Carbon and Nutrient Interactions in Cereal-Legume Intercropping Systems: Impacts of Phosphorus Fertilization

Abstract

Phosphorus (P) deficiency is a major constraint to agricultural productivity in low-input systems where nutrient availability limits crop growth and resource-use efficiency. Although cereal-legume intercropping is widely promoted to improve nutrient acquisition and productivity, the mechanisms by which P availability regulates nutrient uptake, biological nitrogen fixation (BNF), nitrogen competition, and belowground carbon (C) allocation remain poorly understood. This thesis investigated the effects of P fertilization on productivity, nutrient acquisition, and carbon-nitrogen interactions in cereal-legume systems through a global meta-analysis and isotope-based experiments in a P-limited soil.

A meta-analysis showed that P fertilization significantly increased crop yield and N and P uptake in both monocropping and intercropping, while also improving land-use efficiency in intercrops. Experimental studies using wheat-chickpea intercrops demonstrated that P availability strongly regulated belowground C allocation and BNF. Phosphorus fertilization increased chickpea biomass, P uptake, and BNF, while reducing root C allocation, indicating a shift towards symbiotic N acquisition. In contrast, wheat maintained root C investment regardless of cropping system. A 15N-labelling experiment revealed that wheat was more competitive for nitrate uptake than chickpea, particularly under P fertilization, whereas ammonium acquisition was similar between species. Fine-root traits were important predictors of nutrient uptake under P limitation. A 13CO2 pulse-chase study showed contrasting P-acquisition strategies, with chickpea maintaining high P uptake with minimal changes in rhizodeposition, while wheat relied on increased root C allocation and rhizodeposition. These findings provide new insights into nutrient acquisition and belowground C dynamics in cereal-legume intercropping and highlight opportunities to improve P-use efficiency in sustainable low-input cropping systems.