Abstract
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.
Highlights
Increments in increased carbon (C) sequestration rates in soils of 0.4 % per year have been suggested as a means of compensating for the global emissions of greenhouse gases from anthropogenic sources (Chabbi et al, 2017)
Whereas Soil organic carbon (SOC) stocks did not differ between the agricultural soils and native grassland soils (p>0.05), grazed–seeded grassland soils had 86 % and 77 % less (p
Using the native grassland as a pseudo baseline, we observed SOC stock changes ranging from −44 % to 5 %, with a mean loss of −15 %, and the most considerable negative change occurring in grazed–seeded grassland soils (Fig. 3b)
Summary
Increments in increased carbon (C) sequestration rates in soils of 0.4 % per year have been suggested as a means of compensating for the global emissions of greenhouse gases from anthropogenic sources (Chabbi et al, 2017). Grasslands, on a global scale, sequester around 0.14 Mg C ha−1 year−1, storing 685 Gt C in the upper soil (to 1 m depth). This C pool size is nearly 50 % more than that of forests (346 Gt C) and 70 % more than wetlands (202 Gt C; Grace et al, 2006; Gobin et al, 2011; Conant et al, 2017).
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