During a dry winter, soil nitrogen mineralization rate can be reduced. As a consequence, once the season starts and even if soil moisture is not limited after budbreak, vine nitrogen accumulation can be reduced for lack of early season nitrogen supply following a dry winter as reported by Californian authors.
However, after a wet winter, while the amount of nitrogen may not be limiting, if soil moisture supply is limited after budbreak, vine nitrogen accumulation can be reduced for lack of early season water supply.
Thus, when nitrogen deficit is diagnosed, it can be difficult to say whether low nitrogen supply or low soil moisture supply early season is the culprit. As both conditions can lead to a lack of nitrogen uptake, deciding which corrective action is best is challenging. Today, we discuss how a joint analysis of sap flow and nitrogen accumulation can help discriminate between two situations leading to nitrogen deficit. Such analysis can improve viticulture decision early season and help winegrower selecting the most appropriate action to correct nitrogen deficit.
MATERIAL AND METHOD
We compare sap flow dynamics and nitrogen dynamics obtained in vineyard A and B, located in Napa, California (cv. Cabernet Sauvignon) over two different years. Along sap flow variations; temporal variations in leaf nitrogen concentration are analyzed weekly. Leaf nitrogen readings were performed using leaf fluorescence method. Published guidelines are used to characterize nitrogen level as low, moderate or high.
Figure 1 and 2 show vine water deficit profiles obtained from sap flow analysis over two different years. When nitrogen deficit is diagnosed during the season, sap flow profile is Orange. When there is no nitrogen deficit, sap flow profile is Green.
Figure 1: vineyard A: High sap flow co-occurs with low nitrogen uptake (orange). Foliar spray of nitrogen is recommended to correct nitrogen deficit.
Figure 2: vineyard B: Low sap flow co-occurs with low nitrogen uptake (orange). Irrigation (and maybe fert-irrigation) is recommended to correct nitrogen deficit.
ANALYSIS OF SAP FLOW AND NITROGEN PROFILES
High water supply experienced early season (Figure 1)
In vineyard A, while nitrogen deficit is observed, the water use is high as indicated by high WDI values. However, on the other season, nitrogen uptake is high, in spite of a moderate reduction in sap flow early season as indicated by lower WDI value. Low sap flow is not causing nitrogen deficit in vineyard A.
The “Orange profile” when Nitrogen deficit appears is obtained after a very dry winter. Water supply was not limited during the early season. Dry soil conditions during the winter probably limited nitrogen mineralization rate, lowering the amount of nitrogen available early season. A nitrogen fertilization via a foliar spray is recommended.
Low water supply experienced early season (Figure 2)
In vineyard B, while nitrogen deficit is observed, the water use is low as indicated by low WDI values. However, on the other season, nitrogen uptake is high while water use is also high as indicated by higher WDI value. Low sap flow may be causing nitrogen deficit.
The “Orange profile” when Nitrogen deficit appears is obtained after a wet winter. However, the water supply was limited during the early season. Most likely, reduction in water supply early season contributed to a reduction in nitrogen uptake. Early season irrigation alone or combined with a fertilization is recommended.
- Joint analysis of nitrogen uptake and sap flow improve plant nutritional diagnosis and lead to better-targeted practices.
- Spraying Nitrogen does not conflict with the goal of inducing water deficit pre-veraison and promotes early nitrogen accumulation at the leaf level.
- Early irrigation may solve situation when nitrogen supply seems to be limited when in fact water supply is the culprit.
- Early season, knowing when to spray Nitrogen vs. when to irrigate is critical to optimize the potential of your vineyard production.
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