Benefits of mycorrhizae to soybeans grown on various regimes of nitrogen nutrition¹

Contribution of the Massachusetts Agricultural Experiment Station. Journal Article Number 2830.

Abstract

Nodulating and nonnodulating isolines of soybean (Glycine max Merr ‘Clay') were grown in sand culture in a greenhouse. The plants were cultured with or without mycorrhizal (Glomus mosseae) infection, and nodulating plants were inoculated with Rhizobium iaponicum. Phosphorous was supplied as hydroxyapatite or dicalcium phosphate with N nutrition from nitrate or as combinations of nitrate and ammonium or nitrate and urea. Best growth of the nodulating isoline was with urea nutrition. Best growth with the nonnodulating isoline was with ammonium nutrition. Urea‐treated nodulating plants showed increased growth due to mycorrhizae. Urea‐treated or ammonium‐treated nonnodulating plants showed growth increases due to mycorrhizae. Nitrate‐treated plants did not show increased growth due to mycorrhizae. Mycorrhizal infection was greatest with urea nutrition, and the infection increased the tissue N content of these plants relative to nonmycorrhizal plants. Enhancement of tissue P accumulation through mycorrhizae was greater with hydroxyapatite than with dicalcium phosphate. The efficiency of the symbiotic relationship of Glvcine‐Glomus‐Rhizobium depended on a supply of reduced nitrogen, a high N:P ratio in roots, and a neutral pH in the rhizosphere. Urea nutrition met these requirements best.

Vesicular‐arbuscular mycorrhizal (VM) associations may improve the capacity of higher plants to acquire nutrients. These benefits have been studied extensively in relation to P nutrition since plant requirements for P are high relative to its availability in soils². Significant benefits may occur also in the nutrition of plants with micronutrients³. Reviews of literature suggest that the function of mycorrhizae in the acquisition of N by plants is variable⁴‘⁵‘⁶. The sources of N under various cultural or ecological conditions may account for the conflicting findings among researchers.

Hyphal transport of N is of little importance with NO3^‐nutrition because of the high mobility of this ion⁶ but may be important for the relatively immobile NH₄ ⁺ ion. Roots of NH₄ ^‐nourished plants often are restricted, and mycorrhizae may extend their absorbing surfaces⁷. Mycorrhizae may prefer NH₄‐N over NO₃‐N for their growth and development⁴. In some cases, NH₄ ⁺ ions restrict mycorrhizal infection compared to the effect of NO₃ ^‐ions⁷. The drop in ambient pH associated with NH₄ ⁺ nutrition may be a cause of this inhibition⁷‘⁸‘⁹. Plants grown on urea may not encounter the problems of acidity in the root zone and yet may have access to NH₄ ⁺ nutrition.

The N contents of mycorrhizally infected plants relative to those of uninfected plants are variable¹⁰‘^11,12,13. These diverse results could arise from differences in the levels and forms of N applied to the plants. Crops well‐infected with mycorrhizal fungi may not benefit from the association if N is limiting¹⁴‘¹⁵ although benefits may appear in soils supplemented with N¹⁵‘¹⁷ even if the level of P in the soil is high¹⁸.

An association of fungi, roots, and bacteria exists in nodulating’ legumes. Maximum benefits from this association may be achieved if N and P supplies are balanced properly. In the present study, the preference of the soybean‐Glomus mosseae‐rhizobial system for form of N was investigated to determine if the mycorrhizal benefits to the soybean could be optimized.

Key words:

• Glvcine max
• Glomus mosseae
• Rhizobium japonicum
• phosphorus nutrition
• nitrate
• ammonium
• urea

Notes

Contribution of the Massachusetts Agricultural Experiment Station. Journal Article Number 2830.