Microbial growth and nitrogen immobilization in the root zone of barley (Hordeum vulgare L.), Italian ryegrass (Lolium multiflorum Lam.), and white clover (Trifolium repens L.)

Summary

The effect of roots on microbial growth and N immobilization was investigated in a pot experiment with barley, Italian ryegrass, and white clover. We used a silty subsoil with a low soil organic matter content (0.16%C and 0.012%N), which allowed us to measure N immobilization as an increase in total soil organic N (planted versus unplanted). At sampling, the soil was easily removed from intact roots by gentle washing, with a negligible loss of root material. Plant growth and extra mineral N (in planted soil only) gave increased total counts (fluorescence microscopy) and viable counts (plate dilution) of bacteria, a higher proportion of larger cells, and increased viable counts as a percentage of total counts. Under monocots, 12–17% of the added fertilizer N was recovered as soil organic N. Although this N immobilization was attributed to microbial assimilation, less than 1/4 was actually recovered as microbial biomass N, as measured with the chloroform fumigation/N-extraction method or calculated from total bacterial counts. The white clover accumulated substantial amounts of N due to N₂ fixation. However, microbial N immobilization represented only 3% of the total N accumulation, showing that the microorganisms obtained a smaller share of biologically fixed N₂ than of the N applied as fertilizer. Extra additions of mineral N (monocots) enhanced microbial N assimilation, partly due to increased plant growth. The results also strongly indicated, however, that the microbial growth under monocots was N-limited in the latter part of the experiment and that fertilizer N had a direct effect on microbial growth. In the early phase of plant growth, N immobilization ranged from 33 to 58 mg N g^-1 root C. This level of immobilization required a release of organic C into the soil representing a minimum of 60–100% of that found in intact roots.