Expanding the Root Economics Space With Root Nitrogen Reallocation

Harnessing root nitrogen reallocation (RNR) to enhance plant productivity commences with positioning RNR in the root economics space, about which we still know little. We conducted an inclusive synthesis linking RNR to root traits, combined with a 2-year ¹⁵N-labelling field experiment, to position RNR in the root economics space under acidification. RNR was negatively correlated with specific root length (SRL) and mycorrhizal colonisation in the synthesis, suggesting that RNR is a conservative trait. Sedges, grasses and forbs coordinated root traits (e.g., RD and SRL) from acquisitive to conservative and from low to high RNR reliance (and vice versa for their direct root N uptake) in the ¹⁵N-tracing experiment. Along the collaboration gradient, mycorrhizal symbiosis contributed more to N acquisition in grasses and forbs than in sedges, correlating inversely with SRL but remaining orthogonal to RD and RNR, thus partially reflecting the root economics space. Specifically, sedges and forbs exhibited the lowest and highest RNR that increased and decreased with acidification, respectively. Grasses associated well with mycorrhizal fungi, showing moderate RNR and root traits. Our results demonstrated the significance of RNR in plant growth, and the need to consider RNR as a conservative trait.