Soil nitrogen-related functional genes undergo abundance changes during vegetation degradation in a Qinghai-Tibet Plateau wet meadow

Abstract

Climate change and anthropogenic activities have significantly contributed to the degradation of wet meadows on the Qinghai-Tibet Plateau (QTP). Soil nitrogen (N) availability is a crucial determinant of the productivity of wet meadow vegetation. Furthermore, soil microbial nitrogen functional genes (NFGs) are critical in the transformation of soil N. Nevertheless, the dynamics of NFGs in response to vegetation degradation, as well as the underlying drivers, remain poorly understood. In this study, wet meadows at varying levels of vegetation degradation on the QTP, categorized as non-degraded (ND), slightly degraded (SD), moderately degraded (MD), and heavily degraded (HD), were examined. Soil samples from depths of 0 to 10 cm and 10 to 20 cm were collected during different growth cycles (June 2020, August 2020, and May 2021). The analysis focused on NFGs involved in organic nitrogen fixation (nifH), archaeal and bacterial ammonia oxidation (amoA-AOA and amoA-AOB, respectively), and nitrite reduction (nirK), utilizing real-time fluorescence quantitative PCR. Our findings indicate a significant decline in the abundance of NFGs with intensified vegetation degradation, exhibiting notable spatial and temporal fluctuations. Specifically, the relative NFGs followed the pattern: nirK > amoA-AOA > amoA-AOB > nifH. Redundancy analysis revealed that vegetation cover was the primary regulator of NFGs abundance, accounting for 56.1%-57% of the variation. Additionally, soil total nitrogen, pH, and total phosphorus content were responsible for 38.5%, 28.2%, and 7% of the variability in NFGs, respectively. The (amoA-AOA + amoA-AOB + nirK) ratios associated with effective N transformation indicated that the vegetation degradation process moderately increased the nitrification potential.

Importance: Our research investigates how the degradation of meadows affects the tiny organisms in soil that help plants use nitrogen, which is essential for their growth. In the Qinghai-Tibet Plateau, a region known for its unique ecosystems, we found that as meadows deteriorate-due to climate change and human activities-the number of these beneficial organisms significantly decreases. This decline could reduce soil fertility, impacting plant life and the overall health of the ecosystem. Understanding these changes helps us grasp how environmental pressures influence soil and plant health. Such knowledge is crucial for developing strategies to preserve these vulnerable ecosystems and ensure they continue to sustain biodiversity and provide resources for local communities.

Keywords: nitrogen cycling; nitrogen functional genes; vegetation degradation process; wetland ecosystem.

Fig 1

Daily changes in soil volume moisture content (A and B) and soil temperature (C and D) during the measurement period of 2020 and 2021.

Fig 2

Dynamic changes of soil nitrogen components in four wetland degradation types during the observation period of 2020–2021. On the bar chart, different lowercase letters indicate that there are significant differences between different points at the same time, and different uppercase letters indicate that there are significant differences between the same points at different times, the significance is P < 0.05, and vertical bar is SE (n = 3).

Fig 3

Abundance of NFGs under different levels of vegetation degradation in 2020 and 2021. Different lowercase letters indicate that under the same soil layer, there are significant differences among different vegetation degradation samples (P < 0.05).

Fig 4

Changes of functional gene abundance of soil nitrogen transformation under different vegetation degradation levels during different observation periods in 2020 and 2021. Different lowercase letters indicate that there are significant differences between different points at the same time, different uppercase letters indicate that there are significant differences between the same points at different times, the significance is P < 0.05, and the vertical bar is SE (n = 3).

Fig 5

The relative abundance of soil nitrogen transformation functional genes along vegetation degradation levels. Different lowercase letters indicate significant differences (P < 0.05) between different sites for the same gene. Different uppercase letters indicate significant differences between the same genes under the same treatment (P < 0.05).

Fig 6

Redundancy analysis of the relationship between the absolute abundance of NFGs and environmental factors. OC, organic carbon; COVER, vegetation coverage; ST, sample time.