Analysis of DOC fluorescence spectral structure of black soil in different regions under conservation tillage

The composition, content and turnover rate of soil dissolved organic carbon (DOC) directly affect the composition and activity of microbial communities, thus influencing the mineralization process of soil organic carbon. The present study investigated the effects of conservation tillage practices on the fluorescence properties of soil DOC in different regions, to reveal the effects of conservation tillage on the stability of black soil carbon pools. Furthermore, this research provides a theoretical basis for the scientific selection of tillage methods. In this research, four conservation tillage treatments, including no-tillage (NT), no-tillage + straw returning (NR), subsoiling (ST) and subsoiling + straw returning (SR), were set up in Keshan, Heilongjiang Province (black thick-layer soil ) and Gongzhuling, Jilin Province (black thin-layer soil). The traditional tillage (CT) and tillage + straw returning (CR) were used as the controls. There were 6 treatments in total. Fluorescence spectroscopy was used to analyze the content, composition, and structural characteristics of farmland soil DOC in the 0~20 cm in black thick-layer soil and black thin-layer soil areas under different tillage treatments. The results showed that in black thick-layer soil area, soil TOC content showed SR>NR>CR>ST>NT>CT, DOC content was SR>ST>NT>NR>CR>CT. Both soil TOC and DOC content were the highest in the SR treatment, which were significantly increased (P<0.05) by 6.88% and 25.61%, respectively, compared with the CR treatment. In black thin-layer soil area, the contents of TOC and DOC in NR treatment were the highest, and the DOC content significantly increased (P < 0.05) by 21.30% compared to CR. The results of parallel factor analysis showed that DOC in black thick-layer soil and black thin-layer soil areas were divided into three components: C1 was composed of tryptophan proteins, C2 was a fulvic acid-like substance, and C3 was a humus-like substance. Fluorescence index analysis indicated that in black thick-layer soil and black thin-layer soil areas were affected by both internal and external influences. In black thick-layer soil area, the SR treatment had the lowest autogenic characteristics, which was greatly affected by exogenous input, with the highest humification coefficient, and a more stable DOC structure; In black thin-layer soil area, the NR treatment had the lowest autochthonous source characteristics, low microbial contribution, mainly from exogenous straw input, with stable DOC structure not easily utilized by microbial decomposition. In conclusion, conservation tillage measures have a certain enhancing effect on both TOC and DOC in both thick and thin layers regions of agricultural fields; The SR treatment was more conducive to the accumulation of TOC and DOC and the stabilization of carbon pools in black thick-layer soil area, and the NR treatment was more advantageous for the enhancement and stabilization of organic carbon in black thin-layer soil.