Effects of Pre- and Post-Harvest Practices on the Replenishment of the Nitrogen Reserve Pool in the Permanent Structures of Grapevines

The objective of this study was to quantify the amount of N remobilized and/or taken up from the soil or N fertilizer and put into the N reserve pool (within the trunk and root system) of field-grown grapevines.  This study also determined the effectiveness of a post-harvest N fertilization application on the dynamics of the reserve N pool.  Nitrogen within whole vines was quantified using destructive harvests.  Several treatments were imposed to assess their effect on the replenishment of N reserves independent of remobilization.  They included: 1.) The application of N during the growing season, 2.) The application of N post-harvest and  3.) Fifty percent of the leaves in the canopy removed after harvest to mimic the effects of mechanically harvesting a vineyard.

Petiole NO₃-N at bloom for the non-fertilized treatment averaged less than 100 ppm (dry weight), a value many consider to indicate a N deficiency.  The application of a nitrogen fertilizer one month after budbreak (albeit at only ½ the total amount applied) significantly increased NO₃-N, NH₄-N and total N of the petioles compared to the no N treatment.  The NO₃-N and NH₄– petiole values measured at bloom were in excess of 2500 and 1800 ppm, respectively.  These values would be considered excessive by some.

A N budget for vines in the fertilized (‘+N’) and non-fertilized (‘–N’) treatments was determined at fruit maturity and at the end of the season (after leaf fall).  While it can be surmised from the previous paragraph that the vines in this vineyard may have been N deficient, vines from the ‘–N’ treatment still accumulated 58.9 g N/vine (78 kg N/ha; 69 lbs N/acre) in the leaves, stems, fruiting canes and clusters at fruit maturity with 82.2 g N/vine (109 kg/ha; 97 lbs N/acre) in the trunk and roots.  The amount of N in the leaves, stems, fruiting canes and clusters at fruit maturity for the ‘+N’ treatment was 72.9 g N/vine (96.5 kg/ha; 86 lbs/acre) while that in the trunk and roots was 103 g N/vine (136 kg/ha; 121 lbs/acre). N fertilizer recovery efficiency (RE_N) can be determined by comparing the uptake of N in plants with that of a non-fertilized treatment.  Vines in the ‘+N’ treatment accumulated 35.1 g N/vine more than that of the non-fertilized control at fruit maturity.  Since 34 g N/vine was applied to the fertilized vines the RE_N would be ~100%.

The RE_N was also calculated for data collected at the end of the season, subsequent to leaf fall.  The non-fertilized control would be another ‘–N’ treatment cohort of vines while the fertilized treatment would be the ‘PH +N’ treatment.  The amount of N taken up from the soil by the ‘PH +N’ treatment was 7.1 g/vine greater than that of the ‘–N’ treatment.  Since the ‘PH +N’ treatment was fertilized with 25.5 g N/vine, the RE_N would be ~ 28%((7.1/25.5) * 100).  Therefore, under the conditions of this study, a fertilizer application one month after budbreak and at berry set was more efficient than the post-harvest application of N.