1. Introduction
Increases in population and food consumption result in an increase in agro-industrial wastes [
1]. According to Wang et al. [
2], waste generated from agricultural activities increases approximately 5–10% annually. Agricultural wastes are non-product outputs derived from production and processing of various agricultural materials, including rice husk (RH) and chicken manure. Rice is the second most widely grown crop in the world with a global demand of 477 million tons year
−1 and an average per capita consumption of 57 kg year
−1 [
3,
4]. Apart from rice wastes, chicken production from year 2017 to 2018 in Malaysia increased; total production volume of broiler grew from 1664.9 million metric tons to 1707.6 million metric tons [
5]. Composting is an important approach to transform agro-waste fractions into useful products such as compost [
6] and is generally considered one of the useful methods to sustainably recycle agricultural wastes [
7]. The use of co-composting technology has become a preferred method for recycling many agricultural wastes by-products into safer and more stable materials as soil amendment because matured compost reduces the negative effects of unprocessed agricultural wastes [
8,
9,
10]. Compost production using unwanted agricultural wastes is considered green technology that improves plant nutrition, food security, sustainable agriculture, besides minimizing poverty in most developing countries [
10]. Thus, to ensure high quality of composts is achieved, it is important to determine the maturity, biological, physical, and chemical properties of composts. The co-composting process involves converting solid organic wastes into humus-like materials that have reduced odor, phytotoxic chemicals, weed seeds, and pathogens [
11]. The basic information on suitability of composts before application is found by determining the maturity and stability of composts through a phytotoxicity test [
12]. Evaluation of the feasibility and efficacy of composts derived from agricultural wastes not only serves as means of managing the wastes, but also serves as one of the means of improving chemical manufactured nitrogen (N) fertilizers.
Nitrogen contains organic and inorganic N fractions that are readily transformed to reactive N compounds such as nitrous oxide (N
2O), nitrate (NO
3−), nitrite (NO
2−), ammonia (NH
3), and ammonium (NH
4+) [
13]. Gaseous N emitted from excessive use of urea pollutes the environment, impacts public health, and degrades ecosystems. However, urea has been widely used as the main N fertilizer because its use had contributed to enhancing agricultural productivity in addition to reducing hunger worldwide. Global urea use was heavily concentrated in the United States and Western Europe after which it expanded to regions in Asia and Latin America [
14]. Since 2000, new areas of increased urea use in East and South Asia have emerged [
15]. United Nations Environment Programme (UNEP) highlighted the importance of managing pollution from urea application apart from biodiversity loss and climate change [
16]. Pollution from excessive urea application includes air contamination, soil acidification, soil degradation, water eutrophication, and crop yield reduction [
17]. In addition, urea is an expensive input in crop production, whereas reactive N compounds such as nitrogen oxides (NO
x) and NH
3 are estimated to be major threats to global biodiversity [
17]. Research on sustainable N management has expanded, from understanding aspects of the N cycle and specific effects of reactive N to improving efficient uses of N fertilizer and technologies in farming practices [
18]. To this end, initial expert communities consisting of agricultural and environmental scientists such as soil, plant nutrition, or biochemistry specialists, have expanded to integrate a broader range of policy and social sciences scholars [
19]. The adverse effects of N pollution called for new research questions, including factors of behavioral change among farmers, consumers, and industries, and the need to develop approaches that consider socioeconomic factors and global food chains [
20]. Application of urea is related to the need to achieve high crop production that could sustainably feed the increasing world population [
21]. Therefore, the goal of farmers is that the target crop takes up the applied urea-N with the maximum efficiency and recovery.
In our previous study on estimating N mineralization from rice husk (RH) compost tested in maize production, retention of soil total N and exchangeable NH
4+ were significantly higher in urea amended with RH compost than in urea alone. Also, combined application of urea with RH improved plant N recovery, uptake, use efficiency, and yield of maize grown in tropical acid soils [
9]. Based on these premises, it is expected that co-application of RH compost with conventional urea could minimize NH
3 volatilization by retaining N in the form of NH
4+. Despite being applied as a soil amendment and complement to chemical N fertilizers, there is scanty information on the use of RH compost on mitigating NH
3 volatilization from different rates of urea. Our goals in this study are to address the research questions such as: (i) how much RH compost should be used to mitigate NH
3 volatilization from urea? (ii) how frequently should RH compost be used to reflect the optimum rate of urea and RH compost on NH
3 volatilization? and (iii) how should RH compost be used to accumulate soil total N, exchangeable NH
4+, and available NO
3− following co-application of urea and RH compost?
In this study, the approach was to produce compost from co-composting of RH with chicken manure, chicken feed, and molasses, after which the RH compost was tested in a NH
3 volatilization study. The high sources of organic based N from CDS coupled with high carbon from RH, chicken feed, and molasses reacted during co-composting to produce good quality compost which when applied with urea improves N retention by mitigating N loss via NH
3 volatilization. Ammonia volatilization from the excessive use of urea would contaminate the atmosphere through rapid hydrolysis which catalyzes through the enzyme urease to form NH
3 and carbonic acid. One of the common practices to reduce NH
3 formation is the application of urease inhibitors as additives to urea-based synthetic fertilizers. However, most of the urease inhibitors are acidic, highly corrosive, and expensive. An attempt to produce compost from agricultural wastes is considered a lucrative way to produce cheaper soil organic amendment compared with synthetic urea which has been implicated in environmental pollution. The approach of using RH compost for mitigating N loss from urea via volatilization was because of the acidic nature of the compost (from humic acids formation), besides serving as a source of organic matter (OM). Humic acids (HA) in composts have high cation exchange capacity (CEC) to retain N in the form of NH
4+. In addition, NH
3 volatilization is higher when the soil pH is above pH 7 because of the equilibrium of NH
3 + H
2O ⇌ NH
4+ + OH
− as justified by Sigurdarson et al. [
22]. Rapid hydrolysis of urea could be mitigated using RH composts because of their high CEC, OM, and HA content.
The focus of the study was on minimizing N loss via NH3 volatilization by amending different rates of urea with different rates of RH compost to improve N availability in soil. The N which will hydrolyze from urea in the form of NH4+ is expected to be attracted to the negative charges of the RH compost and this reaction will prevent NH4+ from being converting into NH3. It is hypothesized that RH compost derived from co-composting RH with chicken dung slurry (CDS), chicken feed, and molasses could be used to prevent urea-N from being volatilized. This hypothesis is based on the assumptions that temporary retention of NH4+ is expected to mitigate NH3 volatilization and leaching of NO3−. To confirm the afore-stated hypothesis, this study was carried out to: (i) produce compost through co-composting RH with CDS, chicken feed, and molasses, (ii) determine the effects of optimum rate of urea and RH compost on NH3 volatilization, and (iii) determine accumulations of soil total N, exchangeable NH4+, and available NO3− following co-application of urea and RH compost.
The implications of using RH compost as supplemental material for N retention from urea application is an attempt to delay urea hydrolysis from urea in addition to minimizing formation of NH
3 and carbonic acid in soil systems. In our previous work, we confirmed that N in soil is predominantly in the form of organic fractions, which are stored in OM, with less than 5% present in the inorganic forms (NH
4+ and NO
3−), and are available for crop uptake [
9]. Owing to the aforementioned findings, our approach was not only limited to determining the effectiveness of the RH compost in mitigating NH
3 volatilization, but was also focused on nourishing soil with OM with its application. We also confirmed that the use of RH compost restored and rejuvenated soil N fertility instead of providing essential nutrients directly to the plants. Balanced used of RH and urea have been achieved by determining the optimum rate of RH compost in mitigating NH
3 volatilization from different rates of urea. With the reduction of N added from conventional urea application and the inclusion of RH compost at optimum amount, dissolution rate of urea could be prevented by providing negative surface charges from OM complexes of the RH compost to capture N in the form of exchangeable NH
4+. Retention of N in the form of exchangeable NH
4+ will retard rapid volatilization of N in the form of NH
3 and NH
4+ will be retained in soil for crop uptake. N will then be released gradually from the urea due to it being temporarily fixed in the form of exchangeable NH
4+, which enables synchronization of N supply with crop requirement. The novelty of our research contributes to the scientific literature because incorporation of RH compost with urea at optimum rate provides opportunities for N fertilizer management to accomplish the task of mitigating N loss through NH
3 volatilization. Considering that farmers may not be able to afford the high cost of urea, the use of RH compost or the combination with urea could form a major complement to replenish N deficiencies and ensure N availability in soil. Including RH compost in N fertilizer management in a manner that is environmental friendly, abundantly available, and easily degradable in soils to prevent N loss from urea is considered a way to conserve public health, improve the quality of the environment, and promote sustainable agriculture, all at a low cost. Consistent with an increase in the population growth and unprecedented lockdown over the world due to COVID-19 pandemic, incorporation of RH compost in N fertilization management could increase food crop production. The benefits of N management for sustainable development goals are supporting livelihoods by improving N fertilizer efficiency and reducing N loss, using N fertilizer efficiency and biological N fixation to sustain food production, improving public health through better N air and water quality, educating and training in sustainable N management, decreasing NO
3− contamination of drinking water and rivers, and decreasing NH
3 and NO
x emissions to help protect terrestrial biodiversity [
16]. This study will also provide information on the mechanism of N releases from different rates of urea amended with different rates of RH compost over conventional urea.