Grain Sorghum‐Soybean Rotation and Fertilization Influence on Vesicular‐Arbuscular Mycorrhizal Fungi

Vesicular-arbuscular mycorrhizal fungi (VAMF) can reduce plant stress resulting from nutrient deficiencies, drought, and other factors. The objective of this work was to measure the effect of soybean \Glycine max (L.) Merr. ] and grain sorghum [Sorghum bicolor (L.) Moench] rotation and fertilization on plant response and VAMF root colonization and diversity, and relate effects to soil environment. Fertilizer treatments consisted of no fertilizer, N, and manure. Rooting densities correlated with previous crop, VAMF colonization, and soil NO,. Root colonization by VAMF was affected by previous crop, rooting density, N fertilization, soil P, and water-filled pore space. Root colonization by VAMF ranged from 93% at 15 cm to 15% at the 120-cm soil depth. Root density and VAMF colonization were least when soybean was grown the previous year and manure was applied. Root colonization by VAMF for control, N, and manure treatments were 54, 53, and 30%, respectively, for continuous soybean and 61, 55, and 44%, respectively, for soybean from rotation plots. Root colonization by VAMF for control, N, and manure treatments were 69, 59, and 54%, respectively, for continuous grain sorghum and 56, 48, and 31%, respectively, for grain sorghum from rotation plots. These agricultural soils contained a diverse mixture of 26 VAMF species, which is probably a major factor in the region's soil productivity. Plants stressed due to cropping system or fertilizer practice have greater VAMF colonization and VAMF activity. A diverse VAMF population could increase the ability of VAMF to respond to different stresses. C OF PLANT ROOTS by VAMF often reduces adverse effects of plant stresses (water, nutrient, and environmental factors) and has beneficial effects in maintaining crop yields. Grain sorghum and soybean are colonized by VAMF. Rotating crops and application of N fertilizer may affect VAMF colonization and final crop yield. Clegg (1982) and Roder et al. (1989a) showed that rotating these crops improved crop yield, compared with continuous cropping. They were unable, however, to explain the rotation effect based solely on N availability. Since VAMF colonize the root system and serve as a link between plant roots and soil, changes in cropping systems that affect the soil environment should affect both roots and VAMF. The rate and patterns of root growth vary with soil water, soil strength, bulk density, soil aeration, soil temperature, chemical and biological soil properties, plant genetic potential, and climate (Brown and Scott, 1984; Eavis, 1972; Jones, 1983; Robertson et al., 1980). Although soil fertility, soil water, organic matter, soil aeration, soil temperature, and plant host have J.R. Ellis, USDA-ARS, Keim Hall, East Campus, and S.C. Mason, Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583; and W. Roder, International Rice Research Inst. , Vientiane, Laos. Contribution from the USDA-ARS in cooperation with the Agricultural Research Div. , Univ. of Nebraska, Lincoln. Published as Paper no. 9787 Journal Series. Partial financial support was provided by the International Sorghum and Millet Collaborative Research Program (INTSORMIL) and USAID Grant DAN-1254-G00-0021. Received 3 June 1991. * Corresponding author. Published in Soil Sci. Soc. Am. J. 56:789-794 (1992). been shown to affect the host-symbiont relationship (Busse and Ellis, 1985; Harinikumar and Bagyaraj, 1989; Menge et al., 1980; Raju et al., 1988, 1990a; Ross, 1971; Saif, 1981), the effect of most of these properties on VAMF colonization and development under field conditions is limited. This may be a result of problems in sampling roots under field conditions (Brown and Scott, 1984) to examine the relationship between VAMF colonization, root growth, and soil environmental conditions. Harinikumar and Bagyaraj (1988) found that growing a nonmycorrhizal plant in soil for one season reduced mycorrhizal colonization by 13% and a fallow period reduced colonization by 40%. Long-term continuous cropping, depending on plant species, may increase or decrease VAMF colonization and spore numbers (Kruckelmann, 1975; Strzemska, 1975; Black and Tinker, 1979; Harinikumar and Bagyaraj, 1988, 1989). Soil fertility practices also may affect VAMF colonization of roots and root development (Menge et al., 1980). Low-organic-matter soils tend to enhance VAMF root colonization when green manure is applied (Harinikumar and Bagyaraj, 1989). However, Hayman (1980) and Strzemska (1975) showed N and P fertilizers depress root colonization by VAMF. The objectives of this work were to examine the diversity of VAMF in a crop-rotation system and determine the effect of soybean-sorghum crop rotation, fertilizer N, and manure on VAMF colonization and root development of soybean and grain sorghum, and to relate these effects to the soil environment. MATERIALS AND METHODS Research plots were established in 1980 on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudoll) at the University of Nebraska Agricultural Research and Development Center, Mead, NE. The treatments consisted of soybean and grain sorghum growing in rotation and in continuous cropping receiving no fertilizer (control), cattle manure (15.8 Mg dry matter hayror 160-250 kg N hayr-), and fertilizer N (44 kg hafor soybean, 90 kg hafor grain sorghum). Nitrogen was broadcast as NH4NO3 (33% N) 3 to 4 wk after planting. The experiment was conducted as a randomized complete-block design with a split-plot treatment arrangement. The cropping system was the whole plot and N treatments were subplots. Subplots were 12 rows wide (0.75-m spacing) and 7.8 m long. Four replicates were used and all observations were limited to the four center rows, using the middle 5.4 m of row length. Manure was applied in spring. Tillage was uniform for all treatments and consisted of disking after manure application and double disking before planting. Crops were planted with a six-row John Deere Max-Emerge planter. In 1986 and 1987, soybean (Pella 86 foundation seed, Univ. of Nebraska, Lincoln) inoculated with BradyrhizoAbbreviations: VAMF, vesicular-arbuscular mycorrhizal fungi; WFPS, water-filled pore space; HSD, highly significant differ1 Mention of companies or commercial products does not imply recommendation or endorsement by the USDA or the University of Nebraska-Lincoln over others not mentioned.