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.