Succession of soil aggregate associated carbon and nitrogen amid the reclaimed mine soils pedogenic process in an opencast coal mine within the Loess hilly region

Reclaimed Mine Soil (RMS) is the type of high compacted and nutrient exhausted soil, due to the mining and reclamation operations. The soil aggregate associated carbon (C), nitrogen (N), and isotope can be a platform to understand the pedogenic process of reclaimed mine soil. This study aims to monitor the soil aggregate recovery and Soil Organic Matter (SOM) formation. Different sorts of age chronosequence soil (0, 3, 12, 14, 17, 25, 28, and 32 a) were collected from the Robinia pseudoacacia L. forests in the Pingshuo opencast coal mine in China. Moreover, the soil was selected from adjacent and undisturbed Populus simonii forest as a reference (CK) as well. The wet-sieving method was used to separate the fractions of soil aggregate, and the aggregate-associated C, N, 13C, and 15N isotopes. The results showed that: 1) The proportion of >250-2 000 μm aggregate in the 32 a soil was 737.02% higher than that in the 0a soil, whereas, the proportion of 53-250 μm in the 32 a soil was 19.25% lower than that in the 0a soil, indicating that the reclamation operations destroyed the structure and stability of water-stable aggregate. Moreover, the mean weight diameter of soil aggregate in the 32 a soil was 133% higher than that the in 0a soil, indicating the excellent recovery of the structure and stability with the ages. 2) Soil aggregate associated C and N increased with the reclamation time. The >2 000 μm and >250-2 000 μm associated C and N were dominated in the total C and N. In addition, the aggregate-associated C, N, and C/N ratio increased with the time. Specifically, the large-size aggregate (>250 μm) associated C/N was significantly higher than that of the microaggregate (P<0.05). 3) The δ13C values were differed from -15.14%0-26.40%0, while the δ15N values were ranged from 1.13%0-10.87%0. More importantly, the δ13C values decreased with the time in the soil aggregate >2 000 μm, >250-2 000 μm, and lower than 53 μm, the δ15N values decreased in the >2 000 μm as well. The 13C isotopes were significantly enriched in the soil aggregate 53-250 μm and lower than 53 μm fractions, compared with the large-size aggregates (P<0.05). It infers that the soil C was incorporated into the RMS from the large-size to small-size aggregate fractions during the pedogenic process. 4) The proportion of the new C was accounted for over 80% of the total C in the soil aggregate >2 000 and >250-2 000 μm, without the significant temporal variation (P>0.05). However, the proportion of the new C increased significantly in the lower than 53 μm fraction over 17 years after reclamation, whereas the proportion remained stable after that, indicating the nearly saturated recalcitrance pool (<53 μm C) after 17-year reclamation. Overall, the reclamation operations significantly increased the proportions of small-size soil aggregates. However, the vegetation restoration can be expected to facilitate the recovery of aggregate structure, stability and aggregate associated C and N. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.