- Fruits: apples
- Crop Production: nutrient cycling
- Education and Training: on-farm/ranch research
- Soil Management: nutrient mineralization, organic matter, soil analysis
The evaluation of three ground floor management systems for organic apple orchards suggests that the Swiss Sandwich System (SSS) is the most promising for Michigan and other states with similar climate. Mulching (alfalfa hay) increases significantly the amount of available nitrogen and organic matter in the soil compared with the other two systems. Flaming and SSS release the same. Mulching actually releases too much nitrogen risking leaching problems.
Root dynamics were affected from the treatments under evaluation behaving differently depending on time of the year, position related to the tree, and depth in the soil profile.
The systems affect the soil food web with mulching and SSS creating a comparable environment while flame has the lowest population numbers.
Organic agriculture focuses on soil management systems and a common phrase used to characterize organic growing is, “Feed the soil, not the plant”. Organic matter plays a central role in the building of a healthy and productive soil (Magdoff and van Es, 2000). In organic fruit production, in contrast to conventional production that relies on regular fertilization, one of the main goals is to build organic matter in the soil (Bloksma, 2000). Organic matter is also an indicator of carbon sequestered in the soil (Stevenson, 1994).
A goal in organic agriculture is the development and implementation of an integrated approach to agriculture that considers potential impacts on the environment and the soil (FAOa).
Consideration of biological, chemical and physical implications of land use and management practices and of ecological principles will allow agricultural productivity to be sustained in low and high input agricultural systems. Effective soil biological management will provide opportunities for enhancing productivity and for the restoration of degraded soils (FAOb). Orchard floor management systems are the main tool to implement these concepts.
Unlike herbaceous crops, limitations exist in woody perennial crops that restrict the incorporation of soil amendments. Also, in organic fruit production there is the need for a more sustainable approach to weed management compatible with organic protocols that provides careful utilization instead of suppression (Sooby, 1999).
Orchard floor management systems (OFMS) affect soil conditions and consequently nutrient availability, tree growth and yield (Yao et al, 2005). Organic growers have to rely on the soil food web to obtain the nutrient availability for the plants since the N release from different sources varies with the quantity and quality of the material used and the soil environment (Myers et al., 1997; Wardle and Lavelle, 1997; Magdoff and van Es, 2000; Brady and Weil, 2002). Orchard floor management systems will change the microbial composition of the soil (Yao et al, 2005).
There have been several studies on the effects of soil management systems on soil microbes and the structure of the soil food web (Mader et al., 2002; Ferris et al., 2004; Yao et al., 2005). Most of these studies were on herbaceous crops (Ferris et al., 1996; Robertson et al., 1997) and very few on apples (Forge et al., 2003; Yao et al, 2005).
Rhizosphere microorganisms may affect the hormonal balance and competitive ability of the plant, which will modify the soil biology community and the soil ecosystem (El-Shatnawi and Makhadmeh, 2001). Factors that are more likely to enhance stability of the soil microbial biomass are more likely to enhance nutrient conservation in the soil (Wardle and Nicholson, 1996).
Alleviating stress on the microbial community has stabilizing effects (Wardle, 1998). Factors which stabilize the microbial biomass reduce turnover, and are likely to have important consequences for soil nutrient dynamics and ultimately plant growth and ecosystem productivity (Wardle, 1998).
The microorganism activity in the organic system influences the release of available nitrogen to the trees in the soil (Forge et al., 2003).
Orchard floor management systems can provide organic forms of C and N which are quickly metabolized to inorganic nitrogen and other nutrients, primarily by bacteria and fungi (Ferris et al., 1998). Nitrogen is also mineralized as predators of bacteria and fungi, such as protozoa and microbivorous ("microorganism eating") nematodes, graze on prey which contain more N than required by the predators (Ferris et al., 1998). Although more research attention is given to the plant parasitic nematodes, microbivorous organisms make up a large portion of the nematode community. Excess N generated by grazing is released to the soil and becomes available for plant uptake (Ferris et al., 1998). Nematodes feeding types play an overall positive contribution to soil and thus ecosystem processes (Yeates, 1987; Bongers and Ferris, 1999; Bardgett et al., 1999).
Nematodes can be used as indicators of soil functional and biodiversity aspects (Yeates, 2003) since they reflect the soil and ecological processes (Yeates, 1999). Most of the studies on the benefits of nematodes on nutrient release and availability in the soil have been done on the bacterial and fungal feeder nematodes. Bacterial feeders are responsible for a greater release of N when soil conditions are closer to optimal (Ferris et al., 1996; Ferris et al, 1997; Laakso et al., 2000; Forge et al., 2003) while fungal feeders have a higher effect when soil conditions are not optimal (Ingham et al., 1985; Ferris et al., 1998; Ferris and Chen, 1999; Ferris et al., 2004). A food web becomes more beneficial with increased structure and diversification (Power, 1992; Paine, 1996; Sugihara et al., 1997; Garlaschelli, 2004).
The interaction between the OFMS and the soil food web has an impact on edaphic conditions and nutrient availability. The degree to which kinetics of nutrient uptake or other potential adjustments are expressed would ultimately depend on soil nutrient availability and soil factors that determine nutrient transport to the root surface (Bassirirad, 2000). The retention of nutrients within an ecosystem depends on temporal and spatial synchrony between nutrient availability and nutrient uptake (Tierney et al., 2001).
Orchard floor management systems that support the most diversified food web will be able to reach a balance in time with an increased probability of a self sustainable environment, thus increasing the long term productivity of the plants.
Organic horticulture is becoming one of the fastest growing sectors in the agriculture economy (Dimitri, 2002). There is a worldwide growing interest in the development of sustainable production systems for food production (Yussefi, 2004).
Organic agriculture limits the inputs used to those considered environmentally and economically sustainable when compared with conventional methods. It becomes then, a challenge to overcome issues such as pest, weed control, and fertilization.
There is the need to identify management systems that are productive under these constraints.
In tree fruit production, Orchard Floor Management Systems (OFMS) are developed to create the best environment for tree growth allowing the maximization of its performance (Weibel, 2002). A successful OFMS needs to increase soil fertility, physical and biological properties, to supply nutrients to the trees, while suppressing the competitive effects of weeds without the use of traditional herbicides, and minimizing insect and disease pressure.
Orchard floor management practices have been developed and adopted by commercial fruit growers to satisfy practical needs. Mulching (with organic or inorganic material) keeps the soil free from vegetation competition, conserves soil moisture, keeps temperature constant, increases organic matter through its decomposition (in the case of organic mulches), releases nutrients to the soil, and improves the soil environment enhancing the microbial activity (Merwin and Stiles, 1994; Merwin et al., 1995; Marsh et al., 1996; Lloyd et al, 2000).
Tillage also keeps the soil free from vegetation competition but can impede internal water drainage, cause surface organic matter losses (Merwin and Stiles, 1994) and disrupt surface roots (Cockroft and Wallbrink, 1996).
Additionally, tillage can cause surface water run-off and soil erosion. It is still widely used globally even if it is expensive, uses precious petrol fuel and requires specialized machinery.
Recently a modified tilling system has been implemented in Switzerland, called the Swiss Sandwich System. It consists of a strip where spontaneous vegetation is allowed to grow on the tree row and two shallow tilled strips at each side of the tree row. This system encourages predator insects to complete their cycle in the volunteer vegetation that grow on the tree row, becoming more efficient in limiting pests and diseases, and increasing biodiversity (Luna and Jepson, 1998; Horton, 1999; Schmid and Weibel, 2000; Galoach, 2002). The resulting vegetation in the tree row can be considered as cover crops, contributing to the system their significant benefits to organic production systems aiding in the prevention of soil erosion, increased soil organic matter, facilitation in the recycling of soil nutrients, and reduction in the amount of nitrate runoff and leaching from the soil (Miles and Chen, 2001). The Swiss system is easy to manage since there is no need to reach under the tree canopy to mow weeds or till (Schmid and Weibel, 2000; Weibel, 2002; Weibel and Haseli, 2003). The two strips of shallow tilled bare soil have the effect of reducing vegetation competition for water and nutrients (Merwin and Ray, 1997).
Flaming is another effective practice in use by organic growers (Gourd, 2002; Robertson, 2003), with relative little known regarding the effects of this system, besides vegetation suppression. It has, however, drawbacks associated with its practice, such as fires, damage to the trees (Zoppolo, 2004) and the need of special equipment.
All of these systems achieve the goal to keep a certain amount of soil surface free from competitive vegetation which can have a negative impact on tree growth (Parker, 1990; Welker and Glenn, 1991; Merwin and Ray, 1997). Without the use of herbicides, vegetation management requires more thought and management skills by growers in organic production (Webster, 2000).
It is clear, that all of these OFMS have a different approach and will have different results, since they are difficult to standardize or compare, even if they somewhat achieve the same results. It will then be left to the trees to overcome eventual stressful situations created in the soil from the OFMS.
Another form of management that growers can use to overcome this problem is the selection of appropriate rootstock, irrigation and nutrient management. There is a wide variety of specifically selected apple rootstocks that have been developed and released over many years. Each rootstock differs in its ability to adapt to soil conditions (Ferree and Carlson, 1987), disease resistance, and influenced vigor and production characteristics of the scion. There are extensive research programs that center on rootstock evaluation for the above mentioned characteristics. The evaluation is performed with conventional practices and under optimal growing conditions unless different conditions are strictly required. This factor alone reduces the utility of the rootstock outside of those conditions, thus inducing the growers to adapt their orchard conditions to the optimal one in which the rootstocks were tested. Environmental factors seem to be more influential on the uptake of nitrogen and phosphorus than the rootstock genotype (Kennedy et al., 1980) but not enough is known on the subject. There is a strong relationship between genetic (vigor) and environmental factors in determining the adaptability of the root system and consequently its capability of nutrient uptake and tree performance under adverse conditions.
Since organic OFMS do create environmental conditions that are different from the conventional practices in which rootstocks are evaluated, perhaps rootstock selection can compensate and overcome these differences.
It will then be of great value to obtain information on rootstock performance as a response to OFMS. In this study we evaluated three rootstocks of different vigor managed with three different organic OFMS (mulching, flaming and the new Swiss sandwich system).
One of the major challenges in organic fruit production is the implementation of successful orchard floor management systems (OFMS). The lack of herbicides in organic horticulture requires OFMS that limit or restrict ground cover competition so tree performance does not suffer.
Whichever the OFMS selected, organic growers rely primarily on soil microbial processes to obtain the nutrients for plants; thus, OFMS are extremely important because they have an effect on soil conditions and consequently on nutrient availability, tree growth and yield (Yao et al, 2005). Orchard floor management systems will change the microbial composition and food web of the soil (Yao et al, 2005). The absorption of nutrients within an ecosystem depends on temporal and spatial synchrony between nutrient availability and nutrient uptake (Bassirirad, 2000; Tierney et al., 2001).
Plant roots can alter their water and nutrient acquisition capacity by adjusting their physiological longevity, morphological and/or architectural characteristic to meet changes in shoot nutrient demand (Chapin, 1980; Clarkson and Hanson 1980; Clarkson 1985). Therefore, it is useful to evaluate root characteristics that play a role such as lifespan and turnover, as indicators of growth potential of the plant and its actual nutrient acquisition (Bakker, 1999).
Roots, and fine roots in particular, play a central role (Schulze et al., 1997) in soil chemicals (pH, O2, CO2 and other ions), physical (moisture and aeration), and biological (soil pathogens, beneficial microorganisms and allelopathy) composition (El-Shatnawi and Makhadmeh, 2001) with important consequences for plant growth and productivity, plant competition, biological activity, and carbon and nutrient cycling at an ecosystem scale by releasing in the soil a wide variety of exudates (Makhadmeh and El-Shatnawi, 2001; Bertin et al., 2003; Walker et al., 2003). Plants expend a substantial proportion of photosynthate below ground in the annual production of fine roots (Eissenstat et al., 2000) and release of exudates (Walker et al., 2003). In many cases, more than 50% of annual net primary production (NPP) is allocated below ground, and nutrient return to the soil by tree fine root death may exceed that by the above ground litterfall (Kasuya N., 1997). Tree root turnover may return four to five times more carbon to the soil than above ground litter (Zech and Lehrmann, 1998). Consequently there is a need to include the effects of root turnover in models of carbon and nutrient cycling (Cox et al., 1978; Vogt et al., 1986; Hendricks et al., 1993; Jackson et al., 1997; Norby and Jackson, 2000).
Moreover, through the exudation of a wide variety of compounds, roots regulate the soil microbial community in their immediate vicinity, cope with herbivores, and encourage beneficial symbioses (Nardi et al., 2000, Walker et al., 2003).
Assessing root turnover becomes then, very important as an indicator of plant productivity and a measurement of carbon return to the soil. In the last decade the most common technique to measure fine root turnover is by minirhizotrons installed in the ground. They can be used to characterize fine root production, phenology, growth, mortality and lifespan, and are useful in developing ecosystem carbon budgets (Hendrick and Pregitzer, 1996; Majdi, 1996). Their reliability depends on the accuracy of assessing physiological status of the roots (alive or dead) (Wang et al, 1995; Tingey et al, 2000) mostly based on color (Hendrick and Pregitzer 1992a, b; Comas et al, 2000).
However, the determination of fine root physiological status is a common problem (Comas et al., 2000). Also, there is a certain inconsistency in the definition of the proper diameter for fine roots. Several authors reported that fine roots, for woody perennials, should be ≤ 1 mm in diameter (McCrady and Comerford, 1998; Eissenstat et al, 2000; Comas and Eissenstat 2001).
Processes and responses vary not only with root diameter but with other root characteristics as well (Norby and Jackson, 2000). It is necessary to couple the total size of the root system with physiological information on the response of root specific nutrient uptake efficiency (Norby and Jackson, 2000). The size of the root system and its efficiency to deploy roots at the time and place nutrients are present (Fitter et al., 1991), as well as the efficiency by which a particular root segment can take up a nutrient from the soil solution (Norby and Jackson, 2000), are fundamental for the plant uptake capacity (Tjeerd et al., 2001). The degree to which kinetics of nutrient uptake or other potential adjustments are expressed would ultimately depend on soil nutrient availability (Tjeerd et al., 2001) and soil factors that determine transport to the root surface (Bassirirad, 2000). Nutrient adsorption, and eventually absorption within an ecosystem, depends on temporal and spatial synchrony between nutrient availability and nutrient uptake, and disruption of fine root development (Tierney et al., 2001).
In general, increased N concentrations in the soil decrease fine root turnover because of their increased lifespan (Pregitzer et al., 1993; Ostonen et al., 1999; Burton et al., 2000; Hendricks et al., 2000; King et al., 2002) allowing the plant to reduce carbon costs. However, several authors reported that with increased N concentration in soil, root turnover rates increase, reducing fine root lifespan (Aber et al, 1985; Nadelhoffer et al., 1985; Persson and Ahlström, 2002). Burton et al. (2000), in an experiment on fine roots (<1 mm) dynamics within and across forest species with different N availability, found that, across species root lifespan decreases with increased N availability, while within species there is the opposite trend. Tjeerd et al. (2001) found that in P depleted soils citrus fine roots life span was diminished.
In any case, all the authors agree on the importance of the interaction between N availability and fine root dynamics, especially for the effects on organic matter (McClaugherty et al, 1982), on the organic N pool in the soil (Ehrenfeld et al., 1997), and on the control of the substrate quality (Hendricks et al., 2000).
Root turnover has been extensively studied in grasslands and forests ecosystems but there is not much literature on fruit trees and none in organic tree production.
The established interaction between OFMS, food web, and root processes could create a different response from the roots that needs to be studied and compared between conventional and sustainable conditions.
In this experiment we evaluated three different OFMS for their rationale and input of organic material to the soil. One has a constant addition of N containing organic material (alfalfa hay mulch added twice yearly), another has the constant destruction of organic material on the soil surface (flaming), and the last one is a combination between modified cover crop (natural vegetation never mowed) and tillage.
We hypothesize that these differences in management will have an impact on the soil N concentration and organic matter content and therefore will impact the soil food web structure and diversity, tree productivity, rootstock adaptation, and fine root dynamics.
Investigate the behavior of apple trees fine roots (timing and rate of growth, and their turnover) subjected to two different ground managements under the organic protocol.
Amount of carbon sequestered in the soil by the trees.
Effects of fine root turnover on nutrient cycling, food-web and soil sustainability.
Introduction of the best ground floor management system for the desired growing conditions.