Simulation of Potassium Availability in the Application of Biochar in Agricultural Soil

Round 1

Reviewer 1 Report

The paper „Simulation of potassium availability …” deals with the relevant topic of nutrient availability from biochar. As it has been shown frequently that biochar from nutrient-rich feedstock may serve as a slow-release nutrient source when used as a soil amendment, it is useful to know more about the nutrient release dynamics. As potassium is faster bioavailable than other nutrients in biochar, it makes sense that the authors have focused on the potassium release.

However, the reader will be disappointed to discover that the paper contains only simulation results without any validation data. But these would be necessary to verify the assumptions made by the authors and the results based on them

·        How will the authors know how near to real conditions HP1 will be? This requires at least a few soil column experiments with K measurements in the leachates.

·        How will the authors know that the exchangeable K from biochar will not be supplemented by other, less soluble K pools from the biochar? This would require sequential extraction analyses of biochar to get an overview of the different pools of K in biochar, their sizes, and their solubilities.

·        How will the authors know to which degree weak organic acids in the rhizosphere of plants growing in that soil will modify the solubility of the K pools?

·        When assuming irrigation, why do the authors select so high irrigation doses (Fig. 2) that most probably will lead to surface runoff losses (including the K from biochar), especially when the selected loam soil will not be very quick in absorbing the water?

 

When the authors could supply answers to these questions, ideally by supplementing the simulations with experimental data, the paper will be a valuable contribution to the knowledge about the contribution of biochar to plant nutrient supply.

Author Response

1. Comment

The paper „Simulation of potassium availability …” deals with the relevant topic of nutrient availability from biochar. As it has been shown frequently that biochar from nutrient-rich feedstock may serve as a slow-release nutrient source when used as a soil amendment, it is useful to know more about the nutrient release dynamics. As potassium is faster bioavailable than other nutrients in biochar, it makes sense that the authors have focused on the potassium release.

However, the reader will be disappointed to discover that the paper contains only simulation results without any validation data. But these would be necessary to verify the assumptions made by the authors and the results based on them

Response

We thank the reviewer for his/her efforts to provide valuable comments improving our manuscript. We disagree that the potential readers will be disappointed; on the contrary, we believe that they will find very interesting these new findings related to the effect of soil moisture conditions (saturated/unsaturated) and ion-exchange mechanisms (equilibrium vs kinetic) to potassium availability in agricultural soil/biochar mixtures.

Please find below, an answer to each comment.

 

2. Comment

• How will the authors know how near to real conditions HP1 will be? This requires at least a few soil column experiments with K measurements in the leachates.

Response

We agree with the comment of the reviewer that modelling results using HP1 might not be near to real conditions. However, we argue that also soil column experiments with K measurements in the leachates experimental might not be near to real agronomic conditions. Actually, this was the motivation of our  research, as many experimental works adopt saturated soil conditions, which is far away from real conditions in agricultural fields where unsaturated soil conditions exist. Our modelling work show how important is the soil moisture conditions to study potassium availability, and to our opinion gives new insights how the future experimental works should be designed.

 

3. Comment

• How will the authors know that the exchangeable K from biochar will not be supplemented by other, less soluble K pools from the biochar? This would require sequential extraction analyses of biochar to get an overview of the different pools of K in biochar, their sizes, and their solubilities.

Response

Indeed, sequential extraction analyses would provide great information regarding the speciation of K and the available K pools. Based on the current literature (Nguyen et al., 2020; Wang et al. 2023), water-leachable and exchangeable K are the main forms of K relevant to agricultural applications, so we focused on them in the mathematical models. These pools consist of 25% of the total K present in biochar (see Lines 366-369).

The following lines were added in the discussion section (lines 302-312):

“Potassium in biochars is present in water-soluble, exchangeable and insoluble forms. Water-soluble K is released during the dissolution of K-rich salts, such as KCl, K₂SO₄, KNO₃ (Angst & Sohi, 2013). Exchangeable K is associated with the organic functional groups of biochar, whereas insoluble K is in stable phases, such as silicate minerals or incorporated within the amorphous phase (Nguyen et al., 2020; Wang et al. 2023). Among them the water-soluble and exchangeable K comprise more than 90% of the total K content, and are the main pools of K in biochar-amended soils (Wang et al. 2023). In our study, the leached K comprise the 25% of the total K present, and is related to the presence of KCl in biochar. KCl is highly water-soluble and the main K source in the biochar-amended soils, explaining the great K amounts eluted (Table 2). Other K pools, may also add K in the soils in a long-term basis, due to biochar degradation (Allohverdi et al., 2021), following a kinetic approach as the one simulated.”

New references in the manuscript:

1. Wang, M.; Xu, D.; Bai, Y.; Yu, G.; Zhang, J.; Zhang, S.; Xu, J.; Zhang, H.; Zhang, S.; Wei, J. Dynamic Investigation on Potassium Migration and Transformation during Biochar Combustion and Its Correlation with Combustion Reactivity. Fuel 2023, 340, 127540, doi:10.1016/j.fuel.2023.127540.
2. Nguyen, A.T.Q.; Bui, T.A.; Mai, N.T.; Tran, H.T.; Tran, S. V.; Nguyen, N.H.; Tsubota, T.; Shinogi, Y.; Dultz, S.; Nguyen, M.N. Release Kinetics of Potassium from Silica-Rich Fern-Derived Biochars. J. 2020, 112, 1713–1725, doi:10.1002/agj2.20209.

Existing references in the manuscript [10, 23]:

3. Allohverdi, T.; Kumar Mohanty, A.; Roy, P.; Misra, M.; Ibáñez, E.; Antoniotti, S. molecules A Review on Current Status of Biochar Uses in Agriculture A Review on Current Status of Biochar Uses in Agriculture. 2021, doi:10.3390/molecules26185584.
4. Angst, T.E.; Sohi, S.P. Establishing release dynamics for plant nutrients from biochar. GCB Bioenergy 2013, 5, 221–226, doi:10.1111/gcbb.12023.

 

4. Comment

• How will the authors know to which degree weak organic acids in the rhizosphere of plants growing in that soil will modify the solubility of the K pools?

Response

We agree with the reviewer, that plant and microbial activity modifies biochar and thus the distribution of the different K pools in the amended soils. Many variables, including the kind and quantity of the weak organic acid, the chemical composition of the soil, and the plant species, might affect how easily potassium pools dissolve in the rhizosphere of plants (Jones et al., 2004; Neumann et al., 2001). Weak organic acids in plants' rhizospheres like citric acid, malic acid, and oxalic acid chelate cations like K, increasing the solubility of K pools in the soil, thus making it more available to the plant, while they reduce rhizosphere pH at the same time. However, we consider that biochar degradation will not significantly change the available potassium amounts, since most of it is in the presence of salts.

References:

1. Jones, D.L.; Hodge, A.; Kuzyakov, Y. Plant and mycorrhizal regulation of rhizodeposition. New Phytol. 2004, 163, 459–480.
2. Neumann, G.; Romheld, V. The release of root exudates as affected by the plant’s physiological status. In The rhizosphere; CRC press, 2000; pp. 57–110 ISBN 0429116691.

 

The following lines were added in the discussion section (lines 313-321):

“Biochar enhances the microbial and flora activity in soils, and modifies the available potassium pools (Wang et al., 2018). These bacteria as well as the rhizosphere of the plants excrete organic anions that may destroy the K-bearing mineral phases in soil. Additionally, degradation of biochar with time, contribute to the available nutrient pools. The microbial activity, therefore, also aids in enriching the soils in K (Wang et al., 2018). However, the amount of K released due to biochar decomposition seem to be of secondary importance compared to the one released from water-leachable and exchangeable pools. The reported biochar decomposition rates (and therefore of mobilization of insoluble K) reported are 0.0093% per day (Wang et al., 2015), whilst the leaching rates of water-soluble K exceed 0.15% per day (Limwikran et al., 2018).”

New references in the manuscript:

1. Wang, J.; Xiong, Z.; Kuzyakov, Y. Biochar Stability in Soil: Meta-Analysis of Decomposition and Priming Effects. GCB Bioenergy 2016, 8, 512–523, doi:10.1111/gcbb.12266.
2. Limwikran, Kheoruenromne, I., Suddhiprakarn, A., Prakongkep, N., Gilkes, R.J. Dissolution of K, Ca, and P from biochar grains in tropical soils. Geoderma 2018, 312, 139-150, https://doi.org/10.1016/j.geoderma.2017.10.022.

Existing references in the manuscript [42]:

3. Wang, L.; Xue, C.; Nie, X.; Liu, Y.; Chen, F. Effects of biochar application on soil potassium dynamics and crop uptake. Plant Nutr. Soil Sci. 2018, 181, 635–643, doi:10.1002/jpln.201700528.

 

5. Comment

• When assuming irrigation, why do the authors select so high irrigation doses (Fig. 2) that most probably will lead to surface runoff losses (including the K from biochar), especially when the selected loam soil will not be very quick in absorbing the water?

Response

The assumed irrigation dose (Fig. 2) is the typically applied one in agricultural fields of Northern Greece, i.e. around 50 mm at every irrigation is applied for 24 hours (duration of irrigation) and 450-500 mm for the whole irrigation period. Moreover, our HYDRUS model runs with a variable time step from 1 sec up to 1 day. When the irrigation dose is applied as boundary condition in the model, a smaller time step is used to allow water infiltration in the soil and avoid surface runoff. In addition to that, a surface storage component of 2 mm is applied in the model to avoid surface runoff and ensure that all applied water will be infiltrated. Finally, from our model results, we have verified that there is not surface runoff loss.

 

6. Comment

When the authors could supply answers to these questions, ideally by supplementing the simulations with experimental data, the paper will be a valuable contribution to the knowledge about the contribution of biochar to plant nutrient supply.

Response

We thank the reviewer for the valuable comments to improve our manuscript. We would like to clarify that the objective of our manuscript is to study potassium availability in agricultural soil/biochar mixtures under different soil moisture conditions (saturated vs unsaturated) and under different ion exchange release mechanism (equilibrium vs kinetic).

Modelling is the research tool to answer these research questions. Experimental data would be valuable, indeed, to clarify some aspects of potassium availability in agricultural soils, such as which ion exchange release mechanism is more appropriate or prevail, and therefore, should be followed to study potassium availability in numerical assessments. To this direction, our modelling results clearly show the significant effect of ion exchange mechanism, as well as the significant effect of soil moisture conditions, to future long-term research studies (simulation or experimental oriented) of potassium availability in agricultural soil.

We strongly believe that these conclusions from our modelling work show how important is the soil moisture conditions to study potassium availability and how potassium release rate is highly associated with ion exchange mechanisms, and to our opinion gives new insights how the future experimental works related to potassium availability in agricultural soils should be designed.

Reviewer 2 Report

The manuscript titled ‘’ Simulation of Potassium Availability in the Application of Biochar in Agricultural Soil’’, is a well-written paper with an interesting topic and valuable findings. However, there are some small needs to be corrected.

The title should be written in lower letters!

The abstract is well-structured and represents all parts of the study. Just in line 10: the word Nutrients is written in bold. Please correct it.

There are some straight statements in the text that are left without any reference! Please bring at least one reference for each statement. It can strength the scientific soundness of your work!

Here is one example without any ref:

Lines 160-161: ‘’ Pyrolysis temperature is an important parameter that influences not only the biochar production yield, but also its physico-chemical characteristics. For agricultural valorization purposes, pyrolysis temperatures between 400 °C and 500 °C are generally preferred in order to get respectable biochar yields (>25–30%) and sufficient bioavailable nutrient contents.’’.

Here is one suggestion you can add: https://doi.org/10.3390/su142214722

Again in discussion lines 302-307 without any ref!

Lines 310-311: ‘’ The presence of other cations in the system plays an important role to potassium availability and mobility. Calcium is the main cation that competes with potassium in the exchange sites of solids.’’ -> Reference?

The conclusion is too long! Make it shorter in a single paragraph.

Good Luck!

Author Response

1. Comment

The manuscript titled ‘’ Simulation of Potassium Availability in the Application of Biochar in Agricultural Soil’’, is a well-written paper with an interesting topic and valuable findings. However, there are some small needs to be corrected.

The title should be written in lower letters!

The abstract is well-structured and represents all parts of the study. Just in line 10: the word Nutrients is written in bold. Please correct it.

Response

We thank the reviewer for his/her efforts to provide valuable comments improving our manuscript. We corrected the title and abstract as suggested.

 

2. Comment

There are some straight statements in the text that are left without any reference! Please bring at least one reference for each statement. It can strength the scientific soundness of your work!

Here is one example without any ref:

Lines 160-161: ‘’ Pyrolysis temperature is an important parameter that influences not only the biochar production yield, but also its physico-chemical characteristics. For agricultural valorization purposes, pyrolysis temperatures between 400 °C and 500 °C are generally preferred in order to get respectable biochar yields (>25–30%) and sufficient bioavailable nutrient contents.’’.

Here is one suggestion you can add: https://doi.org/10.3390/su142214722

Again in discussion lines 302-307 without any ref!

Lines 310-311: ‘’ The presence of other cations in the system plays an important role to potassium availability and mobility. Calcium is the main cation that competes with potassium in the exchange sites of solids.’’ -> Reference?

Response

We revised accordingly, as suggested:

Regarding lines 160-161, we add the new reference:

• Ghorbani, M.; Amirahmadi, E.; Neugschwandtner, R.W.; Konvalina, P.; Kopecký, M.; Moudrý, J.; Perná, K.; Murindangabo, Y.T. The impact of pyrolysis temperature on biochar properties and its effects on soil hydrological properties. Sustainability 2022, 14, 14722.

Regarding lines 302-307, we add the new reference in lines 336-337 of the revised manuscript:

• Angst, T.E.; Sohi, S.P. Establishing release dynamics for plant nutrients from biochar. GCB Bioenergy 2013, 5, 221–226, doi:10.1111/gcbb.12023.

and also, we use the existing reference of the manuscript [26]:

• Kypritidou, Z.; Doulgeris, C.; Tziritis, E.; Kinigopoulou, V.; Jellali, S.; Jeguirim, M. Geochemical Modelling of Inorganic Nutrients Leaching from an Agricultural Soil Amended with Olive-Mill Waste Biochar. Agron. 2022, Vol. 12, Page 480 2022, 12, 480, doi:10.3390/AGRONOMY12020480.

Regarding lines 310-311, we use in line 342 of the revised manuscript the existing reference of the manuscript [37]:

• Kolahchi, Z.; Jalali, M. Simulating leaching of potassium in a sandy soil using simple and complex models. Agric. Water Manag. 2006, 85, 85–94, doi:10.1016/J.AGWAT.2006.03.011

 

 

3. Comment

The conclusion is too long! Make it shorter in a single paragraph.

Response

We revised the conclusion section in two shorter paragraphs.

Round 2

Reviewer 1 Report

The authors have satisfyingly considered the comments of the reviewer and provided illuminating answers to the questions and comments.