Nitrogen Addition Alleviates Microbial Nitrogen Limitations and Promotes Soil Respiration in a Subalpine Coniferous Forest

Abstract

Soil microbes are an important component of soil ecosystems that influence material circulation and are involved in the energy flow of ecosystems. The increase in atmospheric nitrogen (N) deposition affects all types of terrestrial ecosystems, including subalpine forests. In general, alpine and high-latitude ecosystems are N limited. Increased N deposition could therefore affect microbial activity and soil respiration. In this study, four levels of N addition, including CK (no N added), N1 (2 g m−2 a−1), N2 (5 g m−2 a−1), and N3 (10 g m−2 a−1), were carried out in a Sichuan redwood forest at the eastern edge of the Tibetan Plateau. The dynamics of soil respiration, major microbial groups, ecoenzymatic stoichiometry, and microbial biomass carbon and nitrogen (MBC and MBN, respectively) were investigated over a year. The results showed that N application significantly increased soil respiration (11%–15%), MBC (5%–9%), MBN (23%–34%), N-acetylglucosidase (56.40%–204.78%), and peroxidase (42.28%–54.87%) activities. The promotion of soil respiration, N-acetylglucosidase, and peroxidase was highest under the N2 treatment. The carbon, nitrogen, and phosphorus metabolism of soil microbes in subalpine forests significantly responded to N application. In the latter stages of N application, microbial metabolism changed from being N restricted to phosphorus restricted, especially under the N2 treatment. Soil bacteria (B) and gram-positive (G+) bacteria were the dominant microbial groups affecting soil respiration. Structural equation modelling indicated that N application significantly promoted soil respiration and microbial biomass, whereas the main microbial groups did not significantly respond to N application. Therefore, we conclude that short-term N addition alleviates microbial nitrogen limitation and promotes soil respiration in the subalpine forest ecosystem that accelerates soil carbon (C) and N cycling. Continuous monitoring is needed to elucidate the underlying mechanisms under long-term N deposition, which may help in forecasting C, N, and P cycling in the alpine region under global climate change.

Soil Biological Community and Chemical Characteristics of Evergreen and Deciduous Shrub Islands in an Alpine Shrub Meadow

“…Lower litter N and P, higher lignin content, and the production of phenols would reduce the material circulation speed and finally make the microorganisms subject to carbon and nitrogen limitations (Sinsabaugh et al,2008;Mori et al, 2020). The included angles of the vectors of the alpine shrub island and the tundra control site were higher than 45°, showing that the soil in the alpine region of western Sichuan was limited by phosphorus, which was consistent with the previous research results of our team Zheng et al, 2019).…”

“…The high temperature difference, low humidity, and high light in the tundra are not conducive to the survival of soil fauna, while the low temperature, high humidity, and low light under the shrub island can serve more soil fauna individuals (Connell 1977;Wu et al, 2014). The difference in soil enzyme activities between the organic layer and the mineral layer under shrub islands showed that the shrub effect significantly affected the decomposition of organic matter in topsoil, while the lack of a difference in control site might be due to the lack of enzyme from the organic layer to the mineral layer due to the difference in soil environment and the fewer number and species of underground biological communities, so that there was no significant difference (Zheng et al, 2019).…”

“…For the soil nutrient and microbial analyses, the organic soil layer was collected as intact 20 × 20 cm blocks to the depth of the mineral soil, and the underlying mineral soil was sampled to 10 cm depth in the same locations using a steel auger (2.5 cm diameter). The samples were collected, stored in a 4 °C cooler and shipped to the laboratory immediately (Zheng et al, 2019). For the soil fauna community, samples of the organic and mineral soil layers with a 5 cm radius and volume 100 cm 3 were collected at each sampling point with a cutting ring and then stored in a sealed and ventilated black sack at low temperature and transported to the laboratory immediately (Wu et al, 2014).…”

“…The region has a typical alpine climate (Zhou et al, 2020;Wang et al, 2021 (Liu et al, 2010). Based on the United States Department of Agriculture Soil Taxonomy, the soils in the alpine shrub land are classified as Cryumbrepts, and the soils in the alpine meadows are Histosols (Zheng et al, 2019). The experimental plot is located in an alpine shrub meadow area with elevations ranging from 4150 to 4200 m (31°51'27'' N, 102°41'12'' E), a slope of 28, a slope oriented to the southeast, and two types of shrub islands dominated by RL and SR.…”

The morphological and chemical properties of fine roots respond to nitrogen addition in a temperate Schrenk’s spruce (Picea schrenkiana) forest

“…The fine root samples were oven-dried at 65 ℃ for 48–72 h to a constant weight and then pulverized with a ball mill. The fine root N content was measured using the Kjeldahl method 60 . To analyze the P content, the fine roots were digested with H 2 SO 4 –H 2 O 2 and the molybdenum antimony colorimetric method was applied 61 .…”