Besides that which is essential for photosynthesis, namely carbon dioxide, water, and sunlight, grapevines also require a range of nutrients in order to grow, survive and prosper. These nutrients are split into two groups depending on scale of requirement, macro and micronutrient, the former being those required in larger amounts. Nitrogen is the most abundant soil‐derived macronutrient in the grapevine. It plays a major role in all processes and a significant amount of nitrogen is essential for normal vine growth. In viticulture a nitrogen deficiency may affect key metabolic functions and retard shoot development and bunch formation. In winemaking a shortage of yeast assimilable nitrogen can result in problematic fermentations. In this article I will explore nitrogen in viticulture from soil to bottle.
Nitrogen in the soil
Nitrogen (N) constitutes about four-fifths of Earth’s atmosphere and is abundant in the Earth’s crust at around 0.3 part per 1,000. Nitrogen constitutes on average about 16% by weight of complex organic compounds known as proteins, present in all living organisms.
Plants require more nitrogen than any other nutrient; however, as is the case with potassium, only a small portion of the nitrogen in soil is available to plants, 98% is in organic form. These organic forms consist mainly of plant residue, animal manures, sewage and soil organic. In these organic forms nitrogen is present as part of proteins, amino acids and other plant and microbial materials. Whilst abundant in nature, these organic forms are not soluble and thus not available to plants.
In contrast, plants can readily take up mineral forms of nitrogen, including nitrate (first nitrite) and ammonium. Both nitrate and ammonium are ions, nitrate (NO3–) is a negatively charged anion and ammonium (NH4+) a positively charged cation. It is in part the net electrical charge of these ions that helps drive chemical reactions. Through active transport (plants also take up nutrients through passive transport), these ions are absorbed by the plant. Generally in aerated soils, nitrate predominates, and if it is not absorbed by plant roots or utilised by microorganisms, it is available for leaching. Nitrate leaching occurs because nitrate has a very weak affinity to form surface complexes with soil minerals (anion exchange capacity) and most soils more strongly adsorb cations (cation exchange capacity) compared with anions.
The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into various chemical forms (specifically those available to plants) as it circulates amongst atmospheric, terrestrial, and marine ecosystems. Although nitrogen is abundant in the atmosphere as diatomic nitrogen gas (N2), it is extremely stable, and conversion to other forms requires a great deal of energy. The nitrogen cycle has five primary steps. Nitrogen fixation, nitrification, assimilation, ammonification and dentrification.
Nitrogen fixation is the process by which gaseous nitrogen (N2) is converted to ammonia (NH3 or NH4+) via biological fixation or nitrate (NO3) through high-energy physical processes. Gaseous nitrogen (N2) can be converted directly into nitrate (NO3-) through processes that exert a tremendous amount of heat, pressure, and energy. Such processes include combustion, volcanic action, lightning discharges, and industrial means. However, a greater amount of biologically available nitrogen is naturally generated via the biological conversion of gaseous nitrogen (N2) to ammonia (NH3/ NH4+). A small group of bacteria and cyanobacteria are capable using the enzyme nitrogenase to break the bonds among the molecular nitrogen and combine it with hydrogen. Nitrogenase only functions in the absence of oxygen. The most important soil dwelling bacteria, rhizobium, live in oxygen-free zones in nodules on the roots of legumes and some other woody plants.
Nitrification is a two-step process in which ammonia (NH3/ NH4+) is converted to nitrate (NO3-). First, nitrifying bacteria convert ammonia (NH3 to NO2-), and then another soil bacterium oxidises NO2- to NO3-. Assimilation is the process by which plants and animals incorporate the nitrate (NO3-) and ammonia (NH3 or NH4+) formed through nitrogen fixation and nitrification. Plants take up these forms of nitrogen through their roots (passive or active transport) and incorporate them into plant proteins and nucleic acids. Animals are then able to utilize nitrogen from the plant tissues.
Assimilation produces large quantities of organic nitrogen, including proteins, amino acids, and nucleic acids. Ammonification is the conversion of organic nitrogen into ammonia. The ammonia produced by this process is excreted into the environment and is then available for either nitrification or assimilation. Denitrification is the reduction of nitrate (NO3-) to gaseous nitrogen (N2) by anaerobic bacteria. This process only occurs where there is little to no oxygen, such as deep in the soil near the water table. Hence, areas such as wetlands provide a valuable place for reducing excess nitrogen levels via denitrification processes.
Neither plants nor animals can obtain nitrogen directly from the atmosphere. Atmospheric reactions and slow geological processes controlled Earth’s earliest nitrogen cycle. Around 2.7 billion years ago, a linked suite of microbial processes evolved to form the modern nitrogen cycle with robust natural feedbacks and controls. Over the past century the development of new agricultural practices to satisfy a growing global demand for food has drastically disrupted the nitrogen cycle.
Availability of nitrogen is not static across sites, studies have shown that there are a range of factors which influence the availability of nitrogen to plants. More specifically there are a number of factors which inhibit nitrogen fixation, the stage of the nitrogen cycle in which gaseous nitrogen (N2) is converted to NH3 or NH4+ or NO3-.
Leaching – Leaching is the loss of water-soluble plant nutrients from the soil due to rain and irrigation. There are a range of factors which may affect the rate and likelihood of leaching, namely soil structure, cover crop planting, and type and application rates of fertilisers. Ammonium (NH4+) is a cation, most soils have a relatively high cation exchange capacity, as such they are able to hold on to cations fairly well. In contrast nitrate (NO3-) is an anion, most soils have poor anion exchange capacity and as such nitrate (NO3-) is more readily available for leaching.
Soil type should also be considered in regards leaching potential, coarse-textured soils (sandy) have a lower water-holding capacity and therefore are more likely to lose nitrate to leaching compared to fine-textured soils (silt loam or clay loam). Nitrate can be leached from any soil if rainfall or irrigation moves water through the root zone. Soil type is also important in determining soil moisture and temperature, studies have shown that soil moisture and temperature influence soil mineral nitrogen availability. For this reason irrigation and soil water-holding capacity should be considered when comprehending nitrogen in the soil.
Soil pH – The cation exchange capacity of a soil depends on the pH of the soil. At low pH (acidic soil) the CEC decreases. This is because at low pH hydrogen ions displace other exchangeable ions from the soil (more competition for CEC sites). Once these hydrogen ions have displaced other exchangeable ions they are more easily leached from the soil, making them unavailable. At high pH (basic soils) the CEC increases. At high pH the OH- ions remove the hydrogen ions (which previously displaced other exchangeable ions) from the clay, creating a negative charge and less competition for CEC sites.
Additionally, low pH soils have been shown to inhibit nodulation rate of legumes, these nodules play a critical role in the symbiotic interaction which fixates soil nitrogen. Further studies have shown that high pH soils are often related with high levels of salinity which is known to reduce nitrogen fixation (possibly due to inhibited osmosis). For this reason it is important to monitor and understand soil pH levels to achieve optimum nitrogen fixation and limit availability of minerals toxic in large quantities such as manganese.
Soil fertility and herbicide/pesticide use – Throughout several steps of the nitrogen cycle successful conversion depends on the presence of beneficial microorganisms in the soil. The use of herbicides and pesticides in conventional farming has been commonplace for a number of years, indiscriminate use may seem harmful; however, over time studies have shown that common landscape herbicides inhibit soil bacteria and glyphosate reduces the growth and activity of free-living nitrogen-fixing bacteria in soil
For many years little was known about the true impact of extensive herbicide and pesticide use, particularly in the field of soil microbiology. However, it is now clear that even indiscriminate use over a number of years can greatly impact soil fertility and nitrogen fixation. If winemakers wish to maintain healthy yields for decades to come, whilst minimising the cost of supplementation, and reducing their impact on the nitrogen cycle they ought to be economic with their use of herbicides and pesticides and consider a shift toward organic viticulture, focusing on biodiversity.
Cover crop – Intrinsically linked to soil fertility is the , selection planting and maintaining of cover crop, thus discouraging monoculture in the vineyard. As has already been demonstrated, the nitrogen cycle is complex and relies upon many interconnected factors. A monoculture can inhibit the efficacy of the cycle (thus increasing the likelihood of supplementation needs) by inhibiting the survival of vital cover crop. Planting cover crop has many benefits, whilst legume crops fix soil nitrogen via their nodulation, non-legume crops are beneficial in their ability to scavenge and trap soil nitrogen, discouraging and preventing to some extent leaching. Cover crop can affect nitrogen fertiliser uptake, this should be considered when developing a fertiliser program.
Function in the vine
Nitrogen in viticulture has a major impact on vine development, shoot growth, yield and sensitivity to fungal diseases such as botrytis. Moreover, nitrogen influences the synthesis of primary metabolites such as sugar and organic acids, as well as that of secondary metabolites such as amino acids, total phenolics, flavonoids and aroma compounds such as volatile thiols and their precursors.
The reason that nitrogen is so important in the grapevine is that grapevines use nitrogen to build essential compounds including proteins, enzymes, amino acids, nucleic acids, and pigments including chlorophyll and anthocyanins. These compounds could be considered the building blocks of the grapevine. It is for this reason that nitrogen is inherently linked to vine vigour. Too much nitrogen results in excessively vigorous vines and too little results in insufficient vigour. Research is limited on nitrogen concentration in grapevines; however, studies have shown that the grapevines translocates and remobilises nitrogen throughout the growing seasons with concentration varying through flowering, berry-set, berry growth, veraison, ripening, and harvest.
Nitrogen concentration has been shown to be highest in the leaves of the secondary shoots at flowering. A decrease of nitrogen content has been measured in the primary leaves after flowering, indicating a remobilisation toward the clusters. Later in the season, during veraison, leaves translocate nitrogen to permanent organs and primary stems. For this reason pruned wood and fallen leaves account for the largest nitrogen removal from the vine after clusters. This movement through the vine at key stages of requirement further highlights the importance of nitrogen to the proper function and development of the grapevine.
Deficiency and excess in viticulture
Generally speaking more nitrogen in viticulture than is ‘necessary’ is associated with vines displaying excessive vigour. High or excessive nitrogen can have an adverse effect on productivity of the vine in terms of cluster growth and ripening, this is due to vigorous growth of the canopy leading to shading and subsequent reduction in fruit set and poor bud fertility. Research has shown cluster weight, cluster length and yield values increase depending on the nitrogen dose, so finding a balance between too much and too little nitrogen is key for optimum yield and growth.
Vines low in nitrogen generally display low vigour and poor production, as a result of reduced protein synthesis and photosynthesis. Vines deficient in nitrogen will also display a yellowing of all leaves and green tissue. This symptom is indicative of a lack of chlorophyll content in the leaves, which is evidence of reduced photosynthetic capacity. Leaves in this state are known as chlorotic. Yellowed leaves may defoliate mid-season, which can lead to delayed ripening and in extreme cases defoliation and loss of bunches. Nitrogen deficient vines may produce smaller bunches with fewer and smaller berries as they do not possess sufficient nitrogen to perform required growth.
Nitrogen is mobile within the grapevine, as such it can, and does, move from mature organs to areas of new growth. Because of this, deficiency symptoms often appear first on older leaves, symptomatic leaves may also fall prematurely.
Management in the vineyard
Where deficiencies or excess exist, they must be managed, in order to effectively manage, vineyard managers must first test for plant nutrient status. Whilst nitrogen deficiency can be identified via leaf discolouration and inhibited growth, this is reactive opposed to proactive, by the time leaf discolouration has occurred the vine is deficient and the damage has to some extent already been done. Whilst soil organic matter and nitrogen can be tested for it is worth performing annual tissue sampling in order to take a proactive approach.
Every year vast amounts of nitrogen are removed from the vineyard in both pruning and fruit. Fruit contains about 0.18% nitrogen and pruning around 0.25%. This means that for a vineyard cropping at 4 ton/acre with a 5:1 yield to pruning weight ratio, the site would lose around 18.4lbs of nitrogen per acre. A general rule of thumb is that around 30-50lb of nitrogen per acre are required at bloom time and this can be applied as broadcast fertiliser but is more efficient if applied via fertigation. Typically nitrogen is applied to soil as calcium nitrate due to the acidifying nature of a number of other nitrogenous fertilisers. Further information regarding nitrogen fertilisation can be found here.
Timing of nitrogen application is extremely important for fertiliser use efficiency, application should coincide with periods of maximum soil nitrogen uptake. The vine tends to utilise internal nitrogen reverses from bud break to bloom. The most effective time to apply nitrogen fertiliser to the soil is around bloom. For further information regarding nitrogen management in the vineyard, Lodi Growers offer a comprehensive guide.
When applying nitrogen supplementation optimum YAN status of the must ought to be considered. A range of nitrogen supplementation options are available, each demonstrating varying influence on the vine, some influencing vegetative growth and some YAN status, this will be explored below.
Yeast assimilable nitrogen
In addition to its critical role in grapevine growth and fruit development, nitrogen also plays a key role in fermentation. Yeast assimilable nitrogen is a critical grape nutrient for yeast growth and fermentation activity and affects the rate and completion of fermentation, fermentation bouquet and style of wine.
There are several nitrogenous compounds found in the must including peptides, larger proteins, amides, biogenic amines, pyridines, purines and nucleic acids. However, not all of these can be directly used by yeast for metabolism. The lack of protease enzymes, which break down larger peptides into smaller components, limits the size of the molecules that yeast can use as a source for nitrogen. The amount of YAN that winemakers will see in their grape musts depends on a number of components including grape variety, rootstock, vineyard soils and viticultural practices as well as the climate conditions of particular vintages.
Low levels of YAN (less than 150ppm) can lead to the production of hydrogen sulphide, stuck and sluggish ferments, less fermentative volatiles and increased higher alcohols. Excessive YAN (more than 400ppm) can lead to biogenic amine formation, high protein content, overly vigorous fermentation and increased risk of spoilage microbes. An ideal level of YAN is between 150 and 400ppm but should be taken in context of grape variety and desired style. The AWRI provide further information on YAN levels.
When vineyard managers apply nitrogen to the soil it will generally have a greater impact on the vegetative components of the vine. However, when applying nitrogen via foliar spray it has a dramatic impact on YAN concentrations. Work out of Virginia Tech has shown that nitrogen applied in the form of urea foliar spray achieved YAN of above 150ppm where as a range of soil applications were not able to achieve this minimum requirement. Application time of urea foliar spray is important, foliar urea spray should be applied around veraison with low dose, high water rates and be applied early in the morning to avoid sunburn. As a footnote, high YAN fruit is susceptible to botrytis and so this should be monitored.
Deficiency and management in winemaking
Where deficiencies or excess exist, they must be managed, in order to effectively manage, winemakers must first test the juice for nutrient status. Juice can be sent to a laboratory for analysis where tests will determine whether YAN is sufficient to achieve ‘successful’ fermentation.
A common practice amongst winemakers is to make a standard addition of DAP to the juice or must (100-300mg/L) at inoculation without measuring the nitrogen concentration. However, DAP addition can have significant flavour consequences and measuring initial nitrogen concentration provides the opportunity to adjust DAP addition appropriately. Furthermore, DAP supplementation is more expensive than foliar spray and so application of foliar in the vineyard is more economical. Where DAP supplementation is required the AWRI DAP calculator is a useful resource.
Not only is foliar urea spray cheaper than DAP, Virginia Tech has shown that wines made from grapes to which foliar urea were applied resulted in wines with greater aromatic complexity.
For further information on YAN and DAP supplementation, the AWRI resource is detailed and well referenced. It is commonplace for producers to be inclined to avoid intervention in the winery where necessary, opposed to adjusting the must here, the preference will almost always be to make these corrections in the vineyard before the grapes reach the winery, here corrections are more integrated and less prone to error.