Biochar is produced by pyrolyzing biomass, ideally from organic waste products which have no further commercial purpose, into stable biochars. This process removes carbon from the short term carbon cycle [1
]. In Asia, rice husks are a waste material of abundant quantity that could provide a biomass source for the production of biochar. Indeed, in 2011, an estimated 592,477 tons of rice husk and 32,000 tons of rice husk biochar were generated from rice milling operations in Malaysia [2
]. In large rice processing mills in Malaysia, fresh rice husks generated as a byproduct of the process are utilized as fuel. They are gasified using cyclone furnaces to generate heat for the rice drying process and the rice husk biochar produced (at a recovery rate of approximately 30%) is predominantly considered as a waste byproduct. Rice drying is a batch process and is carried out over a period of three months per year during each paddy harvest period. Rice husk biochar has been used in agricultural practices since the beginning of rice cultivation in Asia several thousands of years ago. In the state of Kedah (in the northern area of Peninsular Malaysia), rice husk biochars produced by rice mills are utilized as a seedling bed for the rice transplanting technique practiced in that state. In Kedah farmers are able to acquire the rice husk biochar free of charge. However, in other areas of Malaysia such as Kelantan (on the east coast of Peninsular Malaysia), rice husk biochar is considered a problematic waste material and mill owners pay approximately 1 USD to dispose of 3.38 tons of the material due to ineffective utilization. A small proportion of farmers in Kelantan have recently started utilizing rice husk biochar as a potting media for vegetables under fertigation systems, replacing commonly used cocoa peats.
Amending biochar to soil is being promoted as a sustainable practice that simultaneously mitigates climate change and improves the quality of marginalized agricultural land in impoverished regions. The amendment of small doses of biochar to soils in such regions has been carried out in many continents (summarized in [1
]). Biochar amendment to soil can result in an increase in pH, increase in cation exchange capacity (CEC) [3
], improved water holding capacity, and improved soil structure [4
]. Amendment studies have demonstrated both positive and negative results following biochar addition. A meta-analysis of 16 biochar studies considering 177 different treatments showed that the effect of biochar on crop yield is variable, ranging from −28% to +39%, with an overall mean of +10% [5
]. The study cited plant-available water, pH and nutrient retention increases as the most important factors explaining the effect of biochar addition on plant growth. However, in many studies it was not completely clear what factor was decisive for the observed biochar effects, or lack thereof, on crop yield. In addition, a further meta-analysis was carried out in order to investigate the effect of biochar amendment on crop growth both for pot and field experiments [6
]. These authors found that when the biochar dose was <30 t/ha, crop productivity was increased by 11% on average, with greater responses found in pot experiments than in field experiments, in acidic soils compared to neutral soils, and in sandy soils compared to loamy and silty soils.
Malaysian soils can be broadly divided into the sedentary soils formed in the interior areas of Malaysia on a wide variety of rock types, and soils that are found in the coastal alluvial plains. The coastal alluvial soils can be classified as Entisols, Histosols, Inceptisols, and Spodosols and fall in to four main categories; fine textured clay soils, peat and organic soils, acid sulfate soils and sandy soils [7
]. It is often the acid sulfate soils and the sandy soils that are most problematic from an agricultural perspective. Acid sulfate soils occur almost exclusively in the coastal plains of Malaysia [8
]. These soils are characterized by high levels of pyrite (FeS2
) which produce high acidity (soil pH < 4) when they are exposed to the atmosphere due to drainage, resulting in the release of high amounts of Al3+
into the environment [10
]. The sandy spodosols (>85% sand) are considered problematic due to excessive water drainage as well as low organic matter contents, clay contents, cation exchange capacities (CEC) and nutrient contents. Water and nutrients are easily leached out of the soil due to the low field capacity and the low CEC respectively, and thus water and nutrient stress are common.
Reported studies probing the effects of the addition of biochar to Malaysian soils are scarce. However, Malaysian universities and research agencies (personal communications) have carried out several unpublished studies. Of those that have been published, the use of biochar has been shown to be one possible way to ameliorate poor agricultural soils. For example, the addition of 4% wt of rice husk biochar to compost accelerated the composting process as the rice husk biochar acted as a bulking agent and promoted higher decomposition rates due to a larger microbial population being present at the thermophilic stage, as well as higher moisture and nutrient retention [2
]. Panhwar et al.
] studied the effects of biochar and organic matter amendments on rice grown on acid sulfate soil from Kelantan. The application of 4 t/ha biochar in combination with 9 t/ha biofertilizer resulted in a significant increase in soil pH and crop yield as compared to the control, non-amended soil. Syuhada et al.
] studied the effects of biochar amendment on nutrient uptake of corn planted under sandy Malaysian spodosols. The results showed that the addition of biochar at 20 t/ha in combination with fertilizer significantly increased the uptake of nitrogen by corn compared to the control. However, when applying biochar to soil as an amendment with the aim to improve the agronomic quality of the soil, the effect of co-formed polycyclic aromatic hydrocarbons (PAHs) and potentially toxic elements (PTEs) must also be considered. Previous research has shown that the level of PAHs in a broad suite of biochars produced using different methods and from different feedstocks does not result in biochars containing total PAH concentrations or bioavailable PAH concentrations above guideline values set by relevant biochar standards [13
]. Specifically for biochars produced form rice husk, previous studies have also reported low concentration of PTEs [14
] that do not exceed guideline values set by the UK for the use of sewage sludge application to soil [15
]. One additional environmental issue specifically related to the addition of rice husk biochar to soil is the high amount of silicates in the biomass which may lead to the production of crisobalite, an element with associated health concerns, during the gasification process [15
Previous biochar trials have been carried out for similar problematic soils to those in Malaysia, in other countries. The application of rice husk biochar together with mineral fertilizers and lime significantly enhance the root nodule formation, growth and yield of maize in Indonesia [16
]. In Thailand, Oka [17
] reported the positive effects of 10 t/ha rice husk biochar application on nitrogen fixation rates, growth and yield of soy bean planted in a low fertile sandy soil. The authors explained the positive effects based on an improvement of soil physical properties, porosity, water holding capacity, pH and CEC resulting from the amendment of biochar. Jaafar et al.
] studied the response of arbuscular mycorrhizal towards biochar soil amendment and reported that application of 50 t/ha of biochar gave significantly higher colonization of mycorrhizal root for subterranean clover and wheat. Kameyama et al.
] reported that the addition of 1%–10% wt sugarcane bagasse biochar produced at 800 °C to two types of sandy soils in Japan increased the water retention capacity of the soils proportionally to the amount of biochar added. Steiner et al.
] reported charcoal from secondary forest wood addition to compost at 5 t/ha and 11 t/ha, respectively, gave higher stover and grain yield compared to mineral fertilization alone and its presence in the soil was as measured by carbon loss, was more stable compared to other organic amendments such as chicken manure and compost.
Within this study, the amelioration effects of adding rice husk biochar to two problematic soils from Kelantan, namely sandy spodosols and acid sulfate soils over two cropping season was investigated in greenhouse pot trials. The rice husk biochar was applied just once and residual effects resulting from the application were evaluated in a second cropping cycle. Rice husk biochars were produced locally in two manners: (i) in a rice mill using a cyclone gasifier and a gasification process (the above mentioned waste product) and (ii) in a controlled manner using an upscaled Belonio device [21
]. Corn (Zea mays
) and rice (Oryza sativa
) were selected as test crops as they are currently grown under sandy spodosols and acid sulfate soils but often have a low average yield. The working hypothesis was that these biochars could increase crop yields in these soils by favorable changes in soil physico-chemical properties, specifically water stress relief in the sandy soil and acid stress relief in the acid sulfate soil. This study extends knowledge related to the use of biochar as a soil amendment to Malaysian soils by considering two biochars applied at two rates to two problematic soils planted with two types of crop over two cropping seasons. The study is among the first to probe the possible use of gasifier rice husk biochar in Malaysia, a readily and cheaply available biomass waste material from rice mills that promotes positive waste management strategies.