[Effect of Soil Moisture and Temperature on the Soil Inorganic Carbon Release of Brown Limestone Soil in the Karst Region of Southwestern China].

In order to understand the influence of environmental factors on the carbonate conversion of the Karst soil, typical brown limestone and red soil samples were collected from the Karst ecosystem, and a 100-day incubation experiment was conducted. The characteristics of inorganic carbon release from the soil under three temperature gradients (15, 25, and 35℃) and water contents (30%, 65%, and 100% WHC) were studied by adding 14C-CaCO3 for 100 d. The results showed that under the different soil moisture and temperature conditions, the maximum rate and the cumulative amount of inorganic carbon release from the soil over 100 days varied between 0.7-16.8 mg·(kg·d)-1and 5.9-29.4 mg·kg-1, respectively, in the brown limestone soil, and varied between 39.7-103.3 mg·(kg·d)-1 and 83.3-135.1 mg·kg-1, respectively in the red soil. Under drought conditions (30% WHC), the cumulative amount of inorganic carbon release was the highest for the two soils and increased with increasing temperature. At 65% WHC and 100% WHC, increasing temperature can still promote inorganic carbon release from the soil. The temperature sensitivity of the soil inorganic carbon release in the brown limestone soil is greater than that of the red soil, which is significantly affected by soil moisture. The soil pH and MBC content were remarkably increased after adding CaCO3, and the difference between the two soils was significant. The variance partition showed that temperature and soil moisture can explain 7.6% and 2.0% of the soil inorganic carbon release variability, respectively. In conclusion, warming and drought aggravate inorganic carbon release from brown limestone soil in the southwestern Karst region. Therefore, in the context of global warming and more frequent extreme precipitation events, the effects of soil moisture and temperature on inorganic carbon conversion in soil should be fully considered when studying the soil carbon cycle and its dynamic changes in southwestern Karst. This research can provide a scientific basis for further understanding the influence of climate change on the global carbon cycle.

[Straw Returning Plus Nitrogen Fertilizer Affects the Soil Microbial Community and Organic Carbon Mineralization in Karst Farmland].

The use of straw returning plus nitrogen fertilizer on farmland is one of the important agronomic practices for adjusting soil organic carbon (SOC) transformations. To explore the mechanisms of straw and nitrogen fertilizer application on straw and SOC mineralization in long-term fertilized soils, an incubation experiment with the 13C isotope tracing technique was conducted, which involved three long-term fertilized models in typical karst soils (no fertilization, inorganic fertilization, and a combination of inorganic fertilization and straw). To study the mechanisms of 13C-labeled straw and SOC mineralization, four treatments were designed as follows:no straw and nitrogen (control), and straw combined with three levels of nitrogen fertilizer (0, 214.0, and 571.0 mg·kg-1 soil). The results showed that cumulative mineralization amounts of straw-derived organic carbon in long-term fertilized soils were markedly higher than those in non-fertilized soil. Straw-derived organic carbon mineralization was significantly affected by nitrogen fertilizer levels. The positive priming effects (PE) in long-term fertilized soils were much lower than those in non-fertilized soil. The PE was decreased at the low nitrogen fertilizer level but increased at the high nitrogen fertilizer level. The principal component analysis (PCA) of phospholipid fatty acids (PLFAs) indicated that the soil microbial community structure was greatly affected by the long-term fertilization models and combined straw and nitrogen fertilizer application. Moreover, the content of PLFAs in soil microorganisms, namely, bacteria and fungi, were remarkably increased by the straw plus nitrogen fertilizer (values increased by 40.3%-53.0%, 41.1%-62.6%, and 60.5%-148.6% compared with control), but levels were not significantly affected by nitrogen fertilizer levels alone. The ratios between PLFAs of soil gram-positive and gram-negative bacteria (G+/G-) decreased and were stable at around 0.8. The structure equation models (SEM) demonstrated that the combination of straw and nitrogen affected the soil gram-positive and gram-negative bacteria structure and increased the soil DOC content, which promoted the decomposition of straw and affected the mineralization of SOC. These results indicate that straw returning plus low nitrogen fertilizer can improve the SOC sequestration capacity in karst farmland.