Weaker land-climate feedbacks from nutrient uptake during photosynthesis-inactive periods

Abstract

Terrestrial carbon(C)-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 CMIP5 represented nutrient dynamics poorly, rendering predictions of 21^st century C-Climate feedbacks highly uncertain. Here we use ELMv1 to quantify 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 process 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 leads to large positive biases in annual N leaching (96%) and N₂O emissions (44%). This N₂O emission bias has a Global Warming Potential equivalent to ~1.5 PgC y^-1, which is comparable 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.