Spatial variability of agricultural soil carbon dioxide and nitrous oxide fluxes: Characterization and recommendations from spatially high-resolution, multi-year dataset

Abstract

Mitigating agricultural soil greenhouse gas (GHG) emissions can contribute to meeting the global climate goals. High spatial and temporal resolution, large-scale, and multi-year data are necessary to characterize and predict spatial patterns of soil GHG fluxes to establish well-informed mitigation strategies, but not many of such datasets are currently available. To address this gap in data we collected two years of in-season soil carbon dioxide (CO₂) and nitrous oxide (N₂O) fluxes at high spatial resolution (7.4 sampling points ha⁻¹) from three commercial sites in central Illinois, one conventionally managed continuous corn (2.8 ha in 2021; 5.4 ha in 2022) and two (one site 5.4 ha in 2021 and 2.0 ha in 2022, another site 2.7 ha both years) under conservation practices in corn-soybean rotations. At the field-scale, the spatial variability of CO₂ was comparable across sites, years, and management practices, but N₂O was on average 77 % more spatially variable in the conventionally managed site. Analysis of N₂O hotspots revealed that although they represent a similar proportion of the sampling areas across sites (conventional: 12 %; conservation: 13 %), hotspot contribution to field-wide emission was greater in the conventional site than in the conservation sites (conventional: 51 %; conservation: 34 %). Also, the spatial patterns, especially hotspot locations, of both gases were inter-annually inconsistent, with hotspots rarely occurring in the same location. Overall, our result indicated that traditional field-scale monitoring with gas chambers may not be the optimal approach to detect GHG hotspots in row crop systems, due to the unpredictable spatial heterogeneity of management practices. Meanwhile, sensitivity analysis demonstrated that reliable (< 25 % error) field-scale soil GHG flux estimates are attainable when sampled above certain spatial resolutions (1.6 points ha⁻¹ for CO₂ and 5.6 points ha⁻¹ for N₂O in our dataset). Especially for N₂O, lower spatial resolutions were prone to underestimating its field-wide flux.