Links Between Nitrogen Limitation and Saturation in Terrestrial Ecosystems

ABSTRACT 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 N 2 O 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.