Soil-based cycling and differential uptake of amino acids by three species of strawberry (Fragaria spp.) plants
Introduction
The question of whether plants can utilize organic nitrogen was asked as early as 1935 (Virtanen and Linkola, 1946). Methodology at the time was not sufficiently developed to determine whether all organic molecules were first mineralized before nutrient uptake. Recent research, however, has shown that many plants do have transporters for the uptake of organic forms of nitrogen, particularly amino acids (Soldal and Nissen, 1978, Jones and Darrah, 1994). In addition, plant uptake of organic nitrogen can be quite important to total plant production in nutrient limited systems such as the arctic tundra (Lipson and Näsholm, 2001). The significance of these findings to agriculture is controversial since agriculture uses great inputs of inorganic N and contains only small pools of free amino acids in the soil solution (Owen and Jones, 2001, Jones et al., 2005a).
Since organic and low-input agriculture rely primarily on green manures and/or composts and animal manures to meet crop N demands, amino acid-N may be present in greater supply in these systems. There is currently little published research on available pools and amino acid cycling in these systems. Relying on N mineralization from organic inputs can limit plant uptake and growth, especially at times of peak crop demand or in cool or dry soil conditions where mineralization is limited. Even small contributions of organic N to total N uptake at critical periods could improve yields in these systems. Moreover, crop varieties developed under high soluble N conditions could have inadvertently selected against the ability to use organic N. Murphy et al. (2007) showed that wheat cultivars selected under organic management perform better in organic systems, and cultivars selected under conventional management perform better in conventional systems. Selecting varieties with the ability to utilize a significant portion of their N as amino acid-N could be important when developing crops suited to organic and low-input systems.
The goal of this research was to compare amino acid pools and turnover in organic and conventionally managed strawberry fields and to determine amino acid uptake ability in two wild species and a domestic variety of strawberry. We tested the following hypotheses: (1) fine-textured and organically managed soils have higher concentrations of amino acids present in the soil solution than coarse-textured and conventionally managed soils; (2) amino acid turnover is slower in fine-textured and organically managed soils; and (3) wild species of strawberry take up more organic N than domestic varieties.
Section snippets
Soil sampling and analyses
Soil was sampled in April 2005 at 0–10 cm from eight paired organic and conventional strawberry fields in the area of Watsonville, CA. Paired fields were carefully selected for the same soil type, strawberry variety, and all other environmental conditions except management, according to the method described by Reganold (1988). Surface soil texture of each field pair was either loamy sand or sandy loam (referred to as coarse-textured) or silty clay loam (referred to as fine-textured). Each sample
Amino acid turnover
Fine-textured soils contained significantly greater total C and N and supported significantly greater native and potential protease activity than coarse-textured soils (Table 1, Table 2). There were no significant differences in C:N ratio, NO3−, NH4+ or total mineralizable N. Organically managed soils contained significantly higher total C and N than conventionally managed soils, but there were no significant differences in C:N ratio, NO3−, NH4+, free amino acids, protease activity or total
Amino acid turnover
While a growing body of literature has focused on the amino acid uptake ability of plants, less research has measured the dynamics of amino acid cycling in soils (Kielland et al., 2007), particularly agricultural soils. Contrary to Scheller and Raupp (2005), we found no significant differences in soil amino acid pools as a result of organic management, despite significantly higher total C and N in organically managed soils. Use of a stronger HCl based extractant by these authors could explain
Acknowledgments
This research was supported by The Organic Center and USDA-ARS. Many thanks to Marc Evans for statistical consulting and to Debbi Bikfasy for much needed advice and assistance in the laboratory. A special thanks to Stewart Higgins for invaluable help trouble shooting SAS code and debating statistical procedures in general, and to Ron Bolton for assistance with numerous computer related problems and questions.
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