 1991 was used for both years.

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Yield data were analysed by three-factorial ANOVA followed by F-test, using a latin square design for the main plots and a split-block design for the subplots as described in Little and Hills (1978) (Table 2). With significant F-tests, mean separations were made by least significant difference tests. The cowpea yields were log-transformed because of the strong correlation between means and standard errors. The same design was used for the combined soil data of 1990 and 1991, considering years as subplots in a split-block scheme (Steel and Torrie, 1980). An additional computation was made for the 1990 data alone to check for initial differences between treatments. Such 'pseudo-treatment effects' were found for some chemical soil parameters as explained below. For this reason, significant main plot effects had to be considered as the result of initial soil heterogeneity, unless a significant interaction of these treatments with time

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23

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cations and high variability, the trenching effect was not significant. Root trenching led to a reduction of water tensions in the field at 4 m from the trees, particularly when short dry spells caused them to rise (Fig. 3), and a similar trend could still be observed at 5.6 m distance (data not shown). In the trenched plots, the maize yields were practically constant at distances greater than 5.4 m from the trees. This indicates strong reduction of tree competition by trenching and supports the common assumption of the high competitiveness of the Senna root system (Van Maydell, 1986). Positive yield responses to trenching in agroforestry associations have been reported earlier (Singh et al., 1989; Ong et al., 1991; Rao et al., 1991). However, a limitation of the present trial as well as of several of the cited experiments was that the trenching was carried out when the tree roots were already established in the plots; thus the effects of eliminating root competition and some nutrient release from the decomposing tree roots were confounded. The results can thus not be used to quantify accurately root competition. In contrast, the reduction of soil water tensions in the trenched field plots gave clear evidence for the utilisation of soil resources in the fields by tree roots. However, the low water tensions during most of the time made it unlikely that water competition was the main reason for maize yield reductions in the untrenched plots.

3.2. Senna root distribution and effects of main plot treatments At the end of the 1991 cropping season the roots of Senna had reached subplot IIb in all main plots. In most cases, root densities in the topsoils lay between 3 and

20 roots dm-2 at 13 m from the tree row, i.e. beyond the limit of the plots. By soil coring, roots were even found at low quantities until 20-30 m from the tree rows, i.e. at more than twice the tree height (Fig. 4). The decrease of root mass and length with increasing distance from the trees could be approximated with logarithmic relationships. It seems likely that the lateral development of the tree roots was favoured by the shallow soils. Rao et al. (1993) reported a lateral root extension of 4.5-year-old Senna siamea trees of 9 m. However, root extensions of 30 m or more have also been observed for other tree species, including the tropical genera Acacia, Adansonia and Bombax (Kessler and Breman, 1991; Stone and Kalisz, 1991). This implies a considerable area of influence around borderline plantings or isolated trees and partly explains the frequently observed nutrient enrichment under savanna trees (Kessler and Breman, 1991 ). The distribution of Senna roots within the soil profile was examined in the central part of each experimental plot, i.e. at about 7 m from the nearest tree row. Here, root densities in the topsoil lay between 6 and 20 roots d m - 2. In the hoe-ridged plots, the height of the ridges was about 13 cm when these examinations were conducted. On average, the upper 5-6 cm of the ridges and also of the plough-tilled soil were free from tree roots. Apparently this was not so much the effect of a tillage method as an adaptation to site conditions, possibly indicating reduced N-mineralisation (Pilbeam and Warren, 1994) and mobility of nutrients and increased danger of root death in the surface layer during dry spells. However, the rare observation of tree roots directly underneath the soil surface in the ridges showed that their colonisation by upward-growing

136

G. Schroth et al. / Agriculture, Ecosystems and Environment 52 (1995) 129-140

Table 3 Cowpea yields (kg ha J) during 1990 in subplots 1 and I1, at 5.3 m and 10.1 m a v e r a g e d i s t a n c e from the trees, r e s p e c t i v e l y

organic matter, stimulating root growth, or because of higher root mortality from ploughing, resulting in a higher mass of dead roots in the soil. In the first case, even a preferential ingrowth of tree roots into ridges would be expected, as these are the only tilled parts of a ridged plot. However, the profile wall method as used in this study was too insensitive to detect this.

(means+ SE)

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manure manure

111

112

15

Fig. 6. Grain weight per cob as a function of the distance from the tree row (means and standard errors).

roots was possible and that crop roots in the ridges were not generally protected from the competition of tree roots coming from outside the plots. Considerably higher root length densities and root masses were found in tilled than in untilled soil (Fig. 4), either because of higher mineralisation rates of soil

Low cowpea yields resulted from low soil fertility, absence of fertilisation corresponding to local practice and the wide inter-row distances (Table 3). The large variability between the replications indicates a marked reaction of the cowpeas to the small-scale variability of the site. The effect of distance from the trees (tree competition) was highly significant (P < 0.01), with average yields in subplot I only 22% of those in subplot II. There were no significant effects of the methods of soil tillage and biomass application. Maize yields decreased by 61% from subplot IIb to subplot Ia (P<0.01, Fig. 5). The yields showed no tendency to level off at a certain distance from the trees, suggesting that tree competition may have still affected the subplots at 11.3 m mean distance. This was in agreement with the observed tree root extension. Ploughing increased grain yields significantly ( P < 0 . 0 5 ) by 14.3% compared with hoe-ridging. In subplot IIb alone the difference was 12.7%. The method of biomass application had no effect on yields. The question if the increase of maize yields by ploughing was mediated by reduced tree root competition can be answered when looking at the mechanisms of both effects. The distance from the trees affected the grain weight per cob over the whole area of the main plots (P < 0.01 ), but with the exception of the nearest subplots (4.1 m) the cob number per row was not affected (Figs. 6 and 7). In subplot Ia, feeding of birds on seeds and shading were most intensive, and the maize plants were often stunted and sterile. The tillage method, however, only affected the cob number per row, which was significantly (P < 0.05) higher in the ploughed treatments than in the hoe-ridged treatments, but had no effect on the grain weight per cob. Neither of these parameters was affected by the method of biomass application, nor by interaction between distance from the trees and any of the main plot treatments.

G. Schroth et al. / Agriculture, Ecosystems and Environment 52 (1995) 129-140

100

I

--q--

I

d

80

;

o c

~

t

I b

P

I

II o

. ~f : i , ~ 1 ~

40

O O A A O--O A--A

20#

I

II. b

Io

do

I

-, Plough/mulch Plough/green monure Daba/mulch D(]b(]/green monure

f 0 i 2

i-5

i 4

I 5

I 6

Distonce from

I 7

I 8

i 9

the t r e e row

lP0

I-11

ll2

[ m ]

Fig. 7. Number of fertile cobs as a function of the distance from the tree row (means and standard errors).

Thus, the yield advantage of the ploughed plots over the ridged plots was not mediated by a difference in tree competition, but was an independent effect. This result is in line with the observation that ridging did not effectively protect the crops from tree root competition. A possible explanation of the tillage effect on yields is that, according to estimations at flowering, lodging of maize was higher in the ridged treatments than in the ploughed treatments, as also observed by other authors (Kowal and Stockinger, 1973; Leyenaar and Hunter, 1977; Lal, 1986). Lodging very often led to complete loss of the cobs to seed predators, like chicken and mice, or mould. Increased cob initiation because of higher nutrient release from soil organic matter after ploughing seems also possible. The absence of a yield effect of the method of biomass application is in contrast to reports of more efficient use of biomass-N from green manure than from mulch (Wilson et al., 1986), possibly because of higher N volatilisation losses from the latter (Kang and Mulongoy, 1992). However, no difference between these methods has been found in N-utilisation from labelled Leucaena prunings by maize (Xu et al., 1993 ). In view of the favourable effects of mulch on soil protection against erosion and overheating (Wilson et al., 1986) and the lower labour requirements for mulching in agriculture with low mechanisation, mulching should be preferred in situations where green manuring has no clear yield advantage.

3.4. Development of soil conditions Averaging both sampling dates, the daba treatments were superior to the ploughed treatments in C, N, Ca,

13

137

K, Mg, Zn and pH (KCI) without showing significant interactions with time (Table 1). This indicates that site heterogeneity rather than treatment effects may have been responsible for part of the observed differences. For pH (KCI), K, Ca and Zn the difference between tillage treatments was already significant at the beginning of the experiment. Uniform changes during the experiment without a significant differentiation from treatments were found for K and Mg (both decreasing) as well as soil texture. Tillage effects on soil properties were found for pH (H20), which decreased in all treatments, although significantly less in daba than in plough treatments, and P, which decreased in plough treatments and increased in ridged treatments. For C, N, Ca, Zn and pH (KCI), there was also a tendency for the traditional tillage method to have more favourable effects than ploughing, resulting in decreasing levels in the plough treatments but approximately constant or increasing levels in the daba treatments. Assuming a bulk density of the topsoils of 1.5 g cm -3, the nutrient losses during two cropping seasons amounted to 62 kg ha- ~ of K, 25 kg ha- i of Mg and, in the ploughed treatments, 180 kg ha-~ of Ca. The available P contents decreased by 7 kg hain the ploughed treatments, but increased by 10 kg hain the ridged treatments. On average over all treatments, the topsoils lost 3.8 t ha-~ of silt+clay. The biomass treatments had no detectable effect on soil properties. The observed nutrient losses from the plots were not a result of removal with harvest products because these were fully compensated by nutrient additions in mineral fertilisers and prunings, even without taking N2 fixed by the cowpeas and atmospheric inputs into account (Table 4). Seasonal variations of nutrient availability may result from equilibrations between different soil pools (Haby et al., 1990), but should be rather low in sandy kaolinitic soils. Decreasing nutrient levels can be mainly attributed to leaching, erosion and surface runoff as well as nutrient uptake by the trees bordering the plots. Leaching was facilitated by the shallow root systems of the crops on this site in combination with moderately high infiltration rates of the hardened subsoil of 15-19 mm h - ~. Erosion and surface runoff were commonly observed during heavy rains especially in the ploughed plots, whereas much of the surplus water was retained in the furrows of the daba treatments. The soil-conserving effect of ridging on slopes less than 3-3.5% is

G. Schroth et al. /Agriculture, Ecosystems and Environment 52 (1995) 129-140

138

Table 4 Nutrient additions to the system in mineral fertilisers and prunings, losses with crop harvests and nutrient storage in crop residues at soil sampling in 1991 (kg h a - t ) N

Additions Fertilisation Prunings Sum of additions Harvest losses Maize (1.7 t ha -~ ) Cowpeas a (200 kg h a - ~) Sum of losses Storage in residues Maize straw Additions-losses-storage

P

K

Ca

Mg

45.5 88.8 134.3

9.8 6.2 16.0

18.7 55.9 74.6

28.1 28.1

10.6 10.6

28.9 10.0 38.9

4.3 1.5 5.8

8.1 8.0 16.1

0.7 2.3 3.0

1.4 1.8 3.2

23.5

2.9

34.8

8.0

4.9

+71.9

+7.3

+23.7

+17.1

+2.5

a Both grains and straw were removed from the field at harvest.

well established (Fournier, 1967). Erosion is known to be an important factor in the P balance of savanna soils (Roose, 1981) and may partly explain why the surplus P from Table 4 was entirely retained in the ridged plots, whereas the ploughed treatments lost much of their available P contents. The apparent conflict between differing erosion rates in the tillage treatments and the uniformity of the textural changes may be explained with a considerable micro-erosion of fine particles into the furrows in the daba treatments, from which nutrients could be recycled by crop roots, whereas the eroded material was entirely lost from the ploughed plots. By sampling the topsoil at the side of the ridges, the fine particle fractions and probably other fertility parameters may have been slightly underestimated in the ridged plots. Apart from increased erosion, ploughing may have accelerated cation losses and soil acidification by increasing mineralisation and thus nitrate formation because of the tillage of the whole topsoil compared to

only half of the topsoil in the ridged plots. Nitrate leaching is closely associated to losses of Ca and Mg in sandy savanna soils (Pied, 1989). 3.5. Competition and treatment effects on maize nutrition

Despite the application of considerable nutrient quantities in mineral fertiliser and prunings (Table 4), the foliar nutrient concentrations of maize indicated deficiencies of N, P and Zn in comparison with literature values (Lou6, 1987; Jones et al., 1990) (Table 5). V-shaped chloroses and necroses on older leaves confirmed N-deficiency (Bergmann, 1988). This reflects the high danger of nutrient losses by surface runoff, erosion and leaching as well as the difficult nutritional situation of the crops on these very shallow soils and stresses the need to conserve the inherent soil fertility as far as possible. Leaf nutrient concentration in subplot Ia (data not shown) were either higher or similar to those in subplot IIb, because biomass production decreased simultaneously with nutrient uptake. Only Zn decreased slightly, but significantly (P < 0.01) to an average concentration of 11 mg kg-~ in subplot Ia, without significant interactions with the main plot treatments. This may indicate root competition for this element between trees and crops which was unaffected by tillage, in agreement with the results presented above. The difficulty in detecting competition for nutrients for other elements by foliar analysis may reflect multiple rather than a single mechanism of competition, including competition for water and light and possibly allelopathic effects. Leaf nutrient concentrations in subplot IIb were not affected by treatments except for K, which was significantly (P < 0.05) higher in the mulched than in the green manured plots. The reason is not clear. If the

Table 5 Nutrient concentrations in the dry matter of maize ear leaves at silk taken from subplot lib. For statistical results see text Treatment

N ( m g g -J )

P ( m g g -1)

K (mgg -t)

Ca (mgg -l)

Mg (mgg -l)

Zn ( m g k g -1)

Cu (mgkg -I)

Mn ( m g k g -~ )

PM PG DM DG

18.4 16.4 17.9 17.3

1.9 2.1 2.1 2.0

24.2 21.7 23.6 22.6

2.8 2.6 3.1 2.9

1.5 1.6 1.8 1.7

12 12 12 13

17 15 15 17

33 33 35 37

G. Schroth et al. / Agriculture, Ecosystems and Environment 52 (1995) 129-140

mulch had increased the availability of fertiliser nutrients by creating more humid conditions in the surface layer of the soil, this should have also affected the uptake of P. Owing to the already high K status of the maize, better K uptake in the mulched plots did not affect crop yields.

4. Conclusions This study was conducted under conditions of heavy tree competition to which root processes have probably contributed substantially, Although indications were found for competition for water and nutrients, the actual mechanisms of competition remain unclear. The inefficiency of the tested treatments to reduce tree root competition indicates that this common problem of agroforestry associations has to be alleviated by appropriate species selection and eventually wider distances between trees. Knowledge of tree root characteristics as influenced by management and site conditions is clearly necessary for the optimisation of agroforestry systems. Ploughing increased maize yields independently of tree competition. However, the observed effects of the tillage methods on soil conditions indicate a trade-off between short-term yield improvements and mediumterm degradation of soil fertility by level ploughing compared to traditional hoe-ridging. Increased nutrient losses from ploughed plots are likely to result in yield decreases, so that the observed advantages in crop yields may be progressively lost. With regard to the inherently poor chemical and physical characteristics of the soils in question, it is clear that the conservation of soil fertility must play a dominating role in management considerations for these sites. For this, ridging techniques, either traditionally by hand-hoe or with appropriate tools drawn by oxen, are better suited than the currently recommended technique of level ploughing and should be applied. Alternatively, minimumtillage techniques as described by Charpentier et al. (1991) should be tested.

Acknowledgments The authors thank the project 'Developpement Integr6 de la R6gion Centrale du Togo' of the 'Deutsche

139

Gesellschaft fiir Technische Zusammenarbeit ( G T Z ) ' for financing this research work, and the 'Direction R6gionale du D6veloppement Rural' at Sokod6 for permission to work at the research station at Kazaboua and for the generous and uncomplicated support provided. D. Kolbe and J. Lehmann carried out most of the field work for the soil coring study. The first author is indebted to T. Kitt for his hospitality and his interest in this work. Drs. P. Drechsel, M. Kaupenjohann, two anonymous referees and an Editor-in-Chief made valuable comments on earlier versions of the text.

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