Microbial carbon mineralization in tropical lowland and montane forest soils of Peru

Abstract

Climate change is affecting the amount and complexity of plant inputs to tropical forest soils. This is likely to influence the carbon (C) balance of these ecosystems by altering decomposition processes e.g., "positive priming effects" that accelerate soil organic matter mineralization. However, the mechanisms determining the magnitude of priming effects are poorly understood. We investigated potential mechanisms by adding (13)C labeled substrates, as surrogates of plant inputs, to soils from an elevation gradient of tropical lowland and montane forests. We hypothesized that priming effects would increase with elevation due to increasing microbial nitrogen limitation, and that microbial community composition would strongly influence the magnitude of priming effects. Quantifying the sources of respired C (substrate or soil organic matter) in response to substrate addition revealed no consistent patterns in priming effects with elevation. Instead we found that substrate quality (complexity and nitrogen content) was the dominant factor controlling priming effects. For example a nitrogenous substrate induced a large increase in soil organic matter mineralization whilst a complex C substrate caused negligible change. Differences in the functional capacity of specific microbial groups, rather than microbial community composition per se, were responsible for these substrate-driven differences in priming effects. Our findings suggest that the microbial pathways by which plant inputs and soil organic matter are mineralized are determined primarily by the quality of plant inputs and the functional capacity of microbial taxa, rather than the abiotic properties of the soil. Changes in the complexity and stoichiometry of plant inputs to soil in response to climate change may therefore be important in regulating soil C dynamics in tropical forest soils.

Keywords: cloud forest; decomposition; ecosystem function; microbial community composition; priming; respiration; soil organic matter.

Figure 1

Basal and substrate-induced respiration of 10 tropical forest soils incubated at standard temperature (20°C) and moisture (75% max. WHC). Data represent mean ± SE (n = 5). Control (•), xylose (◦), vanillin (∆), hemicellulose (■), glycine (▼).

Figure 2

Mineralization of four C substrates [xylose (A,E), vanillin, (B,F), hemicellulose (C,G), glycine (D,H)] by 10 tropical forest soils (elevation m asl). Comparison of cumulative substrate C (A–D) and primed C (E–H) (μg g⁻¹ soil dwt) respired over 7 days at standard temperature (20°C) and moisture (75% max WHC). Data represent mean ± SE (n = 5). Three-Way anova is presented in Table 4 and Supplementary Tables 1, 2.

Figure 3

The proportion of primed C relative to substrate C respired by 10 tropical forest soils (elevation m asl) after 168 h incubation with four C substrates (xylose, vanillin, hemicellulose, glycine). Data represent mean ± SE (n = 5). Three-Way anova is presented in Table 4.

Figure 4

Microbial abundance and assimilation of ¹³C substrates into PLFAs in four tropical forest soils after 7 days incubation: (A) Total PLFA concentration (B) ¹³C incorporation into PLFAs (μg g⁻¹ soil dwt) (C) % of PLFA-C derived from substrate. Data represent mean ± SE (n = 3). Two-Way anova is presented in Table 5.

Figure 5

Proportion of ¹³C substrate assimilated into biomarker PLFAs of key microbial groups (% of total incorporation into all PLFAs): (A) xylose; (B) vanillin; (C) hemicellulose and (D) glycine. Fungi = SP + ECM fungi; GN = gram-negative bacteria; GP = gram-positive bacteria; Unspecified = unspecified microbial PLFAs. Data represent mean ± SE (n = 3). Two-Way anova is presented in Table 5.

Figure 6

Microbial carbon use efficiency (¹³C substrate incorporation into PLFAs relative to respired ¹³C-substrate) in four gradient soils in response to four C substrates. Data represent mean ± SE (n = 3). Two-Way anova is presented in Table 5.