Amino acids from soil do enter roots but are not important for trees in nature

Background/Question/Methods

The source of N for trees in forest and tundra is a major question of plant ecology. Many scientists believe an important source is the amino acids (AA’s) in soils. However there is a paradox here; forests and tundra are strongly N-limited yet relatively large amounts of organic N and free AA’s exist in soil. When these amounts are added back to soil in labeled form or added to hydroponic cultures, the AA’s do enter tree roots. The ecological question, however, is not “do AA’s from soil enter plants” but “do important amounts of soil AA’s enter plants and affect plant N budgets?”.

In laboratory experiments, the kinetics of uptake of 14C-labeled leucine were measured in cucumber seedlings grown in sterile sand and in pine roots with and without mycorrhizal fungi. Leucine final concentrations for 30 min uptake incubations were 0.1, 1, 10, 100, and 150 microM. Samples were counted for radioactivity with liquid scintillation.

Results/Conclusions

In the experiments, uptake of leucine into microbe-free plant roots and plant roots with and without mycorrhizal fungi was directly related to concentration. This diffusion-like uptake operated at all concentrations added from lowest to highest. The conclusion is that dissolved amino acids always enter plant roots but when added concentrations are at nM concentrations the very low amounts entering are not ecologically significant. When added concentrations are at high microM concentrations, then relatively high amounts of AA enter the roots. Therefore, the actual concentration available to plant roots in the soil is all important.

Recent publications demonstrate that soil microbes, present in high abundance, readily take up and respire dissolved AA’s and thereby hold the amounts available to plant roots at very low concentrations. In fact, soil microbes immediately take up all concentrations of added amino acids up to the mM level. Yet, microM concentrations are routinely measured in soil research after various extraction and preparation techniques. It appears that these measured AA’s are released from micropores and other structures. The answer to the question of ecological importance is, therefore, that in undisturbed soil AA concentrations available to plants are so low, in the nM range, that little AA enters plant roots and this amount does not significantly contribute to plant nutrition.