Impact of long-term nitrogen fertilization and rotation with soybean on the diversity and phosphorus metabolism of indigenous arbuscular mycorrhizal fungi within the roots of maize (Zea mays L.)

Abstract

A clear understanding of the importance of arbuscular mycorrhizal fungi (AMF) to phosphorus (P) nutrition in intensively managed maize cropping systems will impact how we currently manage these systems, particularly tillage, fertilization and crop rotation decisions. The influence of nitrogen (N) fertilizer (0, 50, 100, 150 and 300kgha−1) and crop rotation of maize (Zea mays L.) with soybean (Glycine max L.) versus monoculture maize on AMF communities and P metabolism in maize roots were investigated in a 13-year irrigated field trial located in SE Nebraska, USA. Maize roots were sampled at four growth stages (V6, V12, R1 and R4) and analyzed for AMF biomass and species composition using AMF-specific fatty acid biomarkers and DGGE-cloning of 18s rDNA, respectively. The activities of AMF-derived alkaline phosphatase (ALP), acid phosphatase (ACP) and polyphosphate (Poly P) were measured as indicators of P metabolism. The biomass of AMF in maize roots increased from V6 to R4 stage, and was not significantly influenced by N fertilizer addition or by rotation with soybean. The Shannon–Weiner diversity index (H′) and richness of AMF phylotypes varied with phenology, but was not influenced by crop rotation. Richness and H′ of AMF phylotypes decreased mainly under the highest N rate. Detection of AMF-derived ALP, ACP and Poly P in maize roots provided indirect evidence for P uptake via AMF in highly productive maize agroecosystems. AMF-associated P uptake was not affected by N application, but was higher under maize monoculture. We conclude that N fertilization at agronomic rates has minimal impact on overall AMF diversity as measured by H′ and colonization of maize roots; however, the frequency of AMF phylotypes varied with N fertilization level and may have important implications for agroecosystem management.