Plants in nutrient‐deficient soils often benefit when colonized by vesicular‐arbuscular mycorrhizal fungi (VAMF). Plants vary, however, in responsiveness or mycorrhizal dependency (MD). The objective of this work was to evaluate MD within corn and soybean cultivars. Three improved and three unimproved corn (Zea mays L.) and three improved and two unimproved soybean [Glycine soja Siebold & Zucc. and Glycine max (L.) Merr.] cultivars were evaluated for growth response, nutrient uptake (N, P, K, Ca, Mg, and Zn), and root phosphatase activity. Greenhouse studies were conducted with VAMF (Gigaspora margarita or Glomus intraradices) and without inoculation in a Iow‐P soil. The VAMF colonization of roots with inoculation ranged from 62 to 87% for soybean and 49 to 68% for corn. Soybean had a higher MD than corn, but considerable variation occurred within soybean cultivars. Relative growth of the two unimproved soybean cultivars was significantly greater with VAMF colonization than without (Soja, >1900%; Mandarin, >400%). Among improved cultivars, relative growth was less enhanced with colonization (BSR 201, Richland, and Swift cultivars averaged ≈ 200% greater growth with VAMF than without VAMF colonization). An unimproved corn cultivar, Reid Yellow Dent, was unresponsive to mycorrhizal colonization, whereas another unimproved cultivar, Argentine Pop, increased growth 400% with colonization. Total uptakes of N, P, K, Ca, Mg, and Zn were significantly greater (P < 0.001) in mycorrhizal plants, but the concentrations (mg g−1) of N, Mg, and Ca were lower in mycorrhizal plants than in nonmycorrhizal plants. These results suggest that considerable variability exists in MD of corn and soybean cultivars when grown in P‐deficient soils, and the variability extends across both improved and unimproved cultivars.

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of propagules, and grow rapidly (Menge, 1985). Arbuscular mycorrhizal fungal species that adapt to a wide Survival of arbuscular mycorrhizal (AM) fungi in soil may be afrange of hosts and edaphic contingencies presumably fected by the presence or absence of crops and by the crop being grown. Field studies were conducted in central Iowa during three have a better chance of long-term survival. One means growing seasons with cropping to continuous corn (Zea mays L.) by which AM fungi survive is the production of spores (two cultivars), continuous soybean [Glycine max (L.) Merr.] (two although the length of survival of spores in field soils cultivars), or fallow in three soils to determine AM selection and under varying conditions is not well documented. Presurvival. The initial numbers of spores (all following soybean) in May sumably, when conditions are favorable, these spores 1996 averaged 0.9 g Ϫ1 soil in Clarion (well drained), 1.1 in Nicollet germinate, grow, and produce other fungal structures. (somewhat poorly drained), and 3.6 in Webster (poorly drained) soils. Many cornfields, especially in low-lying areas, exhib-In May 1998, the highest spore count average was 11.2 g Ϫ1 soil in Webster under corn and 3.0 spores g Ϫ1 under soybean. Nicollet soil ited stunting and purple coloration characteristic of seaveraged 6.8 spores g Ϫ1 for corn and 0.9 for soybean in May 1998, vere P deficiency in the fall following the central U.S. whereas Clarion soil had 6.3 for corn and 2.0 for soybean. The fallow floods of summer 1993 (Fixen et al., 1994). Plants treatments consistently had low spore counts, ranging from 0.7 to 1.0 seemed not to be absorbing sufficient P for fast early spores g Ϫ1 for all three soils. After 3 yr under the same cropping growth even though soil tests indicated adequate P. regime, spore numbers in soil were corn Ͼ soybean Ͼ fallow; no Farmers were advised to use 35 to 40 kg P ha Ϫ1 as starter significant differences were found between cultivars of the same crop. fertilizer in areas that had been flooded the previous Most probable number counts were correlated with spore counts and averaged 11% of spore counts, suggesting that only a portion of the year. Ellis (1998) conducted a survey to determine the spores were viable (or culturable in our determination). By the end status of AM fungal populations in areas that were of the study, Glomus albidum and G. etunicatum dominated under flooded in 1993 and reported a decline in AM fungal corn, whereas G. constrictum dominated under soybean.

A potential concern about the use of fast pyrolysis rather than slow pyrolysis biochars as soil amendments is that they may contain high levels of bioavailable C due to short particle residence times in the reactors, which could reduce the stability of biochar C and cause nutrient immobilization in soils. To investigate this concern, three corn ( L.) stover fast pyrolysis biochars prepared using different reactor conditions were chemically and physically characterized to determine their extent of pyrolysis. These biochars were also incubated in soil to assess their impact on soil CO emissions, nutrient availability, microorganism population growth, and water retention capacity. Elemental analysis and quantitative solid-state C nuclear magnetic resonance spectroscopy showed variation in O functional groups (associated primarily with carbohydrates) and aromatic C, which could be used to define extent of pyrolysis. A 24-wk incubation performed using a sandy soil amended with 0.5 wt% of corn stover biochar showed a small but significant decrease in soil CO emissions and a decrease in the bacteria:fungi ratios with extent of pyrolysis. Relative to the control soil, biochar-amended soils had small increases in CO emissions and extractable nutrients, but similar microorganism populations, extractable NO levels, and water retention capacities. Corn stover amendments, by contrast, significantly increased soil CO emissions and microbial populations, and reduced extractable NO. These results indicate that C in fast pyrolysis biochar is stable in soil environments and will not appreciably contribute to nutrient immobilization.

While water quality functions of conservation buffers established adjacent to cropped fields have been widely documented, the relative contribution of these re‐established perennial plant systems to greenhouse gases has not been completely documented. In the case of methane (CH4), these systems have the potential to serve as sinks of CH4 or may provide favorable conditions for CH4 production. This study quantifies CH4 flux from soils of riparian buffer systems comprised of three vegetation types and compares these fluxes with those of adjacent crop fields. We measured soil properties and diel and seasonal variations of CH4 flux in 7 to 17 yr‐old re‐established riparian forest buffers, warm‐season and cool‐season grass filters, and an adjacent crop field located in the Bear Creek watershed in central Iowa. Forest buffer and grass filter soils had significantly lower bulk density (P < 0.01); and higher pH (P < 0.01), total carbon (TC) (P < 0.01), and total nitrogen (TN) (P < 0.01) than crop field soils. There was no significant relationship between CH4 flux and soil moisture or soil temperature among sites within the range of conditions observed. Cumulative CH4 flux was −0.80 kg CH4–C ha−1 yr−1 in the cropped field, −0.46 kg CH4–C ha−1 yr−1 within the forest buffers, and 0.04 kg CH4–C ha−1 yr−1 within grass filters, but difference among vegetation covers was not significant. Results suggest that CH4 flux was not changed after establishment of perennial vegetation on cropped soils, despite significant changes in soil properties.