Soil Carbon Stocks prediction using NIRS: Simplifying Quantification for MRV Protocols

Accurate quantification of soil carbon stock is essential for assessing the capacity of agricultural systems to sequester carbon and contribute to climate change mitigation. Reliable MRV protocols depend on precise, scalable, and cost-effective methodologies. Innovative technologies are critical to enable robust soil carbon assessment in the field. We evaluated soil carbon stocks using conventional methods in a long-term field experiment under different tropical pasture management systems, including extensive pasture (EX), recovered pasture (RP), intercropped pasture with pigeon pea (CON), and a native forest (FO). Additionally, we propose a fast and accurate near infrared spectroscopy (NIRS) approach to estimate volumetric soil carbon (carbon × bulk density), enabling straightforward calculation of soil carbon stocks. A total of 576 soil samples were collected from 1 m deep trenches, bulk density was determined using the volumetric ring method, carbon content was measured by elemental analysis, and NIRS was applied to predict volumetric soil carbon. Soil carbon stocks ranged from 121 to 134 Mg C ha^-1, with no statistically significant differences among systems despite higher absolute values in managed pastures. These results demonstrate that the pasture systems maintained soil carbon stocks, the CON system enhanced sustainability by operating without nitrogen fertiliser inputs through biological nitrogen fixation. The innovative application of NIRS for soil carbon stock quantification showed strong predictive performance (R² = 0.85, RMSEP = 4.09 kg C m^-3, RPIQ = 3.63), with NIRS carbon stocks statistically indistinguishable from reference measurements, highlighting its potential as a rapid, cost-effective, and scalable alternative to conventional methods.