Análisis del nivel de efectividad de la empresa

Observed topsoil temperatures ranged from -7.16 to 33.61 ˚C (figure 4.6), with a mean temperature for the duration of the experiment of 11.33 ˚C. There was an associative relationship between TOC normalised total lignin concentrations detected in topsoil

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leachates from mixed grass, R. repens, and F. excelsior through time with the topsoil seasonal temperature (figure 4.6).

Figure 4.6. Mean (± standard error of the mean, n = 3) normalised total lignin

concentrations in topsoil leachates from decomposing litter (mixed grass, Ranunculus repens, Fraxinus excelsior, and Quercus robur) through time. Mean (n = 3) topsoil temperature is shown in red.

Greatest total lignin concentrations with respect to bulk TOC were detected in late spring/early summer. This seasonal pattern was displayed most strongly in R. repens. Q. robur showed a gradual increase in total lignin concentration detected in leachate

through time which appeared to be independent of temperature.

At the compound specific level, some phenols followed a similar seasonal trend through time (figure 4.7) to total lignin concentration (figure 4.6): P1 for all vegetation types; P2 for mixed grass, R. repens, and Q. robur; G1 for R. repens, F. excelsior, and Q. robur; G2, S1, and G3 for F. excelsior; P6, G6, P18, and S6 for mixed grass, R. repens, and F. excelsior; and finally G18 for R. repens.

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Figure 4.7. Mean (± standard error of the mean, n = 3) concentrations of selected lignin phenols detected in topsoil leachates from decomposing litter (mixed grass, Ranunculus repens, Fraxinus excelsior, and Quercus robur) through time. Phenol abbreviations are defined in figure 1.10.

Some phenols deviated from the seasonal trend: P2 for F. excelsior; G2, S1, and G3 for mixed grass and R. repens; G5, G24, and G18 for mixed grass; and finally, S5 for mixed grass, R. repens, and Q. robur.

Figures 4.8 to 4.14 below show the distributions of lignin phenols that constitute the total lignin concentration in leachates (figure 4.6) for each litter type at each sampling time, respectively.

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Figure 4.8. Mean lignin phenols and aromatic acids (n = 3) detected in soil leachates with different vegetation treatments applied using TMAH/GC-MS at time 1. Phenol abbreviations are defined in figure 1.10.

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Figure 4.9. Mean lignin phenols and aromatic acids (n = 3) detected in soil leachates with different vegetation treatments applied using TMAH/GC-MS at time 2. Phenol abbreviations are defined in figure 1.10.

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Figure 4.10. Mean lignin phenols and aromatic acids (n = 3) detected in soil leachates with different vegetation treatments applied using TMAH/GC-MS at time 3. Phenol abbreviations are defined in figure 1.10.

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Figure 4.11. Mean lignin phenols and aromatic acids (n = 3) detected in soil leachates with different vegetation treatments applied using TMAH/GC-MS at time 4. Phenol abbreviations are defined in figure 1.10.

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Figure 4.12. Mean lignin phenols and aromatic acids (n = 3) detected in soil leachates with different vegetation treatments applied using TMAH/GC-MS at time 5. Phenol abbreviations are defined in figure 1.10.

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Figure 4.13. Mean lignin phenols and aromatic acids (n = 3) detected in soil leachates with different vegetation treatments applied using TMAH/GC-MS at time 6. Phenol abbreviations are defined in figure 1.10.

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Figure 4.14. Mean lignin phenols and aromatic acids (n = 3) detected in soil leachates with different vegetation treatments applied using TMAH/GC-MS at time 7. Phenol abbreviations are defined in figure 1.10.

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At times 1 and 2, mixed grass, R. repens and F. excelsior leached more total lignin than Q. robur. Dominant phenols at time 1 (figure 4.8) for all litter types were P1 and P2. Mixed grassleached significantly more G3 (F pr. = 0.043) and G18 (F pr. = 0.012) than other litter types. At time 2 (figure 4.9), R. repens leached more total lignin, comprising principally: P1, S1, and P6. Mixed grasswas significantly different from other litter types for G3 (F pr. = 0.011), and mixed grass and F. excelsior were significantly different (F pr. = 0.015) from R. repens and Q. robur in G18 concentration.

At times 3 and 4 total lignin concentrations for mixed grass, R. repens, F. excelsior reduced to converge with Q. robur (figure 4.6). Dominant phenols in leachates for each litter at time 3 and 4 are shown in figures 4.10 and 4.11 respectively. At time 4, mixed grass was significantly different from other litter types for G2 (F pr. = <0.001), S1 (F pr. = 0.002), and G3 (F pr. = <0.001). G3 was also dominant in mixed grass at time 3 (figure 4.10), but not significantly different as there was none detected for other litter types.

At time 5 there was a dramatic increase in total lignin phenols leached for R. repens, F. excelsior and mixed grass, whereas only a slight increase for Q. robur (figure 4.6). R. repens had the greatest total lignin concentration which comprised mainly carboxylic acid compounds (figure 4.12) including: P24, G6, P6, S6, P1, and other non-lignin derived aromatic acids such as benzoic acid, methyl ester and benzeneacetic acid, methyl ester. Many of these compounds were detected in R. repens leachate at a greater concentration than in fresh or degraded R. repens litter. At time 5, F. excelsior was significantly different from other litter types in G1 (F pr. = 0.037) and G2 (F pr. = 0.027). Mixed grassand F. excelsior were significantly different from R. repens, and Q. robur in G3 (F pr. = 0.044).

At time 6, there was a sharp reduction in total lignin leached for all vegetation types except Q. robur which remained at a similar level as time 5 (figure 4.6). Total lignin concentration in leachate for R. repens was significantly different from other litter types (F pr. = 0.030). Dominant lignin phenols leached at time 6 (figure 4.13) were: G6, P1, P2, G18, S6, and G3. Mixed grasswas significantly different from other litter types for G3 (F pr. = 0.008). There were multiple significant differences between litter types for P18 (F pr. = <0.001) and G18 (F pr. = <0.001): For both compounds, R. repens was similar to Q. robur, and these were different from mixed grass, which was also different from F. excelsior.

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At time 7 (after 671 days), total lignin phenol concentrations leached from F. excelsior and mixed grass increased sharply whereas R. repens and Q. robur showed no change (figure 4.6). F. excelsior was significantly different from other litter types in total lignin concentration (F pr. = 0.038). Dominant lignin phenols leached (figure 4.14) were: G6, P1, G1, G18, and P2. R. repens was significantly different from mixed grassand F. excelsior for P1 (F pr. = 0.046). For G6, R. repens was similar to Q. robur, and these were different from F. excelsior (F pr. = 0.045). For P18, R. repens was similar to Q. robur, and mixed grasswas similar to F. excelsior, although these two groups were significantly different from each other (F pr. = 0.005). F. excelsior was different from other litter types in G18 (F pr. = 0.015).

Additionally, from time 5 onwards, there was an increase in relative abundance of higher molecular weight phenols in leachates. This may arise if there are differences in retention of different phenols in soils, which is explored in Chapter 6.

4.4 Discussion

The hypotheses tested in this experiment were: dominant lignin phenols found in topsoil leachate DOM are the same as those in the source litter type (H3); and total lignin concentrations determined in leachate increase with increasing topsoil temperature (H4). The following discussion explores the main findings and concludes with acceptance of H3 for specific phenols for certain litter types and rejection of H3 for other phenols. H4 was accepted for mixed grass, R. repens, and F. excelsior because total lignin, and many specific phenol concentrations, increased as a proportion of TOC at higher seasonal temperature, although it was rejected for Q. robur.