Monoculture and improper management may reduce soil fertility and deteriorate soil structure in Black soils (Mollisols) of Northeast China. The experiment was carried out from 2015 to 2016 in Black Soils comprising five cropping systems: continuous corn (CC), soybean-corn rotation (SC), cornsoybean rotation (CS), fallow-corn (FC), and fallow-soybean (FS). Our results showed that CS and FS treatments significantly increased mean weight diameter (MWD) and fractal dimension (D) in mechanical stability aggregates (MSAs), and increased MWD and geometric mean diameter (GMD) in water-stable aggregates (WSAs) compared with CC treatment. These two treatments were also significantly increased water-stable aggregates stability rate (WSAR), but decreased percentage of aggregates destruction (PAD) than CC treatment. Meanwhile, CS and FS treatments exhibited a higher carbon accumulation than cc treatment in bulk soils. Soil organic carbon (Soc) concentration in WSA 0.106-0.25 ,WSA 2-5 mm and WSA 0.5-1 mm had a dominant effect on aggregate stability. Simutaneously, Soc in WSA >5 mm affected SOC concentration in bulk soils. As a whole, the CS and FS treatments can increase the percentage of macro-aggregates, enhance aggregate stability, as well as increase SOC concentration in bulk soils and all soil aggregate sizes. Soil organic carbon (SOC) plays a key role in forming and stabilizing soil structure, enhancing soil physical properties, and nutrient recycling 1-3. Soil aggregate, the basic unit of soil structure, mediates many physical and chemical processes in soils 4-8 , such as soil compaction, soil nutrient recycling, soil erosion, root penetration, and crop yield 9. Aggregate stability is frequently used as an indicator of soil structure 10-12 because better soil structure and higher aggregate stability are vital to improve soil fertility, soil sustainability, and productivity 13,14. SOC influenced aggregate stability and soil structure 15,16. The stability of organic carbon in different size aggregates is different. Organic carbon in the micro-aggregates is less susceptible to change than it is in the macro-aggregates 17. The soil organic matters of different cropping systems differed based on the quantity and quality of the crop residue coverage and the environment, affecting the organic carbon contents of the soil and the aggregate stability 18. The cropping systems mainly create conditions for the decomposition and transformation of soil organic matter by changing the distribution of soil organic carbon and the active habitat of microorganisms, thereby causing changes in soil aggregates 19. Soil mean weight diameter (MWD), geometric mean diameter (GMD), fractal dimension (D), percentage of aggregates destruction (PAD) and water-stable aggregates stability rate (WSAR) are all indicators of soil aggregate stability. The larger the MWD and GMD values are, the higher the average particle size agglomeration of soil aggregates are, and the stronger the stability of soil structure is 20. Castrignano and Stelluti 21 found that ...
Abstract. Using a succession of 24 h Weather Research andForecasting model (WRF) simulations, we investigate the sensitivity to initial soil moisture of a short-range hightemperature weather event that occurred in late July 2003 in East China. The initial soil moisture (SMOIS) in the Noah land surface scheme is adjusted (relative to the control run, CTL) for four groups of simulations: DRY25 (−25 %), DRY50 (−50 %), WET25 (+25 %) and WET50 (+50 %). Ten 24 h integrations are performed in each group.We focus on 2 m surface air temperature (SAT) greater than 35 • C (the threshold of "high-temperature" events in China) at 06:00 UTC (roughly 14:00 LT in the study domain) to analyse the occurrence of the high-temperature event. The 10-day mean results show that the 06:00 UTC SAT (SAT06) is sensitive to the SMOIS change; specifically, SAT06 exhibits an apparent increase with the SMOIS decrease (e.g. compared with CTL, DRY25 generally results in a 1 • C SAT06 increase over the land surface of East China), areas with 35 • C or higher SAT06 are the most affected, and the simulations are more sensitive to the SMOIS decrease than to the SMOIS increase, which suggests that hot weather can be amplified under low soil moisture conditions. Regarding the mechanism underlying the extremely high SAT06, sensible heat flux has been shown to directly heat the lower atmosphere, and latent heat flux has been found to be more sensitive to the SMOIS change, resulting in an overall increase in surface net radiation due to the increased greenhouse effect (e.g. with the SMOIS increase from DRY25 to CTL, the 10-day mean net radiation increases by 5 W m −2 ). Additionally, due to the unique and dynamic nature of the western Pacific subtropical high, negative feedback occurs between the regional atmospheric circulation and the air temperature in the lower atmosphere while positive feedback occurs in the midtroposphere.Using a method based on an analogous temperature relationship, a detailed analysis of the physical processes shows that for the SAT change, the SMOIS change affects diabatic processes (e.g. surface fluxes) more strongly than the adiabatic process of subsidence in the western Pacific subtropical high in the five groups of simulations. Interestingly, although diabatic processes dominate subsidence during the daytime and night-time separately, they do not necessarily dominate during the 24 h periods (e.g. they are dominant in the WET and CTL simulations only). Further, as the SMOIS decreases, the SAT06 increases, which is largely due to the reduced cooling effect of the diabatic processes, rather than the warming effect of subsidence.Unlike previous studies on heatwave events at climate timescales, this paper presents the sensitivity of simulated short-term hot weather to initial soil moisture and emphasises the importance of appropriate soil moisture initialization when simulating hot weather.
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