Contrasting effects of nitrogen and phosphorus addition on soil respiration in an alpine grassland on the Qinghai-Tibetan Plateau

Abstract

High soil organic carbon content, extensive root biomass, and low nutrient availability make alpine grasslands an important ecosystem for assessing the influence of nutrient enrichment on soil respiration (SR). We conducted a four-year (2009-2012) field experiment in an alpine grassland on the Qinghai-Tibetan Plateau to examine the individual and combined effects of nitrogen (N, 100 kg ha^-1year^-1) and phosphorus (P, 50 kg ha^-1year^-1) addition on SR. We found that both N and P addition did not affect the overall growing-season SR but effects varied by year: with N addition SR increased in the first year but decreased during the last two years. However, while P addition did not affect SR during the first two years, SR increased during the last two years. No interactive effects of N and P addition were observed, and both N addition and P addition reduced heterotrophic respiration during the last year of the experiment. N and P addition affected SR via different processes: N mainly affected heterotrophic respiration, whereas P largely influenced autotrophic respiration. Our results highlight the divergent effects of N and P addition on SR and address the important potential of P enrichment for regulating SR and the carbon balance in alpine grasslands.

Figure 1

Seasonal and annual variation in (a) air temperature and soil temperature at a depth of 5 cm (ST_5) and soil temperature at a depth of 10 cm (ST_10); (b) precipitation; (c) relative humidity (Rh_air), volumetric soil moisture at 5 cm depth (VWC_5) and volumetric soil moisture at 10 cm depth (VWC_10); and (d) soil respiration rates under different treatments from July 1^st in 2009 to September 30^th in 2012. Shaded areas represent the non-growing season.

Figure 2

Effects of nitrogen and phosphorus addition on soil respiration (a,c) and changes (%) in soil respiration (b,d) during the growing seasons from 2009 to 2012. ^*P < 0.05, ^**P < 0.01. Error bars indicate the standard errors.

Figure 3

Effects of nitrogen and phosphorous addition on (a) autotrophic respiration (SR_A), (b) the ratio of SR_A to soil respiration (SR_A/SR), (c) heterotrophic respiration (SR_H) and (d) the ratio of SR_H to soil respiration (SR_H/SR) in 2012. ^*P < 0.05. Error bars indicate the standard errors.

Figure 4

Effects of nitrogen and phosphorus addition on (a) aboveground biomass (AGB), (b) belowground biomass (BGB), (c) root nitrogen content (Root [N]) and (d) root phosphorus content (Root [P]) in 2012. ′P < 0.1, ^*P < 0.05, ^**P < 0.01, ^*** P < 0.001.

Figure 5

Model of the effects of nitrogen (a) and phosphorus (b) addition (over four years) on soil respiration. The thickness of the arrows represents the strengths of the treatments; the solid arrows indicate where data were measured, and dashed arrows indicate where data were not measured in this study. Up arrows represent positive effects, and down arrows represent negative effects, while dashes represent neutral effects. The white boxes are the results from the present study, and the grey boxes are the results from previous studies.