Urea has become one of the most commonly used N fertilizers in the world due to its high N content, stable nature, easy application, and other characteristics (Subbarao et al., 2012). However, urea applied to the soil surface can be rapidly hydrolyzed into ammonia (NH3) or ammonium (NH4+) by soil urease, which easily causes NH3 volatilization and leads to high concentration of NH3 in soil microzone, thus harming germinating seedlings and young plants (Subbarao et al., 2013). In addition, NH4+ can be easily transformed into nitrate (NO3−) via nitrification and NO3− may be lost in harmful forms of nitrous oxide (N2O) and nitric oxide (NO) by denitrifying bacteria or leaching through the soil to contaminate groundwater (Arango et al., 2014). Overall, the losses of N fertilizers may exceed 70% of the total application amount and cause serious environmental issues (Upadhyay et al., 2011).
Therefore, to ensure continuous and optimal N supply, improve use efficiency of N fertilizers, and protect the environment, regulation strategies should be adopted based on reducing urea hydrolysis and nitrification. Adding urease inhibitors into urea or nitrification inhibitors into NH4+-form and amide N-form fertilizers is an appropriate choice (Zaman et al., 2009). A large number of studies have been conducted on urease inhibitors such as N-(n-butyl)-thiophosphoric triamide (NBPT), phenyl phosphorodiamidate (PPD), and hydroquinone (HQ), as well as nitrification inhibitors such as dicyandiamide (DCD), nitrapyrin (N-serve), and 3,4-dimethylpyrazole phosphate (DMPP), some of which have been confirmed to have excellent potential to improve use efficiency of N and reduce environmental risks of applying N fertilizers (Arora and Srivastava, 2013). However, due to their high prices, poor availability, and adverse influences on beneficial microorganisms in the soil, most of synthesized urease and nitrification inhibitors are limited to the experimental stage and are not suitable to be popularized in agricultural production practice, especially in developing countries.
Phenolic compounds isolated from certain plants influence N cycling, which results in a decrease of N losses and represents a key issue for environmental protection worldwide. Plant metabolites and plant-based biochemical metabolic inhibitors have been considered as the best alternatives to synthetic urease and nitrification inhibitors due to their minimal environmental impact and biological damage (Patra et al., 2006). Therefore, the search for inhibitors from natural sources has received much attention, and efforts have been made to identify new natural resources for N efficiency improving compounds. Previous studies have shown that a number of extracts derived from higher plants can effectively inhibit UA and NP. Many empirical studies have pointed out that some plants and their by-products can inhibit urea hydrolysis or nitrification or both. To be specific, karanja (Pongamia glabra), neem (Azadirachta indica), and mint (Mentha spicata) are important sources of natural inhibitors (Abbasi and Manzoor, 2013; Kiran and Patra, 2003; Opoku et al., 2014).
A wide variety of tropical medicinal plants that are traditionally used to cure diseases, or to maintain and improve health, have been developed and applied to disinfection, sterilization, virus inactivation, antioxidation, and cosmetics (Schmidt, 2012). The aim of this study was to evaluate the inhibitory effects of extracts obtained from a small part of medicinal plant species that grow in Hainan Island, southern China, on urea hydrolysis and nitrification in soil, to obtain some biodegradable, eco-friendly, and inexpensive natural compounds capable of increasing N-use efficiency in agricultural soils. Moreover, this study is focused on the relationship between phenols in the extracts of selected medicinal plants and their urease/nitrification inhibitory effects. The phenolic compounds constitute a large group of secondary plant metabolites that are ubiquitous among higher plants. They are natural compounds, which generally help plants for defense, and widely known antibacterial agents that inhibit the activity of bacteria and fungi (Rauha et al., 2000).
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