Amino acids produced during enzymatic hydrolysis of proteins in soils are important sources of nitrogen to microbes and, ultimately, to plants. To measure turnover and fate of amino acids, 14C-labeled amino acids are added and removal from solution and production of 14CO2 are followed. Yet neither concentration nor time-course effects are a usual part of the experiments. To investigate effects of concentration and time, we added 14C- leucine to temperate forest soils to give nanomolar final concentrations; 14CO2 production was followed over time-scales of minutes to hours.
A comparison of effects of different added concentrations show remarkable agreement of nanomolar level results with previous amino acid studies at micromolar and millimolar concentrations. At all concentrations, labeled amino acids were completely removed from the soil solutions in minutes and a fixed percentage of the added isotope appeared as 14CO2. It is likely that most of the soil bacteria and fungi have low activity but are poised for a rapid increase in activity when substrate becomes available. These low activity microbes are so abundant that they immediately take up all of the amino acids that are added at concentrations covering many orders of magnitude. Based on this evidence, we conclude that the same effect happens in undisturbed soils: all the amino acid is taken up as soon as it is formed from the breakdown of soil protein. Also, the immediate respiration of amino acid carbon, rather than the microbes using the amino acids to construct proteins, implies that soil microbes are starved for carbon and energy. Immediate uptake of amino acids by microbes should result in very small pools of free amino acids. Yet, water extracted from soil yields concentrations of free amino acids of hundreds of micromoles per liter. These amounts are very high when compared with the nanomolar concentrations of free amino acids in lakes and oceans. Several possible explanations exist for the relatively high measured amounts of free amino acids in soils. One possibility is that during sample preparation and extraction, amino acids are released from fine roots or lysed from microbial cells. Another likely explanation is that the micromolar concentrations of free amino acids measured in the soil are not available to microbes; the amounts measured in soil water could be protected from microbes by organic matter, hidden in soil microaggregates or micropores, or attached to clay leaves.