Abstract
Soil P pools are strongly driven by microbial activities, and vice versa, P pools shape bacterial communities and their functional potential. Biological soil crusts (biocrusts) represent a microbial hotspot for nutrient turnover. We compared biocrusts and bulk soil samples from different temperate beech (Fagus sylvatica L.) forests representing a gradient in soil texture, nutrient concentrations, and pH values at biocrust peak biomass. We measured the total and plant-available P and N concentrations and assessed the bacterial potential to mineralize (phoD, phnX), solubilize (gcd), and take up P (pstS and pitA) and mineralize (chiA, apr) and fix N (nifH) by quantifying the respective marker genes (qPCR). We found an increase of absolute and relative bacterial abundance involved in P turnover in biocrusts, but the strategy to acquire P differed between the regions as bacteria harboring the starvation-induced pstS gene were most abundant where labile P was lowest. In contrast, the region with lowest total P concentrations has a higher potential to utilize more stable phosphonates. N mineralization was strongly correlated to P turnover at regions with increased labile N and P concentrations. Interestingly, the potential to fix N was highest in the bulk soil where total P concentrations were highest. Even though the correlation of N and P turnover is strongest if their ratio is low, the acquisition strategy strongly depends on soil properties.
Highlights
Many terrestrial habitats are limited in the major nutrients P and N (Elser et al 2007)
We hypothesize that (i) microorganisms colonizing biocrusts are involved in solubilization of mineral P and its transformation to biomass and abundances of those microorganisms are higher in biocrusts compared with bulk soil. (ii) As it was supposed that P and N turnover are closely linked, we further hypothesize that similar to P mineralization, the potential for N mineralization is more pronounced in biocrusts, because of the higher microbial abundance, while the energy demanding fixation of N is less important in biocrusts from nutrient-rich forest soils. (iii) The strength of correlations between N and P turnover strongly depends on the N/P ratio of the bulk soil
Our data demonstrate that the potential to biogeochemically turnover P is an essential microbial function in biocrusts, as such microbes were more abundant in the biocrusts compared with bulk soil at all investigated regions on the level of absolute and relative abundance
Summary
Many terrestrial habitats are limited in the major nutrients P and N (Elser et al 2007). Biocrusts have been mostly studied in arid and nutrient-poor habitats (Belnap et al 2001), where they are the predominant vegetation form and their growth is limited by water and nutrient availability Biocrusts from those regions contribute about half of the terrestrial N2 fixation (Elbert et al 2012). The role of biocrusts in P transformation in temperate forests has been described by Baumann et al (2017), who found that, similar to biocrusts from arid regions, the P concentrations in biocrusts are enriched compared with adjacent bulks soils. They demonstrated that especially the concentration of P-containing minerals decreased and of organic P concentrations increased in biocrusts compared with bulk soil. We hypothesize that (i) microorganisms colonizing biocrusts are involved in solubilization of mineral P and its transformation to biomass and abundances of those microorganisms are higher in biocrusts compared with bulk soil. (ii) As it was supposed that P and N turnover are closely linked, we further hypothesize that similar to P mineralization, the potential for N mineralization is more pronounced in biocrusts, because of the higher microbial abundance, while the energy demanding fixation of N is less important in biocrusts from nutrient-rich forest soils. (iii) The strength of correlations between N and P turnover strongly depends on the N/P ratio of the bulk soil
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