The exchange of inorganic phosphorus between soil solution and matrix might largely affect the model predictions of terrestrial carbon cycle
Authors: Yu Lin , Ahrens Bernhard , Wutzler Thomas , Schrumpf Marion , Helfenstein Julian , Pistocchi Chiara , Zaehle Sönke 
Published: March 2020

&lt;p&gt;Phosphorus (P) availability may influence the response of terrestrial ecosystems to environmental and climate change. Soil biogeochemical (organic) and geophysical (inorganic) P cycling processes are the key players in this regulation. There has been a continuous effort to include P cycling processes into terrestrial biosphere models (TBMs) and many modelling studies agreed on the significance of organic P cycling processes to terrestrial ecosystems. However, the role of inorganic P cycling processes remains unclear. Although the model representations of inorganic P cycling in most TBMs are similar, their parameterisations differ greatly, and none of TBMs have been validated against soil P measurements.&lt;/p&gt;&lt;p&gt;In this study, we developed a new algorithm based on the two-surface Langmuir isotherm to describe the inorganic P exchange between soil solution and soil matrix in the QUINCY TBM, and tested both the novel and conventional models at five beech forest sites in Germany along a soil P stock gradient, which are the main study sites of the German Research Foundation (DFG) funded priority programme 1685.&lt;/p&gt;&lt;p&gt;We conducted a literature review on Langmuir P sorption parameters, which indicates that the P sorption capacity (S&lt;sub&gt;max&lt;/sub&gt;) is strongly correlated with soil texture and the Langmuir coefficient (k&lt;sub&gt;m&lt;/sub&gt;) is strongly correlated with soil pH and organic matter (OM) content. We divided soil P sorption sites into the OM-rich clay and silty sites and OM-poor sandy sites and extracted empirical equations to calculate their S&lt;sub&gt;max&lt;/sub&gt; and k&lt;sub&gt;m&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;The two-surface Langmuir isotherm approach was implemented to QUINCY, and both the novel and conventional (one-surface Langmuir isotherm) models were applied to the study sites. The models were evaluated with observed soil inorganic P fractionations, foliar N and P contents, and normalized vegetation carbon (C) without calibration. The novel model significantly improved the goodness of model fit to P fractionation measurements at all sites. Both models were able to adequately capture the observed foliar N and P contents, but only the novel one reproduced the observed pattern of vegetation C along the soil P gradient.&lt;/p&gt;&lt;p&gt;We further tested the effect of both models on the responses to CO&lt;sub&gt;2&lt;/sub&gt; addition, P addition and C&amp;P addition at all study sites. The conventional model showed stronger ecosystem responses to P and C&amp;P additions than the two-surface Langmuir one, especially at P-poor sites. It is probably due to that plants store more added P in the conventional model than the novel one. We also tested the sensitivity of both models to the P cycling parameterisation at one low-P site. Despite better model fit to the observed soil P fractionation, the novel model also produced higher and more robust gross primary production, foliar P content and vegetation C than the conventional one.&lt;/p&gt;&lt;p&gt;In summary, we showed that the two-surface Langmuir isotherm approach adequately reproduced the observed soil P fractionations and the pattern of vegetation C along a soil P gradient, owing to its better representation of inorganic P cycling and thus C-P interactions, particularly at low-P ecosystems.&lt;/p&gt;