Placing cropping systems under suboptimal phosphorus conditions promotes plant nutrient acquisition and microbial carbon supply without compromising biomass

Instead of viewing low soil nitrogen (N) and phosphorus (P) availability as a global limitation to plant carbon (C) assimilation, it has been recently suggested that crops placed under slight to moderate nutrient limitation optimize root activity and root-associated ecological benefits. However, the implementation of a new fertilization paradigm is limited by our poor understanding of how suboptimal soil nutrient conditions influence ecological benefits associated with enhanced root exudation. We reviewed the impacts of N and P fertilization rates on plant biomass, root exudation, and nutrient uptake. We found that a <50% decrease in N fertilization reduced plant biomass by circa 17% but increased root exudate amount by 6.7%. For a <50% decrease in P fertilization, root exudate amount increased by up to 29% and plant P uptake by 13%, while biomass production reduced by only 2.2%. These findings demonstrate that enhancing root exudation, while simultaneously improving nutrient uptake and reducing fertilizer dependence in agroecosystems, is an achievable goal that does not compromise plant biomass. This entails identifying the level of soil P availability that optimizes carbon transfer to roots without compromising shoot growth, tailored to the specific ecological context. The increased production of root exudates, a primary source of labile carbon input to the soil, is anticipated to enhance microbial biomass turnover and promote the formation of necromass, a key contributor to soil organic carbon accumulation. Testing cropping systems under suboptimal P conditions across diverse soil types and climatic conditions is crucial for assessing their impact on grain yields, nutrient use efficiency, and soil carbon storage. • Suboptimal P conditions raise root activity and C supply to microbes in crops. • Suboptimal P conditions do not compromise plant biomass. • Moderate N limitation increases root exudation but decreases plant biomass.