Low assimilate partitioning to root biomass is associated with carbon losses at an intensively managed temperate grassland

Aims: This study aimed to investigate how efficiently assimilated carbon (C) is incorporated in plant biomass at an intensively managed old permanent grassland, how C is partitioned between shoots and roots and what are the implications for C sequestration. Methods: Using the eddy covariance technique, the atmosphere-biosphere exchange of CO₂ was measured for two years at a sandy grassland site in northern Germany. In addition to aboveground net primary production (ANPP), belowground NPP (BNPP) was observed using the ingrowth core method. Results: The grassland showed a high productivity in terms of biomass yield (14.8 Mg dry matter ha⁻¹ yr⁻¹) and net CO₂ uptake (−2.82 Mg CO₂-C ha⁻¹ yr⁻¹). Photosynthetically assimilated C was converted to biomass with a high carbon use efficiency (CUE) of 71% during the growing season. However, a comparably low fraction of 17% of NPP was allocated to roots (f_BNPP). Consequently, the main fraction of NPP was removed during harvest, turning the site into a net source of 0.29 Mg C ha⁻¹ yr⁻¹. Conclusions: Our study showed the flexibility of grass root growth patterns in response to alterations in resource availability. We conclude that highly fertilized grasslands can lose their ability for C sequestration due to low belowground C allocation.