the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Can models adequately reflect how long-term nitrogen enrichment alters the forest soil carbon cycle?
William R. Wieder
Melannie D. Hartman
Edward R. Brzostek
William T. Peterjohn
Abstract. Changes in the nitrogen (N) status of forest ecosystems can directly and indirectly influence their carbon (C) sequestration potential by altering soil organic matter (SOM) decomposition, soil enzyme activity, and plant-soil interactions. However, model representation of linked C-N cycles and SOM decay are not well-validated against experimental data. Here, we use extensive data from the Fernow Experimental Forest long-term, whole-watershed N fertilization study to compare the response to N perturbations of two soil models that represent decomposition dynamics differently (first-order decay versus microbially-explicit reverse Michaelis-Menten kinetics). These two soil models were coupled to a common vegetation model which provided identical input data. Key responses to N additions measured at the study site included a shift in allocation to favor woody biomass over belowground carbon inputs, reductions in soil respiration, accumulation of particulate organic matter (POM), and an increase in soil C:N ratios. The vegetation model did not capture the often-observed shift in allocation with N additions, which resulted in poor predictions of the soil responses. We modified the plant C allocation scheme to favor wood production over fine root production with N additions, which significantly improved the vegetation and soil respiration responses. To elicit an increase in the soil C stocks and C:N ratios with N additions, as observed, we also modified the decay rates of the particulate organic matter (POM) in the soil models. With all of these modifications, only the microbially explicit model captured a positive soil C stock and C:N response in line with observations. Our results highlight the importance of accurately representing plant-soil interactions, such as rhizosphere priming, and their responses to environmental change.
Brooke A. Eastman et al.
Status: open (until 11 Apr 2023)
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RC1: 'Comment on bg-2023-36', Anonymous Referee #1, 21 Mar 2023
reply
The manuscript by Eastman et al. represents a relevant attempt to improve current modelling efforts to more adequately reflect responses of forest ecosystems to changes in local nitrogen levels. One of the main outcomes of the study is the significance of including an explicit representation of soil microbial physiology in existing biogeochemical models that link C-N cycles and SOM decay.
My overall impression is that this manuscript presents an interesting study for the biogeochemical community and for future predictions of how changes in nitrogen inputs may alter forest C cycling. The study is well designed and the data is clearly presented. The results support the hypothesis that including realistic estimates of plant biomass partitioning, soil organic matter decomposability and microbial physiology in biogeochemical models can improve predictions of ecosystem responses to environmental change. In particular I enjoyed the discussion and found that the authors did a good job in highlighting the caveats related to the model structures and lack of representation of key soil variables. Yet some aspects of the study still need to be clarified and some terminology should be better explained.
Please note that as an experimentalist, my review is has largely focused on the conceptual aspects of the study more than on the choice and parameterization of the biogeochemical models (namely section 2.2 in the Methods).
Main comments:
-The authors talk about plant C allocation throughout their manuscript, although they are mainly referring to shifts in plant biomass partitioning. I suggest they adapt this terminology.
-When introducing ‘microbial physiology’ and ‘microbial activity’, it would be helpful to explicitly provide some examples of the parameters that are commonly used to incorporate these concept into modelling efforts.
-Line 58: do the authors mean reductions in microbial catabolic activity?
-Line 115: It would be important to specify in which years the observed biomass of trees was measured. At the moment it is quite vague, especially for the fine root assessments. Was this done as in a previous paper by the authors: doi: 10.1111/nph.17256? Moreover, it is not clear if the aboveground and belowground biomass assessments where done for all the dominant plant species present in the forest or only for certain tree species. Since not all plant species respond similarly to nitrogen additions (especially in terms of fine root growth) I think this aspect should be better clarified in the method section and also briefly discussed.
Additional comments:
Some sentences are quite long and could be shortened. E.g. Line 51-54: Consider splitting this long sentence into two. Line 55 – 61: I also found this sentence to be very long, and advise the authors to split it in more than one.
Line 57: I assume the authors meant ‘fresh inputs of organic matter to the soil’
Line 60: change ‘microbial models’ to ‘microbially explicit models’.
Line: 239: possible typo: did the authors want to write ‘away from roots and towards wood production’?
Citation: https://doi.org/10.5194/bg-2023-36-RC1
Brooke A. Eastman et al.
Model code and software
wwieder/biogeochem_testbed: v2.0.0 William Wieder and Melannie Hartman https://doi.org/10.5281/zenodo.7636495
Brooke A. Eastman et al.
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