Oil palm production, particularly in Indonesia, is the world's largest producer of palm oil and is a significant contributor to deforestation. While highly economically attractive due to short-term returns and global demand, conventional industrial oil palm plantations (>50 ha) rely heavily on high fertilizer and herbicide use, compromising ecosystem health and multifunctionality. Unlike forests, these plantations are structurally simplified and less capable of providing multiple ecosystem services simultaneously. Although the negative impacts of oil palm expansion on forests and biodiversity have been extensively studied, a comprehensive assessment of various management strategies on ecosystem functions, biodiversity, and economic productivity is lacking. This study addresses this gap by experimentally evaluating the impact of reduced management inputs (reduced fertilization and mechanical weeding) compared to conventional practices on a mature oil palm plantation.

High fertilization rates in oil palm plantations, while boosting productivity, lead to significant nutrient leaching, greenhouse gas emissions, and negative soil impacts (reduced microbial biomass, pH changes, etc.). Economically, over-fertilization can reduce profitability due to yield saturation. Reduced fertilization, compensating only for nutrients removed through harvest, coupled with organic fertilization, can promote nutrient retention and soil health, potentially improving profitability. Herbicide use, the conventional weed control method, reduces understory vegetation diversity and impacts biodiversity. Mechanical weeding presents a more sustainable alternative, supporting faster vegetation regrowth, improved nutrient cycling, and enhanced habitat complexity, benefiting animal diversity. However, the higher labor costs associated with mechanical weeding might offset these ecological benefits. Existing studies mainly focus on individual aspects (e.g., herbicide impact or reduced fertilization). A holistic evaluation of multiple ecological and economic impacts is needed for more effective management recommendations.

A 2² factorial field experiment was conducted in a mature (≥16 years old) state-owned industrial oil palm plantation in Jambi, Indonesia. The experiment compared four treatment combinations: (1) conventional fertilization with herbicide, (2) reduced fertilization with herbicide, (3) conventional fertilization with mechanical weeding, and (4) reduced fertilization with mechanical weeding. Each treatment was replicated four times on 50 m × 50 m plots. Over four years (2016-2020), indicators of eight ecosystem functions (GHG regulation, erosion prevention, decomposition, soil fertility, pollination potential, water filtration, plant refugium, biological control), seven biodiversity indicators (taxonomic richness across trophic groups), and six economic indicators (yield, yield stability, costs, profit, gross margin) were measured. The reduced fertilization rate was based on the calculated nutrient export through fruit harvest. Statistical analyses (linear mixed-effects models and linear models) were used to assess treatment effects, with z-standardization applied to indicators to prevent the dominance of certain indicators.

Mechanical weeding significantly increased ecosystem multifunctionality compared to herbicide treatment (P=0.03), even though individual ecosystem functions did not show significant differences. This effect was driven by improvements in litter decomposition, soil fertility, water filtration, and plant refugium. Biodiversity, particularly understory plant species richness, was significantly higher (P=0.01) under mechanical weeding (33% more species). Cumulative yield did not differ significantly between treatments, indicating that reduced management maintained high productivity. The reduced fertilization and mechanical weeding treatment showed a 41% reduction in material costs and a 10% increase in labor costs. However, the overall profit was 12% higher and the relative gross margin was 11% higher under reduced management.

The results demonstrate that reduced management, specifically combining reduced fertilization and mechanical weeding, promotes ecosystem functioning and maintains high yield and profit. Mechanical weeding enhanced multifunctionality and biodiversity likely through increased organic matter input and a more favorable microclimate for soil biota. The lack of significant treatment effects on individual ecosystem functions might be due to the legacy effect of long-term conventional management. The similar yields across treatments suggest efficient fertilizer use in the reduced management system. The increased profitability stems from substantial reductions in material costs. Sensitivity analysis suggests the economic superiority of reduced management is robust against changes in labor costs, remaining favorable unless labor costs increase dramatically.

This study provides strong evidence that integrating mechanical weeding and reduced, compensatory fertilization in mature industrial oil palm plantations offers a win-win scenario, boosting profitability while enhancing ecosystem multifunctionality and biodiversity. Future research should investigate the long-term impacts of this management approach and explore the optimization of fertilization rates across the oil palm life cycle. Furthermore, the economic robustness of the proposed system should be evaluated under varying scenarios of fertilizer price fluctuations.

The four-year timeframe of this study might not capture all long-term ecological impacts. The study is confined to a specific industrial plantation in Jambi, Indonesia, limiting the generalizability to other regions or plantation types. The economic analysis focused solely on yield and production costs, excluding other economic factors such as variations in producer prices or external market influences.