Standardized Data to Improve Understanding and Modeling of Soil Nitrogen at Continental Scale

• Published in: Earth's future Vol. 11; no. 5
• Main Authors: Weintraub‐Leff, Samantha R., Hall, Steven J., Craig, Matthew E., Sihi, Debjani, Wang, Zhuonan, Hart, Stephen C.
• Format: Journal Article
• Language: English
• Published: Bognor Regis John Wiley & Sons, Inc 01.05.2023; American Geophysical Union (AGU); Wiley
• Subjects: Biodiversity; Biogeochemistry; Carbon; CESM2; Climate; Cycles; Datasets; Ecosystems; Mineralization; Modelling; NEON; nitrification; Nitrogen; nitrogen mineralization; Observatories; Prediction models; Sampling; Sampling designs; Seasonal variations; soil; Soil analysis; Soils; Supply & demand; Terrestrial ecosystems; Trends; Variables; Variation; Vegetation; United States > US
• ISSN: 2328-4277; 2328-4277
• DOI: 10.1029/2022EF003224

Nitrogen (N) is a key limiting nutrient in terrestrial ecosystems, but there remain critical gaps in our ability to predict and model controls on soil N cycling. This may be in part due to lack of standardized sampling across broad spatial–temporal scales. Here, we introduce a continentally distributed, publicly available data set collected by the National Ecological Observatory Network (NEON) that can help fill these gaps. First, we detail the sampling design and methods used to collect and analyze soil inorganic N pool and net flux rate data from 47 terrestrial sites. We address methodological challenges in generating a standardized data set, even for a network using uniform protocols. Then, we evaluate sources of variation within the sampling design and compare measured net N mineralization to simulated fluxes from the Community Earth System Model 2 (CESM2). We observed wide spatiotemporal variation in inorganic N pool sizes and net transformation rates. Site explained the most variation in NEON’s stratified sampling design, followed by plots within sites. Organic horizons had larger pools and net N transformation rates than mineral horizons on a sample weight basis. The majority of sites showed some degree of seasonality in N dynamics, but overall these temporal patterns were not matched by CESM2, leading to poor correspondence between observed and modeled data. Looking forward, these data can reveal new insights into controls on soil N cycling, especially in the context of other environmental data sets provided by NEON, and should be leveraged to improve predictive modeling of the soil N cycle. Plain Language Summary Nitrogen (N) is not only a key limiting nutrient that often constrains plant growth but also a major pollutant in places where supply exceeds demand. However, we lack the ability to accurately predict and model rates of soil N cycling. This paper introduces a first of its kind, standardized and publicly available data set of soil inorganic N pools and net transformation rates spanning the United States. We describe the data set in detail, then examine spatiotemporal trends in N pools and fluxes and how those measurements compare to predictions from an Earth system model. Key Points The National Ecological Observatory Network (NEON) measures soil inorganic nitrogen (N) pools and net flux rates at continental scale The most N cycle variation is observed at the site level, followed by plots within sites. Organic horizons have high N pools and flux rates The Community Earth System Model 2 poorly predicts temporal trends in net N mineralization. NEON data can be used to improve predictive power