Potassium deficiency reduces mycorrhizal colonization.

Potassium (K) deficiency reduced arbuscular mycorrhizal fungi (AMF) biomass in roots by up to 50% in a 70-year experiment in managed pasture in Austria. Nitrogen (N) and phosphorus (P) primarily influenced soil communities. Potassium determined root colonization. The data broaden the understanding of K's role in mycorrhizal symbiosis in agroecosystems.

The experiment began in 1946, with three annual cuts and continuous removal of biomass. Treatments included isolated or combined omission of N, P, and K, in addition to plots with liming and organic fertilization. The prolonged absence of replenishment generated nutritional deficiencies and imbalances in the soil.

Soil arbuscular mycorrhizal fungi (AMF) biomass increased under nitrogen deficiency. The absence of phosphorus slightly reduced this biomass. Potassium deficiency, however, did not significantly alter AMF in the soil, but it did significantly reduce biomass in the roots. This effect occurred mainly when nitrogen was available.

Quantification

The authors quantified arbuscular mycorrhizal fungi (AMF) using lipid biomarkers and 18S rRNA gene sequencing. Soil communities responded to N and P deficiencies. Root communities responded to K deficiency. Microbial composition varied between soil and roots.

The arbuscular mycorrhizal (AMF) families showed specific responses. Under phosphorus deficiency in the soil, there was a reduction in the rhizophyll group and an increase in the ancestral group. Under potassium deficiency in the roots, there was also a reduction in rhizophylls, with a relative increase in ancestral families. The Glomeraceae family, dominant in the roots, showed a decrease in the absence of potassium.

The study also identified a strong association between soil AMF composition and plant community. pH and dissolved inorganic nitrogen explained a significant portion of the variation in plant and fungal communities. Liming raised the pH and increased AMF biomass in the soil, especially in plots without mineral fertilization.

The results indicate that potassium plays a central role in root colonization by arbuscular mycorrhizal fungi (AMF). Prolonged deficiency of this nutrient alters the functional structure of the fungi, with potential impacts on nutrient cycling and the productivity of managed pastures.

Further information at doi.org/10.1111/nph.70969