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.

中文翻译：

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氮的可利用性决定了土地利用变化对加纳南部草原土壤碳储量的长期影响

提高农业土壤抵抗土壤退化和缓解气候变化的能力，需要对土地利用系统进行长期评估。这种长期评估，特别是关于低投入牲畜系统的长期评估是有限的。在缺乏合适的长期实验的情况下，这项研究评估了经过50多年持续土地利用的热带稀树草原的耕作和永久系统中一系列植物功能组的碳输入和输出结果。从可耕作物田，放牧的草场，已砍伐的永久性作物和原生草地上采集土壤样品（深度为0–30 cm）。土壤有机碳（SOC）储量范围为17至64 Mg SOC ha ^-1（平均值 ±  sd  =  32.9  ±  10.2 Mg ha^-1）。与割草，豆科植物和灌木相比，草木牧场的SOC数量较低。因此，虽然在这段时期内，将原始草地转换为草场导致SOC损失约44％，但转换为木质豆科植物却仅获得了5％的增量收益。在播种系统中，氮（N）的可用性似乎是决定SOC储量命运的最关键因素，土壤氮浓度和SOC高度相关（ r – 0.86； p  < 0.001）。总氮，磷和钾是土壤中SOC浓度的重要预测指标。此外，豆科植物中的次生植物代谢产物，即单宁被确定对SOC有影响。这项研究的结果为检验以下假设提供了理论基础：改善土壤肥力管理和使用富含单宁的植物具有促进大草原生态区长期SOC储存的潜力。我们的研究还表明，豆科植物/灌木饲草物种作为一种环境可持续土地使用方案的潜力，可以减轻加纳南部草原低投入畜牧系统的农业CO ₂排放。