Nitrogen availability determines the long-term impact of land use change on soil carbon stocks in grasslands of southern Ghana

Abstract

Enhancing the capacity of agricultural soils to resist soil degradation and to mitigate climate change requires long-term assessments of land use systems. Such long-term evaluations, particularly regarding low-input livestock systems, are limited. In the absence of suitable long-term experiments, this study assessed the outcome of C inputs and outputs across an array of plant functional groups in arable and permanent systems of a tropical savannah after more than 50 years of consistent land use. Soil samples were taken (0–30 cm depth) from arable crop fields, grazed–seeded grassland, cut–use permanent crops and native grassland. Soil organic carbon (SOC) stocks ranged from 17 to 64 Mg SOC ha−1 (mean ± sd = 32.9 ± 10.2 Mg ha−1). SOC stocks were lower for grazed–seeded grassland relative to cut–use grass, legume trees and shrubs. Accordingly, while the conversion of the native grassland to grazed pastures caused an estimated loss of 44 % of SOC over the period, the conversion to woody legumes resulted in slight (5 %), incremental gains. Within sown systems, nitrogen (N) availability seemed to be the most critical factor in determining the fate of the SOC stocks, with the soil N concentration and SOC being highly correlated (r – 0.86; p < 0.001). In total N, P and K were significant predictors of SOC density in the soils. Moreover, secondary plant metabolites in legumes, namely tannins, were identified as having an impact on SOC. The results from this study provide the theoretical basis for testing the hypothesis that improved soil fertility management and the use of tannin-rich plants have the potential to promote long-term SOC storage in the savannah ecological region. Our study also shows the potential of legume tree/shrub forage species as an environmentally sustainable land use option to mitigate agricultural CO2 emissions from low-input livestock systems in the grasslands of southern Ghana.