Mycorrhizal Fungi And Cultural Diversity and Their Significant Increase in No-Till Systems
Originally written by Claudia Maurer et al. and published in TCS in February 2015
The Swiss trial site at “Oberacker” has been comparing tillage and ploughing systems for over 20 years and continually assess the quantity and diversity of mycorrhizal fungi in various different establishment types. The results reveal significantly increased species richness and diversity of mycorrhizal fungi in plots under long term direct seeding.
Since 1994, the long-term monitoring site “Oberacker” in Zollikofen (Switzerland), has had the objective to develop a method of cultivation of farmland which is economically, ecologically and socially compliant. Direct seeding and tillage systems should therefore be optimized taking into account the selection of varieties and crop rotation, the type and amount of fertilizer, selection and application of plant protection products as well as management of straw and green manure.
Soil organisms play a central role, especially in the success of a sub-till cropping system. Like earthworms, which contribute significantly to the structuring of the soil and the decomposition of organic substances, bacteria and fungi act as a “hub” for nutrition and plant health.
Nearly 80% of plants benefit from fungi living in symbiosis with their roots: these mycorrhizal fungi facilitate access to nutrients for plants, especially phosphorus, but also by making nitrogen and water available – thanks to their hyphae – giving access to even the smallest pores of the soil normally unattainable for plant roots. In return, plants provide the fungi some of the carbohydrates they have assimilated (energy). Most species of crops and pastures live in very specific symbiosis with AMF (Arbuscular Mycorrhizal Fungi). Nearly 270 species have been investigated worldwide. Their presence depends mainly on the soil type and the operating methods.
That is why Mycorrhizal Fungi are considered bioindicators of good agricultural soils. Promotion of specific communities of mycorrhizal fungi may be an important factor to a bio-system that ensures efficient absorption of water and nutrients. The objective of this study was to compare the diversity of mycorrhizal fungi on plots operated for several years under direct seeding and of the ploughed plots, to determine the effect of cultivation, to identify the indicator species and to compare the results with the knowledge currently available.
The long-term monitoring site “Oberacker” is located on deep brown soil. Six adjacent land parcels are seeded half on a no-till system and half ploughed. The crop rotation – winter pea protein crops, winter wheat, beans, winter barley, sugar beet and maize silage – lasts six years. In February 2011, soil samples were collected from a depth of 0-10 cm on the twelve subplots. For each sub-parcel, a sample was selected from 20 collected over the entire surface (about 1 kg).
The main crops sampled were: winter pea protein crops, winter wheat and winter barley, two parcels of a mixture of green manures composed of several species and succeeding previous crops of wheat volunteers and winter barley and finally a bit of peas and field beans. Arbuscular mycorrhizal fungi spores were then isolated and identified by light microscopy. The spore density was determined for each case in number of spores per 100 g of soil air dried. Diversity has been characterized as an index for each type of crop and subplot.
Direct sowing leading to stable numbers of species and high diversity
A total of 39 arbuscular mycorrhizal fungi species were identified, including 38 in the no-till system and 25 in the ploughed system (Tables 1 and 2). The number of species identified in the various cultures (subplots) was between 15 and 21 under direct seeding, between 10 and 17 under ploughing.
Comparing averages also showed that the number of MA species is significantly higher under direct sowing (average 18.5) than under tillage (average 13.2). In both systems, a higher number of species (21/17) was identified in winter protein pea plots than in winter wheat (17/15) of green manure mixtures succeeding in winter wheat (17/14) and barley stubble (15/11). In the green manure mixtures succeeding winter barley sowing and harvesting before following by the sugar beet, the number of species identified in the no-till system was also higher than that of pea protein plots, with respectively 21 and 20 species, whereas it was only 12 or 10 species in the ploughing system.
In the case of planting following a preceding sugar beet crop, the explanation may lie in the fact that sugar beet is a species incapable of mycorrhization and during the beet harvest, the soil is strongly stirred in the top 10 cm. This effect appears, however, to be apparent in the ploughed fields regularly for the sowing of the main crop. In the case of direct sowing, the number of species remains high and the interaction between plant and fungi seems to be more stable. Not only the number of species but also their frequency and density of spores are important criteria in the variety description (Table 2).
The comparison of the average values of all types of crops (six subplots) shows a higher diversity index in the no-till system (H = 2.49) than under tillage (H = 2.17; Table 1) although the difference is not significant. In direct seeding system, the specific values vary between 2.12 and 2.86 depending on the cultures, and between 1.77 and 2.56 in the ploughed system. The H values calculated for direct seeding are comparable to those of previous studies in Central Europe with organic cultivation or grassland.
More species found in no-till
The list of species shows that about a third of species may be present regularly in both cultivation systems (Table 2, group A with 13 species and 12 species groups, gray background).
The majority of species, 24, however, were found primarily or exclusively in the direct seeding (no-till) system. Among them, 11 species showed a relatively high density of spores (group B, green background) and 13 species of spores rather low density (group C, yellow backdrop). In the latter group, they have been mostly typical species of extensive exploitation and soil conservation work, or before any typical grassland environments. Among the 39 species or groups of species identified, only two were found primarily or exclusively in the ploughed plots (group D, brown backdrop).
Multivariety analysis clearly distinguished spore communities of both no-till and ploughed systems from each other. Organic carbon in soil (Corg), the cropping system and microbial mass are the variables that individually have the greatest influence on the composition of Arbuscular Mycorrhizal Fungal (AMF) communities. Among the chemical parameters, the Corg and pH were significant. The influence of the cropping system on the AM fungi community is shown indirectly through other parameters, particularly through the highest organic carbon content of the upper layer of soil (0-10 cm) in the ploughed system.
A high diversity of sub-till AM fungi can have a positive impact on the uptake of nutrients by plants, especially phosphorus. Some of the observations presented in Table 2 were confirmed by the redundancy analysis: Funneliformis, Caledonius and Paraglomus sp. BE12 are grouped near the ploughed plots, while the majority of AM fungi species were significantly denser under direct drilling. Other species that were present everywhere (Table 2) showed a more or less close relationship with the no-till system (p. Ex. Fu. Geosporus or Glomus aureum), less with the ploughed system (p. ex. Fu. mosseae and Claroideoglomus claroideum).
These observations concur with those of studies conducted in central Europe. The characteristic species that have been able to show for the long-term monitoring site “Oberacker” are Septoglomus constrictum for plots in long term direct seeding and Funneliformis Caledonius for ploughed plots.
Same species found in grassland
The type of use and the operating intensity have great influence on the AMF communities in agricultural soils: grasslands usually have a higher diversity than crops, extensive cropping increases the number of species, intensive rotations reduce them, and there are more species of AM fungi in uncultivated soil than in land which is cultivated. This latter finding confirms the results of previous studies on the long-term monitoring site “Oberacker”: there is an increase in species richness and diversity of AMF in plots under direct seeding since they ceased ploughing in 1994.
Several species are characteristic of a no-till soil and some are also typical of grassland habitats. One can designate Septoglomus constrictum as an indicator species of direct seeding long-term on this site. For ploughed plots, the characteristic species is Funneliformis Caledonius. Among the various cultures, we find that the number of AM fungi species is usually lower in plots of winter cereals (barley and winter wheat) than in those of intermediate crops (green manure mixtures, sown before harvest). A successful direct sowing system depends on a fertile living soil. Promoting AM fungi, particularly specific species and AM fungi groups could make an important contribution to this fertile living soil.