Soil disturbance reduces the efficacy of mycorrhizal associations for early soybean growth and N 2 fixation

Abstract

The aim of this paper is to study how soil disturbance affects the interaction between indigenous arbuscular mycorrhizal (AM) fungi, Bradyhizobium japonicum and soybean, specifically effects on nitrogen fixation. To generate a differential inoculum potential of indigenous AM fungi, two cycles of maize, each of 3 weeks, were grown in sieved or undisturbed soil before soybeans were sown. The maize plants from undisturbed soil were taller and produced twice as much biomass (1.2 g dry weight per plant) as plants from sieved soil (0.6 g dry weight per plant). Soil disturbance decreased P uptake from 2.7 to 0.8 mg P per plant, and decreased N accumulation from 39 to 18 mg N per plant. These results were consistent with those from previous studies that indicated that improved growth was related to the presence of an intact, extraradical mycelium in the soil at the start of AM fungal colonization of maize roots. Soybean plants grown in undisturbed soil developed faster and accumulated 42% more dry matter by pod-fill than did plants from sieved soil. There was a positive interaction between the two microbial symbionts in undisturbed soil that resulted in more rapid colonization of soybean roots by both microbes, as indicated by nodule number and frequency of arbuscules. Although nodule numbers became the same after 23 days, irrespective of the degree of disturbance, at pod-fill dry weight of nodules was almost four times greater in undisturbed than in sieved soil (42 mg per plant compared with 12 mg). The effect of disturbance on the mass of N 2 fixed was determined using 15N dilution, and reflected the difference in nodule weight. The percentage of plant N derived from the atmosphere decreased from 32% in undisturbed soil to 12% in sieved soil. Colonization of soybean roots by AM fungi was still greater at pod-fill in undisturbed soil. Plants apparently started taking up P only after 10 days from emergence in undisturbed soil, and only after 23 days in sieved soil. These results establish that tillage can modify the interaction between indigenous AM fungi, B. japonicum, and soybean, and that all of the effects appear not to be solely a function of P uptake.