|Review of MS bg-2020-150 “Simulation of soil carbon dynamics in Australia with Rᴏᴛʜ C” by Lee et al.|
I reviewed the revised version of the MS, also taking note of the previous reviews and changes that were made to the new version.
This is a very interesting, technically demanding study where the controlling parameters input rate and input quality for soil carbon stocks in Australian ecosystems were evaluated by using RothC and an elaborated set of external data. The overall structure of the work, its general quality, and the fact that it seamlessly follows a number of previous Australian studies on the same topic, where different data sets and methods have been developed that were also used here, is appreciated.
In line with a previous comment, I see a major drawback in that the authors did not use climate change (CC) scenarios for calculating the effect of increasing C inputs to SOC stocks. Considering the well documented effect that CC will have on future SOC in previous modeling approaches from other regions, it seems not plausible to rely on a repetition of time windows of past climate to simulate the response of SOC to changing conditions which, by nature, can only take place in the future. I therefore suggest that the authors use CC scenarios for evaluating the effect of higher C input on SOC storage. In this context, also the discussion starting in line 362 could be extended towards climate as an important determinant of long-term SOC changes.
Estimating or simulating belowground input is crucial for every SOC modeling study. On pages 6-7, the approach used for estimating these inputs is described. A fixed shoot:root ratio is considered to get the belowground part from the crop model (whether or not shoot:root ratios vary for a specific crop depending on external conditions such as management is a matter of debate in the literature [e.g. Hirte et al. 2018], but such a simplified approach seems justified given the poor availability of belowground input data from experiments). The authors then change monthly inputs (page 8) to better understand the response of the SOC pool to it. In consequence, shoot:root ratios will change as well. I suggest to display and discuss the resulting shoot:root ratios, as they provide a means of quality control to the overall approach. Reviews such as the one from Bolinder et al. (2007) might be helpful to put the derived ratios into context.
The selected range of inputs of between 0.25 – 6 times the equilibrium input seems wide, and Fig. 4 indicates inputs of up to 10 t C ha-1 per year. Even though the authors acknowledge that allocating inputs to sites/soils where higher additional storage might be achieved (line 360) is aimed for, an increase by more than 2-fold seems highly unrealistic given issues of transport or nutrient input. For comparison, a recent study from the temperate zone came up with an estimate of c. 3.7 t C ha-1 for croplands and grasslands (Jacobs et al. 2020). Therefore, I consider these input scenarios as partially unrealistic and suggest to downscale them a bit.
Bolinder, M.A., Janzen, H.H., Gregorich, E.G., Angers, D.A., Vandenbygaart, A.J., 2007. An approach for estimating net primary productivity and annual carbon inputs to soil for common agricultural crops in Canada. Agriculture Ecosystems & Environment 118, 29-42.
Hirte, J., Leifeld, J., Abiven, S., Oberholzer, H.-R., Mayer, J., 2018. Below ground carbon inputs to soil via root biomass and rhizodeposition of field-grown maize and wheat at harvest are independent of net primary productivity. Agriculture, Ecosystems & Environment 265, 556-566.
Jacobs, A., Poeplau, C., Weiser, C., Fahrion-Nitschke, A., Don, A., 2020. Exports and inputs of organic carbon on agricultural soils in Germany. Nutrient Cycling in Agroecosystems 118, 249-271.