Using nitrogen-loaded biochar for soil improvement to decrease applied nitrogen and stabilize rice yield under alternate wet-dry irrigation

Abstract

Nitrogen (N) loss from paddy fields is an important cause of eutrophication and yield decrease. In order to minimize the negative impact of N loss, biochar was used as a medium for the sorption of ammonia from simulated wastewater and subsequently as a N releaser in the cultivation of rice plants under the N deficiency environment during two years. The experiment was conducted in a completely randomized block design, with a conventional fertilization treatment (N1C0), 10 t/hm2 or 20 t/hm2 N-loaded biochar and 25% less N fertilizer (N3/4C1, N3/4C2), and 10 t/hm2 or 20 t/hm2 N-loaded biochar and 50% less N fertilizer (N1/2C1, N1/2C2). The NH4+-N and NO3--N concentrations in surface water, the number of rice tillers, dry matter weight, and N uptake were measured. Results showed that the adsorption efficiency of the prepared N-loaded biochar to NH4+-N was 30.8%; reducing the amount of N fertilizer coupled with N-loaded biochar application reduced the average surface water concentration of NH4+-N by 15.44–43.62% within one week after rice transplantation; it also reduced the average surface water concentration of NO3--N during the whole growth period by 18.76–25.19%. In the late rice growth stage, the average concentration of NH4+-N in surface water was shown in descending order: N3/4C2 ＞N3/4C1 ＞ N1C0＞ N1/2C2 ＞ N1/2C1. Compared with N1C0, N3/4C2 did not clearly impacted the rice tiller numbers, the dry matter weight and N uptake of rice stems, leaves and panicles. In sum, under alternate wet-dry irrigation, 20 t/hm2 N-loaded biochar purified the eutrophic water in the irrigation area and permitted N fertilizer input to be reduced by 25%, which stabilized rice yield and was conductive to improving a spatial balance of “misplaced” N resource.