Effects of biochar and irrigation regime on soil carbon and nitrogen distribution in wheat fields: a two-year field...

To clarify the responses of soil carbon and nitrogen to biochar amendment under deficit irrigation, a two-year field experiment was conducted in wheat fields of northern Xinjiang with a completely randomized block design. Four biochar rates (0, 10, 20, 30 t/ha, namely B0-B3) and three irrigation regimes were set. Irrigation volumes were 4500, 4050 and 3600 m³/ha in 2021 (namely W0, W1, W2), and adjusted to 4500, 4050 and 3825 m³/ha in 2022 (namely W0, W1, W2'). Two-way ANOVA was used to analyze the main and interactive effects of biochar dosage, irrigation level and experimental year on all indicators. The results revealed that biochar application significantly increased soil organic carbon (SOC) and total nitrogen (TN) contents. Biochar and irrigation exerted significant interactive effects on SOC in the second year (P < 0.05). SOC and TN were higher in the 0-20 cm soil layer than in the 20-40 cm layer. The B2W1 treatment had the highest SOC and TN levels, which were 23.33% and 22.46% higher than those of B0W1, respectively. Biochar significantly regulated SOC and TN contents in 0.25-2 mm soil aggregates across both soil layers. The TN contribution proportion of soil aggregates was markedly affected by biochar addition and irrigation reduction. In the second year, more than 40% of soil TN was distributed in 0.25-2 mm aggregates, and its contribution trend was consistent with that of SOC. Both treatments significantly affected wheat yield (P < 0.05). Proper biochar application combined with moderate irrigation reduction improved grain yield, which increased first and then decreased with reduced irrigation and increased biochar input. Collectively, the optimal biochar application rate was 10-20 t/ha, and the recommended irrigation volume was 4050 m³/ha with a 10% reduction. This matching pattern can achieve efficient water saving and stable soil carbon sequestration in irrigated farmlands.