Long-term nitrogen fertilization reduces extraradical biomass of arbuscular mycorrhizae in a maize (Zea mays L.) cropping system

Arbuscular mycorrhizal fungi (AMF) form symbiotic relationships with most plant roots in natural and agroecosystems. The benefit of the relationship to AMF, host plant and soil is realized through the production of extraradical hyphae important to nutrient acquisition and soil carbon (C) storage. Maize grown in rotation with soybeans is a major agroecosystem in the Midwest and northern Great Plains of the United States and its potential to sequester C is the subject of much research. The goal of this study was to determine the impact of crop rotation and N rate on the production of extraradical AMF biomass in a long-term, minimal till field in eastern Nebraska (USA) where maize had been grown continuously (M-M; current crop in bold) or in rotation with soybeans (M-S; S-M) and treated with five nitrogen (N) rates (0, 50, 100, 150 and 300 kg N ha−1) for 12 years. The amount of extraradical AMF biomass was measured in the top 20 cm of soil using fatty acid methyl ester (FAME) analysis of the AMF biomarker C16:1cis11. AMF biomass was highest under M-M with significant rotation by date by N rate interactions. At peak crop biomass in August the AMF biomarker under 0 N addition declined from 37.5 nmol g−1 in M-M to 16.7 nmol g−1 in M-S to 8.0 nmol g−1 in soybean following maize (S-M). In M-M extraradical AMF biomass declined sharply as N rate increased from 0 to 100 kg ha−1. A similar trend was found for M-S, but significant only at p < 0.1. Declining soil pH with increasing N rate could not account for this difference indicating a more direct effect of N on soil AMF biomass. This is in contrast to maize roots sampled the same year from the same field site where intraradical AMF structures were non-responsive to N rate (Tian et al., 2013). During soybean growth (S-M), there was no relationship between soil AMF biomass and the previous year's maize N rate. The effect of crop rotation and long-term N application on the extraradical AMF biomass in soil has implications for nutrient cycling, crop growth promotion, soil tilth, and C storage.