Nutrient Management Research Database

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.

• Growers face the challenge of supplying crops with sufficient N fertilization to obtain high yields while avoiding N pollution to the environment.
• Soil carbon plays an important role in controlling soil N availability and transformations such as mineralization, nitrification, and denitrification.
• This study examined yields, plant-soil N cycling, and changes in root consumption of N in soils on organic farms of differing soil C levels.

• Fields that showed highest levels of soil C generally had more efficient N cycling, meaning that N losses were limited while high yields were still obtained.
• These fields were part of systems that utilized inputs with high C:N ratios coupled with targeted use of organic fertilizers with high levels of plant available N, such as fish emulsion, at times of max crop N demand.
• Despite significant yield decreases generally associated with organic farming, the average yield of the most efficient systems studied (42.8 tons/ac) exceeded the average county yield (38.5 tons/ac), which included mostly conventional operations.
• In these fields, genetic analysis showed increased potential for root N uptake and high levels of soil microbial activity.
• These results show the importance of measuring not only seasonal nitrate and ammonium levels, but also soil microbial activity and root genetics to capture   a more accurate picture of soil N supply.