Soil nutrients affect sweetness of sugar maple sap

• Sap sugar concentrations in maple sap were higher in plots with greater native soil nitrogen availability.
• Addition of 30 kg N ha yr−1 increased sap sweetness by 11% two years after initial treatment.
• Foliar P of sugar maples had a negative correlation with sap sweetness (p = 0.02).
• Sugar maples with higher foliar N:P had sweeter sap (p < 0.001).

Understanding how soil nutrients affect sap sweetness of sugar maples (Acer saccharum Marsh.) is important for producing maple syrup, an economically important non-timber forest product in the northeastern USA and southeastern Canada. Sugar maples were sampled for sap sweetness in 21 plots distributed across five stands in the White Mountains of New Hampshire. Sugar concentrations in maple sap were higher in plots with greater native soil nitrogen availability, indicated by N mineralization in laboratory incubations (p = 0.01). To test whether nutrient additions can improve sap sweetness, treatment plots were fertilized with N, P, N and P, or Ca. Addition of 30 kg N ha−1 yr−1 increased sap sweetness two years after initial treatment. Foliar P had a negative correlation with sap sweetness (p = 0.02) while trees with higher foliar N:P had sweeter sap (p < 0.001). By selecting sites with higher soil nitrogen or fertilizing N-limited sites with N, maple sugar producers may be able to collect sweeter maple sap.