Soil microorganisms concentrate in larger numbers near the soil surface in areas of plant-root interface. Conditions in the zone around the plant roots (rhizosphere zone), such as availability of organic substances, favour microbial activity by providing an energy source. Likewise, soil microorganisms can affect the availability of nutrients available for plants, stimulating the growth and development of plants and associated microbes. Farming practices such tillage, fertilizer and pesticides applications can affect the way soil microbiota function in any given agro-ecosystem. These practices can affect nutrient availability and, therefore, fertilizer management decisions.
A beneficial type of association formed between plant roots and a group of soil fungi are known as "Mycorrhizae". Around 96% of plant species can form this type of partnership. Mycorrhizae consist of structures formed by fungal filaments around or inside the roots of plants (Figure 1). Nutrient movement occurs through the fungal network present inside the roots and in the soil zone surrounding the roots. These fungal networks help transport of nutrients and water directly into the plant roots. The plant provides the fungal network with carbon-based nutrients. The network of fungal filaments in the root-surrounding soil greatly increases the absorbing surface area of the roots. "Arbuscular mycorrhizal fungi" (AMF) is a type of mycorrhizae important to most agricultural plants. Inside the plant roots, the fungus form structures responsible for nutrient exchange which are called "arbuscules". The term "arbuscules" refers to the formation of microscopic tree-shaped structures (Figure 2). Several crops important to agriculture such as flax, corn, wheat and barley can form mycorrhizae.
Some crops, for example canola, do not form this type of association and might affect the condition of the fungal network for the following crop. Depending on tillage system, the fungal network might be readily active as seedlings germinate allowing symbiosis to start immediately or it might have to form from fungal spores, in which case symbiosis will start at a later date. Under zero tillage, the fungal filament network left intact from the previous growing season can be readily functional as soon as seedlings germinate. Consequently, these plants could have competitive advantage and start obtaining nutrients and water from fungus filaments at a very early growth stage in the spring. Studies conducted in the past have estimated 6 to 30 yards of fungal filaments are produced per inch of colonized root (Isaac, 1992).
In the field, several factors can affect plant responses to mycorrhizal fungi colonization such as a host crop dependency to mycorrhizal colonization, fertilizer application, and inoculum's potential of the mycorrhizal fungi such as spores and fungi filaments from the previous crop (Jakobsen, Joner, and Larsen, 1994).
Flax is a highly mycorrhizal crop (Figures 1 and 2). It is possible that mycorrhizal associations could be responsible both for part of the positive response that flax shows in no-till systems and for the limited P response observed in recent studies. If so, P fertility requirements in flax could be greatly affected by tillage system and by whether the preceding crop is mycorrhizal or not. Phosphorus fertilization could possibly be reduced or eliminated in flax grown in no-till following a mycorrhizal crop and optimized in flax grown on summer fallow, after a non-mycorrhizal crop, or under conventional tillage management. By more clearly defining the P requirements of flax, canola and wheat grown under different management systems, we may be able to reduce inputs while maintaining or improving crop yield and quality.
Direct seeding practices are used on the prairies to conserve soil moisture, increase crop yield potential, improve soil quality and reduce time, labour and equipment costs in farming operations. While many research studies have evaluated the impact of tillage systems on N fertility requirements, there has been little information collected on the impact of tillage management on P phytoavailability.
Early season P supply is critical to the determination of optimum crop yield. Withholding P during early plant growth will limit crop production and cause a restriction in crop growth from which the plant may not recover. Phosphorus limitation later in the season has a much lower impact on crop production than limitations experienced early in growth. No-till systems reduce early season soil temperature and can increase soil compaction (Grant and Lafond 1994), which may reduce the availability of phosphorus during early growth. Low P supply and slow root growth may combine to cause severe P stress early in the season when plant demand for P will outstrip the soil's ability to supply the nutrient. This may occur more frequently under reduced tillage, where the soil is slightly slower to warm up in the spring and where bulk densities in the soil surface may be increased to some extent (Grant and Lafond 1994). However, it may be that in soils with a history of phosphorus fertilization, as most of our soils have now, starter phosphorus to optimize crop yield may be less important than in the past, if management practices encourage availability of residual phosphorus from the soil. Information on the impact of tillage system and past phosphorus fertilizer management on phosphorus response of crops is limited.
Canola, wheat and flax are three annual crops important to agriculture in the Canadian Prairies. Flax and wheat tend to respond very well to reduced tillage systems, frequently producing higher yields under no-till as compared to conventional till management (Lafond et al. 1994). According to both research trials (Lafond et al. 1994) and producer experience, canola may not respond as beneficially to no-till management as cereal crops or flax. If part of the reason for lower relative yield of canola under no-till is the change in nutrient dynamics, optimization of soil fertility could lead to significantly higher canola yields.
Canola and wheat have a high demand for crop nutrients, including phosphorus (Grant and Bailey 1993). Deficiencies of P are common and frequently limit crop yield. Therefore, proper P fertilization is important in optimising crop production. Although canola requires a large amount of P for growth, maximum responses are often attained at lower rates of P than for wheat, corn or barley. Kalra and Soper (1968) evaluated the efficiency of a number of crops in absorbing soil and fertilizer phosphorus, under greenhouse conditions. Rapeseed and flax used about equal amounts of soil P, but rapeseed absorbed fertilizer P in large amounts. Rapeseed was much more effective than flax in extracting fertilizer P. This is because rapeseed, a non-mycorrhizal plant, could modify its root structure and root hair number, proliferating roots in fertilizer reaction zones and decreasing pH in the rhizosphere. However, rapeseed (canola) is sensitive to damage from seed-placed P and quantities of P required to optimize yield may lead to seedling damage.
Phosphorus fertilization of flax can be problematic, since flax is very sensitive to seed-placed applications of monoammonium phosphate (Nyborg and Hennig 1969). Banded applications of P fertilizer are not generally used effectively by flax unless they are positioned within 2.5 to 5.0 cm of the seed-row (Sadler 1980) and broadcast applications of P tend not to increase flax seed yield (Grant and Bailey 1993). Therefore, unless a producer has access to seeding equipment capable of side-banding fertilizer, P fertilization of flax is frequently ineffective. Most of the studies conducted on P fertilization of flax were conducted under conventional tillage. Cooperative studies being conducted by Agriculture and Agri-Food Canada, coordinated by Guy Lafond (Pers. Comm.), show responses of no-till flax to P fertilizer were generally quite low, in the order of 0 to 2 bu acre, which was generally not statistically significant. In 14 site years of research in Manitoba and Saskatchewan, the P response of the "best" treatment in the experiment exceeded three bu/acre in only three instances. Producers frequently avoid P application in flax and increase the P supply in the preceding crops, in order to supply residual P for use by the subsequent flax crop.
Plant roots-arbuscular mycorrhizae fungi associations can develop under low-P situation, enhancing the uptake of P by the crop. Tillage disrupts the mycorrhizal network. Research at Guelph (Miller 1998) and Agassiz (Bittman et al. 1998) showed that corn produced on summer fallow or under intense tillage was restricted in its ability to access P, while corn which followed a mycorrhizal crop, particularly under no-till, showed improved early season P nutrition. The greater P absorption is largely a result of the undisrupted mycelium present in an undisturbed soil. The mycelium remains viable over extended periods in frozen soil and so can acquire P from the soil and deliver it to the plant immediately upon becoming connected to a newly developing root system in the spring. Phosphorus status of the crop in the first 4-6 weeks of growth has a major impact on final crop yield. Flax is a highly mycorrhizal crop. It is possible mycorrhizal associations could be responsible both for part of the positive response that flax shows in no-till systems and for the limited P response observed in recent studies. If so, P fertility requirements in flax could be greatly affected by tillage system and by whether the preceding crop was mycorrhizal or not. Phosphorus fertilization could possibly be reduced or eliminated in flax grown in no-till following a mycorrhizal crop and optimized in flax grown on summer fallow, after a non-mycorrhizal crop, or under conventional tillage management. By more clearly defining the P requirements of flax, canola and wheat grown under different management systems, we may be able to reduce inputs while maintaining or improving crop yield and quality.
A study was initiated in 1999 to determine the early season accumulation of P and yield response by flax as influenced by preceding crop, P fertilizer application and two tillage system, conventional (CT) and no-till (NT). Since the year 2000, research is being conducted to determine the effect of P-fertilizer, crop rotation and tillage on flax production and its relationship to arbuscular mycorrhizae fungi (AMF) colonization.
Two field locations were selected north of Brandon; both are on Newdale clay loam soils. The Manitoba Zero Tillage Farm site (MZTRA Farm) had been under no-till (NT) for the past 6 years. The Brandon Research Centre site (BRC Farm) had been under conventional tillage. In 1999, canola and wheat were seeded at both sites at three rates of P fertilizer (0, 25, and 50 Kg of P2O5 ha-1). P was side banded as monoammonium phosphate. Both crop and phosphate treatments were randomized within the tillage system, in 2 m by 5 m plots, with 2 sub-plots of each treatment in each of four replications. Flax fertilized with side banded MAP at 0 or 25 Kg of P2O5 ha-1 was seeded on the plots that had been seeded to canola or wheat the previous year.
In 2001, arbuscular mycorrhizae root colonization (%) and biomass yield of flax tissue (Kg ha-1) were determined at five weeks after seeding. Flax roots were sampled by randomly selecting ten flax plants within each experimental plot. Flax plants with soil shoot-root system intact were placed in plastic bags and brought to the laboratory for analysis.
For the years 2000 and 2001, flax biomass yield was consistently higher when grown on wheat stubble than on canola stubble. Part of the effect of preceding crop may be due to the high density of volunteer canola in the flax during early growth, as early season weed competition is particularly damaging in flax. However, mycorrhizal colonization might be a factor.
For the year 2001, the percent area of flax roots colonized by arbuscular mycorrhizal fungi (AMF) indicates higher mycorrhizal formation after wheat than canola, which may have enhanced the biomass yield of flax after wheat in both study sites (Figure 3).
Effect of P fertilization of the flax crop varied with site, and year, with small increases occurring in some situations. Seedling damage may have reduced crop response in some situations. Similarly, P fertilization of the preceding crop did not consistently influence biomass yield.
In 2001, the percent area of flax roots colonized by arbuscular mycorrhizal fungi (AMF) decreased with P fertilization in the preceding crop at the no-till farm (MZTRA Farm site) for flax after canola and flax after wheat (Figure 4). However, the results were the opposite at the BRC Farm site that showed increased AMF colonization in flax at increased P rates where canola was the preceding crop (Figure 5).
Biomass yield was generally higher under NT than CT. For the year 2001, the percent area of flax roots colonized by arbuscular mycorrhizal fungi (AMF) was higher for NT than CT, after canola and after wheat (Figure 6). The biomass yield increase due to NT was greater after wheat than after canola.
Seed yield of flax was higher when grown after wheat than after canola at both locations under both tillage systems. This may reflect early season weed competition from the volunteer canola. Flax seed yield was generally similar in NT and CT. Seed yield of flax was not increased by P application to the flax crop at either location. However, there was a tendency for seed yield to decrease with application of P to flax under NT at the Research Centre location. Flax does not tend to proliferate roots in fertilizer reaction zones and so is relatively ineffective at absorbing P from fertilizer applications. At the No-Till Farm location, P fertilization of the preceding crop led to higher flax seed yield the following year, with the effect being greater when wheat was the preceding crop as compared to canola. Increased residual P from previous fertilizer applications may be as or more available to flax than side-banded P applications.
Phosphorus is needed by the crop early in the growing season to ensure optimum crop yield. In 2000, early season tissue P concentration was positively correlated with biomass yield at 5 weeks (r=0.22, p<0.03). Early-season P concentration in flax tissue was higher after wheat than
canola at the Research Centre farm, but not at the no-till farm. Biomass yield was also higher after wheat than canola, so the difference in Tissue P concentration was not due to dilution effects. Tissue P concentration increased when P was side-banded at seeding with the flax at both locations. Phosphorus applied to the preceding crop also increased P concentration in the flax at both locations, with the effect being greater after wheat than canola at the no-till farm.
In 2000, at both study sites, tissue P was higher under CT than NT after canola, but did not differ with tillage after wheat. At the No-till Farm, P applied in the previous crop had a greater effect under NT than CT when the preceding crop was wheat, while the effect was greater under CT than NT when the preceding crop was canola.
In 2000, concentration of P in the seed at the Research Centre was higher when flax followed wheat than when flax followed canola. This was not due to dilution, as flax seed yield was higher after wheat than canola. Concentration of P in the seed increased with side-banded application of P at both sites. Seed P also increased to a similar extent when P had been applied to the preceding crop. There was an interaction between preceding crop and tillage system, with higher seed P occurring under NT than CT in wheat at both locations. Seed P tended to be slightly higher under NT than CT at the Research Centre, but not at the No-till Farm.
Flax seed yield in 2000 was higher when grown after wheat than after canola. This may reflect early season weed competition and the effect of increased colonization by AMF after wheat. Flax yield in 2000 was generally similar under NT and CT.
At both sites, flax tissue P was higher under CT than NT after canola, but it did not differ with tillage after wheat. At the No-till Farm, P applied in the previous crop had a greater effect under NT than CT when the preceding crop was wheat, while the effect was greater under CT than NT when the preceding crop was canola.
Seed yield of flax was not influenced by P application to the flax crop at either location. However there was a tendency for seed yield to decrease with application of P under NT at the Research Centre Study site. At the No-till Farm, P fertilization of the preceding crop resulted in higher flax yield in the following year, with the effect being greater when wheat was the preceding crop as compared to canola.
In 2001, biomass yield of flax was higher after wheat than canola and after NT than CT at both sites. At the Research Centre, the difference between NT and CT was higher after canola than wheat. Response of biomass to P application was small and inconsistent.
In 2001, arbuscular mycorrizal fungi colonization of flax roots was higher after wheat than after canola at the Research Centre site for both tillage treatments. At the No-till farm, the preceding crop had a smaller effect on percentage AMF colonization in flax, but there was a significant effect of P fertilizer application on preceding crop.
In order to increase P availability for flax, it might be a useful strategy for producers to increase P application in the preceding crop, rather than fertilize the flax crop, and at the same time eliminating the risk of seedling damage.
This project was funded by Potash and Phosphate Institute, Saskatchewan Flax development Commission, Agrium Ltd, Westco Fertilizers, Ltd., United Grain Growers, Cargill, Ltd. and the matching Investment Initiative of Agriculture and Agri-Food Canada.
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Figure 1. Hyphal filaments and spores of arbuscular mycorrhizal fungi (AMF) present around and inside flax roots sampled five weeks after seeding (2001).
Figure 2. Hyphal filaments and arbuscules (tree-shaped structures) of arbuscular mycorrhizal fungi (AMF) present inside flax roots sampled five weeks after seeding (2001).
Figure 3. Percent area of flax roots colonized by arbuscular mycorrhizal fungi (AMF) and flax biomass yield at five weeks (kg/ha), when seeded after a canola (Can) and wheat (Wht) at the BRC and MZTRA Farm sites for P-Till Study 2001.
Figure 4. Effect of P application (0, 25, and 50 Kg of P2O5 ha-1) in the preceding crop on percent area of flax roots colonized by arbuscular mycorrhizal fungi (AM) and flax biomass yield at five weeks (kg/ha), when seeded after a canola (Can) and after wheat (Wht), at the MZTRA Farm Site.
Figure 5. Effect of P application (0, 25, and 50 Kg of P2O5 ha-1) in the preceding crop on flax roots colonized (%) by arbuscular mycorrhizal fungi (AM) and flax biomass yield at five weeks (kg/ha), when seeded after a canola (Can) and after wheat (Wht), at the BRC Farm site.
Figure 6. Effect of ZT and CT on flax arbuscular mycorrhizae root colonization (%) and on flax biomass yield (kg/ha) at five weeks after seeding date at the on Biomass (kg/ha) BRC and MZTRA sites.