Soil Amino Acid Utilization among Species of the Cyperaceae: Plant and Soil Processes

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Amino acids are released during the decomposition of soil organic matter and have been shown to be utilized as a nitrogen source by some non-mycorrhizal species in the family Cyperaceae (the sedge family). Twelve out of 13 Cyperaceae species examined in the current study were capable of absorbing soil amino acids in the non-mycorrhizal state. With two exceptions (two species in the genus Kobresia), species from subalpine or alpine habitats exhibited lower rates of total nitrogen uptake compared to species from more temperate habitats, which is possibly explained by lower growth rates in the alpine and subalpine species and a lower overall demand for soil nitrogen. The alpine and subalpine species exhibited higher rates of glycine uptake relative to NH4+ and NO3− uptake, compared to species from the more temperate habitats. This may reflect specialization toward the uptake of organic N in the alpine and subalpine species. In three ecosystems where sedges commonly occur, amino acids were present in the soil pore water, but in highly variable amounts. Seasonal maximum amino acid concentrations in alpine soil pore water ranged from 13 to 158 μmol/L in four dry meadow sites. The most common amino acid in these alpine soils was glycine. In a subalpine fen habitat, aspartate was the most commonly observed amino acid, and total amino acid concentrations were 15–20 μmol/L. In a shortgrass steppe habitat, glutamate was the most abundant amino acid, and total amino acid concentrations were 25–45 μmol/L. Soil protease activities correlated positively with soil protein content when all three ecosystems (alpine, subalpine fen, and shortgrass steppe) were considered together. The addition of protein (as casein) significantly increased soil protease activity, indicating that soil protein content contributes to the capacity to generate soil amino acids. Soil protein contents correlated positively with total soil N content when the alpine and shortgrass steppe sites were considered together. The retention of NH4+ and amino acids on the alpine soil was high compared to soils of lower organic matter content that have been investigated in past studies. We conclude that (1) the potential to take up amino acids in non-mycorrhizal roots of the family Cyperaceae is widespread among species native to a variety of different habitats, (2) amino acids are available for uptake (though most abundant in the alpine), (3) the potential to generate soil amino acids is proportional to soil protein concentration, and (4) soil amino acids can be retained in the soil at high levels, leading to the conclusion that the soil binding affinity of amino acids may constrain uptake by plant roots and microorganisms.