Element interactions limit soil carbon storage

Abstract

Rising levels of atmospheric CO2 are thought to increase C sinks in terrestrial ecosystems. The potential of these sinks to mitigate CO2 emissions, however, may be constrained by nutrients. By using metaanalysis, we found that elevated CO2 only causes accumulation of soil C when N is added at rates well above typical atmospheric N inputs. Similarly, elevated CO2 only enhances N2 fixation, the major natural process providing soil N input, when other nutrients (e.g., phosphorus, molybdenum, and potassium) are added. Hence, soil C sequestration under elevated CO2 is constrained both directly by N availability and indirectly by nutrients needed to support N2 fixation.

Fig. 1.

The effect of elevated CO₂ on soil C contents, root biomass, and N₂ fixation. (a) Change in soil C contents as affected by N fertilization. There is a significant difference between N fertilization classes (P = 0.02). The values for <30, 30–150, and >150 kg·ha⁻¹·yr⁻¹ of N are based on 43, 25, and 12 observations, respectively. (b) Change in root biomass as affected by N fertilization. There is a significant difference between N fertilization classes (P = 0.03). The values for <30, 30–150, and >150 kg·ha⁻¹·yr⁻¹ of N are based on 29, 17, and 10 observations, respectively. (c) Change in N₂ fixation as affected by nutrient additions. There is a significant difference between studies that received additional non-N nutrients (43 observations) and studies that did not (49 observations) (P = 0.02). All observations are weighted by experiment duration and number of replicates. All error bars represent 95% confidence intervals.