Abstract:
The NASA SMAP (Soil Moisture Active Passive) mission was launched in January 2015 and provides global soil moisture observations every 1-3 days using low frequency (1.4 GHz) microwave radiometer brightness temperature retrievals. The SMAP mission also produces a set of model-enhanced Level 4 (L4) products for daily surface (0-5 cm depth) and root zone (0-1m depth) soil moisture and terrestrial carbon (CO2) fluxes. The SMAP L4 Carbon (L4C) product employs a coupled light-use efficiency and 3-pool soil decomposition model combining MODIS (MODerate resolution Imaging Spectroradiometer) vegetation and SMAP L4 Soil Moisture (L4SM) observations with GMAO GEOS-5 daily surface meteorology. The L4C outputs represent a comprehensive terrestrial carbon budget, including net ecosystem CO2 exchange (NEE), component carbon fluxes for gross primary production and ecosystem respiration, and surface soil organic carbon (SOC). The SOC stock estimates are derived at 1-km resolution consistent with MODIS vegetation inputs and represent the balance between photosynthetic carbon uptake and ecosystem respiration losses. The SOC parameter thus provides a climate-sensitive metric of soil health that tracks variations in the size and residence time of soil carbon storage due to perturbations in ecosystem productivity and respiration, including drought.

Here, we use the SMAP L4C product to investigate global patterns and recent trends in SOC attributes, as well as their underlying drivers. We also assess the response and recovery of SOC in light of recent record warm temperatures and anomalous drought events. Soil health stability, represented by SOC enhancement or degradation, is also assessed against other independent observations, including soil and vegetation surveys, tower carbon flux measurements, and the US Drought Monitor. Our results establish SMAP L4C as a new, operational tool for global monitoring of key soil health attributes.