Microbial carbon use efficiency along an altitudinal gradient
Mganga, Kevin Z.; Sietiö, Outi-Maaria; Meyer, Nele; Poeplau, Christopher; Adamczyk, Sylwia; Biasi, Christina; Kalu, Subin; Räsänen, Matti; Ambus, Per; Fritze, Hannu; Pellikka, Petri K.E.; Karhu, Kristiina
(2022) Soil Biology and Biochemistry, volume 173, pp. 1 - 12
(Article)
Abstract
Soil microbial carbon-use efficiency (CUE), described as the ratio of growth over total carbon (C) uptake, i.e. the sum of growth and respiration, is a key variable in all soil organic matter (SOM) models and critical to ecosystem C cycling. However, there is still a lack of consensus on microbial
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CUE when estimated using different methods. Furthermore, the significance of many fundamental drivers of CUE remains largely unknown and inconclusive, especially for tropical ecosystems. For these reasons, we determined CUE and microbial indicators of soil nutrient availability in seven tropical forest soils along an altitudinal gradient (circa 900–2200 m a.s.l) occurring at Taita Hills, Kenya. We used this gradient to study the soil nutrient (N and P) availability and its relation to microbial CUE estimates. For assessing the soil nutrient availability, we determined both the soil bulk stoichiometric nutrient ratios (soil C:N, C:P and N:P), as well as SOM degradation related enzyme activities. We estimated soil microbial CUE using two methods: substrate independent 18O-water tracing and 13C-glucose tracing method. Based on these two approaches, we estimated the microbial uptake efficiency of added glucose versus native SOM, with the latter defined by 18O-water tracing method. Based on the bulk soil C:N stoichiometry, the studied soils did not reveal N limitation. However, soil bulk P limitation increased slightly with elevation. Additionally, based on extracellular enzyme activities, the SOM nutrient availability decreased with elevation. The 13C-CUE did not change with altitude indicating that glucose was efficiently taken up and used by the microbes. On the other hand, 18O-CUE, which reflects the growth efficiency of microbes growing on native SOM, clearly declined with increasing altitude and was associated with SOM nutrient availability indicators. Based on our results, microbes at higher elevations invested more energy to scavenge for nutrients and energy from complex SOM whereas at lower elevations the soil nutrients may have been more readily available.
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Keywords: Carbon use efficiency, Ecological stoichiometry, Enzyme activity, Microbial growth, Soil microbial carbon pump, Microbiology, Soil Science
ISSN: 0038-0717
Publisher: Elsevier Limited
Note: Funding Information: This research was supported by funds from the Academy of Finland (grant numbers 319952 , 316401 ), Helsinki Institute of Life Science (HiLIFE) (a HiLIFE Fellow Grant to K.K covering e.g. the salary of N.M.) and Maj and Tor Nessling foundation (personal grant to O.-M.S.) We thank Marjut Wallner for her assistance with laboratory analysis. We would also like to sincerely thank staff at the Taita Research Station, University of Helsinki , for their support and assistance while undertaking the fieldwork. Three anonymous reviewers are gratefully acknowledged. Funding Information: We found that along our gradient SOM nutrient availability (as indicated by extracellular enzyme activity and qM) and soil pH were more strongly correlated with microbial 18O-CUE than soil stoichiometric controls. This finding was similar to a study from a wide latitudinal gradient in western Siberia by Takriti et al. (2018). There, the range of soil C:N ratios was wider than in our study, but still SOM quality and pH were stronger determinants for substrate use efficiency than stoichiometry. Other studies have also demonstrated a strong link between SOM quality and microbial CUE (Manzoni, 2017; Soares and Rousk, 2019). These results support the view that soil C:N ratios alone are not always good predictors of microbial CUE (Sinsabaugh et al., 2016). However, other studies have demonstrated that the effect of N-availability becomes more evident in soils with comparable SOM properties (e.g. Silva-Sánchez et al., 2019; Soares and Rousk, 2019).Based on our variance partitioning analyses, labile-C degradation related enzyme activities were more associated to microbial growth on SOM, whereas nutrient availability of the substrate, as reflected by chitinase and phosphatase activities, was positively associated to microbial 18O-respiration. Soil C:N ratio correlated negatively with 18O-CUE and positively with 18O-respiration along the altitude gradient. Soil C:N ratio contributed to explaining the observed variation both in microbial respiration and growth. A recent meta-analysis by Hu et al. (2022) indicates that N addition causes increases in microbial 18O-growth rate, and thus increases in 18O-CUE estimates, which may have been the case in also our gradient. Our results from variance partitioning also support the microbial carbon pump theory where labile C is easily taken up by the microbes and transformed into microbial biomass through microbial growth, whereas modification of recalcitrant lignin compounds and scavenging SOM for nutrients causes CO2 emissions from soil (Liang et al., 2017). However, some of these CO2 fluxes can also originate from extracellular enzyme activity, and thus may not be directly correlated with microbial growth and microbial CUE. We suggest that the low 18O-CUE in higher elevations of our gradient indicates greater need of microbes to allocate energy for nutrient acquisition related enzyme production than for growth compared to microbes at low elevations.This research was supported by funds from the Academy of Finland (grant numbers 319952, 316401), Helsinki Institute of Life Science (HiLIFE) (a HiLIFE Fellow Grant to K.K covering e.g. the salary of N.M.) and Maj and Tor Nessling foundation (personal grant to O.-M.S.) We thank Marjut Wallner for her assistance with laboratory analysis. We would also like to sincerely thank staff at the Taita Research Station, University of Helsinki, for their support and assistance while undertaking the fieldwork. Three anonymous reviewers are gratefully acknowledged. Publisher Copyright: © 2022 The Authors
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