Effects of Dissolved Organic Carbon Leaching and Soil Carbon Fractions Under Intercropping Dactylis glomerata L.–Medicago sativa L. in Response to Extreme Rainfall

Climate change aggravates the frequency of extreme rainfall events, resulting in carbon (C) loss. For the special climate of the highlands, cultivating the land underneath orchards increases C reservation. Systematic research on the impact of extreme rainfall on soil organic carbon compositions and (dissolved organic carbon) DOC leaching is limited, especially regarding the response to different cropping patterns underneath orchards, requiring a deeper understanding. The results showed that the DOC-leaching fluxes for the cropping patterns under rainstorms and heavy rainstorms were in the order Dactylis glomerata L. monocropping (13.5, 4.4 kg/hm2) > Medicago sativa L. monocropping (11.2, 3.8 kg/hm2) ≥ D. glomerata. + M. sativa. (10.4, 3.6 kg/hm2). The DOC-leaching fluxes during heavy rainstorms were reduced with D + M, and the root morphology showed a significant correlation with DOC concentration. Compared to the D, SOC in layers 40–60 cm of the M and the D + M increased by 68.36% and 64.24%, respectively. TP and POC of the D + M increased with soil depth. Relationships between cropping pattern and rainfall intensity for particulate organic carbon (POC) and mineral-associated organic carbon (MOC) were observed. Heavy rainstorms reduced MOC, including the decomposition of substances related to the MOC, such as ROC and DOC, then POC in layers 40–60 cm increased; compared with 0–20 cm of D and M, the content of readily oxidizable carbon (ROC) in layers 40–60 cm reduced by 56.90~77.64%, and the POC increased by 38.38~87.00% in the D + M. Therefore, it was suggested that the decomposition of deeper MOC due to heavy rainstorms is the main source of soil POC and leaching DOC. This will provide a reference basis for research on assessing soil carbon-leaching fluxes and carbon stocks under extreme rainfall events.