Interpretive Summary of the Technical Paper:
“An evaluation of nitrogen indicators for soil health in long-term agricultural experiments”
Cycling of nitrogen (N) in soil is vital to maintaining soil health and agricultural productivity. While N research related to optimal fertilization rates and nutrient management has been conducted for decades, traditional soil fertility tests only provide a snapshot of a soil’s N content and may not provide a comprehensive understanding of the capacity of a soil to supply N over the long term. In the context of soil health, nitrogen indicators focus on the biological processes that determine a soil’s ability to cycle and supply N over time, rather than measuring the nitrogen available to plants in a given soil at a single time point. Many indicators have been proposed to measure organic N pools, N processes, or proxies for N pools/processes, but limited information is available on how these indicators are related to each other, to soil properties, or to management practices.
To identify the most suitable indicators for characterizing N cycling at a continental scale, the Soil Health Institute collaborated with approximately 100 partners to assess soil health measurements for a range of management practices at 124 research sites across North America. We evaluated several measurements that quantify different types of N or the potential of soil microbes to cycle N, including five indicators related to organic N pools and microbial activity (soil organic N, potential N mineralization, water extractable organic N, autoclavable citrate extraction protein, and N-acetyl glucosaminidase enzyme activity), and two commonly used fertility measurements (extractable nitrate and ammonium). We focused our evaluation on three aspects: the relationship of each N indicator to soil properties and climate variables, their response to management practices, and their correlation with other indicators and carbon measurements.
We found that across North America, N indicators of soil health were most consistently related to temperature, with higher indicator values observed at colder sites. The N indicators generally responded to the soil health practices investigated, including reduced tillage, cover cropping, organic nutrient application, and crop residue retention, but did not show a response to changes in cropping system diversity. In contrast, the commonly used fertility measures of extractable inorganic N were not related to the soil health management practices investigated. The N indicators evaluated in this study were strongly correlated with each other and with carbon indicators previously evaluated by SHI, with both C and N indicators being sensitive to management practices that increased organic matter inputs to the soil. Given these findings, we propose that the carbon indicators previously recommended by SHI (24-hour carbon mineralization potential and soil organic carbon) can also be used to capture N cycling.
Learn more about this study by reading the peer-reviewed manuscript here: https://doi.org/10.1002/saj2.20558