Reallocation of nitrogen and phosphorus from roots drives regrowth of grasses and sedges after defoliation under deficit irrigation and nitrogen enrichment

First published: 04 September 2021Abstract 1. Reallocation of nutrients from roots to shoots is essential for plant regrowth in grasslands, particularly in nutrient-poor conditions. However, the response of root nutrient reallocation to changes in nitrogen (N) and water availability remains largely unknown. 2. Using a novel 15N and 32P labelling technique, we quantified the contribution of N and phosphorus (P) to shoot regrowth from either root reallocation or direct soil uptake for perennial grasses exposed to high-frequency deficit irrigation (HFDI) and N addition. 3. Without N addition, HFDI showed no impact on uptake and reallocation of N and P, likely due to unaffected soil N availability and a greater diffusion barrier offsetting increased accumulation in plant-available soil P. With N addition, HFDI increased plant N rather than P uptake, because of increasing soil N availability instead of P under combined HFDI and N addition. The HFDI decreased both N and P reallocation with N addition, possibly due to exhaustion of nutrient reserves in roots that were re-allocated above-ground. Reallocation contributed 48%–97% of N and 58%–79% of P required during the first 2 weeks of shoot regrowth. 4. Synthesis. Our results highlight the importance of N and P reallocation from roots to buffer against changes in soil N and P availability and to maintain N:P ratio in shoot regrowth.Ruzhen Wang, Tom Cresswell, Mathew P. Johansen, Jennifer J. Harrison, Yong Jiang, Claudia Keitel, Timothy R. Cavagnaro, Feike A. Dijkstr