Nitrogen transformation mediated by artificial root exudates derived from young alder and English oak trees

Root exudates account for up to 17% of the carbon fixed from photosynthesis and are allocated belowground, where they significantly influence microbial communities that drive nutrient cycling, particularly nitrogen in the rhizosphere. Root C exudation for nitrogen acquisition may differ between tree types. This study aimed to investigate how root exudates from English oak (Quercus robur) influence nitrogen cycling in rhizosphere soils compared to soils under alder (Alnus glutinosa). We hypothesized that oak root exudates would prime faster N transformation, given that alder tree roots host nodules for biological nitrogen fixation and thus will not invest exudate C in nitrogen acquisition. We experimented to measure gross and net nitrogen mineralization rates in soils subjected to simulated oak- and alder-specific carbon exudation rates. The study was designed using three artificial root exudate concentrations: 0, 77, and 359 µg C g⁻¹ soil day⁻¹ for alder, and 0, 187, and 814 µg C g⁻¹ soil day⁻¹ for oak. Soils were collected from the top 15 cm of the mineral layer from a four-year-old monoculture plantation of oak and alder trees in Staffordshire, England. The artificial root exudates were based on the actual root exudate rates from alder and oak trees collected during the Summer of 2022 and Spring of 2023 and contained carbohydrates, amino acids, and organic acids. Nitrogen transformation responses in the incubated soils were measured on days 15 and 30. On day 15, half of the soils were recovered from the incubation chambers and subjected to ¹⁵N-N tracer addition to determine gross N mineralization. The study revealed that higher concentrations of root exudate significantly (p<0.001) enhanced microbial activity. This was evidenced by increased soil respiration (21-fold in the oak simulation and 10-fold in the alder), microbial biomass carbon (3-fold in both tree species), and microbial biomass nitrogen (6-fold in oak and 2-fold in alder simulations) compared to the control after 30 days of incubation. These changes contributed to a 282% increase in total dissolved nitrogen in the oak and a 140% increase in the alder simulations. Root carbon inputs altered both gross and net mineralization and nitrification rates. Higher exudate concentrations over longer incubation periods elevated gross mineralization rates by up to 20-fold in the oak but reduced by up to fivefold in the alder compared to controls. Net mineralization rates increased with exudate concentration in both species. In gross nitrification, oak exudates enhanced tenfold, while alder exudates increased eightfold compared to controls after 15 days. Gross mineralization strongly correlated with net mineralization (R²_oak=0.92, R²_alder=0.76) but showed weaker correlations with net nitrification (R²_oak = 0.30, R²_alder = –0.47). Oak root exudates exhibited higher responses across gross mineralization (lnRR=3.08), net mineralization (lnRR=2.50), and gross nitrification (lnRR=1.57) compared to alder. Our results demonstrate that higher oak exudation rates enhanced nitrogen cycling compared to alder, underscoring the importance of species-specific traits in shaping carbon allocation strategies and nutrient cycling in the rhizosphere. This research highlights the critical role of root exudation in regulating soil nutrient dynamics and has broader implications for forest management.