the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Sep 2024
Including the Phosphorus cycle into the LPJ-GUESS Dynamic Global Vegetation Model (v4.1, r10994) – Global patterns and temporal trends of N and P primary production limitation
Abstract. Phosphorus (P) is a critical macronutrient for plant growth, often limiting plant production in areas where plant demand is higher than soil supply. In contrast to nitrogen (N), P cannot be sourced from the atmosphere, therefore where it is rare, it becomes a strong constraint of primary production. Due to this, most DGVMs are incorporating a prognostic P cycle in addition to N, improving their ability to correctly predict stocks and fluxes of carbon, and how climate change may impact N and/or P limitations to soil processes and plant productivity.
We included the P-cycle into an individual-based DGVM, LPJ-GUESS (v4.1, r10994), in order to improve the model performance with regard to observations of vegetation and soil N and P stocks and fluxes in comparison to the N-only (LPJ-GUESS-CN) model version. The new model version (LPJ-GUESS-CNP) includes soil organic P dynamics, P limitation of organic matter decomposition, P deposition, temperature and humidity-dependent P weathering, plant P demand and uptake, and P limitations to photosynthesis. Using the CNP version of LPJ-GUESS we also estimated global spatial patterns of nutrient limitation to plant growth, as well the temporal change in plant N and P limitation during the 20th and early 21st centuries, evaluating the causes for these temporal shifts.
We show that including the P-cycle significantly reduces simulated global vegetation and soil C and N stocks and fluxes, in particular in tropical regions. The CNP model simulation improved the fit to global biomass observations in relation to the CN simulation. The CNP model predicted predominant P limitation of plant growth in the tropics, and N limitation in the temperate, boreal, and high altitude tropical regions. The CNP model also correctly predicted the global magnitude (~ 50 PgP) and the spatial pattern of total organic P stocks. P limited regions cover less land surface area (46 %) than N limited, but are responsible for 57 % of global GPP and 68 % of vegetation biomass, while N limited regions store a larger portion of total carbons stocks (55.9 %). Finally, the model showed that globally primary production limitation to N availability decreased and limitation to P increased from 1901 to 2018, with N being more responsive to temperature, and P to CO2 changes. We conclude that including the P-cycle in models like LPJ-GUESS is crucial for understanding global-scale spatial and temporal patterns in nutrient limitation and improving the simulated carbon stocks and fluxes.
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RC1: 'Comment on egusphere-2024-2592', Anonymous Referee #1, 16 Oct 2024
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This manuscript clearly explains the method and performance of integrating the phosphorus cycle into the LPJ-GUESS Dynamic Global Vegetation Model. The writing is logically structured, and the model development process is well-reasoned. The authors also provide a thorough discussion of the results. I have only a few detailed comments, which are outlined below.
Introduction:
The introduction does a good job in establishing the need for incorporating the phosphorus cycle into dynamic vegetation models, highlighting its importance as a plant growth constraint. However, it would benefit from a more thorough discussion of past challenges in phosphorus modeling and recent advancements, especially regarding prior efforts in Earth system models, where phosphorus cycling, though less common than nitrogen, has been integrated in models like ORCHIDEE-CNP and CASA-CNP. Clarifying research gaps and this study’s unique contributions compared to these models would enhance its impact. Additionally, providing context for the four scientific questions would help readers understand their relevance, as they currently appear somewhat abruptly.
Methodology:
In Figure 1, consider adding some equations (or other elements) to illustrate the principles behind phosphorus limitation, such as how soil phosphorus cycling regulates available phosphorus levels and how soil phosphorus uptake limits plant photosynthesis. This would help readers better understand the model's configuration, especially as this is a central focus of the study. Additionally, is the leaching path missing from the figure?
I recommend moving the content related to P soil processes from the appendix into the main text, particularly sections on P sorption and P weathering. For readers less familiar with these processes, equations can facilitate understanding.
Integrating phosphorus limitation into the model is indeed a significant advancement. However, we know that plant growth may also be limited by other elements, such as potassium or calcium. Therefore, there is still the potential to overestimate plant growth in this model (at least from a nutrient supply perspective). I suggest adding a few sentences in the discussion to address this limitation.
Results:
The results are well-documented.
Discussion:
The discussion section provides a thorough evaluation of the model's performance and areas for improvement. I suggest more discussion of potential limitations. For example, the model lacks certain processes in the phosphorus cycle, such as the impact of plants and microbes on weathering, and there is uncertainty in data sources, as phosphorus content in rock is typically based on sample averages that may not fully represent the mineral composition across all regions. Highlighting these limitations would inspire future research to address these aspects.
Citation: https://doi.org/10.5194/egusphere-2024-2592-RC1 -
AC1: 'Reply on RC1', Mateus Dantas de Paula, 25 Oct 2024
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Many thanks for your comment to the manuscript, the suggestions have been duly appreciated:
Introduction
The paragraph starting in line 62 was extended to include additional arguments in favor of including the phosphorus cycle in DGVMs, as well as a larger list of existing models such as ORCHIDEE and CASA-CNP which include this cycle. In addition, the research gaps in these previous studies were included, namely a global spatial and temporal analysis of N and P limitation, also through a factorial analysis, which in addition provide more context for the four scientific questions which were presented.
Methodology
References to the equations illustrated by Figure 1 were added, as well as further details on how P in soil may affect decomposition rates. Hopefully this increases the understanding of the model´s configuration. The leaching path that was absent was included.
Due to text size constraints, the equations were kept in the appendix. It is believed this would not hinder access to this information, since in the journal´s format the Appendix are embedded into the main document, and not a separate one.
Model limitations in relation to absent processes were added in the manuscript´s final paragraph.
Discussion
A few lines on the uncertainty of P release estimations based on the Chemical Weathering Model and due to the limited amount of sampling was added to the last paragraph of section 4.2.
Citation: https://doi.org/10.5194/egusphere-2024-2592-AC1
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AC1: 'Reply on RC1', Mateus Dantas de Paula, 25 Oct 2024
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RC2: 'Comment on egusphere-2024-2592', Anonymous Referee #2, 20 Dec 2024
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My recommendation is minor revision or rather technical corrections. The manuscript is well written and structured, and most importantly, the transparency of the model assumptions and evaluation. The aim with the extended LPJ-GUESS-CN with the P cycle was to addresses the following research questions (1) Does including the P-cycle improve model agreement to biomass and gross primary production (GPP) observations? (2) What drives N and P limitation across climate zones? (3) How has that changed during the 20th and early 21st century? and (4) Which environmental factors are more relevant for N and P limitation change? Questions 1-3 are clearly answered, and the answers form also the major contribution of the novelty value of the manuscript. Concerning research question (4), I recommend a specification which environmental factors are studied or in mind. Do you mean drivers of plant growth such as N deposition, temperature, precipitation and ambient CO2-concentrations or do you have other factors in mind?
Concerning the P cycle concept, I have one question or minor concern considering the effect of fire on P cycle. L 124 It says “we consider burnt P to be completely retained in the soil”. Do you mean burnt P in the soil or burnt P total plant? Earlier, the authors mention that in the tropical region, most of P is found in the biomass. Hence, I rather expected an addition of both burnt fine and burnt coarse woody debris. Adding specific pools and/or cohorts for burnt debris, would also allow to include the impact of burnt debris or charcoal on decomposition and sorption processes. I have the impression that disturbance by fire was not applied in this study, and thus the results shown are independent of the fire concept. Yet, fires are expected to become more frequent and intensive, and for future use of LPJ-GUESS-CPN, it maybe of importance.
Detailed comments
L 65 Cramer, W., Bondeau, A., Woodward, F.I., Prentice, I.C., Betts, R.A., Brovkin, V., Cox, P.M., Fisher, V., Foley, J., Friend, A.D., Kucharik, C., Lomas, M.R., Ramankutty, N., Sitch, S., Smith, B., White, A., Young-Molling, C., 2001. Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models. Global Change Biology 7, 357e373. Is a relevant reference here.
L 69 Wrong reference for He et al. 2021. Change to He et al. 2021a and add He H., P-E Jansson, A.I. Gärdenäs. 2021. CoupModel (v6.0): an ecosystem model for coupled phosphorus, nitrogen and carbon dynamics – evaluated against empirical data from a climatic and fertility gradient in Sweden. Geoscientific Model Development 14, 735–761, doi.org/10.5194/gmd-14-735-2021.
The later references to He et al. 2021 such be changed to He et al. 2021b2.5 Driving data
L184 Add the ranges in N and P deposition, so that the reader get an impression of magnitude.
Other driving data?
Table 1 Heading, denote CN and CNP-versions same as in text.
L281 Please clarify is water runoff considered important for transport and losses of weathered P or for weathering process?L356 Here He et al. 2021a is relevant. Please clarify why plant root-symbiosis maybe more important for plant P than for plant N acquisition. Slight overlap with paragraph L425.
Figures A5 and A6 incomplete figure legend; dots, lines and area denote …
Citation: https://doi.org/10.5194/egusphere-2024-2592-RC2
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