High Temporal and Spatial Nitrate Variability on an Alaskan Hillslope Dominated by Alder Shrubs

In Arctic ecosystems, increasing temperatures are driving the expansion of nitrogen (N) fixing shrubs across tundra landscapes. The implications of this expansion to the biogeochemistry of Arctic ecosystems is of critical importance, yet many details about the form, location, and availability of N from these shrubs remain unknown. To address this knowledge gap, the spatiotemporal variability of nitrate (NO3) and its environmental and edaphic controls were investigated at an alder (Alnus viridis spp. fruticosa) dominated permafrost tundra landscape in the Seward Peninsula, Alaska, USA. Soil pore water was 15 collected from locations within alder shrubland growing along a well-drained hillslope and compared to soil pore water collected from locations outside (upslope, downslope, and between) the alder shrubland. δ15N and δ18O of soil pore water were consistent with the predicted range of NO3 produced through microbial degradation of N-rich alder shrub organic matter. Soil pore water collected within alder shrubland had an average NO3 concentration of (4.27 ± 8.02 mg L-1) and differed significantly from locations outside alder shrubland (0.23 ± 0.83 mg L-1; p < 0.05). Temporal variation in NO3 within and 20 downslope of alder shrubland corresponded to precipitation events, where NO3 accumulated in the soil was flushed downslope during rainfall. Enrichment of both δ15N and δ18O isotopes at wetter downslope locations indicate that denitrification buffered the mobility and spatial extent of NO3. These findings have important implications for nutrient production and mobility in Nlimited permafrost systems that are experiencing shrub expansion in response to a warming Arctic.

A few more specific section comments: Introduction: The language could use an overhaul, mainly a condensation of the text, where some points are repeated and some sentences/sections come out of context (see specific comments below).
Materials and methods: The sample design is very comprehensive and complicated and as such benefits from a detailed description. However, the information could be more closely related to Figure 1b and 1c for clarity and condensed. The description of isotopic calculations is clear and useful. There is a lack of a quantitative estimate of precipitation (now currently addressed simply as "Precipitation events") from e.g. a micrometeorological station, as the precipitation downslope movement of NO 3is such a central part of the results.
Discussion: The storyline of the discussion is not clear and appears more as a list of results related to literature than a use of results to illuminate your research questions. As an example, the discussion of δ 18 O related to precipitation events (line 328-339) comes a bit disconnected from the NO 3 --story, but I suspect there is a point related to N transport, which needs to be clarified. The discussion needs to be restructured and condensed to tell the study story based on the results.
Because of the large revisions needed in the communications of the results, I recommend that the manuscript can be reconsidered after major revisions.

Specific comments with line numbering
Line 35-37: Which links and why is it important? Give one or two examples for a more engaging story.
Line 51-56: This second half of the paragraph seems a bit out of place, because the text introduces alder effects on soil chemistry above and continues below. Consider moving it and even skipping line 51-52 or replacing the sentence in line 39-40 as they say much the same.
Line 68: Alternatively "situated in a hillslope landscape" Line 91-91: I don't understand the function of this sentence in relation to the next sentences.
Line 160: A sentence on how δ 18 O from H 2 O (soil solution) in your NO 3is derived would be useful here.
186-190: Iron, sulfate and Manganese enter the story a bit abruptly here. If they have a function in the study design (as it is later clear that they have), please add a sentence earlier when explaining the study scope and strategy, adding the function of measuring those parameters. Lines 328-339: This section is interesting and coherent in its argumentation, but its place in the story of the manuscript is not clear. The point may be that there is a connection to the NO 3transport and -source, however, this link needs to be clearer for this section to be relevant to the overall story.
Line 375-385: This section is a good example of clear, well-written communication/discussion of the results. !

Technical comments with line numbering:
Line 19: The parentheses around NO 3concentrations are not necessary and should either be removed or the sentence restructured Line 32: Consider using "near-surface hydrologic conditions" in order to exclude e.g. subpermafrost groundwater Line 181: a comma is likely missing between "bags" and "frozen".
Line 184: Soil temperature at which depth?
Line 185: Introduce DO as Dissolved Oxygen before abbreviating Line 200-201: Back up this statement with a reference?
Line 214: The beginning of this sentence should be reformulatedfor once, the comma seems misplaced before "2017" Figure 4: the lower part of the figure is cut off by the caption