Tian Tian and R. Paul Schreiner*
* USDA-ARS Horticultural Crop Research Laboratory, 3420 NW Orchard Avenue, Corvallis, OR 97330 (Paul.Schreiner@usda.gov)

Grapevines form associations with arbuscular mycorrhizal fungi (AMF) that enhance nutrient uptake, especially of phosphorus (P). Studies in other crops indicate that AMF can increase plant nitrogen (N) acquisition, but whether AMF improve N uptake in grapevines is unclear. To understand whether AMF play a role in N uptake, responses of mycorrhizal and non-mycorrhizal Pinot noir grapevines were compared using a range of N and P inputs in the greenhouse. A soil with moderate P was used to ensure vines were not P-deficient, which would mask the ability to test whether AMF increase N uptake. A factorial experiment with four rates of N (0, 3, 6, or 12 mM N as ammonium nitrate), three rates of P (0, 1, or 2 mM P as potassium phosphate),and two AMF treatments (mycorrhizal, non-mycorrhizal) was conducted using potted vines. Shoot growth and mycorrhizal colonization of roots were periodically assessed, then vines were destructively harvested to determine biomass and nutrient uptake after exposure to different N and P treatments for nine weeks. Shoot and root bio- mass increased with increasing N supply, but shoot length and dry weight were both suppressed by AMF at intermediate levels of N. P supply did not alter vine biomass. Vine N uptake increased with N supply, but N uptake was not improved by AMF at any level of N or P supply. Across all treatments, AMF enhanced uptake of P and zinc. Root colonization by AMF was reduced at the lowest rate of N supply, while arbuscules in roots were more abundant at intermediate rates of N coincident with reduced shoot growth. These findings indicate that AMF do not enhance N uptake when vines receive inorganic N and that AMF may compete with shoots for carbon when N supply is moderate and P is ample for growth.

Funding Support: USDA-ARS