General soil properties
Soil total organic C content increased by a factor of approximately 1.3 relative to values at the beginning of the field trial in 2001, but total N content remained constant in both the fertilised and unfertilised treatments (Fig. 2 and Table 2). However, among Salix varieties, no significant differences in soil organic C or N contents, or pH were observed in both the fertilised and unfertilised treatments (Table 2).
Table 2
Soil characteristics of the long-term field trial. Means of total N and organic C contents and medians of pH are presented for four field replicates. Means and medians suffixed by a different letter are significantly different at P < 0.05
Soil treatment | Salix varieties | Total organic C* content (%) | Total N* content (%) | pH (H2O) |
Unfertilised | Björn | 1.4 a | 0.12 a | 6.4 a |
| Tora | 1.4 a | 0.12 a | 6.6 a |
| Tordis | 1.4 a | 0.12 a | 6.4 a |
| Jorr | 1.4 a | 0.12 a | 6.6 a |
| Gudrun | 1.5 a | 0.13 a | 6.5 a |
| Loden | 1.5 a | 0.13 a | 6.4 a |
Fertilised | Björn | 1.4 a | 0.12 a | 6.0 a |
| Tora | 1.5 a | 0.13 a | 6.2 a |
| Tordis | 1.4 a | 0.11 a | 6.0 a |
| Jorr | 1.4 a | 0.12 a | 5.9 a |
| Gudrun | 1.3 a | 0.11 a | 6.0 a |
| Loden | 1.3 a | 0.11 a | 5.9 a |
Unfertilised | Main plot 1 | 1.4 b | 0.12 b | 6.5 ab |
| Main plot 2 | 1.1 a | 0.10 a | 6.3 a |
| Main plot 3 | 1.7 c | 0.14 c | 7.8 b |
| Main plot 4 | 1.5 b | 0.13 cb | 6.5 ab |
Fertilised | Main plot 1 | 1.5 b | 0.13 b | 5.8 a |
| Main plot 2 | 1.3 a | 0.11 a | 5.9 a |
| Main plot 3 | 1.4 ab | 0.11 a | 6.1 a |
| Main plot 4 | 1.3 a | 0.11 a | 6.1 a |
* At the start of the experimental field trial in 2001, total organic C and total N contents were on average 1.1% and 0.12% respectively and the bulk density was on average 1.3 g cm-1 (n = 4) across the trial (Weih and Nordh, 2005) |
At the species level, a small but significant effect of Salix species on soil organic C content was observed in the fertilised treatment (P = 0.035). On average S. dasyclados varieties had slightly more soil organic C (0.1%) than S. viminalis varieties. No effect was observed on the soil N content (P = 0.071).
Among the main plots there were, however, significant differences in soil total N and organic C contents in the unfertilised treatment (P < 0.0001), with less pronounced but significant differences in the fertilised treatment (P < 0.05) (Fig. 2 and Table 2). Similarly, soil pH varied significantly among the main plots, but only in the unfertilised treatment (P < 0.01) (Table 2). Positive and significant relationships between soil pH, soil N and organic C contents (adjusted R2 > 0.5, P < 0.0001) were found in the unfertilised treatment plots, but not in the fertilised treatments.
Composition of soil organic matter below Salix species: mid-IR spectra
The Between Class Analysis (BCA) of the mid-IR spectra did not reveal any significant separation between Salix varieties in the unfertilised treatment (Fig. 3a), but a significant separation was found in the fertilised treatment (P < 0.01, Fig. 3b). The variable “Salix varieties” explained 22.3% and 24.4% of the overall inertia of the principal component analysis of the unfertilised and fertilised treatments, respectively (Fig. 3).
Overall, in the fertilised treatment, the composition of soil organic matter below Loden and Jorr varieties differed from the other four varieties (Fig. 3b):
Loden and Jorr were separated from Tora along the horizontal axis due to less abundant carboxylic acid C-O stretch, OH deformation, ester, phenol C-O asymmetric stretch (wavenumbers ranging from 1200 to 1280 cm-1) and C = O stretch (wavenumbers ranging from 1645 to 1717 cm-1). Furthermore, soil organic matter below Loden was enriched, relative to Tora, Björn, Tordis, in aromatic compounds, with more abundant aromatic C-H out-of-plane bend, aromatic C = C stretch and/or conjugated ketone C = O stretch, and/or amide N-H bend and C = O stretch (amide II) (wavenumbers ranging from 894 to 1036 cm-1 and from 1503 to 1639 cm-1). Additionally, Jorr also differed from Tordis, Björn and Gudrun due to less abundant alcohol, ether, phenol C-O-C stretch and poly OH stretch assigned molecular functional groups (wavenumbers ranging from 1160 to 1190 cm-1).
The composition of soil organic matter below Jorr separated along the vertical axis due to higher abundances of aliphatic C-H asymmetric stretches (wavenumbers 2950–2917 and 2865–2849 cm-1) compared to Tordis and Gudrun (Fig. 3b).
In the unfertilised treatment, the mid-IR spectra did not differ among the six Salix varieties (Fig. 3a). Yet, the composition of soil organic matter signifivantly differed among the four main plots (P < 0.01) (data not shown). Here, the aliphatic C-H asymmetric stretches as well as carbonate and calcite stretches were least abundant in the second main plot and most abundant in third main plot. Changes in mid-IR spectra between the four main plots were similar to changes in other general soil properties such as pH and soil organic C (adjusted R2 > 0.5; P < 0.0001).
Composition of soil organic matter below Salix species: pyrolysis-GC/MS
The BCA of the pyrolysis-GC/MS did not reveal any significant separation Salix varieties in the unfertilised treatment (Fig. 4a), but a significant separation in the fertilised treatment was found (P < 0.03, Fig. 4b). The variable “Salix varieties” explained 16.5% and 34.8% of the overall variation in the unfertilised and fertilised treatments, respectively (Fig. 4). The composition of soil organic matter below Gudrun differed the most from the varieties Tora and Jorr in the fertilised treatment (Fig. 4b; Table 3).
Table 3
Biochemical categories of identified pyrolysis products from the soils in the fertilised treatment. The relative abundances expressed as per mille (‰) are shown as the mean ± one standard deviation for four replicates (except for Björn that has one NA value). Polyaromatic compounds, cutin and/or suberine originated compounds, short chain alkane and long chain alkane are not presented because their relative abundances were similar among varieties
Most probable origin | Biochemical categories | Björn | Tora | Tordis | Jorr | Gudrun | Loden |
Higher plant | Lignin | 113 ± 14 | 96 ± 16 | 111 ± 38 | 133 ± 27 | 142 ± 10 | 152 ± 39 |
| Long chain fatty acids | 25 ± 6 | 23 ± 5 | 28 ± 9 | 30 ± 4 | 18 ± 7 | 19 ± 9 |
Microbial community | Short chain fatty acids | 44 ± 9 | 55 ± 10 | 47 ± 21 | 48 ± 12 | 42 ± 8 | 39 ± 5 |
Multiple origins | Nitrogenous compounds | 57 ± 32 | 53 ± 20 | 53 ± 19 | 61 ± 8 | 79 ± 21 | 71 ± 17 |
| Carbohydrates | 55 ± 17 | 32 ± 8 | 38 ± 21 | 38 ± 10 | 53 ± 11 | 50 ± 12 |
| Other aliphatics | 117 ± 96 | 179 ± 74 | 154 ± 81 | 303 ± 30 | 208 ± 30 | 207 ± 37 |
| Phenols | 13 ± 7 | 10 ± 8 | 11 ± 7 | 11 ± 2 | 26 ± 3 | 25 ± 10 |
| Benzene derivatives | 162 ± 108 | 190 ± 114 | 198 ± 81 | 101 ± 14 | 184 ± 67 | 198 ± 54 |
| N-heterocyclic compounds | 396 ± 142 | 340 ± 21 | 337 ± 82 | 253 ± 48 | 223 ± 21 | 213 ± 11 |
Gudrun and, to a lesser extent, Loden were separated from the other varieties along the horizontal axis due to a higher abundance of phenol derivatives of unspecific origin. Furthermore, Gudrun was enriched in nitrogenous compounds from proteins and nucleic acids compared to Tordis and Tora. Carbohydrates and lignin derived compounds from higher plants were enriched in Gudrun compared to Tora. Loden was depleted in short chain fatty acids derived from microorganisms compared to Tora. Loden and Gudrun clearly had a lower abundance of N-heterocylic compounds of unspecific origin compared to Tora, Björn and Tordis.
The separation along the vertical axis between Jorr and the other varieties was due to more aliphatic compounds of unspecific origin and more abundant long chain fatty acids of higher plant origin, particularly in comparison to Gudrun and Loden. Furthermore, soil organic matter below Jorr was depleted in benzene derivatives of unspecific origin relative to the other varieties.
Although soil organic matter below the six varieties was composed of similar compound classes differing only in relative abundances, we observed that the diversities of identified pyrolysis products were different among varieties in the fertilised treatment: Gudrun had a significantly higher effective Simpson index compared to Tora and Tordis (P < 0.001) and Jorr (P < 0.05), by a factor of about 1.7 (Fig. 5).
In the unfertilised treatment, we did not observe any differences in pyrolysis-GC/MS profiles or diversity of pyrolysis products between varieties or between the four main plots.