Soil amendments for cost-effective tropical forest restoration

Rebecca J Cole, Gerald Quiros Cedeño, Rakan Zahawi, Lilly Briggs, Leland K Werden 

Introduction: Low-cost soil amendments hold promise for improving assisted forest restoration outcomes and reducing dependence on emission-intensive inputs, yet rigorous field-based comparisons of novel and standard amendments, including cost-effectiveness, remain rare.
Objectives: We evaluated seedling performance and cost-effectiveness under three novel soil amendments: nutrient-rich agricultural biowaste (composted coffee pulp), native soil microbiome inoculation (live primary forest soil), and locally sourced effective microorganisms (LEM), against commercial fertilizer and an unamended control.
Methods: Treatments were tested on one early-successional (Croton tenuicaudatus) and one late-successional (Persea caerulea) seedling species. We assessed survival, relative growth rate, root mass fraction, soil and foliar nutrients, and cost-effectiveness (USD/kg plant carbon gain/ha) over the first year on degraded tropical pasture.
Results: Coffee pulp was the most effective amendment, significantly enhancing soil and foliar nutrients (particularly in Croton), matching fertilizer in Croton growth rate (0.182 vs. 0.160), improving survival by 13–19% over control and LEM treatments for both species, and reducing cost per unit carbon gain by up to 92% (Croton) and 68% (Persea) relative to controls. Native soil microbiome inoculation significantly enhanced Croton growth and cost-effectiveness relative to controls); LEM had no significant effect on growth or survival and significantly increased costs per unit plant carbon gain relative to other amendments for both species.
Conclusions: Composted coffee pulp and native soil microbiome inoculation improved first-year seedling performance and cost-effectiveness on degraded tropical pasture, with coffee pulp more cost-effective than commercial fertilizer; LEM provided no benefit.
Implications for Practice:
• Composted coffee pulp and other nutrient-rich agricultural biowastes significantly improve seedling performance and offer a scalable, low-cost alternative to emission-intensive synthetic fertilizers on degraded tropical land.
• Inoculation with live forest soil may also improve seedling growth, survival and restoration cost-effectiveness.
• We do not recommend LEM as a direct soil amendment as it may increase costs without ecological benefit.
• Soil amendments are a small share of total restoration costs but can greatly improve cost-effectiveness and should be evaluated with financial as well as ecological metrics.
• Future work should test more biowastes and soil microbiome inoculum sources and match amendments to species traits.