Nitrogen saturation thresholds for disturbance-sensitive soil carbon pools

This is the authors' abstract. We don't add key points for this paper.

Environmental thresholds shape soil carbon stocks worldwide. Excess nitrogen (N) from deposition or overfertilization can threaten biological diversity and functions, including altering soil organic carbon (SOC) stocks and challenging climate security. Yet whether SOC stocks respond to N addition in a non-linear pattern driven by N thresholds remains an open question. Here, using a global synthesis of 1561 individual observations of bulk SOC, particulate (POC), and mineral-associated organic carbon (MAOC) stocks, we found that N addition creates global thresholds for POC and MAOC stocks. POC rose nonlinearly with N addition rate, with an abrupt change at 59 kg N ha −1 y −1 , as increased N raised plant-derived carbon inputs and favored POC accumulation. In contrast, MAOC decreased nonlinearly and then stabilized, although mineral protection was observed with a threshold of 110 kg N ha −1 y −1 . This is because high N addition rates stimulated microbial activity and increased necromass production; however, they also probably destroyed mineral protection, which challenges MAOC's capacity to mitigate climate change. Ongoing N addition may cause unforeseen carbon losses, and the protective role of minerals may not offset these effects. Most soils will cross the N threshold in the future, calling for immediate action to minimize the negative impacts of N addition on organic carbon storage. In this regard, maintaining soil carbon sinks requires targeted strategies focusing on MAOC to tackle climate change.