A modified version of RothC to model the direct and indirect effects of rice straw mulching on soil carbon dynamics, calibrated in a Mediterranean citrus orchard

Pesce, Simone; Balugani, Enrico; de Paz, José Miguel; Marazza, Diego; Visconti, Fernando

doi:https://doi.org/10.5194/egusphere-2023-298

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https://doi.org/10.5194/egusphere-2023-298

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14 Mar 2023

| 14 Mar 2023

Status: this preprint is open for discussion.

A modified version of RothC to model the direct and indirect effects of rice straw mulching on soil carbon dynamics, calibrated in a Mediterranean citrus orchard

Simone Pesce, Enrico Balugani, José Miguel de Paz, Diego Marazza, and Fernando Visconti

Abstract. Mulching of agricultural soils has been identified as a viable solution to sequester carbon into the soil, increase soil health and fight desertification; as such, it is an interesting option for carbon farming in Mediterranean areas. Models are used to project the effects of agricultural practices on soil organic carbon in the future for various soil and climatic conditions, and to help policy makers and farmers assess the best way to implement carbon farming strategies. Here, we modify the widely used RothC model to include mulching practices and their direct and indirect effects on soil organic matter input, soil temperature changes, and soil hydraulic balance. We then calibrated and validated our modified RothC (RothC_MM) using the dataset collected in a field mulching experiment described in detail in a companying article, and used the validated RothC_MM to estimate the expected soil carbon sequestration by year 2050 due to mulching for the Valencian community (Spain). Our results show that RothC_MM improved the fit with experimental data with respect to basic RothC, and was able to predict SOC and CO₂ observations taken in the field, and to model the effects of mulch on soil temperature and soil water content.

Received: 20 Feb 2023 – Discussion started: 14 Mar 2023

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RC1:
'Comment on egusphere-2023-298', Anonymous Referee #1, 29 Mar 2023 reply
The paper submitted by Pesce et al is an attempt to calibrate the RothC model over two mulching experiments in the Mediterranean region. The topic is of interest and fits with the scope of the journal but the study suffers from several methodological flaws making this study inappropriate for publication in its present form. I list them below:

In general, a lot of information are missing making the evaluation of the work very difficult. For instance, the RothC_N version is not described and the reader needs to read the Farina et al. paper to understand the starting point of the version used here. Other problem, the author wrote that TSMD is calculated using pedotransfer function based on the Van genuchten equations. This is way too vague, much more details are needed here. In particular, the equations, the parameters values, how the pedotransfer functions were calibrated, etc. Another problematic description is how the M_T parameter was calibrated. This is a key part of the paper and a much better descriptions is needed. Because of all the missing information I doubt that the study can be reproduced by someone else.

Another point that must be clarified is the initialization procedure. The author wrote that they used a spinup to reach equilibrium (section 2.2.2) but then at line 244 they wrote that the proportion of the pools obtained with the spinup is multiplied by the observed stocks. It makes the initialization procedure quite confusing and a better description of what was done to initialize the model is absolutely needed. If the exact procedure is indeed a spinup to reach equilibrium, this is in contradiction with the statement in the introduction claiming at line 35 that cropland soils are loosing carbon.

Something that also surprised me is written at line 284 where the authors explained that they had to increase the input up to 0.1 and 0.5 tC ha-1 for the mulch experiment. It suggests that the extra carbon inputs due to mulching are not measured and therefore the constrain on the stock given by the model is very poor.

Another problematic approach is how they used the SOC data. If I understood well, they used SOC data obtained during 22 months to calibrate the model and the data from month 22 to 36 to evaluate. I think it is inappropriate because this period is way too short to detect any SOC changes and therefore the model calibration and evaluation are basically done using the same value.

The calibration methods is not described enough. How it has been done, simple tunning, least square method, MCMC…?

At line 296, where the gas chamber always on site? If yes how do the authors managed the temperature increase and the absence of evaporation within the chambers?

What are the value of the C input in the model, where do they come from?

About the future scenario, to which IPCC scenario does it corresponds? What is the scenario for C inputs?

Why the starting point in the SOC data for the control and the mulch treatment are different?

The author used different versions of the model but at the end they only present one. Or all the versions needs to be presented or only the final version needs to be described.

When comparing the CO2 fluxes, you are comparing apples and oranges because the model is providing heterotrophic respiration and the data are given soil respiration including roots respiration. This is acknowledged by the authors at line 351 but I think it should be avoided and removed from the manuscript

I disagree with the statement in the introduction saying that RothC is more simple than CENTURY (line 63). The approach in the two models is similar (pool based, first order kinetics, etc.). Much more complex model could have been found to support your statement in particular those which are driven by microbial mechanisms (Millenial, MIMICS, etc.).

I also disagree with the statement at line 378-379. If you use site data to fit empirical parameters to then run local simulations, I don't think you really integrate any mechanisms related to mulching. You just tuned your model regarding local conditions and there is no guarantee that such a set of parameters would work in a different situation even under that same crop management and the same climate.

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Short summary

Covering the soil with plant residues (mulching) decreases soil erosion, insulates the soil to temperature fluctuations, decreases evaporation of soil water, and increases the soil organic matter, sequestering CO₂ from the atmosphere to the soil. We modified an existing computer model widely used to predict changes in carbon content of the soil to include mulching and its effect on soil temperature, water content and carbon for rice straw covering the soil of a Mediterranean citrus orchard.


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