Plants allocate large amounts of photosynthetic carbon metabolites to roots and soil and thereby rely on the root-soil-microbe interactions to acquire soil nutrients such as nitrogen (N) and phosphorus (P) for aboveground growth. 

Studies have shown that plant carbon input for nutrient acquisition varies between N-fixing and non-N-fixing plants and between arbuscular and ectomycorrhizal plants, and the process can be affected by soil nutrient availability. However, quantitative data about the trade-off between carbon input and nutrient acquisition at the root-soil interface are still scarce.

Prof. WANG Peng and assistant researcher LU Jiayu from the Underground Ecological Process Group of the Institute of Applied Ecology (IAE) of the Chinese Academy of Sciences, together with researchers from Dr. Feike A. Dijkstra's Team of University of Sydney, Australia, recently quantified below-ground carbon allocation for supporting root functions and the carbon efficiency for nutrient acquisition (CENA). The CENA refers to the amount of nitrogen or phosphorus nutrients that can be obtained per unit of underground carbon (C) input. Using stable¹³C and¹⁵N isotope tracers, they quantified total plant belowground C input, nutrient uptake and CENA in ryegrass (Lolium perenne) and white clover (Trifolium repens) with and without additional phosphorus fertilizers applied to the soil.

The researcher found that, for both species, nearly half of the underground C input was allocated to rhizosphere respiration, while 37% and 14% of the C input was used for root growth and root rhizodeposition (e.g. root secretion and turnover), respectively. They found that, compared with obtaining nutrients from soil, the legume (i.e.,Trifolium repens), through biological N fixation and stronger rhizosphere priming effect, can acquire N and P with lower C input. In addition, they found that the phosphorus fertilizer application increased the CENA of plants for obtaining P, but decreased the CENA of plants to acquire N.

The researchers urged to better understand below-ground C allocation and its efficiency for N and P acquisition in order to improve global C cycling model predictions and help agricultural management practices to increase yield and fertilizer use efficiency.

This study has been published inFrontiers in Plant Science, entitled “Belowground carbon efficiency for nitrogen and phosphorus acquisition varies between Lolium perenne and Trifolium repens and depends on phosphorus fertilization”, and it was funded by the Natural Science Foundation of China and the Australian Research Council.