# Duality of Biochar and Organic Manure Co-Compositing on Soil Heavy Metals and Enzymes Activity
## 1 Introduction
Inorganic-N (NO\({}_{3}-\)-N, NH\({}_{4}+\)-N) is adsorbed on the surface of biochar and released gradually in the soil solution, thus functioning as a slow-release fertilizer [9]. This suggests that the surfaces of biochar exhibit a higher affinity for the positive NH\({}_{4}+\)-N ions than for NO\({}_{3}-\) ions. One excellent source of plant nutrients is animal manure. It increases soil biological activity and improves soil health when added to agricultural soils [10; 11; 12]. For farmers with limited resources, the availability of animal manure offers a stabilized form of nutrients that support the development and yield of a wide range of crops. An important source of soil nitrogen is animal manure, such as sewage sludge, chicken manure, horse manure, etc. Soil bioremediation can be improved by using sewage sludge as a soil conditioner to improve the physical, chemical, and microbiological characteristics of the soil. Chicken manure improves the proliferation of many kinds of microorganisms, including bacteria, fungi, and actinomycetes, as well as the biological activity and fertility of the soil. The nutrients that are most frequently utilized in inorganic fertilizers, such as N and K, are found in abundance in animal manure, along with other minerals [13; 14]. N is the main nutrient required to increase plant growth and crop yield [15]. N is required for plants and animals due to its role in the formation of several energetic proteins and amino acids. [PERSON] et al. [16] published information on the role of N in eggplant production, and confirmed that eggplants depend on N to supply up to 120 kg of N ha\({}^{-1}\), whereas [PERSON] [17] suggested using up to 200 kg of N ha\({}^{-1}\) for farming. [PERSON] et al. [14] also found that the maximum number of fruits per plant\({}^{-1}\) was obtained when N concentration ranged between 14.0 and 17.0 g of N per plant\({}^{-1}\) (145-177 kg of N ha\({}^{-1}\)). On the contrary, N rates greater than 15.03 g per plant\({}^{-1}\) minimized crop yields. The application of animal manure in agricultural production systems reduces dependence on synthetic inorganic fertilizers. In addition, animal manure application in agricultural systems provides a useful means of waste disposal. However, animal dung and biochar are some of the pathways by which heavy metals enter the ecosystem and accumulate in the soil, and these then discharge the heavy metals into natural water resources captivated by the growing crops, and they are consumed by humans, posing a threat to anthropological health [18; 19]. Therefore, keeping such metal concentrations below legal limits is challenging for farmers and health authorities. Heavy metals influence the microbial population that secretes soil enzymes. Heavy metals can reduce bacterial species abundance and lessen bacterial communities' biomass and diversity in polluted soils [20]. [PERSON] et al. [21] reported that Cd is more harmful to hydrolyzing enzymes than Pb. Additionally, Cu inhibits the activity of \(\beta\)-glucosidase more notably than cellulose due to Cu's higher mobility and lower affinity for soil colloids. In contrast, high levels of Pb in the soil significantly reduce the activities of urease, invertase, catalase, and acid phosphatase. Soil enzymes play a vital role in numerous essential soil functions. They are deeply involved in nutrient recycling, enhancing fertilizer efficiency, reflecting soil microbiological activity, and as indicators of soil transformations. Bioindicators, such as soil enzymes in animal manure, are tools for monitoring soil health [10; 22; 23]. Enzymes are susceptible to conservational stress caused by elevated levels of heavy metals [24; 25], soil hormones, and antibiotics [26] in animal manure, which affect soil biological activity. Urease (urea amidohydrolase, EC 3.5.1.5) converts urea fertilizers into NH\({}_{3}\) and CO\({}_{2}\), which raises soil pH [27] and causes N loss to the troposphere because of NH\({}_{3}\) volatilization. Soil urease is created from plants and microorganisms [28].