Triharmony/Trilemma of Nutrients Assets in tropical peatland

Peatland soils and water are characterised by low nutrient concentrations. Despite this, biomass in native tropical peatlands is abundant and has adequate nutrients. At Hokkaido University, Sumitomo Forestry Co Ltd, Japan, and BRIN, Indonesia, scientists studying nutrient cycling in peatlands have developed an innovative nutrients management system for agriculture. Their approach – AeroHydro culture – involves cultivation using high groundwater levels through a Storage orientated Water-management (SoW) system, with nutrient and oxygen supplementation via natural composts and aerial-like roots. This is in contrast to conventional methods that focus on Drainage orientated Water-management (DoW) systems. In experimental plots, AeroHydro culture drastically improves plant growth.

Tropical peatlands are common in the Maritime Continent of Southeast Asia, the region between the Indian and Pacific Oceans, including Indonesia, Borneo, New Guinea, the Philippines, and the Malay Peninsula (1,2). Peatlands are defined by three main characteristics: carbon-rich peat soils, high water levels, and low soil and water nutrient contents. Their low-nutrient environment arises through a number of factors. Peat is composed of carbon-rich fibres, mainly lignin, which are nutrient poor, with nutrient desorption outweighing adsorption. The low pH of the soil (<4) exacerbates this low cation (positive ion) absorption capacity; even nutrients in fertilizers are applied, resulting in limited exchange efficacy. In addition, peatland geomorphology controls the source of water. Peatland regions form peatland domes, areas of relatively higher topography (approx 10m elevation in the centre, dropping to river level along a radius of approx 10km) created by thick layers of peat soils. As natural water courses (rivers) flow around these structures, they do not offer a water source for the peat soils within the dome. Therefore, precipitation, which is naturally nutrient poor, forms the primary water source. The relatively acidic pH of precipitation (~5.6) also contributes to nutrient leaching (1). An additional confounding factor is oxygen deficiency due to extremely low oxygen solubilisation into the water due to physical traits, resulting in reduced nutrients absorption by plants (Figures 1 and 2).

Despite these factors, biomass productivity in native tropical peatlands is very high when compared with normal mineral soils, and plants themselves have adequate nutrient supplies (1). To achieve such growth, peatland plants have developed adaptation strategies to maximise nutrient utilisation. In particular, they have evolved to perform nutrient recycling from litter, by symbiosis with microorganisms to decompose organic matters on the peatland surface; nitrogen (N₂) fixation by free-living N₂-fixing bacteria in the rhizosphere (the thin layer of soil immediately adjacent to the roots); and oxygen uptake via the development of aerial and mounded roots (1,2,3).

Professor Mitsuru Osaki (Hokkaido University, Japan), and colleagues have studied the cycling of nutrients under high water levels in tropical peatlands in great detail. As proponents of ‘eco-management’ and Nature-based Solutions (NbS) for environmental problems, including those caused by the climate crisis, the researchers are on a mission to revolutionise agricultural practices and peatland eco-management in the peatlands of Maritime Continents, Southeast Asia. In particular, they have developed an innovative Land Surface Management (LSM) in these areas by identifying and then emulating the natural mechanisms that produce abundant biomass (1,2,3,4).

Nutrient and oxygen utilisation in tropical peatlands

Among the key factors for plant growth are oxygen, and nutrients nitrogen (N), phosphorus (P), and potassium (K). The very low oxygen availability in peatlands is a direct reflection of the high groundwater level (because the solubility of oxygen into water is very low). Pristine peatlands forests have adapted to the low-oxygen conditions in two main ways. First, they have evolved aerial roots, which enable plants to absorb oxygen from the air. Second, the formation of mound roots in litter mound allows both oxygen absorption from the air and nutrient absorption from the forest litter that accumulates on mound.

Isotope data from peatland forests in Thailand and Indonesia show that tropical plants fix nitrogen from the air effectively. This is because plants prefer the lighter ¹⁴N isotope over its heavier ¹⁵N counterpart when N₂ gas from air is fixed by microorganisms. Nitrogen in the atmosphere is predominately ¹⁴N; moreover, during the decomposition of litter, ¹⁴N i s released before ¹⁵N, leaving heavier ¹⁵N in soil. As a results of the change in ¹⁵N (delta ¹⁵N) in leaves, in tropical peatlands the main nitrogen source is identified via air through aerial roots by symbiotic N₂-fixing free bacteria (1).

Another critical plant nutrient is phosphorus, with deficiencies resulting in metabolic disorders. In tropical peatlands, phosphorus is mainly concentrated in above-ground biomass and forest litter. Plants source phosphorus from the decomposition of other plant species, with the rate of decomposition and level of available phosphorus dependent on local conditions and plant species. Forest fires severely degrade phosphorus stocks. The main mechanisms for absorption are mycorrhizal symbiosis (phosphorus uptake by symbiotic fungi that colonise the roots) and the production of acid phosphatases and organic acids. Acid phosphatase decomposes organic phosphorus accumulated on the peatland surface or litter mound, while organic acids and acid phosphatases solubilise insoluble phosphorus compounds (Figure 3).

Potassium is the third major plant nutrient. As the cation (mainly K⁺) adsorption capacity of peat soil (organic soil) is extremely low in low pH conditions (less than 4), most potassium, even that from commercial fertilizers, is leached away and only a little is absorbed by plants. Serious potassium deficiencies have been observed in oil palm plants. The severity of such deficiencies is exacerbated because potassium is critical for healthy root development; poor root development in turn leads to even lower potassium absorption, creating a cycle of negative feedback. As research into nutrient uptake in tropical peatlands has largely focused on nitrogen and phosphorus, the potassium cycle is less well understood in peatland.

AeroHydro culture

Armed with their knowledge of nutrient and oxygen cycling in native tropical peatland ecosystems, Osaki and colleagues have developed new approaches that offer the opportunity for cultivation that is both productive and protective. Conventional water management in tropical peatlands (and elsewhere) is based on water drainage (Drainage orientated Water-management) (DoW); that is misleading peatland management. Unfortunately, large-scale severe disasters have been generating such peat fires (hot fire), peat degradation by microorganisms (cold fire), high greenhouse gas emission (CH₄ and N₂O) with chemical fertilizer application, loss of nutrients, and reduction of plant growth.

By mimicking the high water level environment of pristine tropical peatland, the innovative management technology system known as AeroHydro culture, is proposed for cultivation of plants in a high groundwater level environment, which is created by damming irrigation channels to allow water storage (Storage orientated Water-management, or SoW). In addition, rather than applying additional nutrients via commercial fertilizers, which are both environmentally and economically damaging (nutrients pollution and CH₄ and N₂O emission), nutrients and oxygen are added from the land surface setting via bags containing natural materials (eg, natural compost formed of leaves, grasses, and weeds; biochar; microbes and fungi, etc) (1,3,4) (Figure 4).

The application of this AeroHydro culture approach to experimental plots in Indonesia has been hugely successful. Within just two months, the team observed the growth of aerial and mound roots, similar to those observed in native tropical peatlands. Different types of aerial roots were even observed around the trunks of palm oil, similar to the trunk- aerial root in Sago palm (which has high N₂-fixing ability). Leaves and fronds were greener and more numerous. For the species Shorea balangeran, the average plant height increased from 107 to 191cm in just six months. The harvested yield of oil palms increased from 16 tonnes per hectare per year to 21 tonnes per hectare in just one year.

The degradation of tropical peatlands by desiccation (via the application of drainage systems and reduced rainfall owing to climate change) has also resulted in 1) increased CO₂ emissions from these areas, via both microbial decomposition and forest fires, and 2) increased CH₄ and N₂O emissions, via application of large amounts of chemical fertilizers, especially in oil palm plantations. The high water environment of AeroHydro culture has the added benefit of mitigating CO₂ emissions from agricultural land, even reversing this process via significant carbon sequestration due to its greater volume of biomass.

AeroHydro culture has abundant benefits, typical of the Triharmony model. The final challenge is to encourage the adoption of this method by both small-scale and commercial agricultural producers. To implement this Triharmony model, the initial investment in new infrastructure is extremely inexpensive, and the potential benefits in terms of increased productivity and environmental protection are huge. Adoption of this Triharmony model enables us to achieve not just a carbon neutral society but an innovated carbon negative society (5).