Dynamics of topsoil carbon and nitrogen along a tropical forest-cropland chronosequence: Evidence from stable isotope analysis and spectroscopy

Analyses of δ¹³C and δ¹⁵N abundances and near infrared reflectance spectroscopy (NIRS) were used to evaluate the changes in SOC and total nitrogen (TN) content along a forest-cropland chronosequence in the margins of Kakamega forest in Kenya. 300 topsoil samples were collected from 50 paired forest-cropland plots cultivated between 17 and 60 years. Changes in δ¹³C values between forest and cropland soils were used to model the dynamics of forest-derived and maize-derived C. Mean residence times (MRTs) of forest-derived C in bulk topsoil samples was calculated based on changes in δ¹³C with time since cultivation. An ordinal soil fertility classification developed using NIRS was evaluated against SOC and TN concentrations and δ¹³C and δ¹⁵N abundances in the topsoil. Mean δ¹³C isotopic ratios increased from 24.3 ± 0.2‰ in forest to -16.3 ± 0.4‰ under cropland. Similarly, mean δ¹⁵N isotopic ratios increased from 5.9 ± 0.1‰ in the forest sites to 6.8 ± 0.1‰ in soils cultivated sites. δ¹³C and δ¹⁵N enrichment, low levels SOC and TN concentrations were observed in soils that were designated as low fertility based on ordinal fertility classes defined by NIRS. SOC content declined from 7.27 kg C m^-2 in forest soils to 2.67 kg C m^-2 in soils cultivated for 60 years. A nonlinear regression model predicted there was an accretion of maize-derived C₄ carbon, attaining equilibrium at circa 4 kg C m^-2 after circa 70 years. The model also predicted that after about 38 years, maize-derived C₄ carbon was the predominant input-source of bulk topsoil SOC. The minimum, maximum and mean MRTs values for forest-derived SOC were 19, 149 and 60 years, respectively. The results of this study demonstrates that δ¹⁵N and δ¹³C values offer a robust and direct technique for stable isotope techniques for understanding the dynamics of SOC and TN after conversion of forest soils to cropland.