Dear Editor, dear Authors,



This study contributes significantly to the field of enhanced rock weathering and fully deserves its place in this journal. Please find my comments attached. The suggested revision level is moderate.

Congratulations again to the authors.

Regards,

Xavier Dupla

The ms describes a factorial experiment where the impact of basalt and arbuscular mycorrhizal fungi on soil chemical properties and plant performance was assessed. The results show that basalt had very little influence (apart from increasing soil pH, CEC and base saturation as main effects) and even less influence on plant performance (biomass, nutrient mass fractions), whereas AMF did even have less impact on soil chemistry and plant performance. Admittedly, the ms reports somewhat more significant effects, but with the large number of tests applied setting a significance level at p < 0.05 (l. 235) may well be too liberal. It may then be unwarranted to claim synergistic effects (l. 23, 492) – apart from the fact that synergy has not been operationalised here (the authors should look at Larimer et al., Symbiosis 51: 139-148. 2010, where an explicit test for synergy has been proposed), the only parameter with a significant interaction term is DIC; and here the authors explain the problems with the use of DIC as a proxy in section 4.1.1. My reading of that section is that the DIC assessment is complicated and cannot be used as sole proxy for weathering rates.

The paper contains many figures. In my view some of these are superfluous (e.g. Fig. 8 can be omitted if the t-value, degrees of freedom and corresponding p-value are moved to the text). Other figures are difficult to interpret. Fig. 4 (Fe in pore water) shows 4, 3, 2 and 1 data point for the first four assessments over time. I suppose this figure implies overlapping circles, but one cannot see the data points that overlap, unless one studies the supplementary material. (Personally, I am not convinced of the importance of such figures that show changes of metals in pore water over time; one would predict weathering going on over the course of the experiment, and concentrations in pore water are a functioning of continuous weathering, possible uptake (hardly different for most nutrients between the different treatments) and likely soil moisture (if no neutral tracer is used to correct for that).  For most nutrient there is a peak in the middle period and a steep decline towards the end – but the discussion does not engage with potential explanations for this curvilinear relationship. However, the experiment was apparently executed under conditions of no rainfall excess, as there was very little leaching reported (l. 178). (It is also somewhat inconsistent to provide six data points over the course of the experiment and then using the discussion to suggest that the experiment may have lasted too short for an assessment of basalt application.) While the basalt weathered rapidly, one may wonder what would happen under conditions of higher rainfall – with very little plant acquisition of the released cations, one may envisage leaching under field conditions outside the growing season. An issue of potential concern (especially with long-term application) is the fact that basalt releases more Na than K, that Na availability increased and K availability decreased in the soil, and that leaf K mass fractions declined after basalt application. This may lead in the end to an unfavourable K:Na balance that could constrain plant performance. Unfortunately, we cannot assess this potential problem as there are no data on leaf Na (only on corn Na). But a somewhat less optimistic perspective on Na could be useful.

My main problem with the ms is its framing.  (A title like “Limited effect of basalt amendment and mycorrhizal inoculation on soil chemical properties and maize growth on a nutrient-rich soil” does more justice to the results). I think this issue can be easily remedied by looking more at what the data actually show (very little effect of basalt and AMF) and less to general statements that have been made. The concept of context-dependency is mentioned thrice (l. 28, 465, 504), but without specification of the context, the concept remains vague. In this experiment that soil was heavily fertilised and P was adequately available, to judge from leaf N:P (around 8, indicating that despite the high amounts of N added, the system was still N limited; high P availability is also consistent with lack of response to AMF). Note that in the Tables in Supplementary material N is given in %, not in mg/g (otherwise N:P rations would make no sense at all) as the Info to that table suggests; and leaf K contains an evident outlier for pot 18, which could have affected the analysis on plant K.

However, the ms frames the study in use of basalt under very different conditions (nutrient-depleted soils, high acidity) and thereby provides a too optimistic perspective of the use of basalt as soil enhancer in agro-ecosystems of northwestern Europe.  The authors also make sweeping statements that go well beyond basalt weathering, e.g. the link to MaOM (l. 44), a mechanism that works for poorly crystalline material but not for basalt. Also the statements in l. 385 (importance of remedying low soil pH or nutrient depletion as a consequence of intensive agriculture) do not seem to be relevant in relation to the experimental conditions used here. Equally l. 434-436 seem overstated on the basis of the data presented here.

As a mycorrhizal researcher I noted the incorrect claim that hyphal length of AMF can be up to 100 m /cm3. This is a value for ectomycorrhizal fungi, but hyphal length density for AMF is normally an order of magnitude less – as the data by the authors confirm much lower hyphal densities as well.

In l. 138 PO₄^- should be PO₄^3-.