Increasing nitrogen supply to phosphorus-deficient Medicago sativa decreases shoot growth and enhances root exudation of tartrate to discharge surplus carbon dependent on nitrogen form

Abstract

Carboxylate release by roots has been considered a strategy for mobilization and acquisition of phosphorus (P). However, recently, it was argued that carboxylate release may be a way to discharge surplus carbon produced under conditions that limit plant growth. Plant P status may not be the main factor driving carboxylate release by roots. Instead, plant nitrogen (N) status and/or N:P ratio of the soil or plant may play a more important role in enhancing carboxylate release. MethodsA greenhouse pot experiment was performed to grow alfalfa in a P-de cient soil, supplied with two rates of P (0 and 20 mg kg − 1 ) in combination with four forms of nitrogen (N) at ve rates (0, 25, 50, 75, and 100 mg kg − 1 ), to explore the effects of P rate, N form, N rate, and their interactions on plant growth, P and N status, and carboxylate release, and to determine the factors driving carboxylate release. ResultsNitrogen addition weakened the positive effect of P addition on plant growth, and increased plant N ([N]) and P concentrations ([P]); P addition increased plant [P], but weakened the effect of N addition on plant [N]. The amount of tartrate increased dramatically with increasing N rate, which decreased shoot growth, depending on N form. At high P supply, tartrate exudation correlated negatively with shoot biomass. ConclusionsNitrogen addition to P-de cient alfalfa decreased shoot growth and enhanced the release of tartrate, likely to discharge surplus carbon; and the effects varied with N form.

Assessment of Mycorrhizal Fungal Diversity in Legumes (Medicago sativa, Medicago truncatula and Trifolium rubens) from Algeria and Their Influence on Soil Physicochemical and Microbiological Properties

“…MIP50 corresponds to the amount of unsterilized soil (which contains mycorrhizal microorganisms) needed to mycorrhize 50% of plants, which explains the correlation of PIM50 with nitrogen and phosphorus, mycorrhizal symbiosis improves the amount of these two elements in the soil (Vanek and Lehmann, 2014;Dejana el al., 2022). He et al (2021) found that Medicago sativa improves significantly the nitrogen and phosphorus amount in soil rich in mycorrhizal propagules and rhizobia.…”

Sinks for plant surplus carbon explain several ecological phenomena

“…Release of carboxylates is therefore widely considered to be part of a P-acquisition strategy that allows plants to survive in low-P environments (Lambers et al 2006(Lambers et al , 2011. However, several observations are inconsistent with this interpretation, such as increased carboxylate exudation at low N availability (Zhu et al 2016) and inconsistent relationships between rates of carboxylate release and both P uptake and plant growth (Huang et al 2017;He et al 2021;Wang and Lambers 2020). Carboxylate exudation by roots of alfalfa (Medicago sativa) growing in an alkaline soil low in both N and P was studied by He et al (2020He et al ( , 2021.…”

“…However, several observations are inconsistent with this interpretation, such as increased carboxylate exudation at low N availability (Zhu et al 2016) and inconsistent relationships between rates of carboxylate release and both P uptake and plant growth (Huang et al 2017;He et al 2021;Wang and Lambers 2020). Carboxylate exudation by roots of alfalfa (Medicago sativa) growing in an alkaline soil low in both N and P was studied by He et al (2020He et al ( , 2021. Root exudation of carboxylates (particularly tartrate) decreased with increasing P availability but also increased exponentially with increasing shoot N concentration.…”

“…Root exudation of carboxylates (particularly tartrate) decreased with increasing P availability but also increased exponentially with increasing shoot N concentration. The closer association of root carboxylate release with N than with P concentration prompted He et al (2021) to suggest that N addition resulted in increased production of photosynthates, which could not be used for primary metabolism and growth due to the lack of P, and so were discharged as carboxylates.…”

Application of Zinc Oxide Reduces the Bioavailability and Specific Uptake of Cadmium by Alfalfa Grown in a Cadmium-contaminated Alkaline Soil

“…Adding ZnO did not signi cantly in uence the exudation of any of the carboxylates detected, while soil Cd level only signi cantly affected the exudation of tartrate. Increased tartrate exudation by alfalfa is mainly a response to P de ciency, or increased N to P ratio(He et al 2021a;He et al 2021b). The effects of Zn and Cd on carboxylate exudation by alfalfa roots were small, especially when P was supplied(He et al 2020a;He et al 2021a).…”