Soil amino-acid availability across a temperate-forest fertility gradient

Abstract

Despite increasing recognition that free amino acids can be an important source of N for plant uptake, we have a poor understanding of environmental variation in the availability of amino-acid N in soils outside of arctic, alpine and boreal regions. I investigated patterns of amino-acid availability along a temperate forest fertility gradient ranging from low mineral N availability, oak-dominated forests to high mineral N availability, maple-basswood forests (5 sites). I measured standing pools of free amino acids, soluble peptides, ammonium and nitrate, rates of amino acid production (native proteolysis activity) and rates of consumption of a 15 N-labeled leucine tracer. Standing pools of amino acid N decreased consistently along the fertility gradient from the low fertility black oak/white oak system to the high fertility sugar maple/basswood system, with a 25-fold difference in pool sizes between the poorest and richest sites. Standing pools of soluble peptides varied little among sites, instead, the relationship between free amino acids and peptides changed markedly across the gradient. At low fertility sites free amino acids were positively correlated with soluble peptides, whereas free amino acid pools were universally low at high fertility sites, regardless of peptide pools. Assays for native proteolysis activity indicated that amino acid production did not vary significantly among sites. Recovery of leucine tracer in inorganic (NH 4 ? and NO 3 -) pools and in residual soil organic matter both increased with increasing soil fertility; however, total consumption of the added amino-acid tracer did not vary among sites. Results from this study demonstrate that free amino acids can make an important contribution to potentially plantavailable N pools in temperate forest soils, particularly at low fertility sites.

l-Amino acid oxidase of the fungus Hebeloma cylindrosporum displays substrate preference towards glutamate

“…Cellular catabolism of amino acids allows these nitrogen resources to be further distributed into other compounds. In this way, the microbes may better adapt to amino acid resources that display compositional heterogeneity, and are variably available spatially and temporally (Rothstein, 2009). It has been hypothesized that ammonia originating from catabolism of amino acids may be one source of nitrogen transferred from ECM fungi to their symbiotic plant hosts (Chalot et al, 2006).…”

Amino acid uptake by temperate tree species characteristic of low- and high-fertility habitats

“…However, there is growing evidence that amino acid pools are relatively more abundant in temperate forests where N mineralization rates are low (Finzi and Berthrong 2005;Gallet-Budynek et al 2009;Rothstein 2009), providing a pool of potentially plant-available N in what were previously considered low-fertility habitats. Also, a few studies have shown that temperate trees have the ability to use amino acid N (Bennett and Prescott 2004;Hofmockel et al 2007;Warren and Adams 2007).…”

“…However, soluble N pools in the soil contain a wide variety of different amino acids with varying abundances and molecular structures (Kielland 1995;Yu et al 2002;Rothstein 2009). While glycine is often one of the dominant free amino acids in field soil solutions, ranging from approximately 5 to 20% of total free amino acids depending on the ecosystem (Kielland 1995;Rothstein 2009), other amino acids can occur in equal or greater concentrations (Senwo and Tabatabai 1998;Raab et al 1999), suggesting that glycine uptake potential alone does not necessarily represent plant access to amino acid N pools.…”

“…However, soluble N pools in the soil contain a wide variety of different amino acids with varying abundances and molecular structures (Kielland 1995;Yu et al 2002;Rothstein 2009). While glycine is often one of the dominant free amino acids in field soil solutions, ranging from approximately 5 to 20% of total free amino acids depending on the ecosystem (Kielland 1995;Rothstein 2009), other amino acids can occur in equal or greater concentrations (Senwo and Tabatabai 1998;Raab et al 1999), suggesting that glycine uptake potential alone does not necessarily represent plant access to amino acid N pools. Plants have been shown to take up a variety of other amino acids (Kielland 1994;Ö hlund and Näsholm 2001;Persson and Näsholm 2001), sometimes to a greater extent than glycine (Weigelt et al 2005), suggesting that studies evaluating amino acid uptake should use multiple amino acids when trying to determine whether or not amino acid N is important for plant nutrition.…”

“…We used tree species that commonly occur in eastern North American forests, where it has been well documented that tree species composition is correlated with patterns of inorganic N pools in repeatable assemblages across the landscape (Host et al 1988;Zak et al 1986Zak et al , 1989Finzi et al 1998). These predictable associations make this region an ideal study area to investigate whether or not amino acid N availability might also influence tree species dynamics, especially since recent studies have shown that gradients of free amino acids in soil run opposite to those for inorganic N (Gallet-Budynek et al 2009;Rothstein 2009). Our research compared the uptakes of four amino acids, NH 4 ?…”