Indirect estimation of nitrogen reserves contribution to annual growth of grapevine

Perennial crops such as grapevines are reliant on reserves – carbohydrates and nitrogen (N) – to support early seasonal growth stages, particularly during spring growth from bud break to bloom. Thus, reserves mobilization in perennial crops is considered to be a major determinant of vegetative growth and yield. Its estimation, particularly for N, is critical to adapt cover cropping strategy or fertilization management (Conradie, 2005). However, reserves measurements are very difficult to perform. Reserves are mainly stored in the roots (Bates et al., 2002; Zapata et al., 2004) and the practical problems associated with the excavation of whole grapevines make difficult to measure the whole roots biomass. We propose a two-step method to assess the N reserves contribution to annual growth of grapevine, calculating the N balance of a vineyard: (i) soil N balance over the 0-60 cm soil layer and (ii) grapevine N balance. For soil N balance, we take into account: soil N stock variation, net mineralization, denitrification, leaching, intercrop and grapevine N uptake from the 0-60 cm soil layer. Solving this balance leads us to estimate grapevine N uptake from the 0-60 cm soil layer used in the second step. For grapevine N balance, we consider N content of annual aerial parts (leaves, shoots, bunches), grapevine N uptake from the 0-60 cm soil layer, grapevine N uptake deeper than 60 cm, remobilization of reserves, losses through green harvest. Solving this balance leads us to estimate reserves remobilization and grapevine N uptake deeper than 60 cm. The estimation of N reserves contribution to annual growth was calculated over the bud break – bloom and the bud break – veraison periods. We used data from a Mediterrean vineyard (Celette et al., 2008, 2009 and 2010) involving two different types of soil cover management over two years (2005 and 2006): a bare soil obtained by chemical weed control (CWC) and a permanent intercrop (PI) composed of a mix of Festuca arundinacea L. and Lolium perenne. The contribution of N reserves and deeper grapevine N uptake to annual growth ranged from 30 % (CWC, 2005) to 100 % (PI, 2006) which represent respectively 8 kgN.ha-1 and 20 kgN.ha-1. This is consistent with other studies. For the intercropped treatment (PI) contribution of N reserves is always of 100 %. This can be due to the deeper grapevine root system due to the intercrop which allows grapevine N uptake deeper than 60 cm. This method is a simple way to estimate N reserves contribution to annual growth of grapevine without any root measurement, but the maximum depth for grapevine N uptake remains difficult to estimate