Tightly-Coupled Plant-Soil Nitrogen Cycling: Comparison of Organic Farms across an Agricultural Landscape

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.农业生产系统如何在实现高产的同时保障充足氮素（Nitrogen, N）供应并降低氮素损失，是缓解农业氮循环中常见权衡矛盾的核心挑战。农业区域内有机农场的土壤有机质与管理模式存在差异，这可为优化氮循环以及评估氮素有效性的新型指标提供研究思路。本研究针对美国加利福尼亚州某集约化农业区域内种植罗马型番茄（Solanum lycopersicum L.）的一批典型有机田块，评估了产量、植物-土壤氮循环以及氮代谢基因的根系表达情况。这些田块的土壤碳（Carbon, C）与氮素含量跨度达三倍，但土壤类型、质地与pH值均较为相近。有机番茄产量介于22.9至120.1 Mg·ha⁻¹之间，平均产量与该县主要为常规种植番茄的平均水平（86.1 Mg·ha⁻¹）相近。土壤无机氮浓度、番茄氮素含量以及根系基因表达均存在显著差异，表明各田块在产量与氮素损失潜力之间存在多种潜在权衡关系。呈现出植物-土壤氮循环紧密耦合特征的田块——即通过充足氮素供应保障高产且氮素损失潜力极低的理想情景——拥有最高的土壤总碳、活性碳与总氮、活性氮含量，且施加了氮素有效性各异的有机质输入物。在这类田块中，参与根系氮同化的关键基因——胞质谷氨酰胺合成酶GS1（cytosolic glutamine synthetase GS1）——的表达上调，证实即使土壤无机氮库较低，植株的氮同化效率仍处于较高水平。由此可见，若干运营中的有机农场已实现了紧密耦合的氮循环。将氮循环相关指标（即无机氮含量、土壤微生物活性以及氮同化相关根系基因表达）进行新型组合，可为有机农场与常规农场的氮循环优化提供适应性管理依据，在植物-土壤氮循环速率较快的场景中尤其如此。