4.1. Increased Nutrient Availability Following Burning
In this study, Burned areas are located in the peatland margins and the Unburned area is located in the center of the fen. The Burned areas likely burned during the fire due to the margins typically having lower WTs and more feather moss then
Sphagnum moss [
15,
37]. The burning of margins with the center of the fen remaining intact is common in the boreal region [
15]. There are likely to be biogeochemical differences in the margins vs the center of the fen pre-fire; however, there are clear differences in macronutrient concentrations between the Burned and Unburned areas that go beyond the pre-fire chemistry. Averages for the large spatial area of Poplar have been examined by Nwaishi et al. [
36] and Wood et al. [
10], and we see that nutrient concentrations in the Burned are much higher than, and C:N:P ratios are very different from, those found in the pre-fire studies.
This study shows that burning had no effect on NH
4+, and led to increases in both NO
3− and P in the WBF. This concurs with previous findings on P, but contrasts with previous findings on N in other peatland studies that have generally shown a decrease in the total amounts of N, and an increase in the availability of P [
38,
39,
40]. In a previous study [
26] in the same fen (Poplar), pre-fire NH
4+ comprised 80–90% of the TIN concentrations in peat. However, in Poplar post-fire, the average make-up for both the Burned and Unburned areas was, on average, 48–57% NO
3− in peat. The specific mechanisms for this increase in NO
3− are not clear, and additional studies to determine the cause of this are needed. The magnitude of increase in NO
3− concentration observed in the Burned area was unprecedented in this area, even under dry conditions, although previous studies in Poplar [
8,
10,
36] provided evidence on the potential for large oxidation of N following lowering of the WT over the growing season. Indeed, this increase in NO
3− concentration led to a dramatic shift in the ratio of dominant forms of inorganic N, and this has important implications on post-fire N mobility and off-area exports given the high mobility of NO
3− relative to NH
4+. The excess NO
3− observed in the Burned area may have been supplied to the Unburned areas through groundwater-surface connectivity or wind erosion of ash. Our results support this proposition, given that NO
3− was considerably higher in surface leachate in the Burned areas, which are hydrologically connected to the Unburned area.
In the study fen, little net mineralization occurred in the Burned areas, especially when compared to pre-fire studies, which show high levels of net nitrification and net P mineralization for peatlands in the WBF [
8,
10,
36]. The lower rates of mineralization observed in the Burned area could be attributed to the loss of microbial biomass through the burning of the upper peat layer, which represents a critical niche for the microbial communities that mediate nutrient mineralization processes in peatlands [
17,
41]. These findings suggest that peatland fire could indirectly modify the cycling of N and P through the direct effect of fire on the surface peat layer and microbial communities that dominate this critical niche.
Another significant finding of this study was the increase in available P after a fire. Similar to the work of Sulwiński et al. [
38], where a peatland 11 years post-fire had concentrations of available P that were 6 times higher. Our results show that available P was 5 times higher in the Burned areas compared to the Unburned area. The median seasonal values in the Burned areas were 0.0093 mg/g and 0.0017 mg/g of peat for hollows and hummocks, respectively; however, we see that this concentration of P in the Burned areas was highest in the early season and decreased steadily throughout the growing season. The increase of P in peat due to burning was also found in a laboratory study by Wang et al. [
42], where the organic P in peat was converted to inorganic P, increasing the availability of inorganic P more than twofold. In peatlands, P is geochemically reactive, especially in mineral-rich fens, where availability of P is limited by mineral control. Thus, the fate of inorganic P following fire is not clear.
The relationship between fire and P supply are dependent on several interacting factors such as pH, redox conditions and the availability of ions (e.g., Ca
2+, Mg
2+, and/or Fe
3+ in fens) [
38,
43]. For instance, it is well documented in the literature that fire causes the release of P from organic matter, especially following partial combustion of organic matter, where bioavailable P is increased (i.e., sodium bicarbonate-extractable P), which is accompanied by an increase in pH. Moreover, desorption of P may occur from the Fe and Al hydrous oxides as a consequence of this increased pH and/or changes in redox conditions due to hydrophobicity post-fire. On the other hand, in mineral-rich fens, an increase in pH post fire may also lead to reactions between P and Ca
2+ or Mg
2+. Hence, some studies have observed no change in P availability post fire [
44]. The observed increase in concentrations of available P in the Burned areas was consistent across all potential plant nutrient sources (i.e., peat, groundwater, and shallow leachate). Notably, available P was highest in the Burned hollows, which could be a result of greater burn severity in hollows than hummocks [
30,
45]. However, the anoxic conditions that are common in hollows may have further increased the mobility of P [
46]. Indeed, P concentrations in groundwater and peat in hollows in this region have been correlated in previous studies [
21].
Disturbance has been shown to cause increased nutrient input in groundwater due to the oxidation of peat [
10,
21,
36]. Surface water infiltration through the ash layer at the Burned area allows for significantly larger concentrations of available P to be added to the groundwater, which remains apparent in groundwater, where P concentrations are significantly higher than concentrations observed in the Unburned areas. Surface infiltration of available P actually increased over the growing season, but groundwater concentrations decreased, which could be the result of a decreasing WT with a larger portion of the peat column being aerobic for surface water to infiltrate through. This infiltration over greater aerobic depths could have caused increased P retention in deeper peat layers that were unaffected by the fire. The significant increase of P in these nutrient-poor peatlands has the potential to increase P loading to adjacent downstream aquatic ecosystems, or, local vegetation shifts [
46]. Furthermore, vegetation shifts from post-fire regrowth can lead to the development of highly productive species, which have the ability to out-compete typical fen species, especially mosses that are critical for slow decomposition rates and net carbon sequestration function [
38,
47]. The effects of increased P availability can be long-lasting [
48]. When coupled with a warmer and drier climate, the resiliency of both the regrowth of fen species and the persistence of P effects have the potential to change fens like Poplar into a carbon source, changing their pre-fire ecosystem function [
38,
49].
4.2. Wildfire Disturbance on the Nutrient Stoichiometry Balance
Burned area N and P concentrations were higher than those in the Unburned area, which likely affects vegetation, litter quality and the oxidation and turnover of C [
43,
50,
51]. Elevated levels of N and P in the peat could affect microbial breakdown as the mismatch in the stoichiometry of peat and microbes could accelerate decomposition and change C sequestration functionality [
36,
42]. The N:P ratio for vegetation was examined to assess potential changes in nutrient limitation patterns for plant growth relative to N:P imbalances of peat and groundwater nutrient reservoirs for the Unburned and Burned areas (
Figure 5).
Based on the Redfield Ratio, plant growth is commonly classified as N- or P-limited, when the N:P is smaller than 14 or larger than 16, respectively, with the possibility of co-limitation when the ratio falls between 14 and 16 [
52,
53]. In most peatlands, N is the limiting nutrient due to the low mineralization rates and limited exogenous nutrient inputs in often-isolated peatlands [
54,
55]. However, this is probably not the case in our study area, because Poplar is located within the vicinity of active industrial development in the Alberta Oil Sands Region [
36]. A previous study in Poplar, in the same area as this study but pre-fire (where it is denoted as the “Rich Fen”) by Nwaishi et al. [
36], found that both mosses and vascular vegetation were N-limited, while the peat and groundwater nutrient balances were dominated by N. This is consistent with other findings, which show that N:P ratios in plants do not reflect the N and P concentrations in peat or other potential nutrient sources [
56]. Nwaishi et al. [
36] also showed that disturbance associated with road-construction through a peatland modified the pattern of N:P limitation, leading to a shift from N-limitation toward NP co-limitation. Although this was a different type of disturbance compared to Poplar’s natural disturbance, the results agree with the theory the oxidation of organic peat material as result of disturbance (either from drying or from fire) can modify the N:P ratios of plants and potential nutrient sources in peatlands.
In this study, the N:P ratios of mosses at the Unburned and Burned areas were 16 and 15, respectively, while vascular species were 29 and 24, respectively (
Figure 5). This suggests the mosses were co-limited and vascular plants were P-limited in both areas. In contrast, peat had average N:P ratios of 43 and 51, respectively, indicating N limitation (
Figure 5). Both shallow leachate and groundwater were N-limited. For example, shallow leachate in both the Burned and Unburned areas had average N:P ratios of 4 and 6, respectively. Groundwater had higher P concentrations relative to N in the Burned areas, and the reverse was true in the Unburned area, which is indicative of the higher P concentrations in the Burned areas. The stoichiometric ratios shown here contrast with the work of Nwaishi et al. [
36], where vegetation was N-limited and any potential nutrient sources (peat and groundwater) were P-limited. We suggest that the change in nutrient limitation shown by plants is very likely due to the wildfire disturbance, and the shifting stoichiometric ratios in both plants and water are related. Future studies should determine the specific mechanisms behind these shifts.
Clear differences in peat stoichiometry were observed between the Burned and Unburned areas of the peatland. Our results show that C:N and C:P ratios were higher in Burned peat than in Unburned (
Table 1 and
Table 2). Notwithstanding the increase in concentration of available P in the Burned peat, there was no significant decrease in the C:P ratio of peat. This suggests that the P released through the oxidation of burned peat does not affect the C: P ratio of the residual peat, which is mostly comprised of the recalcitrant C fraction. The P released from burned peat facilitates increased biomass productivity. Thus, an imbalance between high biomass production and lower decomposition of recalcitrant residual peat has the potential to support net carbon storage in peatlands post-fire [
36,
57,
58,
59]. The fire also impacted the N stock, decreasing N stock by as much as 33% and changing the C:N ratio [
40]. Additionally, the Burned areas exceeded both critical threshold ratios (>40 and >200 for C:N and C:P, respectively) for microbially mediated substrate mineralization [
60]. The potential presence of this microbial substrate limitation suggests that newly introduced labile substrates from pioneer plants will be allocated towards the recovery of microbial biomass [
36,
60], further increasing potential for short-term carbon storage.