Partial Substitution of Urea with Biochar Induced Improvements in Soil Enzymes Activity, Ammonia-Nitrite Oxidizers, and Nitrogen Uptake in the Double-Cropping Rice System.

Biochar is an important soil amendment that can enhance the biological properties of soil, as well as nitrogen (N) uptake and utilization in N-fertilized crops. However, few studies have characterized the effects of urea and biochar application on soil biochemical traits and its effect on paddy rice. Therefore, a field trial was conducted in the early and late seasons of 2020 in a randomized complete block design with two N levels (135 and 180 kg ha) and four levels of biochar (0, 10, 20, and 30 t ha). The treatment combinations were as follows: 135 kg N ha + 0 t B ha (T1), 135 kg N ha + 10 t B ha (T2), 135 kg N ha + 20 t B ha (T3), 135 kg N ha + 30 t B ha (T4), 180 kg N ha + 0 t B ha (T5), 180 kg N ha + 10 t B ha (T6), 180 kg N ha + 20 t B ha (T7) and 180 kg N ha + 30 t B ha (T8). The results showed that soil amended with biochar had higher soil pH, soil organic carbon content, total nitrogen content, and mineral nitrogen (NH-N and NO-N) than soil that had not been amended with biochar. In both seasons, the 20 t ha and 30 t ha biochar treatments had the highest an average concentrations of NO-N (10.54 mg kg and 10.25 mg kg, respectively). In comparison to soil that had not been treated with biochar, the average activity of the enzymes urease, polyphenol oxidase, dehydrogenase, and chitinase was, respectively, 25.28%, 14.13%, 67.76%, and 22.26% greater; however, the activity of the enzyme catalase was 15.06% lower in both seasons. Application of biochar considerably increased the abundance of ammonia-oxidizing bacteria (AOB), which was 48% greater on average in biochar-amended soil than in unamended soil. However, there were no significant variations in the abundances of ammonia-oxidizing archaea (AOA) or nitrite-oxidizing bacteria (NOB) across treatments. In comparison to soil that had not been treated with biochar, the average N content was 24.46%, 20.47%, and 19.08% higher in the stem, leaves, and panicles, respectively. In general, adding biochar at a rate of 20 to 30 t ha with low-dose urea (135 kg N ha) is a beneficial technique for improving the nutrient balance and biological processes of soil, as well as the N uptake and grain yield of rice plants.