Soil C and N Stocks
Soil C and N stocks were greater in the 0–5-cm depth increment of the TBI system within 1 m of the hardwood row, west of the poplar row, than at the sampling point 1 m east of poplar at St. Edouard (
p = 0.02,
Table 2 and
Table 3). There was no difference in soil C and N stocks in the 0–5-cm depth increment across the TBI system in St. Paulin (
Table 4 and
Table 5). Soil N stocks exhibited a similar pattern as soil C stocks in the 0–5-cm depth, due to significant correlations between soil N and C stocks at all soil depth increments at St. Edouard (
R = 0.87,
p < 0.05,
n = 63) and at St. Paulin (
R = 0.88,
p < 0.05,
n = 63). Soil C and N stocks were not affected by block, tree species and orientation effects (
p > 0.05), suggesting normal soil variability across the two TBI systems.
Soil C and N stocks in the 0–5-cm depth increment were similar in TBI systems and agricultural systems (Positions 1–6 compared to Position 7,
p > 0.05) at St. Edouard and St. Paulin (
Table 2,
Table 3,
Table 4 and
Table 5). We therefore reject our hypothesis of greater soil C and N storage in the soil surface (0–5 cm depth) of the TBI systems compared to agricultural systems producing the same crop.
At St. Edouard, soil C and N stocks in the 0–30-cm depth increment were greater in the agricultural system (11.32 ± 0.48 kg C m
−2 and 1.30 ± 0.07 kg N m
−2) than the integrated value for the TBI system (8.56 ± 0.74 kg C m
−2 and 0.86 ± 0.12 kg N m
−2) (
p = 0.01 and
p = 0.02, respectively;
Table 2 and
Table 3). At St. Paulin, there was no difference in soil C and N stocks of the 0–30-cm depth increment in the agricultural system (10.10 ± 0.79 kg C m
−2 and 1.30 ± 0.17 kg N m
−2) compared to the integrated value for the TBI system (8.57 ± 0.72 kg C m
−2 and 0.95 ± 0.06 kg N m
−2 (
Table 4 and
Table 5). We reject the hypothesis of greater soil C and N storage in the topsoil layer (e.g., 0–30-cm soil depth) of a TBI system compared to an agricultural system producing the same crop.
Comparing the soil C and N stocks in the soil depth increments (0–5, 5–20, and 20–30 cm) of the TBI and agricultural systems revealed greater soil C stocks, but no difference in soil N stocks, in the 20–30-cm depth increment of the agricultural system at St. Edouard (
p = 0.001,
Table 2 and
Table 3), but had no effect on soil C and N stocks in depth increments at St. Paulin (
Table 4 and
Table 5). The reason that the St. Edouard site had a larger soil C stock in the 20–30-cm depth increment of the agricultural system (3.13 ± 0.23 kg C m
−2) than the TBI system (1.65 ± 0.54 to 2.70 ± 0.51 kg C m
−2) is attributed to residue inputs and tillage practices. Compared to agricultural systems, TBI systems have lower crop yields and less biomass in unharvested crop components,
i.e., crop residues and fine roots that act as a source of quickly decomposing C substrates and promote microbial activity [
10]. Fine roots could be important for building soil C stocks in the 0–30-cm depth increment of the agricultural system and intercropped area of TBI systems, since more than 95% of crop fine roots are found within the 0–35-cm soil depth increment of these study sites [
17]. At St. Edouard, the unharvested crop components were plowed into the soil, to a depth of about 15 cm, before planting the forage mix in May, 2012. Poirier [
18] reported that tillage-induced burial of unharvested crop residues increased soil C storage at the bottom of the plow layer, which would correspond to the 20–30-cm soil depth increment in this study.
Table 2.
Average soil C stocks (kg C m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Edouard, southern Quebec, Canada.
Table 2.
Average soil C stocks (kg C m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Edouard, southern Quebec, Canada.
St. Edouard | Position 1 |
---|
TBI System | Ag Sys | Contrast 3 (p-Values) |
---|
Depth (cm) | 1 | 2 | 3 | 4 | 5 | 6 | 7 | TBI vs. Ag Sys |
---|
0–5 | 1.19 ± 0.13 b 2 | 1.50 ± 0.04 ab | 2.21 ± 0.34 ab | 2.31 ± 0.66 a | 1.74 ± 0.17 ab | 2.24 ± 0.35 a | 1.94 ± 0.35 ab | 0.55 |
5–20 | 3.20 ± 0.79 a | 4.76 ± 1.30 a | 5.84 ± 1.40 a | 5.33 ± 1.60 a | 4.23 ± 1.50 a | 4.10 ± 1.60 a | 6.25 ± 0.87 a | 0.08 |
20–30 | 1.86 ± 1.10 a | 1.65 ± 0.54 a | 2.23 ± 0.34 a | 2.43 ± 0.80 a | 1.85 ± 0.16 a | 2.70 ± 0.51 a | 3.13 ± 0.23 a | 0.001 |
0–30 | 6.24 ± 0.67 a | 7.90 ± 0.64 a | 10.28 ± 0.68 a | 10.07 ± 1.00 a | 7.82 ± 0.62 a | 9.04 ± 0.82 a | 11.32 ± 0.48 a | 0.01 |
Table 3.
Average soil N stocks (kg N m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Edouard, southern Quebec, Canada.
Table 3.
Average soil N stocks (kg N m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Edouard, southern Quebec, Canada.
St. Edouard | Position 1 |
---|
TBI System | Ag Sys | Contrast 3 (p-Values) |
---|
Depth (cm) | 1 | 2 | 3 | 4 | 5 | 6 | 7 | TBI vs. Ag Sys |
---|
0–5 | 0.11 ± 0.05 b 2 | 0.16 ± 0.01 ab | 0.28 ± 0.00 ab | 0.38 ± 0.23 a | 0.21 ± 0.03 ab | 0.27 ± 0.05 ab | 0.24 ± 0.05 ab | 0.82 |
5–20 | 0.26 ± 0.11 a | 0.47 ± 0.25 a | 0.65 ± 0.21 a | 0.58 ± 0.12 a | 0.41 ± 0.27 a | 0.41 ± 0.28 a | 0.73 ± 0.08 a | 0.07 |
20–30 | 0.07 ± 0.11 a | 0.08 ± 0.10 a | 0.20 ± 0.09 a | 0.22 ± 0.14 a | 0.15 ± 0.04 a | 0.29 ± 0.15 a | 0.29 ± 0.07 a | 0.22 |
0–30 | 0.43 ± 0.09 a | 0.72 ± 0.12 a | 1.10 ± 0.10 a | 1.20 ± 0.16 a | 0.77 ± 0.12 a | 0.97 ± 0.16 a | 1.30 ± 0.07 a | 0.02 |
Table 4.
Average soil C stocks (kg C m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Paulin, southern Quebec, Canada.
Table 4.
Average soil C stocks (kg C m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Paulin, southern Quebec, Canada.
St. Paulin | Position 1 |
---|
TBI System | Ag Sys | Contrast 3 (p-Values) |
---|
Depth (cm) | 1 | 2 | 3 | 4 | 5 | 6 | 7 | TBI vs. Ag Sys |
---|
0–5 | 1.70 ± 0.49 a 2 | 1.56 ± 0.49 a | 1.69 ± 0.57 a | 1.73 ± 0.02 a | 1.69 ± 0.96 a | 1.52 ± 0.11 a | 1.84 ± 0.45 a | 0.56 |
5–20 | 4.98 ± 1.20 a | 3.71 ± 1.30 a | 4.47 ± 0.99 a | 3.46 ± 0.55 a | 3.43 ± 1.20 a | 3.77 ± 0.60 a | 5.10 ± 1.10 a | 0.10 |
20–30 | 2.36 ± 1.20 a | 2.91 ± 0.57 a | 2.46 ± 0.28 a | 2.71 ± 0.86 a | 2.62 ± 1.10 a | 3.03 ± 0.09 a | 3.18 ± 0.78 a | 0.23 |
0–30 | 9.04 ± 0.98 a | 8.18 ± 0.80 a | 10.28± 0.68 a | 7.90 ± 0.48 a | 7.73 ± 1.10 a | 8.32 ± 0.27 a | 10.10 ± 0.79 a | 0.15 |
Table 5.
Average soil N stocks (kg N m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Paulin, southern Quebec, Canada.
Table 5.
Average soil N stocks (kg N m−2 ± standard deviation, n = 3) for a tree-based intercropping (TBI) system and an adjacent agricultural system (Ag Sys) at St. Paulin, southern Quebec, Canada.
St. Paulin | Position 1 |
---|
TBI System | Ag Sys | Contrast 3 (p-Values) |
---|
Depth (cm) | 1 | 2 | 3 | 4 | 5 | 6 | 7 | TBI vs. Ag Sys |
---|
0–5 | 0.22 ± 0.09 a 2 | 0.20 + 0.10 a | 0.22 + 0.11 a | 0.23 + 0.02 a | 0.21 + 0.20 a | 0.19 + 0.03 a | 0.25 + 0.10 a | 0.52 |
5–20 | 0.65 + 0.27 a | 0.38 + 0.28 a | 0.55 + 0.19 a | 0.33 + 0.13 a | 0.34 + 0.26 a | 0.42 + 0.10 a | 0.66 + 0.24 a | 0.10 |
20–30 | 0.24+ 0.22 a | 0.34 + 0.09 a | 0.26 + 0.06 a | 0.31 + 0.15 a | 0.27 + 0.23 a | 0.36 + 0.04 a | 0.40 + 0.16 a | 0.26 |
0–30 | 1.10 + 0.19 a | 0.92 + 0.16 a | 1.00 + 0.12 a | 0.87 + 0.10 a | 0.82 + 0.23 a | 0.97 + 0.06 a | 1.30 + 0.17 a | 0.13 |
Table 6.
Cumulative carbon content (kg C m−2) of litterfall collected from July–October, 2012, in tree-based intercropping (TBI) systems and adjacent agricultural systems (Ag Sys) at St. Edouard and St. Paulin, southern Quebec, Canada. Values are the mean ± standard deviation (n = 3).
Table 6.
Cumulative carbon content (kg C m−2) of litterfall collected from July–October, 2012, in tree-based intercropping (TBI) systems and adjacent agricultural systems (Ag Sys) at St. Edouard and St. Paulin, southern Quebec, Canada. Values are the mean ± standard deviation (n = 3).
| Position 1 |
---|
| TBI System | Ag Sys |
---|
1 | 2 | 3 | 4 | 5 | 6 | 7 |
---|
St. Paulin | 0.003 ± 0.004 a 2 | 0.002 ± 0.001 a | 0.004 ± 0.004 a | 0.003 ± 0.001 a | 0.002 ± 0.001 a | 0.001 ± 0.002 a | 0.004 ± 0.005 a |
St. Edouard | 0.028 ± 0.020 ab | 0.006 ± 0.004 b | 0.019 ± 0.011 ab | 0.015 ± 0.010 ab | 0.007 ± 0.011 ab | 0.038 ± 0.037 a | 0.008 ± 0.006 ab |
Hay yield at the St. Paulin site was greater 6 m from the tree row and in the agricultural system than at positions 1.5 m and 3.5 m from the tree row (
p < 0.001;
Figure 3). However, hay yields were not correlated with soil C and N stocks in three soil depth increments (0–5, 5–20 and 20–30 cm). Litterfall inputs were similar across the TBI system and agricultural system at St. Paulin (
Table 4), and there was no relationship between litterfall and soil C and N stocks in three depth increments (0–5, 5–20 and 20–30 cm). We attribute this result to the low C input from litterfall at St. Paulin (
Table 4). Greater C inputs from litterfall were documented at St. Edouard, which had significantly (
p < 0.05) more litterfall at the sampling position 1 m west of poplar than at the sampling location 3 m east of poplar (
Table 4). Although previous studies indicated litterfall to be a primary driver of soil C spatial variability within TBI systems [
9], the low C inputs from litterfall across the TBI system resulted in no correlation between litterfall and soil C stocks. Finally, the PTLT was greater in the agricultural system (97% ± 1%) than the TBI system (72% ± 9%) at St. Paulin (
Figure 4); while the variability in PTLT was related to hay yield across the TBI systems (
R = 0.87,
p < 0.05,
n = 21), the PTLT was not correlated with soil C stocks at this site.
Figure 3.
Hay yield (kg C m−2) at sampling positions located 1.5, 3.5 and 6 m from the tree row (pooled amongst the poplar and hardwood trees) and in the center of the adjacent agricultural system (Ag Sys) at St. Paulin. The error bars are the standard deviation of the mean (n = 6). Means with different letters are significantly different at p < 0.05 (Tukey test).
Figure 3.
Hay yield (kg C m−2) at sampling positions located 1.5, 3.5 and 6 m from the tree row (pooled amongst the poplar and hardwood trees) and in the center of the adjacent agricultural system (Ag Sys) at St. Paulin. The error bars are the standard deviation of the mean (n = 6). Means with different letters are significantly different at p < 0.05 (Tukey test).
Figure 4.
Percent total light transmittance (PTLT) at sampling positions located at 0, 1, 2, 3 and 4 m from the hardwood row, located at 0, 1, 2, 3 and 6 m from the poplar row and in the center of an adjacent agricultural system (Ag Sys) at St. Paulin. The error bars represent standard deviation of the mean (n = 3). Means with different letters are significantly different at p < 0.05 (Tukey test).
Figure 4.
Percent total light transmittance (PTLT) at sampling positions located at 0, 1, 2, 3 and 4 m from the hardwood row, located at 0, 1, 2, 3 and 6 m from the poplar row and in the center of an adjacent agricultural system (Ag Sys) at St. Paulin. The error bars represent standard deviation of the mean (n = 3). Means with different letters are significantly different at p < 0.05 (Tukey test).
Note: Position (in meters) is not indicated for Ag Sys because hemispherical photographs were taken at the center of agricultural system plots.