286 B. Kaur et al. Applied Soil Ecology 15 2000 283–294
incubated in 2 l air tight jars at room temperature 27–35
◦
C for 30 days. The CO
2
produced from the soil was absorbed in 0.5 N NaOH and determined titri-
metrically with 0.1 N HCl using phenolphthalein as indicator Gupta and Singh, 1981. The CO
2
evolution rates were determined at an interval of 2–5 days. Car-
bon mineralization rates were determined from CO
2
-C evolved from the soil samples. Specific respiratory ac-
tivity of soil microbial biomass carbon was estimated by dividing the CO
2
-C produced i.e. mineralizable carbon by the value of soil microbial biomass fol-
lowing Campbell et al. 1991. Mineralizable carbon was estimated from the quantity of CO
2
-C produced during 10 days incubation of the soil under laboratory
conditions. 2.4. Soil inorganic N and nitrogen mineralization
rates Inorganic N levels were determined using the
Kjeldahl distillation method. Sieved, moist soil sub-samples equivalent to 50 g oven dry weight
were extracted with 0.5 M K
2
SO
4
4 K
2
SO
4
: 1 soil. Inorganic N levels were determined in the soil fil-
trate by MgO and Devardas alloy distillation. Similar sub-samples of soil were incubated under laboratory
conditions at ambient temperature. After periods of 10, 20 and 40 days the incubated soil samples were
extracted with 0.5 M K
2
SO
4
. Net nitrogen mineral- ization rates were determined by subtracting initial
soil mineral N from final mineral N at 40 days. 2.5. Statistics
Data on soil carbon, nitrogen, microbial biomass and nitrogen mineralization rates were analyzed using
one way analysis of variance ANOVA. Least signif- icant difference LSD values at the 5 levels of sig-
nificance p0.05 and Duncan’s Multiple Range Test values DMRT were calculated following Gomez and
Gomez 1984.
3. Results
3.1. Soil carbon and nitrogen Variations in soil carbon and nitrogen with soil
depth and horizontal spatial variability distance from rows of trees are shown in Table 1. Soil carbon
and nitrogen concentration at 0–7.5 cm depth were higher than at 7.5–15 and 15–30 cm. Soil carbon and
nitrogen concentration varied significantly with soil depth p0.01 and distance from the trees p0.01
in all the treatments. Organic carbon and nitrogen accretion in the soil was significantly higher when
trees were associated with agricultural crops. Max- imum build up of organic carbon in soil occurred
under Acacia based systems followed by Populus and Eucalyptus based systems. The nitrogen content
increased by 8–46 0–7.5 cm soil depth due to integration of trees with crops; the increase being
maximum in the case of Acacia based systems. Av- eraged across the distance from the trees at 0–7.5 cm
soil depth, carbon and nitrogen concentrations were found to be higher in the case of Acacia based sys-
tems 0.57–0.63 C; 0.068–0.077 N and Populus based systems 0.41–0.59 C; 0.047–0.073 N as
compared to Eucalyptus based systems 0.38–0.54 C; 0.039–0.054 N. It is evident that the ameliora-
tive effect of trees was more conspicuous at 1 and 2 m distance from trees and in upper soil layers 0–7.5 cm
soil depth. There were no significant differences due to tree species in organic carbon and total nitrogen at
3 m distance from the trees Table 1.
3.2. Microbial biomass The levels of soil microbial biomass carbon and
nitrogen in rice–berseem cropping, tree plantations and agroforestry systems during July 1995–March
1996 are shown in Fig. 1. Soil microbial biomass C 90.56–168.0 mg g
− 1
soil and biomass N 13.16– 35.59 mg N g
− 1
soil were maximum during the summer month of March 1996. The soil microbial
biomass showed a marked decrease in July as com- pared to values in March and December p0.01.
During the major growth phase of crop plants and the trees in July, the nitrogen released by microbial
biomass could be readily taken up by the plants, thereby causing a decrease in soil microbial biomass.
It is evident that there was low variability in soil mi- crobial biomass within the treatments as the analysis
has been made on pooled soil samples for the various treatments. This could also be attributed to selective
microbial adaptations to alkaline conditions of the soil.
B. Kaur et al. Applied Soil Ecology 15 2000 283–294 287
Table 1 Soil carbon and nitrogen concentrations at different soil depths and distances from rows of tree 1, 2 and 3 m in different systems
a
Treatments Soil carbon
Soil nitrogen 0–7.5 cm
7.5–15 cm 15–30 cm
0–7.5 cm 7.5–15 cm
15–30 cm Control
0.65
c
0.26
cd
0.16
de
0.083
ab
0.036
ab
0.023
b
Rice–berseem 0.47
d
0.28
bc
0.15
de
0.055
c
0.037
ab
0.018
b
1 m Distance Acacia
0.68
bc
0.30
b
0.21
bc
0.083
ab
0.037
ab
0.021
ab
Acacia+rice–berseem 0.77
a
0.36
a
0.27
ab
0.090
c
0.043
a
0.033
a
Eucalyptus 0.48
d
0.19
e
0.14
e
0.058
c
0.029
ab
0.017
b
Eucalyptus+rice–berseem 0.67
bc
0.22
d
0.18
cde
0.073
b
0.035
ab
0.019
b
Populus 0.50
d
0.20
de
0.19
cd
0.060
c
0.031
b
0.018
b
Populus+rice–berseem 0.71
b
0.36
a
0.28
a
0.088
a
0.042
a
0.025
ab
LSD
p 0.05
0.034 0.038
0.033 0.010
0.011 0.008
2 m Distance Acacia
0.57
b
0.20
ab
0.18
ab
0.065
ab
0.025
abc
0.018
a
Acacia+rice–berseem 0.64
a
0.22
a
0.14
bc
0.077
a
0.030
a
0.022
a
Eucalyptus 0.38
c
0.15
c
0.12
e
0.035
bc
0.015
abc
0.014
a
Eucalyptus+rice–berseem 0.55
b
0.19
bc
0.15
abc
0.050
bc
0.021
bc
0.017
a
Populus 0.42
c
0.16
bc
0.14
bc
0.049
bc
0.019
bc
0.015
a
Populus+rice–berseem 0.58
b
0.21
a
0.19
a
0.070
a
0.028
ab
0.021
a
LSD
p 0.05
0.029 0.029
0.028 0.014
0.009 0.010
3 m Distance Acacia
0.48
a
0.13
a
0.11
a
0.057
a
0.019
ab
0.016
ab
Acacia+rice–berseem 0.50
a
0.15
a
0.13
a
0.064
a
0.023
a
0.019
a
Eucalyptus 0.29
b
0.14
a
0.11
a
0.026
b
0.012
ab
0.011
b
Eucalyptus+rice–berseem 0.41
b
0.12
a
0.10
a
0.041
b
0.016
ab
0.014
ab
Populus 0.32
c
0.12
a
0.10
a
0.032
b
0.015
ab
0.012
ab
Populus+rice–berseem 0.49
a
0.14
a
0.12
a
0.061
a
0.020
a
0.017
ab
LSD
p 0.05
0.044 0.029
0.028 0.012
0.007 0.006
a
For each column, values not marked with the same letter are significantly different DMRT; p0.05.
Averaged across the sampling dates, microbial biomass C and N varied from 76.1 to 153.4 mg C g
− 1
soil and 11.1 to 32.6 mg N g
− 1
soil, respectively Table 2. The tree species had a significant effect
on the levels of microbial carbon and nitrogen. The
Table 2 Soil microbial biomass carbon MBC and microbial biomass nitrogen MBN averaged across the seasons and their relationships with
total soil carbon and nitrogen in different management systems
a
Treatments MBC mg g
− 1
soil MBN mg g
− 1
soil Microbial C:N
MBC Soil C MBN Soil N
Control 76.13
f
11.06
e
6.88
a
1.21
c
1.51
d
Rice–berseem 96.14
e
18.61
cd
5.16
cd
2.29
cd
4.04
ab
Acacia 143.40
b
31.45
a
4.56
d
2.31
b
4.08
ab
Acacia+rice–berseem 153.40
a
32.57
a
4.71
d
2.26
d
3.79
bc
Eucalyptus 109.12
d
15.78
de
6.92
a
2.32
ab
3.36
c
Eucalyptus+rice–berseem 133.80
c
21.78
bc
6.14
ab
2.23
b
3.69
bc
Populus 131.10
c
21.70
c
6.04
bc
2.57
a
4.48
a
Populus+rice–berseem 150.13
a
25.62
b
5.86
bc
2.24
b
3.28
c
LSD
p 0.05
4.86 3.38
0.731 0.194
0.525
a
For each column, values not marked with the same letter are significantly different DMRT; p0.05.
integration of trees with crops had a favorable effect on soil microbial biomass as it showed an increase
of 7, 23 and 15 in the cases of Acacia, Eucalyp- tus and Populus based systems, respectively. On the
average, microbial biomass was found to increase by
288 B. Kaur et al. Applied Soil Ecology 15 2000 283–294
Fig. 1. Seasonal variation in soil microbial biomass Carbon a and Nitrogen b in different treatments C=control; RB=rice–berseem; An=Acacia nilotica; An+RB=Acacia nilotica+rice–berseem; Et=Eucalyptus tereticornis; Et+RB=Eucalyptus tereticornis+rice–berseem;
Pd=Populus deltoides; Pd+RB=Populus deltoides+rice–berseem.
42 microbial C and 13 microbial N in the case of tree-based systems as compared to monocropping.
The ratio of microbial C:microbial N varied from 6.92 to 5.86 in Eucalyptus and Populus based systems,
whereas in Acacia based systems the microbial C:N was in the range of 4.56–4.71.
The proportions of microbial carbon and nitrogen to total soil C and N were higher in the case of mono-
cultures indicating higher C and N immobilization in microbial biomass Table 2. The ratio of microbial C
to total soil carbon was comparatively higher in mono- cultures of Eucalyptus and Populus 2.32–2.57. The
proportion of microbial N to total soil N varied from 4.08 to 4.48 in Acacia and Populus plantation.
The effect of soil depth on biomass carbon was sig- nificant, reflecting the steep decline in soil organic
matter content in sub-surface layers of soil Table 3. Distance from trees also had a significant effect on mi-
crobial biomass C and N at 0–7.5 cm and 7.5–15 cm soil depths p0.01. Significantly higher amounts of
microbial biomass carbon and nitrogen were found in soil at 1 m from the trees than at 2 and 3 m. Aver-
aged across the distance from the trees, the microbial biomass carbon and nitrogen were found to be higher
under systems with Acacia 125 –136 mg C g
− 1
soil; 25–27 mg N g
− 1
soil and Populus 95–133 mg C g
− 1
soil; 17–23 mg N g
− 1
soil Table 3. In general, the microbial biomass carbon was significantly higher in
agroforestry as compared to tree plantations. Micro- bial biomass nitrogen did not differ significantly be-
tween soils under trees alone and agroforestry systems Table 3.
At 0–7.5 cm soil depth, the C:N ratio of the mi- crobial biomass was 5.10–6.20. The proportion of
B. Kaur et al. Applied Soil Ecology 15 2000 283–294 289
Table 3 Variations in soil microbial biomass carbon MBC and microbial biomass nitrogen MBN with soil depths in different treatments
a
Treatments MBC mg C g
− 1
soil MBN mg N g
− 1
soil M
C
:M
N
0–7.5 cm soil depth Control
51.54
f
8.51
d
6.06
a
Rice–berseem 97.20
d
17.20
bc
5.65
a
Acacia 125.47
b
24.61
a
5.10
a
Acacia+rice–berseem 136.21
a
26.55
c
5.13
a
Eucalyptus 87.21
e
14.07
c
6.20
a
Eucalyptus+rice–berseem 106.76
c
17.72
c
6.02
a
Populus 94.84
de
16.57
bc
5.72
a
Populus+rice–berseem 133.06
ab
22.51
ab
5.91
a
LSD
p 0.05
7.42 4.76
1.35 7.5–15 cm soil depth
Control 36.33
e
5.78
c
6.28
a
Rice–berseem 40.57
de
7.44
abc
5.45
a
Acacia 50.08
b
9.72
ab
5.15
a
Acacia+rice–berseem 57.56
a
10.65
a
5.40
a
Eucalyptus 37.68
e
5.71
c
6.60
a
Eucalyptus+rice–berseem 43.98
cd
6.99
bc
6.29
a
Populus 37.54
e
6.85
bc
5.48
a
Populus+rice–berseem 48.13
bc
8.00
abc
6.02
a
LSD
p 0.05
4.25 2.87
2.16 15–30 cm soil depth
Control 10.63
c
1.52
b
6.99
a
Rice–berseem 19.20
ab
3.38
a
5.68
a
Acacia 22.55
a
3.89
a
5.80
a
Acacia+rice–berseem 22.38
a
3.93
a
5.69
a
Eucalyptus 17.01
b
2.45
ab
6.94
a
Eucalyptus+rice–berseem 17.22
b
2.63
ab
6.55
a
Populus 15.64
b
2.59
ab
6.04
a
Populus+rice–berseem 21.77
a
3.56
a
6.11
a
LSD
p 0.05
3.42 1.25
1.74
a
For each column, values not marked with same letter are significantly different DMRT; p0.05.
microbial carbon to total soil carbon varied from 1.98 to 2.59 trees with or without crops and from 1.45
to 2.06 rice–berseem cropping. Microbial N ac- counted for 2.61–3.60 of the total soil nitrogen in
the surface layers of soil 0–15 cm. With increasing depth, the proportion of microbial C and N in the to-
tal soil organic pool decreased appreciably, the values being 1.15–1.42 for C and 1.42–1.60 for N.
The proportion of microbial C and N to soil organic C and N was found to be greater in tree-based systems.
Immobilization of nitrogen was 2.61–3.60 0–15 cm soil depth in soils receiving organic inputs in the form
of litterfall and fine roots from the trees. In the case of rice–berseem cropping or the fallow, relatively less
nitrogen was immobilized in microbial biomass in the soil.
3.3. Carbon dioxide evolution and biomass specific activity
The CO
2
-C production from the soil varied with tree species, soil depth and distance from trees.
CO
2
-C production decreased significantly with in- crease in soil depth p0.01. For the various treat-
ments, carbon dioxide evolution rates for the three soil depths mg CO
2
-C g
− 1
soil per day were: 10.75–21.73 0–7.5 cm; 2.92–5.78 7.5–15 cm and
1.94–3.00 15–30 cm. The CO
2
evolution rates were significantly higher at 1 m from the rows of
trees than at 2 m for the soil from 0–7.5 cm depth p0.05. With increase in soil depth, the effect of
distance from the trees on CO
2
-C production was not significant.
290 B. Kaur et al. Applied Soil Ecology 15 2000 283–294
Table 4 Specific soil microbial activity mg CO
2
-C produced mg
− 1
MBC per day in soils of different management systems Treatments
Distance m m
g CO
2
-C produced mg
− 1
MBC per day LSD
p 0.05
0–7.5 cm 7.5–15 cm
15–30 cm Control
– 208.57
85.60 218.25
35.98 Rice–berseem
– 111.94
71.97 39.55
40.36 Acacia
1 122.90
52.81 98.29
14.43 2
105.85 67.44
113.40 17.18
Acacia+rice–berseem 1
125.31 59.17
108.10 29.07
2 123.59
93.31 120.03
17.20 Eucalyptus
1 118.91
57.96 108.24
42.78 2
106.23 67.87
117.29 27.03
Eucalyptus+rice–berseem 1
118.95 56.81
115.21 16.78
2 96.63
64.96 123.61
33.58 Populus
1 114.61
65.23 112.30
19.76 2
100.70 67.67
132.38 43.72
Populus+rice–berseem 1
119.43 72.25
100.07 11.03
2 103.62
93.93 119.47
18.80 LSD
p 0.05
1 14.74
19.84 41.13
2 13.32
22.39 36.23
–
Microbial biomass specific respiratory activity did not vary significantly with tree species Table 4. Spe-
cific respiratory activity was comparatively greater at 0–7.5 and 15–30 cm than at 7.5–15 cm. Biomass spe-
cific respiratory activity was higher in the control soil than in the tree plantations and agroforestry systems.
3.4. Soil inorganic N and nitrogen mineralization There were significant variations in levels of
soil inorganic nitrogen due to treatments p0.01, Fig. 2a. During all three seasons, soil inorganic N
15.89–17.37 mg N g
− 1
soil was maximum in the case of Acacia+rice–berseem. The levels of soil in-
organic N were 41–74, 8–30 and 32–42 higher, respectively, for Acacia, Eucalyptus and Populus
based agroforestry systems than for rice–berseem. Soil inorganic N showed appreciable increases in
the agroforestry systems as compared with the tree plantations. It is interesting to note that this increase
was greater in the case of the Eucalyptus+crop sys- tem 17–47 than in the Acacia+crop 7–13 or
Populus+crop system 6–16.
There were significant variations in nitrogen min- eralization due to tree species, soil depth and seasons
p0.01 Table 5. Nitrogen mineralization was max- imum in July 6.69–31.03 mg g
− 1
soil and minimum in December 1.55–4.26 mg g
− 1
soil Fig. 2b. Maxi- mum nitrogen mineralization during the rainy season
corresponded with the active growth period of plants. In winter months, the presence of dead roots in soil
cores together with low temperature caused low rates of N-mineralization. Nitrogen mineralization rates for
different agroforestry systemsmg g
− 1
soil were: Aca- cia 4.26–31.03, Populus 3.84–26.04 and Eucalyp-
tus 3.13–22.87. In the agroforestry systems nitrogen mineralization increased by 12–37 as compared to
monocultures.
On the basis of seasonal N mineralization rates, to- tal annual nitrogen release has been calculated. Total
N release was highest in the Acacia+rice–berseem system 312.90 kg N ha
− 1
followed by Acacia alone 297.14 kg N ha
− 1
and the other systems studied 182.54–268.58 kg ha
− 1
. Annual release of nitro- gen was 13–95 higher under tree-based systems as
compared to monocropping. Total annual nitrogen mineralization accounted for 24–31 of total soil N
in treetree-cropping systems, 23 in rice–berseem cropping and 6.36 in the control treatment.
Due to higher biological activity, N mineraliza- tion rates were highest at 0–7.5 cm and decreased
with increasing soil depth Table 5. The rates were highest under Acacia 38.17–40.60 and Pop-
ulus 28.31–39.12. The mineral nitrogen NH
4 +
-N and NO
3 −
-N concentrations under the various treatments are shown in Table 6. Significantly,
B. Kaur et al. Applied Soil Ecology 15 2000 283–294 291
Fig. 2. Variations in a initial levels of soil inorganic N and b nitrogen mineralization rates in soils of different treatments C=control; RB=rice–berseem; An=Acacia nilotica; An+RB=Acacia nilotica+rice–berseem; Et=Eucalyptus tereticornis; Et+RB=Eucalyptus
tereticornis+rice–berseem; Pd=Populus deltoides; Pd+RB=Populus deltoides+rice–berseem.
Table 5 Variations in nitrogen mineralization rates at various soil depths in control, cropping system, tree plantation and agroforestry systems
N-mineralization mg N g
− 1
soil LSD
p 0.05
Treatments 0–7.5 cm
7.5–15 cm 15–30 cm
Control 7.39
3.02 1.71
0.87 Rice–berseem
23.02 12.96
4.16 0.78
Acacia 38.17
15.28 5.68
1.34 Acacia+rice–berseem
40.60 16.91
5.89 2.06
Eucalyptus 22.06
10.67 4.77
2.41 Eucalyptus+rice–berseem
30.99 14.74
5.54 1.03
Populus 28.31
13.81 5.65
0.96 Populus+rice–berseem
39.42 15.27
5.69 1.19
LSD
p 0.05
1.38 1.03
0.96 –
Table 6 Variations in levels of NH
4 +
-N and NO
3 −
-N mg g
− 1
soil at two distances from rows of trees averaged across the soil depths in cropping, forestry and agroforestry systems
Treatments NH
4 +
-N NO
3 −
-N NO
3 −
-NNH
4 +
-N 1 m
3 m LSD
∗
1 m 3 m
LSD
∗
1 m 3 m
LSD
∗
Control 2.22
– –
7.53 –
– 3.39
– –
Rice–berseem 2.19
– –
7.71 –
– 3.52
– –
Acacia 7.51
5.30 1.46
3.75 5.36
2.07 0.49
1.01 0.17
Acacia+rice–berseem 9.22
6.22 1.87
4.15 6.00
2.29 0.45
0.96 0.13
Eucalyptus 6.98
4.63 2.99
3.00 4.23
1.61 0.42
0.91 0.24
Eucalyptus+rice–berseem 7.66
5.01 1.95
3.25 4.49
1.86 0.42
0.89 0.19
Populus 7.08
4.34 2.35
3.43 4.95
2.51 0.48
1.14 0.33
Populus+rice–berseem 8.99
5.02 2.25
4.00 5.76
1.99 0.44
1.15 0.43
LSD
p 0.05
2.66 2.24
– 1.68
1.65 –
0.17 0.27
–
292 B. Kaur et al. Applied Soil Ecology 15 2000 283–294
greater amounts of ammonium were present and the NO
3 −
-NNH
4 +
-N ratios were significantly lower closer to the trees p0.05. The consistently lower
NO
3 −
-NNH
4 +
-N ratio near the trees 0.42–0.49 than far from trees 0.89–1.11 in all the treatments
indicated an efficient uptake of nitrate-N by tree roots.
4. Discussion