A systems approach to the management of arbuscular mycorrhiza: Bioassay and study of the impact of phosphorus supply

Summary in the original language of the document

The aim of this study was to find out whether utilization of arbuscular mycorrhiza (AM), in crop production in Nordic conditions, can be promoted through management of the cropping system. P fertilization was chosen as the pilot system to manage because it has a major effect on AM and because it is problematic from the viewpoint of sustainability. Our scant knowledge of AM functioning and its effects in the field is mainly due to the methodological problems of research. Therefore, a bioassay of AM effectiviness was developed. In addition, the work demonstrated the challenge of a systems approach to research on low organizational levels of the agroecosystem. The starting point was the goal of sustainable phosphorus management in the food system, focused down to the AM level. The conclusions were then linked back to the context of the target suprasystem.
The study demonstrated thet, even in research at lower system levels, choices are made regarding the degree of systemism, and the soft dimension of systems approach is also relevant. The higher system level of interest affects the approach to be taken in research on the subsystems, and it is important that the conclusions are related to the system level considered.
Representative results for AM infectivity and effectiviness in field soils were obtained in a bioassay involving sampling of the field soil in the spring, after the thaw, and creating a control with suppressed AM through the incorporation of 20 mg(/kg soil in target moisture)benomyl in the soil immediately before sowing. Flax proved to be an appropriate test plant together with a relevant host. This bioassay also allows standardization for the practical purpose improving the prediction of field soil P availability and its problems, especially in sustainable agriculture.
Understanding the interrelations of the tripartite system of mycorrhiza encompassing plant, soil and AMF is a prerequisite for management of AM. The influence of P on AM formation and effectiviness in terms of growth and nutrient uptake of barley, red clover and flax was investigated. As well, the effect of P on the size, composition and functioning of the field AMF communities was clarified. The impact of the P history on P response and the immediate effect of omitting P application and halving N fertilization were elucidated with flax. Two long-term field experiments representing contrasting soil types with low and intermediate contents of extractable P were utilized. Besides the bioassay for AM effectiviness developed, two other bioassays and a field assay were employed. A conceptual model of the impact of P on AM effectiviness was presented.
P fertilization consistently decreased AMF infectivity and generally AM effectiviness, too. Cumulative P fertilization decreased the size of the AMF communities but did not affect the hyphal P transport capacity, or, in contrast to some other studies, the species composition. Instead, functional intraspecific adaptation of the AMF communities to extreme P conditions was observed. However, the mode of action of cumulative P fertilization of major practical importance was the change in the soil conditions in which the AMF functioned. In some cases the restriction on contribution of AM to crop growth and nutrient uptake in the field seemed to be a low rate of mycorrhization due to incompatibility of the plant with the soil P status or with the AMF community evolved. Most often, however, the benefit appeared to be limited by plant and soil factors which cause AM to be a net cost.
AM appears to be an ecosystem service that is impaired or lost by intensive cultivation. In soil representing the present lower end of the P supply of Finland's field soils, but the average status of the late sixties, the higher AM effectiviness in soil with no added P for 20 years compensated the annual P fertilization of 45 kg/ha(soil PH20 2.5 v. 9.5 mg/kg) for flax. The compensation was not complete for red clover. In contrast to July, at harvest barley received no benefit from AM and only a slight benefit from cumulated P. The management benefit, or increase in relative contribution of AM to growth by omitting the annual P fertilizations of 45 kg/ha, was in the low P soil, for flax, 30 % of seed dry weight, 35 % of seed P content and 35 % of the total P uptake at harvest in the field. The contribution of AM in low P soil was up to 35 % of P uptake and 18 % of dry weight. No benefit from AM was found in soil with an intermediate or high content of extractable P (P H2O 5.4 to 21.3 mg/kg). Instead, yield reductions of as much as 56 % were recorded up to a P level that was sufficient to hinder mycorrhization. The results suggest that AM deserves to be considered in the development of sustainable production systems as well as in breeding and soil quality assessment programmes serving sustainable agriculture.