[Chemical structure characteristics of organic carbon in saline soil aggregates under straw returning condition]

We examined the regularity of distribution and chemical structure characteristics of organic carbon in soda alkaline fluvo-aquic soil aggregates after straw returning. We set up six different straw returning treatments in 2020, including 0 (CK), 2100 (ST₁), 4200 (ST₂), 6300 (ST₃), 8400 (ST₄) and 10500 kg·hm^-2(full straw returning, ST₅). We measured organic carbon (OC) content and infrared spectroscopy characteristics of aggregates and internal different components through physical fractionation method and infrared spectroscopy technology. The results showed that: 1) the OC content of soil and all aggregates increased with the increasing amount of returned straw; 2) different straw returning treatments significantly increased the content of light organic carbon (LOC) in 53-250 μm aggregates. Compared with CK, ST₃ and ST₄ treatments significantly increased the content of mineral-bound organic carbon (MOC) in 250-2000 μm aggregates and the content of fine particulate organic carbon (fPOC) in 53-250 μm aggregates. The OC content of different components in aggregates followed the order of LOC>MOC>POC. The fPOC content in 250-2000 μm aggregates was higher than that of coarse particulate organic carbon (cPOC); 3) the results of principal component analysis showed that OC chemical structure of different components in aggregates was seldom affected by the straw returning, but was mainly affected by particle size; 4) the OCs in >250 μm aggregates were mainly derived from aromatic carbon and polysaccharides. The OCs in 53-250 μm aggregates were mainly derived from carbohydrates, such as monosaccharides and polysaccharides, while the OC in <53 μm aggregates was mainly derived from aliphatic carbon, alkyl carbon, aromatic carbon and phenolic alcohols. Within different aggregates, LOC was mainly derived from aliphatic carbon, aromatic carbon and phenolic alcohols. Particulate organic carbon (POC) was mainly derived from carbohydrates. MOC was mainly derived from alkyl carbon. In summary, straw returning increased organic carbon content in soil aggregates in short term, but did not alter organic carbon chemical structure. The organic carbon chemical structures of the same particle size fractions in different aggregates were similar. The organic carbon content increased with the decreases of particle size, and the chemical structure tended to be stable. Therefore, straw returning promoted the fixation of organic carbon by saline soil aggregates in short term, but did not change their chemistry structural characteristics, indicating that the location and protection degree of soil organic carbon in aggregates were the main factors affecting the chemical structure of organic carbon.