Arbuscular mycorrhizal fungi in spring wheat : impact of waste-based fertilizers

Sammanfattning: Phosphate rock is a diminishing source of phosphorus (P) for crop production. Other sources are being investigated to replace it, for example bio-ash from wood combustion and biogas digestate from house hold wastes. Ash and digestate application have resulted in increased yields, but the effect on the soil biota has been somewhat varied. This study focuses on the effects of bio-ash and digestate on arbuscular mycorrhizal fungi (AM fungi), which live in symbiosis with the studied crop spring wheat (Triticum aestivum). Furthermore, it aims to determine the effect of AM fungi on the cadmium (Cd) uptake in wheat. Soil and plant samples were collected in a field in Östergötland county where seven different treatments had been applied (unfertilized control, digestate from two different production sites, ash, two combinations of digestate and ash, mineral fertilizer). There were three different types of samples collected: soil containing roots, soil without roots, and roots alone. These samples were studied using a quantitative polymerase chain reaction (qPCR) assay; the roots were furthermore studied for AM colonisation in a microscopy investigation and root, straw and kernel samples were also sent for Cd content determination at ALS Scandinavia. The qPCR assay showed that AM fungal biomass in the root samples and the soil samples including roots were positively affected by application of digestate in combination with ash, and negatively affected by mineral fertilizer and digestate alone. The biomass of wheat was affected by the treatments, and the mineral fertilizer addition resulted in the greatest total biomass. The AM fungal biomass in the root free soil was positively correlated with the wheat biomass, indicating that mineral fertilizer can indirectly be beneficial for some type of AM fungi. The microscopy investigation was only done for one block and was hence too small to give a definitive result. In the roots sent for Cd determinations contamination of soil could not be ruled out, instead Cd determinations of the straw and kernel were used for interpretations. The different treatments did not affect the Cd content in the wheat nor was there any correlation between amount of AM fungi and Cd content in the above ground plant parts. The same was true for the pH values in the soil. The conclusion of this investigation is that application of waste-based fertilizers affects AM fungi in various ways. According to my results soil-associated AM fungal biomass is associated with a large host plant biomass, and root-associated AM fungal biomass is positively affected by a combination of digestate and ash. More research is needed in this area, especially with focus on the effect of waste-based fertilizers on AM fungi with different life strategies and functional traits.