Comment on bg-2020-438

There is increasing evidence that hot spots of biogeochemical activity exert profound effects on ecosystems that are disproportionate relative to the physical footprint of the hotspots. Geophysical techniques have a role to play in locating such hotspots. Induced polarization is an interesting method as it may be sensitive to indicators of enhanced biogeochemical activity, such as the precipitation of metallic minerals where anaerobic and aerobic waters interact.

There is increasing evidence that hot spots of biogeochemical activity exert profound effects on ecosystems that are disproportionate relative to the physical footprint of the hotspots. Geophysical techniques have a role to play in locating such hotspots. Induced polarization is an interesting method as it may be sensitive to indicators of enhanced biogeochemical activity, such as the precipitation of metallic minerals where anaerobic and aerobic waters interact.
I found this paper interesting to read and it includes some nice graphics for illustration. My main concern about the work is that the attempt to correlate IP measurements with indicators of enhanced biogeochemical activity (in this case enhanced carbon turnover) is unconvincing based on the presented dataset. The paper focuses on correlating the IPmeasured phase and/or imaginary conductivity with [1] vegetation patterns, [2] chemistry of soil samples, and [3] water chemistry. I could not follow the logic of the argument that the IP measurements are indicative of hotspots and I doubt that the assumed correlations are statistically significant. Figure 8 is presented to argue that the phase is correlated with the vegetation patterning -but how does that represent a hot spot? Figure 10 appears to be presented to show that phase is uncorrelated with fluid DOC, Cl-and Ftot in the pore fluid and therefore is an indicator of biogeochemical hotspots (Lines 374-381). I could not follow this logic at all. Later in the paper, the entire upper layer of the soil is identified as a 'hot spot', which seems inconsistent with the idea that there are localized zones in space of enhanced biogeochemical activity. In summary, I really struggled to understand what how the presented dataset characterizes hotspots in this paper.
Another general problem I have with the paper is that I found myself often failing to see the apparent correlations drawn between the geophysical images and the other available site information. The inferred correlations were often unconvincing at best. As an example, on lines 550-551 the significance of a correlation between polarization response and DOC is emphasized, yet the real part of the conductivity shows the strongest correlation with DOC.
I also noted some significant technical errors that suggest some misunderstanding of induced polarization. One that was particularly concerning is the statement given at Lines 297-300. This is entirely incorrect. Ulrich and Slater (2004) and Kemna et al. (2004) definitely did not say this. This statement is particularly concerning as it implies a possible misinterpretation of the IP measurements throughout the paper, which may explain why I really struggled to follow the arguments being made in most places.

Specific comments:
Line 94: Slater and Binley (2006) is an inappropriate reference -this paper has nothing to do with IP mapping of contaminant degradation Line 288: I don't see the correspondence between the electrical imaging and the contact with the bedrock that is being inferred here Lines 292-293: I don't really see this evidence for higher polarization values in the northern part of the profiles relative to the southern parts Line 296: Why 'roughly'? At the small phase angles you measure it is almost exact -any differences are well below your measurement resolution.
Line 305: What do you mean 'with higher resolution' here? Why would the phase images be higher resolution? This is unlikely given the relative errors in phase measurements relative to electrical conductivity measurements. Lines 468-482: This material belongs in the Introduction/Background to IP rather than in the Discussion Lines 527-528: Not necessarily the case for iron minerals. Point of zero charge for iron is around 8 Line 534-535: Published work supports that in silica-dominated systems pH influence is indeed small Lines 550-551. This makes no sense to me -the DOC correlation is stronger with the real part of the conductivity.
Line 570-571: Is a layer really a hotspot? It rather destroys the concept