3.1 Effects of tree fallow on soils
Figure 2 shows results for soil chemical properties from tree fallows of A. harveyi and A. versicolor at three years old just before cutting to establish the experiment to test the effects of A. harveyi and A. versicolor and management of their fallows on maize yield and soil nutrients. At that age of fallow TN of top soil at a depth of 0–20 cm did not differ significantly (p > 0.05) from that recorded in continuous cropping plots (Fig. 2-a). The TN in tree fallow ranged from 0.073% in A. harveyi to 0.11% in A. versicolor tree fallows compared to 0.06% in the continuous cropping. Though not statistically significant, the values of TN recorded in tree fallows were higher by 21.7–83.3% compared to continuous cropping plots for A. harveyi and A. versicolor, respectively. This indicates a slightly superior ability of A. versicolor fallow to increase TN as compared to A. harveyi.
Organic Carbon was significantly higher (p < 0.05) under the tree fallows compared to continuous cropping plots. In comparison with continuous cropping plots, amount of OC was higher by 41% and 56% in A. harveyi and A. versicolor tree fallows respectively (Fig. 2-b). OC ranged from 0.96% in A. harveyi to 1.06% in A. versicolor tree fallows. This was in contrast to 0.68% OC recorded in continuous cropping plots. Despite the lack of statistically significant differences (p > 0.05), soils under A. versicolor tree fallow contained 67.5% more extractable P compared to the continuous cropping plots (Fig. 2-c).
Effects of tree fallow on ECEC were not significant (p > 0.05) but it was highest (22.04 Cmol kg− 1), intermediate (18.37 Cmol kg− 1) and lowest (17.38 Cmol kg− 1) under A. versicolor fallow, A. harveyi fallow and continuous cropping plots, respectively (Fig. 2-d). This corresponds to improvement in ECEC by 5.7–26.8% as a result of tree fallow management. With the exception of Na, A. versicolor tree fallow recorded significantly (p < 0.05) highest amounts of soil cations (Ca, Mg and K) in 0–20 cm soil depth compared to the continuous cropping (Figs. 2-e, f, g and h). There was no significant interaction (p > 0.05) of the factors for all soil chemical properties assessed except for ECEC, which showed significant interaction between tree species and cutting height (Table 2). However, there were no significant 3-way interactions among the factors.
Table 2
Summary of ANOVA (p > F) testing the effects of tree species, coppice cutting height and coppice thinning on soil chemical properties for the second cropping season at Maseyu, Morogoro, Tanzania.
Source of variation
|
1df
|
TN%
|
OC%
|
P
|
ECEC
|
Ca
|
Mg
|
K
|
Na
|
Block (Blk)2
|
2
|
|
|
|
|
|
|
|
|
Tree species (Spp)
|
1
|
< 0.0001
|
< 0.0001
|
0.8415
|
0.4756
|
0.0045
|
0.0397
|
0.0562
|
0.0007
|
Blk x Spp
|
2
|
|
|
|
|
|
|
|
|
Cutting height (Cut)
|
2
|
0.6768
|
0.3577
|
0.6279
|
0.2948
|
0.9864
|
0.2873
|
0.5441
|
0.7240
|
Blk x Cut
|
4
|
|
|
|
|
|
|
|
|
Spp x Cut
|
2
|
0.4209
|
0.6450
|
0.1252
|
0.0238
|
0.6703
|
0.8297
|
0.3415
|
0.7626
|
Blk x Spp x Cut
|
4
|
|
|
|
|
|
|
|
|
Coppice thinning (Coppthin)
|
1
|
< .0001
|
< 0.0001
|
0.4864
|
0.0209
|
0.4013
|
< .0001
|
< .0001
|
< .0001
|
Blk x Coppthin
|
2
|
|
|
|
|
|
|
|
|
Spp x Coppthin
|
1
|
0.5622
|
0.0524
|
0.9537
|
0.8167
|
0.0965
|
0.1853
|
0.2797
|
0.2511
|
Blk x Spp x Cut
|
4
|
|
|
|
|
|
|
|
|
Cut x Coppthin
|
2
|
0.7373
|
0.3511
|
0.7135
|
0.3037
|
0.9196
|
0.3378
|
0.3344
|
0.4849
|
Blk x Cut x Coppthin
|
4
|
|
|
|
|
|
|
|
|
Spp*cut* Coppthin
|
4
|
0.7764
|
0.5586
|
0.5790
|
0.3522
|
0.3099
|
0.2053
|
0.3764
|
0.9521
|
Blk*Spp*cut* Coppthin
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
Residual error
|
6
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
Corrected total
|
45
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
1df = numerator degree of freedom; 2No test statistics (i.e., F-ratios and probabilities) for replication and replication-by-treatment interactions because these were specified in the error terms of the General Linear Model of SAS for testing main and interaction effects of tree species, coppice cutting height and coppice thinning. |
There were significant effects (p < 0.05) of tree species, stump cutting height and coppice thinning on the assessed soil chemical properties (Tables 2 and 3).
Table 3
Effects of tree species, coppice stump cutting height and coppice thinning on soil chemical properties during the second season of intercropping with maize at Maseyu, Morogoro, Tanzania
Treatment (Unit)†
|
%
|
%
|
mg kg− 1
|
g kg− 1
|
ECEC and bases (Cmole (+) kg− 1)
|
TN
|
OC
|
Extrac. P
|
OM
|
ECEC
|
Ca
|
Mg
|
K
|
Na
|
Tree species
|
A. harveyi
|
§ 0.07c
(0.01)
|
0.66b
(0.02)
|
5.43a
(0.07)
|
12.41a
(0.09)
|
17.37a
(0.08)
|
2.77b
(0.06)
|
1.15b
(0.03)
|
0.45b
(0.01)
|
0.20b
(0.01)
|
A. versicolor
|
0.38a
(0.02)
|
1.27a
(0.03)
|
4.84a
(0.07)
|
11.31a
(0.10)
|
17.66a
(0.08)
|
4.24a
(0.05)
|
1.46b
(0.03)
|
0.56b
(0.02)
|
0.16c
(0.01)
|
Continuous cropping
|
0.11b
(0.24)
|
0.85b
(0.41)
|
3.13a
(0.31)
|
6.6a
(0.44)
|
10.35a
(1.00)
|
2.64b
(0.46)
|
3.98a
(0.69)
|
1.37a
(0.40)
|
5.09a
(0.97)
|
Coppice stump cutting height
|
0 cm
|
0.07a
(0.01)
|
0.67a
(0.04)
|
4.67a
(0.13)
|
11.57a
(0.15)
|
18.36a
(0.14)
|
3.78a
(0.11)
|
1.42a
(0.04)
|
0.51a
(0.03)
|
0.18a
(0.01)
|
30 cm
|
0.53a
(0.01)
|
0.71a
(0.06)
|
5.38a
(0.13)
|
12.27a
(0.19)
|
18.02a
(0.14)
|
3.73a
(0.10)
|
1.16a
(0.06)
|
0.48a
(0.04)
|
0.18a
(0.02)
|
90 cm
|
0.07a
(0.01)
|
0.80a
(0.10)
|
5.55a
(0.08)
|
13.79a
(0.11)
|
16.70a
(0.25)
|
3.68a
(0.12)
|
1.38a
(0.17)
|
0.56a
(0.10)
|
0.19a
(0.24)
|
Tree fallow
|
0.07a
(0.02)
|
1.7a
(0.07)
|
5.39a
(0.30)
|
9.661a
(0.26)
|
16.43a
(0.29)
|
2.14a
(0.19)
|
3.94a
(0.09)
|
1.38a
(0.06)
|
0.18a
(0.03)
|
Continuous cropping
|
0.07a
(0.18)
|
0.54a
(0.31)
|
2.60a
(0.28)
|
5.198a
(0.44)
|
10.35a
(0.75)
|
2.64a
(0.35)
|
1.22a
(0.51)
|
0.46a
(0.30)
|
5.09a
(0.73)
|
Coppice thinning
|
Thinned
|
0.07c
(0.01)
|
0.72b
(0.02)
|
4.736a
(0.09)
|
12.43a
(0.10)
|
17.42ab
(0.10)
|
3.52a
(0.07)
|
1.31b
(0.03)
|
0.52b
(0.02)
|
0.19b
(0.01)
|
Not thinned
|
0.07c
(0.01)
|
0.73b
(0.04)
|
5.67a
(0.09)
|
12.66a
(0.12)
|
17.97a
(0.10)
|
3.95a
(0.06)
|
1.32b
(0.04)
|
0.50b
(0.03)
|
0.19b
(0.01)
|
Tree fallow
|
0.30a
(0.10)
|
1.08a
(0.20)
|
5.34a
(0.15)
|
8.95a
(0.22)
|
14.67c
(0.50)
|
2.67a
(0.23)
|
2.51a
(0.34)
|
0.90a
(0.20)
|
0.21b
(0.48)
|
Continuous maize cropping
|
0.22b
(0.23)
|
0.95ab
(0.38)
|
2.66a
(0.39)
|
6.27a
(0.64)
|
14.27c
(0.45)
|
2.13a
(0.46)
|
1.33b
(0.18)
|
0.49b
(0.15)
|
2.62a
(0.04)
|
†Means for each individual factor are averaged over all other treatments; §Mean of three replicates with standard error in parentheses; within each category means in the same column followed by the same letters are not statistically different at p < 0.05 according to DMRT. |
Significant effects of tree species on soil chemical properties were detected for TN, OC and soil cations (Ca, Mg and Na). On the other hand, coppice thinning had significant effects on TN, OC, ECEC as well as cations (Mg, K and Na). The overlaps of the effects of these two factors are revealing but it is worth to emphasize that there were no any significant interactions between these two factors.
Results showed no significant differences (p > 0.05) between the two levels of coppice thinning for TN and OC but both recorded similar and significantly lower amounts of TN and OC compared to continuous cropping as well as tree fallows. The same pattern was observed for effects of thinning on amounts of Mg, K and Na. However, the pattern was reversed for ECEC, which was significantly highest (p < 0.05) in thinned and unthinned coppices compared to tree fallow and maize cropping. The superiority of the tree fallows over both thinned and not thinned coppice plots with regard to soil TN and OC amounts is conceivable but that of continuous cropping calls for further elaboration.
3.2 Effects on maize growth and yield
For the first cropping season, ANOVA revealed 2-way (stump cutting height x coppice thinning) and 3-way (tree species x stump cutting height x coppice thinning) interactions for maize diameter growth and survival respectively. During the first cropping season of 2008, results showed significant 2-way interaction between stump cutting height and coppice thinning on maize plant diameter growth (p = 0.0443, Table 5), and 3-way interaction between coppice tree species, coppice stump cutting height and coppice thinning (p = 0.0075, Table 4) but they were not significant for yields of maize grain, cobs and stovers (Table 5). However, during the second cropping season of 2009, effects of interactions between the factors on maize growth and yield were no longer significant (Table 5).
Table 4
Summary of ANOVA (p > F) testing the effects of tree species, coppice cutting height and coppice thinning on maize plant growth and survival for the first and second cropping seasons at Maseyu, Morogoro, Tanzania.
Source of variation
|
1df
|
2008
|
2009
|
Height
|
Diameter at 10 cm
|
Arcsine transformed survival
|
Height
|
Diameter at 10 cm
|
Arcsine transformed survival
|
Block (Blk)2
|
2
|
-
|
-
|
-
|
-
|
-
|
-
|
Tree species (Spp)
|
1
|
0.0003
|
0.0513
|
0.0052
|
0.7218
|
0.9961
|
0.2064
|
Blk x Spp
|
2
|
|
|
|
|
|
|
Cutting height (Cut)
|
2
|
0.8318
|
0.7973
|
0.7960
|
0.0258
|
< .0001
|
0.3856
|
Blk x Cut
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Spp x Cut
|
2
|
0.1451
|
0.5077
|
0.7448
|
0.8196
|
0.3967
|
0.8168
|
Blk x Spp x Cut
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Coppice thinning (Copth)
|
1
|
0.6189
|
0.0453
|
0.4533
|
0.0141
|
< .0001
|
0.4791
|
Blk x Copth
|
2
|
-
|
-
|
-
|
-
|
-
|
-
|
Spp x Copth
|
1
|
0.7613
|
0.6056
|
0.4533
|
0.8743
|
0.4550
|
0.6068
|
Blk x Spp x Cut
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Cut x Copth
|
2
|
0.7613
|
0.0443
|
0.1386
|
0.6791
|
0.7773
|
0.0528
|
Blk x Cut x Copth
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Spp*cut*coppthin
|
4
|
0.8258
|
0.6461
|
0.0075
|
0.8151
|
0.8954
|
0.4756
|
Blk*Spp*cut*coppthin
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Residual error
|
5
|
-
|
-
|
-
|
-
|
-
|
-
|
Corrected total
|
44
|
-
|
-
|
-
|
-
|
-
|
-
|
1df = Degree of freedom, 2No test statistics (i.e. F-ratios and probabilities) for replication and replication-by-treatment interactions because these were specified in the error terms of the General Linear Model of SAS for testing main and interaction effects of tree species, coppice cutting height and coppice thinning. |
Table 5
Summary of ANOVA (p > F) testing the effects of tree species, coppice cutting height and coppice thinning on maize grain, stover and cob yields in Mg ha− 1for the first and second cropping seasons at Maseyu, Morogoro, Tanzania
Source of variation
|
df1
|
2008
|
2009*
|
Grain
|
Stover
|
Cobs
|
Grain
|
Stover
|
Cobs
|
Block (Blk)2
|
2
|
-
|
-
|
-
|
-
|
-
|
-
|
Tree species (Spp)
|
1
|
0.0008
|
0.0070
|
0.0163
|
-
|
0.7211
|
-
|
Blk x Spp
|
2
|
-
|
-
|
-
|
-
|
-
|
-
|
Cutting height (Cut)
|
2
|
0.2943
|
0.5423
|
0.1283
|
-
|
< .0001
|
-
|
Blk x Cut
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Spp x Cut
|
2
|
0.2649
|
0.8089
|
0.6408
|
-
|
0.3908
|
-
|
Blk x Spp x Cut
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Coppice thinning (Copth)
|
1
|
0.2414
|
0.8733
|
0.4827
|
-
|
< .0001
|
-
|
Blk x Copth
|
2
|
-
|
-
|
-
|
-
|
-
|
-
|
Spp x Copth
|
1
|
0.6832
|
0.4421
|
0.7777
|
-
|
0.4651
|
-
|
Blk x Spp x Cut
|
4
|
|
|
|
|
|
|
Cut x Copth
|
2
|
0.3975
|
0.2044
|
0.8817
|
-
|
0.7513
|
-
|
Blk x Cut x Copth
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Spp*cut*coppthin
|
4
|
0.0919
|
0.4070
|
0.5921
|
-
|
0.9417
|
-
|
Blk*Spp*cut*coppthin
|
4
|
-
|
-
|
-
|
-
|
-
|
-
|
Residual error
|
5
|
-
|
-
|
-
|
-
|
-
|
-
|
Corrected total
|
44
|
-
|
-
|
-
|
-
|
-
|
-
|
1df = Degree of freedom, 2No test statistics (i.e., F-ratios and probabilities) for replication and replication-by-treatment interactions because these were specified in the error terms of the General Linear Model of SAS for testing main and interaction effects of tree species, coppice cutting height and coppice thinning. *Grain and cobs were not produced in the second cropping season (2009). |
During the first cropping season, ANOVA revealed significant main effects (p < 0.05) of coppice tree species on all of the assessed maize growth and yield variables with the exception of maize plant diameter but this trend was reversed in the second cropping season (Tables 4 and 5). Figure 3 shows the main effects of coppice tree species on maize growth and yield variables for the first and second cropping seasons.
During the first cropping season, yields of grain (1.26 Mg ha− 1), cobs (0.3 Mg ha− 1) and stover (2.43 Mg ha− 1) in maize intercropped with A. versicolor were significantly (p < 0.05) higher compared to that of maize intercropped with A. harveyi as well as continuous cropping treatment (Fig. 3). Corresponding values for continuous cropping treatment were 0.29 Mg ha− 1, 0.06 Mg ha− 1 and 1.06 Mg ha− 1 for yield of maize grain, cobs and stover respectively. This is equivalent to yield gain in maize grain, cobs and stover by 334.5%, 129.2% and 400% relative to continuous cropping treatment as a result of first season intercropping with A. versicolor coppices respectively. The analogous increase in yield, relative to continuous cropping treatment, due to intercropping with A. harveyi coppices were 155.2%, 40.6% and 233%. The results indicate superiority of A. versicolor coppices over that of A. harveyi in improving yields of the intercropped maize. However, this trend was reversed in the second cropping season (Fig. 3) where, though not statistically significant, intercropping with coppices of any of the studied tree species tended to suppress maize growth and yield.
During the second cropping season, there was no maize grain in any of the treatments due to sporadic rainfall. Despite the fact that there were no statistically significant differences, coppices of both tree species tended to suppress maize growth and yield in the second cropping season and the effect was similar for both tree species. Yields of maize stover, height and diameter growth were lower in intercropped maize relative to continuous cropping by 98–98.7%, 14.8–15.3% and 46.4–81.0% respectively.
Coppice stump height and coppice thinning had no significant effects (p > 0.5) on growth and yield of intercropped maize during the first cropping season, whereas their effects became significant (p < 0.05) in the second cropping season (Table 5). Effects of coppice stump cutting height on growth and yield of intercropped maize for two consecutive cropping seasons are presented (Fig. 4).
Though not significant, during the first cropping season, maize yields and growth tended to be higher in maize intercropped with coppices compared to continuous cropping treatment. The general trend was highest maize grain yield in coppices grown from stumps cut at the ground level. A similar pattern was observed for maize cob yields, whereas the pattern for stover yields was not clearly defined being highest in stumps cut at 90 cm above the ground, intermediate in stumps cut at the ground level and lowest in sumps cut at 30 cm from the ground level. During the first cropping season, maize grain yield ranged from 0.98 Mg ha− 1 for coppice stumps cut at 90 cm from the ground to 1.25 Mg ha− 1 for stumps cut at the ground level. This is in contrast to maize grain yield of 0.29 Mg ha− 1 recorded in continuous cropping treatment. These results translate into an increase of maize grain yields ranging from 237.9% for maize intercropped with coppices from stumps cut at 90 cm from the ground to 331% for stumps cut at the ground level.
During the second cropping season, growth and yield of intercropped maize were reduced compared to the first cropping season but similar for all coppice stump height treatments; whereas they become significantly (p < 0.05) lower compared to continuous cropping treatment (Fig. 4). In that season, stover yields ranged from 0.02 Mg ha− 1 for both stumps cut at 30 cm and 90 cm from the ground to 0.04 Mg ha− 1 for stumps cut at the ground level. This was in comparison to stover yield of 1.04 Mg ha− 1 in continuous cropping treatment.
Figure 5 shows effects of coppice thinning on growth and yield of intercropped maize for two successive cropping seasons. During the first cropping season, maize grain yield for maize intercropped with thinned coppice treatment (0.97 Mg ha− 1) was slightly lower than that from no coppice thinning treatment (1.17 Mg ha− 1) but about twice as much as grain yield of 0.29 Mg ha− 1 from continuous cropping treatment (Fig. 5). A similar pattern was observed for cobs and stover yields as well as maize plant growth. The similarities in maize growth and yield between coppice thinning and no thinning treatments continued during the second cropping season but became significantly (p < 0.05) lower than continuous cropping treatment (Table 5; Fig. 5).
In the second cropping season, maize stover yield was reduced in coppice thinning and no coppice thinning treatments at exactly the same rate of about 85% to 0.3 Mg ha− 1 compared to stover yield of 2.03–2.09 Mg ha− 1 in first cropping season. This was significantly (p < 0.05) lower than 1.04 Mg ha− 1 recorded in continuous cropping treatment.