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New, non-chemical weed suppression methods are needed to support multi-tactic integrated weed management (IWM) programs that reduce reliance on herbicides. One such approach is to manage arbuscular mycorrhizal (AM) fungal communities to enhance their weed biocontrol. Successful implementation will require a better understanding of AM fungi-crop-weed relationships under field conditions, especially in the face of soil disturbances imposed by agricultural practices. We examined root AM fungal communities of four weed species and one model crop species (i.e., corn) in a field experiment with contrasting tillage and cover cropping treatments in Illinois, USA. Our objectives were to quantify the impact of AM fungi on the growth of corn and weed species, to assess influences of plant species, tillage, and cover cropping on AM fungal community structure, and to characterize the co-occurrence patterns of AM fungal taxa in weed and corn roots. Our results showed that effects of AM fungal taxa on corn and weed growth varied widely depending on plant species, and several AM fungal taxa had species-specific negative effects on weeds but not on corn. In addition, plant identity was the primary factor affecting root AM fungal community structure and co-occurrence patterns, whereas effects of tillage and cover cropping were relatively limited. We propose that AM fungal taxa that have selectively suppressive effects on weeds but not on corn are candidates for developing new strategies for weed biocontrol as part of IWM. Future approaches to weed management based on direct inoculation with AM fungal taxa or manipulation of AM fungal communities through specific management practices, are promising opportunities to increase the density of weed-suppressive AM fungal taxa in agroecosystems, and our methods provide important insights for AM fungal taxa selection. • The impact of AM fungi on corn and weed growth varied depending on plant species. • Some AM fungal taxa have selectively negative effects on weeds but not on corn. • Plant species drives root AM fungal community structure and interaction patterns. • Tillage and cover cropping have limited effects on AM fungal community structure. [ABSTRACT FROM AUTHOR]

• SOM responded to tillage and cover crops, but patterns varied by site and fraction. • At low-SOM sites, particulate fractions responded most significantly to tillage. • At high-SOM sites, inclusion of cover crops increased silt and clay-associated SOM. Particulate organic matter (POM) is considered an "active" source of nitrogen (N) in cultivated soils, responding readily to management and being more physically accessible to decomposers than mineral-associated forms of organic matter. However, there is increasing evidence that mineral-associated organic matter (MAOM) can also exhibit short-term changes to management that may impact plant and microbial N dynamics. In this study, we investigated how N within soil organic matter fractions responded to three years of tillage and cover crop treatments. We collected soils from a row-crop (maize-soybean rotation) field experiment replicated across three sites in the north central and mid-Atlantic United States: a high-soil organic matter site (3.1% soil organic carbon) in Illinois (IL) and two sites in Michigan (MI) and Pennsylvania (PA) with lower soil organic matter content (1.0% and 1.4% soil organic carbon, respectively). Management treatments included two levels of tillage (chisel plow and ridge tillage) and two levels of cover crop (with and without rye cover crop). Using an optimized sonication method coupled with particle size separation, we isolated and analyzed for N content free POM, occluded POM, a coarse silt fraction, and MAOM. Using partial least squares regression, we also explored broad cross-site relationships between soil organic matter (SOM) fractions, soil N availability, and crop performance. Both particulate and fine fractions responded to tillage and cover crop treatments, but patterns varied by site and fraction. In the low-SOM MI and PA soils, ridge tillage and cover cropping both increased N within POM fractions. The response to ridge tillage was most pronounced, with a 76% and 24% increase in occluded POM N content in MI and PA, respectively. In contrast, at the IL site (high-SOM), the inclusion of cover crops led to higher N, specifically within the fine fractions (coarse silt and MAOM). Cover cropping increased MAOM N content in IL by 24%. When analyzing all sites together, variables associated with fine fractions were more closely associated with N mineralization and crop performance. MAOM can be responsive to short-term management practices and, along with POM, may also be potential sources of N for crops. [ABSTRACT FROM AUTHOR]