COS 89-1
Tightly-coupled plant-soil nitrogen cycling: Insight from root gene expression and biogeochemical indicators on organic farms across an agricultural landscape
Leveraging ecological processes to enhance nutrient cycling and crop productivity can reduce tradeoffs associated with synthetic fertilizer use. Ideally, soil microbial activity releases sufficient nitrogen (N) from organic matter synchronously with plant N demand to support high productivity but with low inorganic N pools and minimal N losses. But achieving tightly-coupled N cycling is challenging in practice given diverse biophysical contexts across agricultural landscapes, necessitating novel assessment tools to support adaptive management. Gene expression of enzymes involved in root N uptake and assimilation are hypothesized to indirectly indicate plant available N in soil, even when typical measures of N availability (e.g. inorganic N) are low. Here, our main goal was to characterize N cycling across farm fields spanning a range of soil organic matter and evaluate how expression of a key gene in root N assimilation, glutamine synthetase GTS1, along with soil assays could help identify situations of tightly-coupled N cycling. We surveyed 13 organic fields growing Roma-type tomatoes in California’s Sacramento Valley across a three-fold gradient of soil carbon (C) and N. Indicators of soil N availability, plant N status, root gene expression, and crop productivity were measured over a growing season.
Results/Conclusions
Nine of 13 fields had similar yields that were above average for the area, but biogeochemical indicators and root gene expression were highly variable. Fields with consistently low inorganic N pools but adequate plant N and yields showed strong evidence of tightly-coupled N cycling, i.e. simultaneously achieving crop productivity, soil N availability, and potential for soil N retention. These fields also had the highest soil C and activities of soil N cycling enzymes. Other fields showed signs of either N excess with greater potential for N loss or N deficiency with low crop productivity. While GTS1 expression was highest where soil nitrate was highest, expression was also elevated in fields with tightly-coupled N cycling where soil nitrate was very low. The lowest levels of GTS1 expression were found in fields with clear N deficiency. Overall, expression of GTS1 was more strongly related to soil bioassays for N availability (e.g. potentially mineralizable N) than to inorganic N pools. Thus, root gene expression may be a “plant’s eye view” of soil N cycling and in conjunction with other indicators of soil organic matter quality and potential for N loss, be used to support an adaptive management process toward tightly-coupled N cycling.