Does Drying Temperature Affect Grape Leaf Nitrogen and Carbon Concentration?

Plant nutrition response studies often report different temperatures and durations for drying plant leaves in preparation for nutritional analysis.  It is possible that drying temperature could have an effect on the final nutrient concentration after analysis.  During two growing seasons we collected a large bulk sample of wine grape leaves from a commercial vineyard at verasion.  Leaves were separated into petiole and blade fractions in the field and the samples were well mixed.  Sub samples of equal volume were dried concurrently at 40, 50, 60 or 70^oC in small drying ovens for 72 hours.  Samples were then ground in a Wiley mill.  All samples were analyzed for total C and N using dry combustion, plus petiole samples were extracted (1:100 wt:vol with DI water) and analyzed for NO₃-N.  Drying temperature influenced total C in petioles in the first year and blades in the second.  Total N concentration in blades decreased with increasing drying temperature in the first year and petiole NO₃-N increased with increasing temperature in the second year.  These results demonstrate that drying temperature does influence grape leaf tissue N and C concentrations, suggesting that a standard temperature needs to be established for sample handling.