Effects of fertilizer and biochar applications on the relationship among soil moisture, temperature, and NO emissions in farmland.

BACKGROUND

Di-nitrogen oxide (NO) emissions from soil may lead to nonpoint-source pollution in farmland. Improving the C and N content in the soil is an excellent strategy to reduce NO emission and mitigate soil N loss. However, this method lacks a unified mathematical index or standard to evaluate its effect.

METHODS

To quantify the impact of soil improvement (C and N) on NO emissions, we conducted a 2-year field experiment using biochar as carbon source and fertilizer as nitrogen source, setting three treatments (fertilization (300 kg N ha), fertilization + biochar (30 t ha), control).

RESULTS

Results indicate that after biochar application, the average soil water content above 20 cm increased by ∼26% and 26.92% in 2019, and ∼10% and 12.49% in 2020. The average soil temperature above 20 cm also increased by ∼2% and 3.41% in 2019. Fertigation significantly promotes the soil N2O emissions, and biochar application indeed inhibited the cumulation by approximately 52.4% in 2019 and 33.9% in 2020, respectively. NO emissions strongly depend on the deep soil moisture and temperature (20-80 cm), in addition to the surface soil moisture and temperature (0-20 cm). Therefore, we established an exponential model between the soil moisture and NO emissions based on theoretical analysis. We find that the NO emissions exponentially increase with increasing soil moisture regardless of fertilization or biochar application. Furthermore, the coefficient a < 0 means that NO emissions initially increase and then decrease. The a < a indicates that fertilization does promote the rate of NO emissions, and the a > a indicates that biochar application mitigates this rate induced by fertilization. This conclusion can be verified by the sensitivity coefficient (SC of 1.02 and 14.74; SC of 19.18 and 20.83). Thus, we believe the model can quantify the impact of soil C and N changes on NO emissions. We can conclude that biochar does significantly reduce NO emissions from farmland.