**4.1 Nitrate leaching risk after biogas digestate amendment**

Anaerobic digestion is the degradation of organic substrates to biogas and produces a by-product "anaerobic digestate" which contains significant amounts of mineral nitrogen (N), which is available for plants [59]. Biogas digestate typically has a high concentration of ammonium (NH4 + ) and relatively little carbon (C), with NH4 + -N accounting for 35–81% of total N and a C/N ratio of 2.0–24.8 [15, 18]. Moreover, it contains other macro- and micronutrients that are necessary for plant growth [7, 60].

The merits and demerits in the application of biogas digestate have been addressed in numerous papers. For example, the benefits are to improve the soil properties by reducing the bulk density, to increase the saturated hydraulic conductivity and the moisture retention capacity [61, 62], to sustain soil organic matter concentrations [63, 64], to enhance biological activities [59, 65, 66], and to suppress pathogenic organisms [15, 67]. In contrast, the demerits are to enhance nitrate leaching risk and to bring chemical and biological contaminations, such as heavy metals, organic pollutants [15, 68–70], *Salmonella*, and *Escherichia coli*, which are the most prevalent pathogenic microorganisms found in manures [71, 72].

Once biogas digestate is applied to a field, the NH4 + -N is subjected to different processes: volatilization, absorption by clay particles, take-up by plants, immobilization into soil organic matter, and/or nitrification [73]. In general, NH4 + -N in biogas digestate is readily nitrified to nitrate (NO3 <sup>−</sup>) in soil [74–76]. Since few NO3 <sup>−</sup> can be absorbed by soil particles, most of excess NO3 <sup>−</sup> moves downward with drainage water and is eventually leached from the soil profile [77]. Many studies have reported the application of biogas digestate enhances NO3 <sup>−</sup> leaching risk in the soil [76, 78–80]. In particular, the nitrate leaching potential is much higher in soil with neutral pH soil than in soil with lower pH [81, 82].

Stumborg [83] reported that dynamics of inorganic N, especially NO3 <sup>−</sup>, is directly influenced by the soil type, climate, frequency of application, and property of the digestate. Rigby and Smith [84] conducted a laboratory experiment to investigate the effect of digestate deriving from different waste types (industrial, agricultural, and municipal solid waste or sewage sludge) on the N dynamics in three types of soil (sandy loam, sandy silt loam, and silty clay) and found that NO3 − concentration was higher in sandy loam and NO3 <sup>−</sup> did not accumulate in silty clay soil due to denitrification. Therefore, it is necessary to consider nitrate leaching risk in applying biogas digestate to an agricultural field from different aspects, such as the properties of digestate, soil type, and moisture content.
