Differentiate Responses of Soil Microbial Community and Enzyme Activities to Nitrogen and Phosphorus Addition Rates in an Alpine Meadow

Abstract

Nitrogen (N) and phosphorus (P) are the dominant limiting nutrients in alpine meadows, but it is relatively unclear how they affect the soil microbial community and whether their effects are rate dependent. Here, N and P addition rates (0, 10, 20, and 30 g m^-2 year^-1) were evaluated in an alpine meadow and variables related to plants and soils were measured to determine the processes affecting soil microbial community and enzyme activities. Our results showed that soil microbial biomass, including bacteria, fungi, gramme-negative bacteria, and actinomycetes, decreased along with N addition rates, but they first decreased at low P addition rates (10 g m^-2 year^-1) and then significantly increased at high P addition rates (30 g m^-2 year^-1). Both the N and P addition stimulated soil invertase activity, while urease and phosphatase activities were inhibited at low N addition rate and then increased at high N addition rate. P addition generally inhibited peroxidase and urease activities, but increased phosphatase activity. N addition decreased soil pH and, thus, inhibited soil microbial microorganisms, while P addition effects were unimodal with addition rates, achieved through altering sedge, and available P in the soil. In conclusion, our studies indicated that soil microbial communities and enzyme activities are sensitive to short-term N and P addition and are also significantly influenced by their addition rates.

Keywords: N and P addition; alpine meadow; phospholipid fatty acid; rate dependent; soil enzyme activities.

FIGURE 1

Effects of nitrogen (N) and phosphorus (P) addition on soil physicochemical. Values are means ± SE (n = 3). Different lowercase letters indicate significant difference among the levels of N or P addition in the same below.

FIGURE 2

Effects of N and P addition on plant community characteristics.

FIGURE 3

Effects of N and P addition on soil enzyme activities.

FIGURE 4

Soil microbial composition along N (A) and P (B) addition rates based on principal component analysis (PCA).

FIGURE 5

Effects of N and P addition on soil microbial biomass (A), PLFA richness (B), the biomass of grenral bacteria (C), fungi (D), gram-positive bacteria (E), gram-negative bacteria (F), and actinomycetes (G).

FIGURE 6

The relative contributions of plants and soils to the variations in soil PLFAs (A,C) and enzyme activities (B,D) along N and P addition rates.

FIGURE 7

Structural equation modeling (SEM) analysis evaluating the effects of N and P addition on soil microbial PLFAs and enzyme activities via pathways of plants and soils. Result of model fitting: chi-squared test = 23.56, df = 21, and P = 0.315. Here, high P-values with the chi-squared tests indicate good model fit to data. Black and blue solid arrows indicate significantly positive and negative effects (P < 0.05), respectively. Dashed arrows indicate the effects are not significant (P > 0.05). Values associated with arrows represent standardised path coefficients. R² associated with response variables indicates the proportion of variation explained by relationships with other variables.