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

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
belowground C reserves, where up to 52% of C in regrowing shoots came from
belowground during the first 30 days after defoliation. Belowground 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 belowground has
profound effects on long-term productivity and C sequestration that needs
to be considered for managing similar grasslands worldwide.

将碳（C）与氮（N）从根与根颈（crowns）重新调动至再生地上部分（regrowing shoots），是草本植物耐受草食啃食的关键策略。尽管学界普遍认为，当植物恢复光合作用能力并可从土壤中摄取氮素后，该养分重新调动过程便会终止，但此类结论尚未得到充分量化验证。
本研究通过在生长室（growth chamber）中培养完整植物-土壤柱体（plant-soil cores）60天，探究了两种刈割频率（defoliation frequencies，每15天、每30天一次）与两种施氮水平（N fertilization levels，0与2.8 g·m⁻²）下，多年生草本群落（perennial grassland）的碳、氮向再生地上部分的重新调动过程。研究采用新型连续13C标记法（13C labelling method），测定了碳从根与根颈向再生地上部分的转移量；同时借助15N示踪法（15N tracer method），量化了氮向再生地上部分的重新调动量以及植物从土壤中吸收的氮素总量。
结果显示，再生地上部分高度依赖地下碳储备（belowground C reserves）：在刈割后的前30天内，再生地上部分中多达52%的碳源自地下碳库。即便植物地上部分完全恢复生长并具备完整光合作用能力，地下碳的重新调动仍在持续。当刈割间隔越短时，植物对碳重新调动的依赖程度越高，该过程以地下生物量（root biomass）的损耗为代价，在未施加氮肥的处理组中尤为显著。
大量碳的重新调动伴随大量氮的重新调动（可达再生地上总氮量的89%），二者呈显著正相关，表明碳与氮从根与根颈向地上部分的转移存在耦合机制。
综合分析表明，植物对碳、氮重新调动的意外高依赖度在刈割后持续存在，并不会在植物完全恢复生长后终止。本研究结果显示，地下碳与氮的重新调动对草地长期生产力与碳固存（C sequestration）具有深远影响，全球范围内同类草原的管理工作需将该因素纳入考量范畴。