The researchers selected a rubber monocultural plantation, three agroforestry systems including rubber-tea intercropping, rubber-orange-tea intercropping and a complex jungle-like rubber-tea agroforestry system (a secondary forest), so as to help improve the sustainability and maximize the potential of rubber-tea agroforestry systems.

The researchers mainly applied stable hydrogen and oxygen isotope techniques (²H and ¹⁸O) to study plant hydrological niche differentiation and stable carbon isotopes (¹³C) to study plant water use efficiency.

To analyze how competition affected plants and soil nutrient status, the researchers measured carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) concentrations of different plant organs (i.e., leaf, stem, and root), litter and soil in these rubber agroforestry systems.

The researchers found that rubber trees absorbed water primarily from deeper soil layers as the number of intercropped species increased and the hydrological niche differentiation between rubber trees and the intercropped plants became more apparent.

In addition, soil nutrient status first improved, but then declined with an increase in the number of intercropped species. However, negative competition effects offset the benefits of intercropping for soil nutrients and water. Rubber trees are the most competitive species in rubber agroforestry systems. P deficiency is a challenge for the growth of diverse intercropped plants.

"We suggest that when establishing or improving rubber-based agroforestry systems, too many species should not be intercropped," said ZHAO Fan, first author of the study.