Nitrogen saturation refers to the phenomenon that the net primary productivity  of an ecosystem, a basic ecosystem function and also a measure of ecosystem carbon (and energy) capture, tends to first increase with the increase of nitrogen input, and does not increase after the nitrogen input amount reaches a certain level.

Decades of research on the nitrogen saturation phenomenon focused only on the aboveground net primary production (ANPP). According to a scientific estimate, belowground net primary productivity (BNPP) of global grassland vegetation accounts for about 60% of its total net primary productivity (NPP) and plays an important role in carbon sequestration. However, empirical data on BNPP are scarce due to the time-consuming and laborious measurement process.

Dr. LV Xiaotao and Dr. YANG Guojiao, the research team members of the Ecostoichiometry Research Team of the Institute of Applied Ecology (IAE) of the Chinese Academy of Sciences (CAS), together with researchers from Lancaster University and the Institute of Botany of CAS, have investigated the response of aboveground and underground components of net primary productivity to nitrogen inputs for years at Erguna (Inner Mongolia) Forest-Steppe Ecotone Research Station.

In one of their recent studies, the researchers found that the nitrogen saturation threshold of ANPP was 13 g N m-2 yr-1, beyond which ANPP would no longer change with the increase of nitrogen input in the temperate steppe. BNPP increased also, but only when the nitrogen input was less than 5 g N m-2 yr-1. Above that threshold, BNPP decreased with the increase of nitrogen input.

The differential responses of ANPP and BNPP to nitrogen input resulted in a much lower nitrogen saturation threshold (6.7 g N m-2 yr-1) for NPP than that for ANPP, which means that the nitrogen saturation study on ANPP might be misused to calculate the response of NPP to nitrogen deposition.

This is another important scientific finding after the research team found the significant difference between interannual stability of ANPP and BNPP in response to nitrogen addition (Global Change Biology, 2022, 28:2711–2720). Both findings promote the understanding of the impact of nitrogen input on grassland productivity.

This study, funded by the National Key Research and Development Program of China and the National Natural Science Foundation of China, has been published in Global Change Biology, entitled "Different nitrogen saturation thresholds for above-, below-, and total net primary productivity in a temperate steppe".