Effects of warming and fertilization on soil organic carbon and total nitrogen contents, and δ¹³C and δ¹⁵N in farmland

Farmland soil is an essential terrestrial carbon and nitrogen pool that is highly sensitive to climate change. However, the response of soil carbon and nitrogen cycles to climate change remains unclear. Understanding the effect of warming on soil organic carbon is particularly important to achieving the goal of carbon peak and carbon neutralization in the context of global warming. Here, the infrared heaters were used to simulate warming and the soil temperature (5 cm depth) increased by approximately 2 °C. The soil organic carbon, total nitrogen, δ¹³C, and δ¹⁵N contents were measured to assess the effects of warming, nitrogen addition, and irrigation on the soil carbon and nitrogen cycle. The experiment consisted of four treatments: no nitrogen addition and no warming (N0T0), no nitrogen addition and warming (N0T1), nitrogen addition and no warming (N1T0), and nitrogen addition and warming (N1T1). The results showed that warming decreased the soil organic carbon content before irrigation. There were significant differences between the N1T1 and no warming (N0T0 and N1T0) treatments at 0–10 cm depth (P<0.05), and between N1T1 and the other three treatments at 10−20 cm depth (P<0.05). Warming tended to decrease soil organic carbon content after irrigation. However, the difference was not statistically significant. Warming enhanced soil δ¹³C in the nitrogen addition treatments (P<0.05) and decreased soil total nitrogen content, but the differences were only significant at the 10−20 cm depth before irrigation and at the 0–10 cm depth after irrigation (P<0.05). Soil δ¹⁵N was enhanced in the warming treatment. However, the defferences were only significant between the N0T0 and warming (N0T1 and N1T1) treatments at 0−10 cm depth before irrigation (P<0.05), between N0T1 and N1T0 at 0−10 cm depth (P<0.05), and between N1T0 and the warming (N0T1 and N1T1) treatments at 10−20 cm after irrigation (P<0.05). The soil organic carbon and total nitrogen contents decreased with increasing soil depth, while δ¹³C and δ¹⁵N increased with increasing soil depth. However, only the changes in total nitrogen and δ¹⁵N were significant. Irrigation had no significant effects on soil organic carbon, total nitrogen, δ¹³C, and δ¹⁵N. The 5-year continuous warming and nitrogen addition experiments suggest that future climate warming may accelerate the decomposition of soil organic carbon and total nitrogen, resulting in a greater loss of the light fraction carbon. Irrigation did not significantly alter the soil organic carbon and total nitrogen contents and the δ¹³C, and δ¹⁵N values in the short term; however, its long-term effects need to be further explored. In addition, future research should focus on the effect of multi-factor interactions on soil carbon and nitrogen cycles.