Links Between Nitrogen Limitation and Saturation in Terrestrial Ecosystems

Nitrogen (N) limitation and N saturation are two fundamental concepts in the field of terrestrial N biogeochemistry. Their ecological links, however, remain unclarified due to their distinct history of origin and the utilization by different research communities. Based on the non-linear responses of soil microbial growth, plant growth, and ecosystem N losses (e.g., gaseous N emissions and soil N leaching) to increasing rates of N supply, we provide a four-stage conceptual model in which (i) microbial growth responds positively to N supply while plant growth and N losses are limited in Stage 1 (denoted as microbial and plant N limitation), (ii) microbial growth is not N limited while plant growth and N losses are both enhanced by N supply in Stage 2 (denoted as microbial N saturation and plant N limitation), (iii) plant growth is not N limited but additional N can be further retained due to plant luxury N consumption and reduced microbial decomposition of soil organic matter in Stage 3 (denoted as plant or ecosystem N saturation), and (iv) plant growth starts to decline and N losses are strongly enhanced by N supply in Stage 4 (denoted as full N saturation). Our model reconciles current concepts of N limitation and saturation and highlights that empirically estimated thresholds of enhanced N leaching and/or N₂O emissions do not necessarily indicate the occurrence of ecosystem N saturation or a removal of plant N limitation, although negative impacts on microbial and plant diversity commonly occur.