Depth effects of long-term organic residue application on soil organic carbon stocks in central Kenya

Summary

This long-term field trial in Kenya demonstrates that manure application was uniquely effective at increasing soil organic carbon stocks in the subsoil (30–70 cm depth), with effects extending to 60 cm, whereas Tithonia diversifolia and maize stover amendments increased SOC only in surface layers (top 20–40 cm respectively). Across the full 0–70 cm profile, all organic treatments significantly increased total SOC, with manure achieving the highest stocks at 120 ± 24 t C ha⁻¹ compared to 91 ± 12 t C ha⁻¹ in the control. The findings underscore that nearly half of soil carbon is stored below the plough layer and that organic amendment selection critically influences where in the soil profile carbon accumulates.

Regional applicability

While this study was conducted in Kenya's tropical climate, the findings regarding organic amendment type and subsoil carbon dynamics are relevant to UK arable management, particularly as UK soil carbon targets increasingly focus on depths beyond the conventional 30 cm sampling layer. UK farmers and policymakers should consider that different organic inputs (farm manure versus crop residues) may have markedly different effects on deep carbon sequestration.

Key measures

Soil organic carbon stocks (t C ha⁻¹) at 0–20 cm, 20–30 cm, 30–70 cm, and 0–70 cm depth layers; subsoil carbon contribution to total profile

Outcomes reported

The study measured soil organic carbon stocks to 70 cm depth across three organic input treatments (manure, Tithonia diversifolia, maize stover) applied over 19 years to maize monoculture. It assessed how different organic amendments and mineral nitrogen affected SOC distribution in topsoil and subsoil layers.

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