AeroHydro Culture: An Integrated Approach to Improve Crop Yield and Ecological Restoration Through Root–Microbe Symbiosis in Tropical Peatlands

Abstract

Tropical peatlands in Indonesia are increasingly degraded by conventional oil palm practices involving drainage and chemical fertilizers. This study evaluates AeroHydro Culture (AHC), a method applying microbe-enriched organic media aboveground, as a sustainable alternative that maintains high groundwater levels while supporting plant productivity. Field trials were conducted at two sites: a managed plantation in Siak and a degraded, abandoned plantation in Pulang Pisau. Ten months after treatment, AHC plots showed development of aerial-like lateral roots, improved chlorophyll levels, and increased arbuscular mycorrhizae colonization (from 0–46% to 22–73% in Siak, and 1.7–20% to 16–60% in Pulang Pisau). In Siak, AHC significantly increased IAA-producing and proteolytic bacteria in the 0–25 cm soil layer and raised oil palm yield by 36% over controls. This yield benefit was sustained in 2025, five years after the initial application. In Pulang Pisau, AHC also enhanced microbial abundance and promoted growth in the native species Shorea balangeran, suggesting its potential for reforestation. Drone imagery confirmed visible long-term differences in canopy color, supporting lasting physiological improvement. These results demonstrate that AHC promotes plant–microbe symbiosis, enhances nutrient acquisition, and sustains oil palm yield under saturated conditions. AHC offers a promising strategy for peatland rehabilitation where ecological recovery and agricultural productivity must be achieved in parallel.

1. Introduction

Tropical peatlands are unique ecosystems characterized by high groundwater levels and rich organic material, predominantly found in Indonesia and Malaysia [1]. These ecosystems play vital roles in global climate regulation, hydrological stability, carbon storage, and biodiversity conservation [2]. In Indonesia alone, tropical peatlands cover over 13.43 million hectares, including 5.85 million ha in Sumatra and 4.54 million ha in Kalimantan [3], and store approximately 55 Gt of carbon while serving as habitat for a range of endangered species [4].

However, rapid land-use changes, particularly the expansion of oil palm (Elaeis guineensis) plantations, have caused severe ecological degradation [5,6]. In conventional oil palm plantations on peat, drainage is commonly applied to lower the water table and facilitate root oxygen access. This approach is typically combined with high inputs of chemical fertilizer, as peat soils are inherently low in available nutrients due to their high acidity and low cation exchange capacity [7,8], and restricted mineral input from rivers [6,9]. Although these practices improves root respiration and boosts productivity in the short term, it also exposes organic peat to oxygen, accelerating decomposition, land subsidence, and greenhouse gas emissions, and increasing fire vulnerability [10,11,12]. Over time, such mismanagement and degradation of peatlands reduce plantation productivity, eventually rendering them economically unviable and leading to abandonment [13]. These challenges highlight the urgent need for ecological and agronomic interventions to restore peatland ecosystem functions and promote farming practices that maintain saturated conditions to prevent further degradation [2,14,15].

Efforts to rehabilitate degraded peatlands have included rewetting, reforestation, and paludiculture (cultivating wetland-adapted crops) [16,17,18]. While Nature-based Solutions (NbS) such as these have gained traction, their implementation faces limitations, especially in regions already impacted by long-term drainage and chemical use. Once microbial communities are disturbed, restoring native species is challenging due to the loss of plant–microbe symbiosis potential [19]. In the case of paludiculture, successful implementation requires prior rewetting of the peat substrate to restore hydrological conditions and is inherently constrained to species adapted to peatland environments. This limits the scalability of paludiculture in meeting both ecological and economic goals in regions where oil palm plays a central role in rural livelihoods and national economies [17].

To address this challenge, AeroHydro Culture (AHC) has emerged as an innovative cultivation method designed to sustain oil palm productivity while maintaining high groundwater levels [20,21,22,23]. The concept of AHC was initially motivated by the innovative farming practices of Mr. Suparjo, a smallholder farmer in Pontianak, West Kalimantan, who has achieved unusually high oil palm yields (40 tons ha⁻¹ yr⁻¹) on shallow peatlands with a high groundwater level without chemical fertilizer inputs. His approach involves piling senescent palm fronds and applying compost composed of grass and chicken manure directly around the base of oil palms. This low-input method, developed under naturally waterlogged conditions, laid the conceptual groundwork for the AHC system. AHC is defined as “plants grown at high GWL with oxygen and nutrients supplied from the land surface to the peatland” [19]. Drawing further on the natural adaptations of native peatland species such as Combretocarpus rotundatus (tumih), Dyera costulata (jelutong), Metroxylon sagu (sago palm), and Melastoma malabathricum (melastoma), AHC mimics key ecological strategies that enable these plants to thrive. These species develop aerial roots and aerial-like lateral roots that extend above the waterlogged zone, allowing access to atmospheric oxygen. The aerial-like lateral roots also penetrate nutrient-rich organic mounds that naturally accumulate at the base of the tree above the soil surface. Within these mounds, the roots absorb nutrients and form symbiotic associations with beneficial aerobic microorganisms such as nitrogen-fixing bacteria, arbuscular mycorrhizal fungi (AMF), and plant growth-promoting rhizobacteria (PGPR) [24]. Through these interactions, plants enhance nutrient mobilization and uptake, enabling robust growth in otherwise inhospitable conditions [25,26].

AHC involves placing organic media enriched with AMF and PGPR on the peat surface near oil palm trees or native plants. This encourages aerial-like lateral roots to grow into the media, promoting microbial colonization, nutrient cycling, and enhanced root function within an aerobic microenvironment. Although some previous studies have described improved plant growth in oil palm and native species when applying AHC, they have largely remained at the observational level, without quantifying its effects on specific aspects of plant productivity or investigating the underlying microbial mechanisms. In particular, little is known about how AHC influences root colonization by beneficial microbes, such as mycorrhizal fungi, or its role in modulating soil–plant–microbe interactions under field conditions.

This study aims to evaluate the long-term effects of AHC on oil palm cultivation and native species restoration in Indonesian peatlands. Specifically, we investigated plant–microbe interactions involving arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR), as well as plant productivity under saturated soil conditions. We hypothesized that AHC promotes plant growth and yield by inducing aerial-like lateral root development and enhancing symbiotic relationships with aerobic microbial communities. Conducted at two plantation sites in Indonesia, this study provides empirical evidence to assess AHC as a sustainable peatland-management strategy with the potential to maintain productivity and ecosystem health over extended periods.

2. Materials and Methods

2.1. Study Sites

The study was conducted at two locations (Figure 1). The first site was located in Koto Ringin Village, Siak Regency, Riau Province, Sumatra Island, Indonesia (0°50′56.400″ N, 102°5′2.400″ E), where the peatlands have been managed as oil palm plantations by the Beringin Cooperative for over ten years. The second site was in Jumpun Pambelom and Tane Pambelom, Pulang Pisau Regency, Central Kalimantan (2°21′17.7″ S 114°06′06.9″ E). This privately owned oil palm plantation, originally a logging concession area used between 1973 and 1993, has remained abandoned for several years.

Peat soil characteristics were assessed at eight randomly selected points at each site. In Siak, peat was classified as shallow with a thickness ranging from 50 to 115 cm, a pH of 3.52, organic carbon content of 40.1%, ash content of 4.30%, and bulk density ranging from 0.21 to 0.36 g/cm³. The peat material was classified as sapric, with a dark brown to black color [27], indicating advanced decomposition and relatively high mineral content suitable for oil palm cultivation [28]. In Pulang Pisau, the peat was classified as deep peat (depth > 3 m) with a pH of 3.35, organic carbon content of 40.1%, ash content of 4.76%, and a bulk density of 0.11 g/cm³ in the 0–50 cm layer. This top layer exhibited moderate to advanced decomposition (hemic), while deeper layers (>50 cm) were fibric with low density. Due to the natural topography, clear stratification of peat layers could not be consistently observed.

2.2. AHC Media Preparation and Application

The AHC media was prepared using a mixture of Lintang Songo NPK (13:6:27) slow-release briquette fertilizer (PT. Prima Mulia Abadi, Gresik, Indonesia), biochar (PalmChar, PPKS, Bogor, Indonesia), zeolite (Bifa Biostab, PPKS, Bogor Indonesia), K-Zeo-G (K₂O 30%), compost, bio-organic fertilizer (VIGANO, PT. Esa Distribusi Nusantara, Bogor, Indonesia), and a microbial activator containing PGPR (VIGADEC, PT. Esa Distribusi Nusantara, Bogor, Indonesia). AMF isolated in previous research (Rhizophagus clarum) were incorporated with zeolite as a carrier [21]. The composition and application rates of each material are summarized in

Table 1. AHC media ingredients.

Dosage/Sack
Ingredients	Unit	Amount
NPK + Mg + B (briquettes)	Kg	0.500
Biochar	Kg	0.125
Zeolite	Kg	0.188
K+ (granules)	Kg	0.160
Compost	Kg	20.00
Bio-organic fertilizer	L	0.500
Arbuscular mycorrhizal fungi + zeolite carrier	Kg	0.250

.

In Siak, compost was made from oil palm bunch waste, grass, and manure (from goats, cows, and chickens), inoculated with bio-activator. In Pulang Pisau, compost was made from grass and manure, also inoculated with bio-activator. Composting followed the windrow method and was incubated for 45 days. The chemical composition of compost from both locations is shown in

Table 2. Chemical analysis of compost in Siak and Pulang Pisau.

Parameter		Result		Unit	Method
		Siak	Pulang Pisau
C—Organic		24.5	30.6	%	Ash/Gravimetry
C/N		15.0	9.00	-	-
Water content		37.6	71.6	%	Gravimetry/Oven
Macro nutrient	N	1.67	3.51	%	Kjeldahl/Distillation
	P2O5	5.12	0.78	%	HNO3/Spectrophotometry
	K2O	1.82	0.60	%	HNO3/F-AAS
Micro nutrient	Fe total	8.15	1.03	ppm	HNO3/F-AAS
	Fe available	310	575	ppm	HNO3/F-AAS
	Zn total	355	132	ppm	HNO3/F-AAS
pH H2O		7.40	8.30	-	Potentiometry/pH Meter
Heavy Metal	As	Not detected	Not detected	ppm	HNO3/F-AAS
	Hs	Not detected	Not detedted	ppm	HNO3/F-AAS
	Pb	36.0	9.00	ppm	HNO3/F-AAS
	Cd	1.10	0.30	ppm	HNO3/F-AAS
	Cr	99.0	3.00	ppm	HNO3/F-AAS
	Ni	32.0	11.0	ppm	HNO3/F-AAS

Detection limit of As: 0.461 ppm; Hg: 0.001 ppm.

.

The media was packed into burlap sacks and placed on the soil surface. In Siak, four sacks were applied per tree, while in Pulang Pisau, three sacks were applied per tree, positioned about 2 m from the trunk. Additionally, two liters of bio-organic fertilizer were applied around the root zones.

2.3. Experimental Design

Both sites included control and AHC treatment plots (Figure 2). Each plot was equipped with a standpipe piezometer (PVC) to monitor groundwater levels biweekly from November 2019 to October 2020. In Siak, only oil palm plots were established, with control plots following conventional management using 6 kg of chemical fertilizer per tree annually. In Pulang Pisau, both oil palm and Shorea balangeran plots were included, with no fertilizer applied to control plots. In both sites, AHC plots received only AHC media nutrients. A buffer of three tree rows was maintained between control and treatment plots in both sites. Each plot covered around 0.18 ha. Siak plots included 25 trees, while Pulang Pisau plots included 16 trees. Due to landowner constraints at both sites, plot arrangements could not follow a fully randomized design.

2.4. Assessment Timeline

Baseline assessments of plant traits and AMF colonization were conducted prior to AHC application: 31 October–1 November 2019 (Siak) and 3–4 December 2019 (Pulang Pisau). Follow-up assessments were conducted 21–25 September 2020 (Siak) and 5–12 October 2020 (Pulang Pisau). Soil microbial assessments were carried out only during the follow-up.

2.5. Plant Trait Assessment

Leaf nutrient content was assessed for oil palm using samples from the 17th frond of oil palm trees. In Siak, chlorophyll content was measured using a SPAD meter on three mature leaves from five randomly selected trees per plot. In Pulang Pisau, chlorophyll content was measured using a Hansatech chlorophyll fluorimeter. The circumference and height of Shorea balangeran were recorded.

2.6. Arbuscular Mycorrhizae Colonization

Approximately 0.5 kg of oil palm root samples were collected from three randomly selected trees per plot at a depth of 0–15 cm. Roots were cleaned and processed using KOH (80 °C, 60 min), HCl (10 min), and Trypan Blue staining (80 °C, 60 min). Stained samples were observed microscopically for AMF colonization.

2.7. Soil Microbial Activity Assessment

Soil samples were collected at depths of 0–25 cm and 25–50 cm from five oil palm trees per plot. The upper layer represents active root and microbial zones; the lower layer reflects more stable substrate conditions [29]. Bacterial isolation was performed using serial dilution and plating on specific media. Plates were incubated at 28–31 °C for 2–3 days in triplicate.

IAA production was assessed using Salkowski reagent, protease activity on milk agar, and phosphate solubilization on NBRIP medium [30].

2.8. Yield Measurement

In Siak, yield was measured from all 25 trees per plot across 13 harvests from 24 February to 24 August 2020. Due to the COVID-19 pandemic, local farmers conducted harvesting, and yields were pooled across two replicate groups. Follow-up yield was measured on 22–23 February 2025 from three randomly selected trees per plot using digital scales.

In Pulang Pisau, yield data were not recorded in 2020 due to the pandemic, and access restrictions prevented the 2025 follow-up.

2.9. Statistical Analysis

All analyses were conducted using R (version 4.4.2). Linear mixed models (LMMs) were fitted using the lmer function (lme4 package). Type III ANOVA tests were performed using the lmerTest package, and post hoc Tukey tests using the emmeans package. Satterthwaite’s approximation was used for degrees of freedom.

The circumference and height of Shorea balangeran, AMF colonization, and chlorophyll levels were analyzed with Treatment and Time (Before, After) as fixed effects. Soil microbial data were analyzed using Treatment and Depth (0–25 cm, 25–50 cm). Yield data from 2020 were analyzed with Treatment and Days, while 2025 yield data used Treatment alone.

Replicate (plot) was included as a random effect in all models. Tree was included as a random effect where applicable, except for the 2020 yield data where tree-level records were unavailable.

3. Results

3.1. Groundwater Level

In Siak, the highest groundwater level (GWL) was recorded in November 2019, and the lowest in March 2020 (Figure 3). In Pulang Pisau, GWL peaked in April 2020 and was lowest in September 2020. GWL fluctuations in both sites corresponded closely with rainfall patterns, resulting in contrasting seasonal trends.

3.2. Plant Morphology

Ten months after AHC application, aerial-like lateral roots were observed emerging above the soil near the AHC media in treated plots at both sites. Quantitative assessment was not feasible because image analysis could not reliably differentiate between roots and surrounding organic material, so representative photographs are presented (Figure 4). Additionally, although leaf count was not quantified due to tree height, visual observation suggested more vigorous new leaf development in treated plots (Figure 5).

In Pulang Pisau, Shorea balangeran showed significant increases in stem circumference and height following AHC treatment (Figure 6). ANOVA showed significant main effects and interaction between Treatment and Time (

Table 3. Statistical analysis result for Shorea balangeran circumference and height.

	numDf	denDF	Sum sq	Mean Sq	f Value	p Value
Circumference
Treatment	1	57	63.72	63.72	14.986	0.0003 ***
Time	1	57	427.81	427.81	100.609	3.364 × 10−14 ***
Treatment: Time	1	57	48.67	48.67	11.446	0.0013 **
Height
Treatment	1	73	76,589	76,589	16.073	0.0001 ***
Time	1	73	337,831	337,831	70.897	2.287 × 10−12 ***
Treatment: Time	1	73	44,053	44,053	9.245	0.0033 **

Significance: *** p < 0.001; ** p < 0.01.

). No differences were observed before AHC application (p = 0.987 for circumference, p = 0.902 for height), but treated trees showed significantly greater growth after AHC application (p < 0.001).

3.3. Leaf Nutrient Content

Composite leaf samples were analyzed from each plot. Because only one composite sample was analyzed per plot, statistical comparisons were not conducted. No major differences were observed between treatments (

Table 4. Leaf nutrient content at the Siak and Pulang Pisau site.

	Siak		Pulang Pisau
Nutrient	Control	Treatment	Control	Treatment
N (%)	2.71	2.54	2.14	1.80
P (%)	0.23	0.24	0.17	0.16
K (%)	0.77	0.80	0.66	0.59
Ca (%)	0.73	0.71	0.49	0.55
Mg (%)	0.59	0.61	0.73	0.82
Cu (ppm)	2.68	2.43	84.5	87.1
Zn (ppm)	13.1	10.5	42.7	73.6
B (ppm)	9.35	9.83	12.2	12.1
Mn (ppm)	392	382	441.1	492

). In Siak, leaf nitrogen and phosphorus levels were within optimal ranges (>2.5% and >0.15%, respectively), while potassium was deficient (<1.00%). Micronutrient levels were sufficient. In Pulang Pisau, macronutrient deficiencies (N < 2.50%, P < 0.16%, K < 1.00%) were observed, and excessive levels of copper and zinc suggested potential toxicity [33].

3.4. Chlorophyll Content

In Siak, chlorophyll content was significantly affected by the interaction between Treatment and Time (Figure 7a,

Table 5. Statistical analysis results for chlorophyll level at the Siak site.

	numDf	denDF	Sum sq	Mean Sq	F Value	p Value
Treatment	1	76	79.142	79.142	3.7463	0.0566
Time	1	76	81.669	81.669	3.8659	0.0529
Treatment: Time	1	76	212.650	212.650	10.0661	0.0022 **

Significance: ** p < 0.01.

). Treated plants maintained stable chlorophyll levels over time (p = 0.8287), while chlorophyll significantly declined in control plots (p = 0.0033). In Pulang Pisau, Treatment had a significant effect on chlorophyll levels (Figure 7b,

Table 6. Statistical analysis results for chlorophyll level at the Pulang Pisau site.

	numDf	denDF	Sum sq	Mean Sq	F Value	(Pr > F)
Treatment	1	57	0.0189062	0.0189062	7.2060	0.0095 **
Time	1	57	0.0090250	0.0090250	3.4398	0.0688
Treatment: Time	1	57	0.0039062	0.0039062	1.4888	0.2274

Significance: ** p < 0.01.

), with treated plots showing higher levels than controls.

3.5. AMF Colonization

In Siak, AMF colonization rates in oil palm roots ranged from 0 to 46% before the application of AHC and increased to 22 to 73% after treatment (Figure 8). Statistical analysis revealed significant main effects of Treatment and Time, as well as their interaction (

Table 7. Statistical analysis result for AMF colonization rate at the Siak site.

	numDf	denDF	Sum sq	Mean Sq	f Value	p Value
Treatment	1	33	3342.3	3342.3	26.0441	1.360 × 10−5 **
Time	1	33	7163.9	7163.9	55.8220	1.374 × 10−8 **
Treatment: Time	1	33	744.7	744.7	5.8025	0.0218 *

Significance: ** p < 0.01; * p < 0.05.

). Prior to AHC application, there was no significant difference in colonization rates between the control and treatment plots (p = 0.245), whereas post-treatment values were significantly higher in the treated plots compared to controls (p < 0.001). Morphological observation of root samples under the microscope further supported this trend. In treated samples, AMF structures were more abundantly developed, with clearly visible arbuscules and vesicles indicative of active symbiosis (Figure 8b,c). In contrast, control roots showed less colonization and less well-defined structures.

In Pulang Pisau, AMF colonization increased from 1.7–20% before AHC application to 16–60% after treatment (Figure 9). Analysis of variance similarly revealed significant effects of Treatment, Time, and their interaction on colonization rates (

Table 8. Statistical analysis result for AMF colonization rate at the Pulang Pisau site.

	numDf	denDF	Sum sq	Mean Sq	f Value	p Value
Treatment	1	33	2322.8	2322.8	25.019	1.834 × 10−5 ***
Time	1	33	2973.1	2973.1	32.023	2.637 × 10−6 ***
Treatment: Time	1	33	1640.7	1640.7	17.672	0.0002 ***

Significance; *** p < 0.001.

). Before AHC application, colonization levels did not differ significantly between control and treatment plots (p = 0.942). After AHC was applied, colonization levels were significantly greater in the treatment group (p < 0.001). Microscopic examination of root samples confirmed enhanced AMF colonization in treated plots, with well-developed arbuscular and vesicular structures not observed in the control group (Figure 9b,c).

3.6. Soil Microbial Activity

In Siak, the abundance of indole-3-acetic acid (IAA)-producing and proteolytic bacteria was significantly influenced by both Treatment as well as their interaction (

Table 9. Statistical analysis result for microbial population at the Siak site.

	numDf	denDF	Sum sq	Mean Sq	f Value	p Value
IAA-Producing Bacteria
Treatment	1	57	14.450	14.450	5.3095	0.0249 *
Depth	1	57	1.653	1.653	0.6074	0.4389
Treatment: Depth	1	57	49.613	49.613	18.2295	7.493 × 10−5 ***
Proteolytic Bacteria
Treatment	1	57	1.7258	1.7258	4.5317	0.0376 *
Depth	1	57	0.6570	0.6570	1.7253	0.1943
Treatment: Depth	1	57	4.3945	4.3945	11.5395	0.0012 **
Phosphate Solubilizing Bacteria
Treatment	1	73	0.0008	0.0008	0.7911	0.3767
Depth	1	73	0.0031	0.0031	3.2295	0.0765
Treatment: Depth	1	73	0.0005	0.0005	0.5012	0.4812
Total Bacterial Population
Treatment	1	73	39.200	39.200	11.2937	0.0012 **
Depth	1	73	5.000	5.000	1.4405	0.2339
Treatment: Depth	1	73	52.812	52.812	15.2155	0.0002 ***

Significance: *** p < 0.001; ** p < 0.01; * p < 0.05.

, Figure 10). Post-hoc analysis revealed significant increases (p < 0.001) in the upper 0–25 cm layer of AHC-treated plots, while no significant difference was observed in the 25–50 cm layer. In contrast, phosphate-solubilizing bacteria were not significantly affected by Treatment, Depth, or their interaction. Total bacterial populations were significantly increased in the upper soil layer of AHC-treated plots compared to controls (p < 0.001), but no difference was detected at the lower depth.

In Pulang Pisau (Figure 11), IAA-producing bacterial populations were significantly influenced by both Treatment and Depth (

Table 10. Statistical analysis result for microbial population at the Pulang Pisau site.

	numDf	denDF	Sum sq	Mean Sq	f Value	p Value
IAA-Producing Bacteria
Treatment	1	44	292.55	292.55	10.2282	0.0026 **
Depth	1	44	391.02	391.02	13.6711	0.0006 ***
Treatment: Depth	1	44	111.02	111.02	3.8816	0.0551
Proteolytic Bacteria
Treatment	1	44	1.7248	1.7248	2.3903	0.1293
Depth	1	44	2.1199	2.1199	2.9378	0.0936
Treatment: Depth	1	44	2.1199	2.1199	2.9378	0.0936
Phosphate Solubilizing Bacteria
Treatment	1	35	0.1725	0.1725	2.5160	0.1218
Depth	1	35	0.0003	0.0003	0.0038	0.9515
Treatment: Depth	1	35	0.0059	0.0059	0.0857	0.7715
Total Bacterial Population
Treatment	1	44	384.22	384.22	32.718	8.694 × 10−7 ***
Depth	1	44	207.29	207.29	17.652	0.0001 ***
Treatment: Depth	1	44	118.91	118.91	10.126	0.0027 **

Significance: *** p < 0.001; ** p < 0.01.

), with significantly higher abundance in AHC-treated plots and in the upper soil layer. In contrast, populations of proteolytic and phosphate-solubilizing bacteria were not significantly affected by Treatment, Depth, or their interaction. Total bacterial populations were significantly influenced by Treatment, Depth, and their interaction, with higher abundance in the upper soil layer of treated plots (p < 0.001), but no significant difference at the lower depth (p = 0.285).

3.7. Yield Analysis

In 2020, maximum yield in treated plots was 710 kg per plot, compared to 520 kg in control plots (Figure 12). Yield was significantly affected by Days (the date of harvest) and by the interactions between Treatment and Days (

Table 11. Statistical analysis results for yield at the Siak site in 2020.

	numDf	denDF	Sum sq	Mean Sq	F Value	(Pr > F)
Treatment	1	51	2324	2324	0.3110	0.5795
Days	1	51	419917	419917	56.1987	8.96 × 10−10 ***
Treatment:Days	1	51	38640	38640	5.1713	0.0272 *

Significance: *** p < 0.001; * p < 0.05.

), with significant treatment differences observed on June 29 (p = 0.016), July 13 (p = 0.015), August 10 (p = 0.016), and August 24 (p = 0.016).

Follow-up yield measurements were conducted four years after the initial AHC treatment (Figure 13,

Table 12. Statistical analysis results for yield at the Siak site in 2025.

	numDf	denDF	Sum sq	Mean Sq	F Value	(Pr > F)
Treatment	1	11	4697	4697	22.532	0.0006 ***

Significance: *** p < 0.001.

). The treatment continued to have a positive impact on yield, as demonstrated by a significant 36% increase in production compared to the control.

4. Discussion

4.1. AHC Application Impact

AHC application significantly influenced multiple biophysical and biological parameters in oil palm plantations at both the Siak and Pulang Pisau sites. Improvements in plant morphological traits, soil microbial populations, and oil palm yield in Siak highlight the potential of AHC as a sustainable approach for rehabilitating tropical peatlands and degraded plantations. Notably, the yield-enhancing effects of AHC persisted five years after initial application, with a 36% increase observed in 2025, despite the cessation of project-led management and the subsequent application of chemical fertilizers by farmers to both control and treatment plots. Such yield improvement is remarkable, considering that under high water table conditions, increased nitrogen input alone has typically failed to enhance oil palm yields [34,35].

One plausible explanation for this yield improvement is adjustments in root architecture toward the aerated AHC medium, which would expand effective uptake area and support resource acquisition, thereby contributing to the observed yield response. In peat soils, root growth can be influenced by hydropatterning, in which auxin distribution may guide root development toward aerated zones [36]. In addition, waterlogged conditions and endogenous ethylene can stimulate root architectural changes, potentially contributing to these traits [37,38]. The formation of white, fibrous, aerial-like lateral roots near the AHC medium observed in this study may reflect such adjustment. Contact with the AHC medium, which is rich in organic matter and beneficial microbes, may further stimulate fine-root proliferation and nutrient uptake. Such proliferation could also support biological nitrogen fixation by free-living nitrogen-fixing bacteria [39], helping to maintain chlorophyll levels as indicated by SPAD measurements.

A second explanation is strengthened plant–microbe associations. In general, higher PGPR abundance and AMF colonization were observed in both sites, which can improve nutrient acquisition [40,41,42,43], help maintain chlorophyll, and promote vegetative growth, thereby supporting higher yield. In Siak, populations of IAA-producing and proteolytic bacteria as well as AMF colonization rate were significantly elevated in treated plots, with the greater AMF response in treated roots consistent with inoculation. In Pulang Pisau, treatment differences in AMF colonization were even more pronounced. Given the site’s long period of abandonment, high acidity, and nutrient imbalance [44,45], the baseline microbial community was likely impoverished; thus, AHC plausibly accelerated re-establishment of nutrient cycling and improved vegetative growth. This microbially mediated recovery is also environmentally favorable, offering a conservation-oriented pathway that relies less on external chemical inputs.

Evidence at the canopy scale of this sustained recovery was also apparent in 2025 drone imagery (Figure 14), showing green canopies in treated palms and yellow foliage consistent with nutrient deficiency in controls. Although quantitative data could not be collected due to institutional constraints, these visual observations further support the long-term benefits of AHC. The enhanced growth observed in Shorea balangeran at this site further indicates that AHC may offer a practical means to facilitate native species reforestation and accelerate ecosystem restoration in degraded tropical peatlands. The AHC medium creates favorable microsites above saturated peat, supporting early seedling growth and microbial re-establishment. By inducing root growth in the near-surface layer and improving microbial activity and nutrient access, both of which would otherwise remain stagnant, AHC may reduce the need for chemical fertilizer and enhance plant resilience on degraded peatlands.

4.2. Hydrological Anomaly and AHC System Resilience

In addition to the observed biological benefits, groundwater-monitoring data revealed contrasting hydrological patterns between the two study sites during the monitoring period. While Siak experienced a period of declining rainfall from November 2019 to March 2020 followed by a sharp increase in April, Pulang Pisau maintained relatively high and stable rainfall levels throughout the same period. This divergence is notable, as Indonesia typically experiences uniform seasonal patterns. The anomaly may be attributed to a strong Indian Ocean Dipole (IOD) event occurring in the absence of El Niño conditions, which is known to trigger southeasterly wind anomalies and offshore Ekman transport along the coasts of Java and Sumatra [46]. Such events underscore the importance of localized groundwater monitoring in determining optimal timing for peatland-management interventions.

A key advantage of the AHC system is its reduced reliance on fluctuating groundwater levels. Since the organic media is applied above the soil surface, the system can remain functional even under elevated groundwater conditions, which often hinder conventional root-zone interventions. This resilience to hydrological variability makes AHC a promising solution for climate-adaptive agriculture in peatland regions, where extreme weather events are projected to increase in frequency and intensity. By decoupling productivity from groundwater constraints, AHC may ease the burden of groundwater-dependent fertilizer scheduling and reduce production losses under erratic climate conditions.

5. Conclusions

This study demonstrates that AHC is a promising and practical method for enhancing oil palm productivity and promoting plant–microbe interactions under high GWL conditions in tropical peatlands. In Siak, AHC led to the formation of aerial-like lateral roots, increased AMF colonization, elevated populations of PGPR, and more stable chlorophyll levels, resulting in significantly higher oil palm yield. Notably, these benefits persisted for at least five years after application, indicating long-term effects on plant resilience and nutrient acquisition even after project-led management had ceased.

At the Pulang Pisau site, which represents a more severely degraded peatland with no prior plantation management, AHC enhanced AMF and PGPR abundance and promoted the growth of Shorea balangeran, a native tree species used for reforestation. Although yield data were not available at this site, the improved microbial and vegetative responses suggest that AHC may accelerate ecological recovery in degraded tropical peatlands.

Despite these benefits, the AHC approach poses practical challenges that must be addressed for widespread adoption. AHC media preparation and placement are labor-intensive, and compost production requires technical knowledge and access to local organic resources. Capacity-building efforts such as farmer training for PGPR inoculation and compost formulation will be essential for upscaling AHC implementation. In addition, cost-effective sourcing and standardization of biochar, zeolite, and other AHC inputs are necessary to ensure economic feasibility.

Future studies should investigate the mechanistic underpinnings of microbial community dynamics in AHC systems using metagenomics. Stable isotope labeling could also be employed to trace nutrient flows and quantify the contribution of microbial symbioses to plant nutrition. Furthermore, long-term monitoring of greenhouse gas emissions from AHC-treated plots is critical for evaluating the net climate benefits of this peatland-management strategy. Lastly, the adaptability of AHC to other crops and reforestation species beyond oil palm and Shorea balangeran warrants further exploration, particularly in the context of integrated NbS for tropical peatland restoration.