30 October 2018

Weaker Land–Climate Feedbacks From Nutrient Uptake During Photosynthesis-Inactive Periods

Standard land modeling approach to represent nutrient constraints leads to large biases in GHG emissions.

Science

The role of terrestrial biogeochemical and plant processes on atmospheric composition is large and uncertain in Earth System Models. In particular, nutrient controls can have very large effects on photosynthesis and microbial processes, thereby affecting atmospheric CO2 and N2O levels. However, most land models participating in CMIP5 and CMIP6 ignore a widely observed phenomenon: plant nutrient uptake in the absence of photosynthesis. Here we apply the Energy Exascale Earth System Model (E3SM) land model (ELMv1), which integrates the Equilibrium Chemistry Approximation, to represent nutrient competition between microbes, roots, and physical processes. We evaluated the effects of applying this new approach and the standard method on global carbon cycling (i.e., CO2) and nitrogen losses (i.e., N2O and nitrate leaching).

Impact

The effects of root nutrient competition with microbes and abiotic processes during periods without photosynthesis is large. Nitrogen and phosphorus uptake during these periods account for 45 and 43%, respectively, of annual uptake, with large latitudinal variation. Simulations show that ignoring this plant uptake, as is commonly done in land models, leads to large positive biases in annual nitrogen leaching (96%) and N2O emissions (44%). This N2O emission bias has a GWP equivalent of ~2.4 PgCO2 yr1, which is substantial compared to the current terrestrial CO2 sink.

Summary

Terrestrial carbon-climate feedbacks depend on two large and opposing fluxes—soil organic matter decomposition and photosynthesis—that are tightly regulated by nutrients. Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 5 represented nutrient dynamics poorly, rendering predictions of twenty-first-century carbon-climate feedbacks highly uncertain. Here, we use a new land model to quantify the effects of observed plant nutrient uptake mechanisms missing in most other ESMs. In particular, we estimate the global role of root nutrient competition with microbes and abiotic processes during periods without photosynthesis. Nitrogen and phosphorus uptake during these periods account for 45 and 43%, respectively, of annual uptake, with large latitudinal variation. Globally, nighttime nutrient uptake dominates this signal. Simulations show that ignoring this plant uptake, as is done when applying an instantaneous relative demand approach, leads to large positive biases in annual nitrogen leaching (96%) and N2O emissions (44%). This N2O emission bias has a GWP equivalent of ~2.4 PgCO2 yr1, which is substantial compared to the current terrestrial CO2 sink. Such large biases will lead to predictions of overly open terrestrial nutrient cycles and lower carbon sequestration capacity. Both factors imply over-prediction of positive terrestrial feedbacks with climate in current ESMs.

Contact
William J. Riley
Lawrence Berkeley National Laboratory (LBNL)
Publications
Riley, W, Q Zhu, and J Tang.  2018.  "Weaker Land–Climate Feedbacks From Nutrient Uptake During Photosynthesis-Inactive Periods."  Nature Climate Change, doi:10.1038/s41558-018-0325-4.