Effects of simulated exudate C:N stoichiometry on dynamics of carbon and microbial community composition in a subalpine coniferous forest of western Sichuan, China

doi:10.17521/cjpe.2015.0045

Abstract

Abstract: Aims

Exudation measurements focus exclusively on total exudate carbon (C) fluxes without considering how root-derived nitrogen (N) inputs and variable exudate stoichiometries may influence microbial activity and biogeochemical cycles. As a result, the biogeochemical consequences of exudate stoichiometry for soil C-nutrient couplings and feedbacks to environmental changes remain largely unknown. Our objective is to explore to what extent N availability modifies soil microbial processes and the dynamics of soil carbon pool induced by labile C.

Methods

We conducted a 50-day laboratory incubation experiment by addition of simulated exudates varying in C:N to two coniferous forest soils: a natural forest and Picea asperata plantation. The five exudate addition treatments are C alone, N alone, and combinations of C and three N levels (C:N ratio of 10, 50 and 100).

Important findings

The addition of labile C exudates decreased soil total C for both natural forest and the plantation by stimulating soil organic matter (SOM) mineralization (i.e. greater priming effect), while the addition of N decreased total C. The decreased soil total C induced by exogenous labile C addition was greater in the plantation than that in the natural forest. The influence of exudate additions produced no significant influence on labile and recalcitrant carbon pools at either soil. The addition of labile C exudate decreased the total phospholipid fatty acid (PLFA), actinomycetic, bacterial and fungal PLFA for the natural forest, but increased them in the plantation. Moreover, the microbial community composition (i.e. the value of bacterial PLFA:fungi PLFA) varied greatly among the treatments. These results indicate that both root-derived N inputs and soil N availability co-regulate the direction and magnitude of priming effects on SOM decomposition by controlling the activity and the relative abundance of bacterial and fungal. Our results provide additional evidences toward a robust theoretical foundation for better understanding the ecological consequences of exudate stoichiometry on soil C cycling in forests.

Key words: exudate stoichiometry, microbial community, root exudation, soil carbon, subalpine coniferous forest

LIANG Ru-Biao,LIANG Jin,QIAO Ming-Feng,XU Zhen-Feng,LIU Qing,YIN Hua-Jun. Effects of simulated exudate C:N stoichiometry on dynamics of carbon and microbial community composition in a subalpine coniferous forest of western Sichuan, China[J]. Chin J Plan Ecolo, 2015, 39(5): 466-476.

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Figures/Tables 7

Table 1 Soil physical and chemical properties in both natural forest and plantation (mean ± SD)

理化性质 Physical-chemical properties	天然林 Natural forest	人工林 Plantation
pH值 pH value	6.97 ± 0.02	6.73 ± 0.03
含水量 Water content (%)	52.52 ± 1.76	37.70 ± 2.46
总碳 Total C (mg·g^-1)	114.87 ± 1.10	49.22 ± 1.18
总氮 Total N (mg·g^-1)	8.46 ± 0.31	3.20 ± 0.12
碳氮比 C:N	13.59 ± 0.51	15.41 ± 0.55

Table 2 Chemical components and contents of simulated root exudates in different C:N stoichiometry

处理 Treatment	葡萄糖 Glucose (g)	柠檬酸 Citric acid (g)	谷氨酸 Glutamic acid (g)	氯化铵 NH₄Cl (g)	去离子水 Deionized water (mL)
对照 Control (CK)	-	-	-	-	1 000
仅添加N N-only	-	-	-	0.022 9	1 000
C:N = 10	0.075 3	0.075 3	0.016 8	0.016 8	1 000
C:N = 50	0.083 0	0.083 0	0.003 4	0.003 4	1 000
C:N = 100	0.084 0	0.084 0	0.001 7	0.001 7	1 000
仅添加C C-only	0.085 0	0.085 0	-	-	1 000

Table 3 Effects of simulated root exudates under different C:N stoichiometry on soil pH in both natural forest and plantation (mean ± SD)

处理 Treatment	天然林 Natural forest	人工林 Plantation
对照 Control (CK)	7.09 ± 0.02^d	7.03 ± 0.05^d
仅添加N N-only	7.26 ± 0.02^c	7.12 ± 0.02^c
C:N = 10	7.32 ± 0.03^b	7.15 ± 0.01^bc
C:N = 50	7.39 ± 0.03^a	7.20 ± 0.02^ab
C:N = 100	7.35 ± 0.03^b	7.27 ± 0.02^a
仅添加C C-only	7.26 ± 0.03^c	7.14 ± 0.02b^c

Fig. 1 Effects of simulated root exudates under different C:N stoichiometry on total carbon content in natural forest and the plantation (mean ± SD). CK, control. Different lowercase letters mean significant differences (p < 0.05) among treatments at a given forest type.

Fig. 2 Effects of simulated root exudates under different C:N stoichiometry on soil labile and recalcitrant carbon pool in natural forest and the plantation (mean ± SD). A, labile carbon pool I (LPI-C). B, labile carbon pool II (LPII-C). C, recalcitrant carbon pool (RP-C). CK, control. Lowercase letters mean significant differences (p < 0.05) among treatments at a given forest type.

Fig. 3 Effects of simulated root exudates under different C:N stoichiometry on ratio of (microbial biomass C : microbial biomass N) MBC:MBN in natural forest and the plantation (mean ± SD). CK, control. Lowercase letters mean significant differences (p < 0.05) among treatments at a given forest type.

Fig. 4 Effects of simulated root exudates under different C:N stoichiometry on contents of soil microbial phospholipid fatty acid (PLFA) in natural forest and the plantation (mean ± SD). A, total PLFA in microbia. B, bacterial PLFA. C, actinomycetic PLFA. D, fungal PLFA. E, ratio of bacterial/fungal PLFA. CK, control. Lowercase letters mean significant differences (p < 0.05) among treatments at a given forest type.

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