Agroforestry, as it relates to viticulture, is concerned with the co-location of trees and vineyards. Trees provide a number of below- and above-ground services which may be of benefit to the viticulturist (documented by Favor). I have previously reported on the impact of trees on vineyard water parameters and continue herein with the effects on soil-based nutrients. The effects portion of this post draws heavily on the Favor work.
Background on Soil-based Nutrients
Adequate amounts of the appropriate nutrients are required to support proper growth of the grape vine, fruit development, and fruit maturity; those nutrients are obtained from the soil by the plant. The table below shows the mineral requirements of the vine plant, the role of each mineral, acceptable ranges of each mineral in the soil, and the impact of mineral deficiency on the vine.
Sources
Rocks
The earth is made up of varying proportions of the 90 or so naturally occurring elements but, according to Alex Maltman (Vineyards, Rocks, & Soils), four of these -- oxygen at 48%, silicon at 28%, aluminum at 8%, and iron at 6% -- are responsible for 88% of its composition. In most geological materials, these elements combine to form minerals -- "a naturally occurring combination of specific elements that are arranged in a particular repeating three-dimensional structure or lattice" (opentextbc.ca, Minerals and Rocks).
In nature, minerals are found in rocks "and the vast majority of rocks are composed of at least a few different minerals." Jackson (Wine Science: Principles and Applications) stipulates that (p. 245) "... the mineral content of soil is primarily derived from the parental rock substrate." The figures below show the weathering of rocks into minerals.
Decaying Organic Material
Jamie Goode (Rescuing Minerality) contends that the bulk of soil mineral content comes "from decaying organic material, not decomposed rock and it is microbial activity in the soil that affects the ability of soil to break down organic matter into mineral ions that can be used by the plant." Maltman agrees with Goode: "... in practice, it's the humus that's more important, indeed essential."
Atmosphere
According to Schwarcz and Schoeninger (Stable Isotope Analysis in Human Nutrition, Yearbook of Physical Anthropology 34, pp. 293-321), almost 100% of exchangeable nitrogen is found in the atmosphere or dissolved in the world's oceans and is transferred from these environments into the biological system through the terrestrial nitrogen cycle.
Cation Exchange
Soil-based nutrients are resident either in the soil solution (water and dissolved minerals in the soil pores) or in the soil matrix (mineral particles and organic matter). Two problems present themselves, however: (i) the concentration of nutrients in the soil solution is low and (ii) the nutrients resident in the soil matrix are immobile. Plant roots have developed adaptions to allow growth into the soil matrix and capture of the nutrients needed for metabolic activity (Dr. Paul Schreiner, USDA-ARS) and we will discuss these later.
Most of the mineral nutrients that the vine needs are cations so the soil's cation exchange capacity (CEC) is a major enabler of it's nutrient acquisition. The positively charged mineral ions bind loosely to the clay and humus colloids in the soil and these minerals are released in exchange for hydrogen ions secreted by the vine roots. (Clay minerals act as harbors for nutrients because the positive ions of the nutrients are trapped by the negative charge of the clay minerals. The abundance and types of minerals determine whether the clay is classed as low- or high-CEC.). The ion that makes the strongest link with the clay is the hydrogen ion "... and its almost as though the vine knows this! The vine's metabolism can prompt its roots to pump out hydrogen ions into the soil water, which then dislodges the other ions held on the clays, thus making them available to the vine roots" (Maltman).
The Effect of Trees on Vine Nutrition Parameters
 |
| Source: Compiled from LGRGP.org and others |
Sources
Rocks
The earth is made up of varying proportions of the 90 or so naturally occurring elements but, according to Alex Maltman (Vineyards, Rocks, & Soils), four of these -- oxygen at 48%, silicon at 28%, aluminum at 8%, and iron at 6% -- are responsible for 88% of its composition. In most geological materials, these elements combine to form minerals -- "a naturally occurring combination of specific elements that are arranged in a particular repeating three-dimensional structure or lattice" (opentextbc.ca, Minerals and Rocks).
In nature, minerals are found in rocks "and the vast majority of rocks are composed of at least a few different minerals." Jackson (Wine Science: Principles and Applications) stipulates that (p. 245) "... the mineral content of soil is primarily derived from the parental rock substrate." The figures below show the weathering of rocks into minerals.
Decaying Organic Material
Jamie Goode (Rescuing Minerality) contends that the bulk of soil mineral content comes "from decaying organic material, not decomposed rock and it is microbial activity in the soil that affects the ability of soil to break down organic matter into mineral ions that can be used by the plant." Maltman agrees with Goode: "... in practice, it's the humus that's more important, indeed essential."
Atmosphere
According to Schwarcz and Schoeninger (Stable Isotope Analysis in Human Nutrition, Yearbook of Physical Anthropology 34, pp. 293-321), almost 100% of exchangeable nitrogen is found in the atmosphere or dissolved in the world's oceans and is transferred from these environments into the biological system through the terrestrial nitrogen cycle.
Cation Exchange
Soil-based nutrients are resident either in the soil solution (water and dissolved minerals in the soil pores) or in the soil matrix (mineral particles and organic matter). Two problems present themselves, however: (i) the concentration of nutrients in the soil solution is low and (ii) the nutrients resident in the soil matrix are immobile. Plant roots have developed adaptions to allow growth into the soil matrix and capture of the nutrients needed for metabolic activity (Dr. Paul Schreiner, USDA-ARS) and we will discuss these later.
The Effect of Trees on Vine Nutrition Parameters
The effect of trees on vine nutrition parameters are summarized in the chart below and described in greater detail in the sections following.
Increased Nutrient Availability
As shown in the chart above, trees increase nutrient availability by increasing organic matter, cycling nutrients from deep to shallow profiles, fixing nitrogen, and transforming nutrients into a more plant-absorbable form.
As regards organic matter (Favor):
- Agroforestry systems have the potential to increase this material by 50 to 100%
- They return an average of 7.4 tons of organic matter/ha/year in the form of prunings
- They also produce organic matter through litterfall, root slough, and root exudates
- Nutrients that take the form of organic matter are released slowly at rates comparable to rates of plant absorption
- They also present in a stable molecular form that is resistant to leaching
- Organic matter produced by trees serves as a source of food for microbes
- Results in increased microbial populations (by as much as 30%)
- Microbes excrete enzymes that mineralize nutrients, stabilize carbon and N in the soil, and decompose organic matter into simple, plant-available forms
- Results in higher plant nutrient uptake
- Results in increased cation exchange capacity
- Translates to a greater ability of soil to hold onto exchangeable cations
- Better retention of applied nutrients
- Resistance from nutrient leaching
- Nutrients that take the form of organic matter are released slowly at rates comparable to rates of plant absorption
Trees cycle nutrients from deeper profiles by accessing those nutrients deep underground, converting them into plant tissue and organic material, and dropping organic material to the ground in the form of leaf-litter and above-ground debris. As this material degrades, it releases nutrients into the upper soil profiles where they become available for use by other crops (Favor).
Almost 100% of exchangeable nitrogen is found in the atmosphere or dissolved in the world's oceans and is transferred from these environments into the biological system through the terrestrial nitrogen cycle illustrated in the figure below.
 |
Source: http://tolweb.org/notes/?note_id=3920 |
Depending on the species, trees can fix nitrogen at average rates of 40 to 200 kg N/ha/yr (Favor).
Finally, the increased microbial life -- resulting from the tree-related increase in organic material -- excrete enzymes that decompose organic material into simple, plant-available forms, resulting in higher plant nutrient uptake.
Reduced Nutrient Loss
By reducing nutrient losses from leaching, erosion, and runoff, trees allow the retention of greater amounts of soil-based nutrients in the vineyard. According to Favor:
- Agroforestry systems increase soil organic by as much as 100% with as little as a 10% increase decreasing soil erodibility by between 13 and 23%
- Litterfall increases ground cover which reduces runoff and erosion
- Surface mulch associated with agroforestry systems reduces the kinetic impact of rainfall, retaining the soil surface structure.
Competition Between Trees and Grapevines
At distances of less than 4 m, there is intense competition for nutrients, especially nitrogen, between plants and grapevines. The negative effects observed were reduced vine vigor and yield with no obvious effect on berry quality. Beyond 4 m, no negative competitive effects were observed.
No comments:
Post a Comment