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

Analyses of delta C-13 and delta N-15 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 delta C-13 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 delta C-13 with time since cultivation. An ordinal soil fertility classification developed using MRS was evaluated against SOC and TN concentrations and delta C-13 and delta N-15 abundances in the topsoil. Mean delta C-13 isotopic ratios increased from 24.3 +/- 0.2 parts per thousand in forest to -16.3 +/- 0.4 parts per thousand under cropland. Similarly, mean 815 N isotopic ratios increased from 5.9 +/- 0.1 parts per thousand in the forest sites to 6.8 +/- 0.1 parts per thousand in soils cultivated sites. delta C-13 and delta N-15 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 C-4 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 C-4 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 delta N-15 and delta C-13 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. (C) 2008 Elsevier B.V. All rights reserved.