Continuous remobilization from below‐ground provides more than half of all carbon and nitrogen in regrowing shoots after grassland defoliation

Abstract

Abstract Remobilization of carbon (C) and nitrogen (N) from roots and crowns to regrowing shoots is an important strategy for grassland plants to tolerate herbivory. While this remobilization is thought to terminate once plants have the capacity to photosynthesize again and to take up N from the soil, this has rarely been quantified. Here we investigated the remobilization of C and N to regrowing shoots of a perennial grassland under two defoliation frequencies (every 15 and 30 days) and two N fertilization levels (0 and 2.8 g m−2) by growing intact plant–soil cores for 60 days in a growth chamber. We measured the C remobilization from roots and crowns to regrowing shoots using a novel continuous 13C labelling method, and quantified the N remobilization to regrowing shoots and plant N uptake from soil with a 15N tracer method. Regrowing shoots relied strongly on below‐ground C reserves, where up to 52% of C in regrowing shoots came from below‐ground during the first 30 days after defoliation. Below‐ground C was still remobilized when plant shoots had fully regrown and retained full capacity to photosynthesize. Plants relied more on C remobilization with shorter times between defoliation events at the expense of root biomass, particularly without N fertilization. Large amounts of C remobilization were accompanied by large amounts of N remobilization (up to 89% of total shoot N) that were positively related, suggesting a coupled C and N transfer from roots and crowns to shoots. Synthesis. The unexpectedly high dependency on C and N remobilization occurred continuously after defoliation and did not terminate once plants were fully regrown. Our results indicate that remobilization of C and N from below‐ground has profound effects on long‐term productivity and C sequestration that needs to be considered for managing similar grasslands worldwide.