Different nitrogen saturation thresholds for above‐, below‐, and total net primary productivity in a temperate steppe

Abstract

Identifying the thresholds for the positive responses of total net primary productivity (NPP) to nitrogen (N) enrichment is an essential prerequisite for predicting the benefits of N deposition on ecosystem carbon sequestration. However, the responses of below‐ground NPP (BNPP) to N enrichment are unknown in many ecosystems, which limits our ability to understand the carbon cycling under the scenario of increasing N availability. We examined the changes in above‐ground NPP (ANPP), BNPP, and NPP of a temperate meadow steppe across a wide‐ranging N addition gradient (0, 2, 5, 10, 20, and 50 g N m ⁻² year ⁻¹) during 5 years. Both ANPP and NPP increased nonlinearly with N addition rates. The N saturation threshold for ANPP ( T _A) and NPP ( T _N) was at the rate of 13.11 and 6.70 g N m ⁻² year ⁻¹, respectively. BNPP decreased with increasing N addition when N addition rates ˃5 g N m ⁻² year ⁻¹, resulting in much lower T _N than T _A. Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, and consequently on the earlier occurrence of N saturation threshold for NPP. Our results highlight the negative effects of soil N enrichment on NPP in natural grasslands super‐saturated with N. Furthermore, by considering ANPP and BNPP simultaneously, our results indicate that previous findings from above‐ground might have over‐estimated the positive effects of N deposition on primary productivity.