Too little or not too little – gross nitrogen mineralization responses to elevated CO2

Primary production is in many terrestrial ecosystems limited by the availability of nitrogen (N). Hence, ecosystems can sustain enhanced growth under global change only when sufficient N is available. Over time a progressive N limitation (PNL) of the response to elevated CO2 might develop. As the N availability for plants is strongly controlled by the internal recycling of N in soil, enhanced gross N mineralization is a potential mechanism to alleviate PNL. Based on meta-analysis we found that gross N mineralization is enhanced by elevated CO2 in N limited, but not in phosphorus limited ecosystems. As it is N limited ecosystems that are prone to PNL, the enhanced mineralization alleviates PNL. However, we found indication that the stimulation of gross N mineralization might diminish over time. More experiments on the soil N cycle in ecosystems exposed to elevated CO2 for decades are urgently needed to develop more robust knowledge. Global change is a multi-factor change. Elevated CO2 in combination with other global change factors led to antagonistic responses of gross N mineralization. This means that the response to combined treatments was not predicted by the sum of single treatment responses. As only few field experiments are multifactorial, there is a great need for more long-term field experiments with multiple global change factors. This meta-analysis highlights the need to study the complex soil N cycle in a holistic way, as different pathways of the N cycle responded differently to global change, complicating the interpretation of how N availability for plants might change.