## 1 Introduction  
Fertilizer nitrogen (N) value, regardless of the N source, can be calculated from the readily mineralizable fractions of N (organic N fractions). The hydrolyzable fractions of organic N, such as hydrolyzable NH\({}_{4}\)-N, amino sugars, and amino acids, are the most active N pools. They are the major sources of organic N fractions that will potentially decompose into available N forms for plant uptake [1,2]. Nitrogen availability is affected by the decomposition of soil organic N fractions which ultimately mineralize into available N (NH\({}_{4}\)\({}^{+}\) and NO\({}_{3}\)\({}^{-}\)). Information on soil organic N fractionsprovides the means to determine soil organic N decomposition [3]. The practices of N fertilizer application, conservation tillage, and the use of cover crops usually increase soil N stocks [4]. However, excessive and unreasonable amounts of inorganic N fertilizers are being used in agriculture to achieve high crop yield. These practices do not only waste scarce or limited resources, but they also pollute or degrade the environment. The excessive use of inorganic N fertilizers, especially in commercial agriculture [5], has adversely affected the quality of the environment. On the other hand, there is underutilization of fertilizers in Africa and parts of Latin America [6] which leads to poor crop yield and low food security. Thus, optimal applications of inorganic or organic N as fertilizers need to be practiced to improve soil N availability and plant N use efficiency. The use of readily available N fertilizer in agriculture may result in high levels of available N and this might exceed crop demand early in the season. Consequently, it could cause N losses and deficiencies. Improving the nitrogen use efficiency (NUE), which is the fraction of the applied fertilizer nitrogen taken up by a plant for its physiological and growth purposes [7], is one of the strategies for reducing N loss in agriculture. With the continued escalating costs of inorganic fertilizers, NUE in field crops should be enhanced to reduce production costs for greater profits to resource-poor farmers. Therefore, it is essential that appropriate N fertilizer management practices are adopted to optimize the use of applied N in cropping systems [8]. In managing the application of inorganic and organic sources of N fertilizers such as urea, ammonium nitrate, ammonium sulfate, and composts, one of the challenges is to synchronize the supply of N with crop demand. This requires an understanding of the mechanisms of N loss and release from the inorganic fertilizers and composts in relation to plant growth patterns. The retention of N applied in fields also needs to be addressed in view of the losses of N partly because of poor soil nutrient holding capacity. Zeolites can be used to retain, for example, ammonium (NH\({}_{4}\)+) from N fertilizers because of their high affinity for this ion. This intervention can minimize N losses [9]. The application of fertilizers, zeolites, and composts was found to enhance NH\({}_{4}\)+ retention due to the high negative charges of zeolites and the humus component of composts [10]. This study is a follow up to a previous greenhouse trial [10], to determine whether the results obtained for urea-N use efficiency, composts, and clinoptilolite zeolite (CZ) could be replicated in a field trial. Clinoptilolite zeolite is a natural zeolite found in soils and sediments and it acts as a molecular sieve that selectively absorbs ammonium ions and other cations such as calcium, potassium, magnesium, and sodium on its negatively charged surface. Clinoptilolite zeolite is commonly used in agriculture as a soil amendment to improve nutrient retention. Applying chemical fertilizers with compost and zeolite resulted in the improvement of macronutrient uptake in maize plants, NUE, and maize ear yield [11].