title: Tightly-Coupled Plant-Soil Nitrogen Cycling: Comparison of Organic Farms across an Agricultural Landscape.
abstract: How farming systems supply sufficient nitrogen (N) for high yields but with reduced N losses is a central challenge for reducing the tradeoffs often associated with N cycling in agriculture. Variability in soil organic matter and management of organic farms across an agricultural landscape may yield insights for improving N cycling and for evaluating novel indicators of N availability. We assessed yields, plant-soil N cycling, and root expression of N metabolism genes across a representative set of organic fields growing Roma-type tomatoes (Solanum lycopersicum L.) in an intensively-managed agricultural landscape in California, USA. The fields spanned a three-fold range of soil carbon (C) and N but had similar soil types, texture, and pH. Organic tomato yields ranged from 22.9 to 120.1 Mg ha-1 with a mean similar to the county average (86.1 Mg ha-1), which included mostly conventionally-grown tomatoes. Substantial variability in soil inorganic N concentrations, tomato N, and root gene expression indicated a range of possible tradeoffs between yields and potential for N losses across the fields. Fields showing evidence of tightly-coupled plant-soil N cycling, a desirable scenario in which high crop yields are supported by adequate N availability but low potential for N loss, had the highest total and labile soil C and N and received organic matter inputs with a range of N availability. In these fields, elevated expression of a key gene involved in root N assimilation, cytosolic glutamine synthetase GS1, confirmed that plant N assimilation was high even when inorganic N pools were low. Thus tightly-coupled N cycling occurred on several working organic farms. Novel combinations of N cycling indicators (i.e. inorganic N along with soil microbial activity and root gene expression for N assimilation) would support adaptive management for improved N cycling on organic as well as conventional farms, especially when plant-soil N cycling is rapid.
authors.affiliation: Department of Land, Air and Water Resources, University of California Davis, Davis, California, United States of America.
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Bowles
authors.affiliation: Information Center for the Environment, Department of Environmental Science and Policy, University of California, Davis, Davis, California, United States of America.
authors.forename: Allan D
authors.initials: AD
authors.lastname: Hollander
authors.affiliation: Crops Pathology and Genetics Research Unit, USDA/ARS, Davis, California, United States of America.
authors.forename: Kerri
authors.initials: K
authors.lastname: Steenwerth
authors.affiliation: Department of Land, Air and Water Resources, University of California Davis, Davis, California, United States of America.
authors.forename: Louise E
authors.initials: LE
authors.lastname: Jackson
mesh_terms: D009584:Nitrogen / Q000032:analysis* / Q000378:metabolism; D058458:Nitrogen Cycle; D058871:Organic Agriculture; D018517:Plant Roots / Q000378:metabolism; D012987:Soil / Q000737:chemistry*
chemical_list: D012987:Soil; D009584:Nitrogen