Arbuscular mycorrhizal fungi (AMF) are a main component of soil microbiota in most agrosystems. As obligately mutualistic symbionts, they colonize the roots of the majority of plants, including crop plants. We used molecular techniques to investigate how different tillage systems (moldboard, shred-bedding, subsoil-bedding, and no tillage) can influence the AM fungal community colonizing maize, bean, and sorghum roots in an experimental site located in northern Tamaulipas, Mexico. Roots from 36 plants were analyzed using AM fungal-specific primers to partially amplify the small subunit (SSU) of the ribosomal DNA genes. More than 880 clones were screened for restriction fragment length polymorphism (RFLP) variation, and 173 of these were sequenced. Ten AM fungal types were identified and clustered into three AM fungal families: Gigasporaceae, Glomaceae, and Paraglomaceae. Glomus was the dominating taxon in all the samples. Four of the 10 identified types were distinct from any previously published sequences and could correspond to either known unsequenced species or unknown species. The fungal diversity was low in the four agriculture management systems, but the multidimensional scaling (MDS) analysis and log-linear-saturated model indicated that the composition of the AMF community was significantly affected by the tillage system. In conclusion, since some fungal types were treatment specific, agricultural practices could directly or indirectly influence AM biodiversity.

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Understanding the effects of long‐term management practices on soil C and N pools and activities is essential for sustaining soil productivity. The objectives of this study were to evaluate long‐term and seasonal changes in soil organic C (SOC), soil microbial biomass C (SMBC) and N (SMBN), and mineralizable C and N in continuous corn (Zea mays L.) under conventional tillage (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no‐tillage (NT) with low (45 kg N, 10 P kg ha−1) and high (90 N kg, 10 P kg ha−1) N fertilization. An Orelia sandy clay loam (fine‐loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas was sampled before corn planting in February, during pollination in May, and following corn harvest in July. No‐tillage and MT retained more corn residue C input as SOC and SMBC than the more intensive tillage systems. Soil organic C, SMBC, SMBN, and mineralizable C and N were greatest in the surface 0 to 50 mm with NT and MT. Seasonal distributions of SMBC and mineralizable C were consistently greater in reduced‐tillage systems (NT and MT), averaging 22 and 34% greater than plowed treatments at planting, 45 and 53% larger at pollination, and 36 and 34% higher at harvest, respectively, at a depth of 200 mm. The greater amount of crop residues remaining with MT and NT may have provided available substrate for maintenance of the larger SMB pool and the higher C and N mineralization in the 0‐ to 200‐mm depth during the growing season. Higher N fertilization increased seasonal mineralizable C and N, but did not consistently affect SOC and SMB. Reduced tillage systems that promote surface residue accumulation provide an opportunity for increasing sequestration of C and mineralizable nutrients within SMB.

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