Abstract

Most Land Surface Models (LSMs), the land components of Earth system models (ESMs), include representation of N limitation on ecosystem productivity. However only few of these models have incorporated phosphorus (P) cycling. In tropical ecosystems, this is likely to be particularly important as N tends to be abundant but the availability of rock-derived elements, such as P, can be very low. Thus, without a representation of P cycling, tropical forest response in areas such as Amazonia to rising atmospheric CO2 conditions remains highly uncertain. In this study, we introduced P dynamics and its interactions with the N and carbon (C) cycles into the Joint UK Land Environment Simulator (JULES). The new model (JULES-CNP) includes the representation of P stocks in vegetation and soil pools, as well as key processes controlling fluxes between these pools. We evaluate JULES-CNP at the Amazon nutrient fertilization experiment (AFEX), a low fertility site, representative of about 60 % of Amazon soils. We apply the model under ambient CO2 and elevated CO2. The model is able to reproduce the observed plant and soil P pools and fluxes under ambient CO2. We estimate P to limit net primary productivity (NPP) by 24 % under current CO2 and by 46 % under elevated CO2. Under elevated CO2, biomass in simulations accounting for CNP increase by 10 % relative to at contemporary CO2, although it is 5 % lower compared with CN and C-only simulations. Our results highlight the potential for high P limitation and therefore lower CO2 fertilization capacity in the Amazon forest with low fertility soils.

Highlights

  • Land ecosystems currently take up about 30% of anthropogenic CO2 emissions (Friedlingstein et al, 2020), buffering the anthropogenic increase in atmospheric CO2

  • We studied the Water Use Efficiency (WUE), as one of the main indicators of Gross Primary Productivity (GPP) changes (Xiao et al, 2013), and soil moisture (SMCL), as one of the main controllers of maximum uptake capacity, in order to better understanding the changes in GPP, P demand and uptake as well as exudates fluxes

  • Our results show an increase in GPP (21%) in response to elevated CO2 (eCO2) which is higher than the average increase of GPP reported in mature eucalyptus forests (11%), growing under low P soils at the free air CO2 enrichment experiment (EucFACE) facility in Australia (Jiang et al, 2020)

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Summary

Introduction

Land ecosystems currently take up about 30% of anthropogenic CO2 emissions (Friedlingstein et al, 2020), buffering the anthropogenic increase in atmospheric CO2. Global process-based models of vegetation dynamics and function suggest a continued land C sink in the tropical forests, largely attributed to the CO2 fertilization effect (Sitch et al, 2008; Schimel, Stephens and Fisher, 2015; Koch, Hubau and Lewis, 2021). Many of these models typically do not consider P constraints on plant growth (Fleischer et al, 2019), which is likely to be an important limiting nutrient in tropical ecosystems, characterised by old and heavily weathered soils. The importance of nutrient cycling representation in Earth System Models (ESMs), and the lack thereof, was highlighted by Hungate et al

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