Tree species composition and nutrient uptake influence soil nutrient availability, even under contrasted levels of water availability

In an 11-year-old plantation in southwestern France, mixed Pinus pinaster–Betula pendula stands shift soil nitrogen and phosphorus availability, with pine dominating N uptake at budburst and birch boosting available P under high water.

• Why it matters: Drought is increasingly affecting forests, but it is unclear whether species-mixing effects on belowground nutrient cycling persist or change when water availability is altered.
• What they did: The study tested stand composition (pine monoculture, birch monoculture, and mixture) under irrigation versus no irrigation, measuring soil N and P availability with ion-exchange resins, net mineralization via in situ incubations every 4–6 weeks, and tree N uptake using two 15N labeling campaigns (budburst and peak leaf expansion).
• The result: Soil nutrient availability differed by composition—pure pine had higher mineral N and pure birch had higher available P under high water—while net mineralization rates were similar; 15N showed pine’s budburst N uptake advantage in mixtures, which disappeared once birch leaves emerged.

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