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 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) which refers to the amount of nitrogen or phosphorus nutrients that can be obtained per unit of underground carbon (C) input.

Using stable ¹³C and ¹⁵N isotope tracers, they quantified total plant belowground carbon 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 carbon input was allocated to rhizosphere respiration, while 37% and 14% of the carbon input was used for root growth and root rhizodeposition (e.g., root secretion and turnover), respectively. Compared with obtaining nutrients from soil, the legume (i.e., Trifolium repens), through biological notrogen fixation and stronger rhizosphere priming effect, can acquire nitrogen and phosphorus with lower carbon input.

In addition, the phosphorus fertilizer application increased the CENA of plants for obtaining phosphorus, but decreased the CENA of plants to acquire nitrogen.

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

This study was published in Frontiers 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.