High latitude and cold ecosystems, which constitute the major environment on Earth, are particularly threatened by global warming. Consequently, huge amounts of SOC stored in these ecosystems may be released to the atmosphere by accelerated enzymatic decomposition. Effects of intensive warming on temperature sensitivity and catalytic properties of soil enzymes were tested in cold-adapted alpine grassland of the Tibetan Plateau. We hypothesized that 1) maximal reaction rate will be insensitive to intensive warming at high temperature range (V max − Q 10 = 1); 2) substrate affinity (K m ) remains constant at elevated temperatures due to expression of enzymes with less flexibility. These hypotheses were tested by examining the kinetics of six enzymes involved in carbon (cellobiohydrolase, β-glucosidase, xylanase), nitrogen (tyrosine-aminopeptidase, leucine-aminopeptidase) and phosphorus (acid phosphomonoesterase) cycles after soil incubation at temperatures from 0 to 40 °C. Q 10 and E a decreased at high temperature (25–40 °C). However, enzymes that degrade low quality polymers remained temperature-sensitive even above 25 °C (V max − Q 10 = 2), which explains the faster decomposition of recalcitrant C compounds under warming. Substrate affinity of all enzymes gradually increased up to 20 °C. At 25 °C, however, K m increased rapidly, leading to an extreme decrease in catalytic efficiency. Above 25 °C, K m of C and N cycles remained nearly constant, while V max gradually increased from 0 to 40 °C. These results reveal two important implications of warming: 1) there are some temperature thresholds (here 20–25 °C) that lead to sudden reductions in substrate affinity, decreasing temperature sensitivity and catalytic efficiency, 2) decoupled temperature sensitivity of V max and K m and the resulting maintenance of stable enzyme systems at high temperatures ensured efficient enzymatic functioning and persistent decomposition of SOM at temperatures much higher than the common adaptation range of the ecosystem. Thus, the temperature thresholds of strong changes in enzyme-based processes should be considered and included in the next generation of models in order to improve the prediction of SOM feedbacks to warming.
Recently fixed plant carbon (C) being released as rhizodeposits is a major resource fueling soil food webs. Soil microorganisms predominate in incorporating root-derived C and subsequently transfer it to higher trophic levels. However, variation in microbial community structure between cropping systems and its consequences for the incorporation of root-derived C into soil microbivores remain unclear. In the present study, we used 13CO2 to pulse label a crop monoculture (oilseed rape, Brassica napus L.), a mixed grass community (dominated by Lolium perenne L. mixed with clover Trifolium repens L.), and a young tree plantation (willow, Salix schwerinii E.L. Wolf and Salix viminalis L.). During 28 days, we traced the incorporation of root-derived 13C into phospholipid fatty acids (PLFAs) of soil microorganisms and neutral lipid fatty acids (NLFAs) of five Collembola species belonging to three functional groups: epedaphic (surface-dwelling), hemiedaphic (litter-dwelling), and euedaphic (soil-dwelling). The contribution of bacterial and fungal channels to the incorporation of root-derived C into Collembola varied considerably between cropping systems. Collembola incorporated more 13C from the bacterial channel in rape than in grass and willow, where fungi were the major C source. This corresponded to a similarly higher 13C incorporation into bacterial marker PLFAs in rape compared to grass and willow. By contrast, while the proportion of bacterial and fungal biomarkers in Collembola NLFAs was related to the 13C incorporation into microbial PLFAs, it did not correlate with the proportion of microbial PLFAs in the different cropping systems. This suggests that Collembola rely on specific microbial pools, presumably related to recent plant inputs. Within the same cropping system, hemiedaphic species incorporated more root-derived 13C from the bacterial channel compared to euedaphic and epedaphic species. The results demonstrate the remarkable importance of cropping system for the flux of root C into microorganisms and microbivore soil invertebrates. Changes in root C flux into bacterial and fungal resources among cropping systems resulted in differential utilization of these resources by soil microbivores, suggesting that in particular microorganisms fueled by rhizodeposits are vital resources for the nutrition of higher trophic levels in soil food webs.
Vegetation restoration affects belowground microbial diversity and trait-based life strategies, as well as soil organic matter (SOM) accumulation. Despite the growing focus on microbial diversity, the relationship between SOM accrual and life strategies following vegetation restoration remains unclear. We used three independent but complementary approaches to relate six soil parameters to SOM accrual and further connected them with microbial diversity and life strategies. Sites with two vegetation restoration strategies (after 15 years of vegetation recovery): i) actively planted forests and ii) passive naturally regenerating forests, were compared with croplands and established 60-year-old secondary forests. Data from seven sites along a large climatic gradient (Δ temperature >9 °C) in subtropical karst regions showed that the average index of SOM accrual increased by 47% in plantations and by 60% in natural regeneration forests compared to that in croplands, but remained lower than that of secondary forests. Related soil parameters (water holding capacity, organic carbon and total nitrogen contents, and bacterial and fungal biomasses) were comparable. Compared to croplands, vegetation restoration decreased the sensitivity of the soil parameters to climate warming. Vegetation restoration reduced bacterial diversity and shifted the community towards K-strategy, as evidenced by i) lower 16S rRNA operon copy number and ii) higher ratios of phyla classified a priori as oligotrophic versus copiotrophic bacteria. These changes were mainly attributed to the decline in labile nutrient content and increase in carbon stability in calcium-rich karst soils after vegetation restoration. Bacterial diversity was negatively associated with SOM accrual at low levels of functioning, whereas the prevalence of K-strategists showed a strong positive association with it, especially at near-peak capacity. Consequently, i) bacterial rather than fungal diversity and life strategies are associated with SOM accrual, and ii) linking bacterial life strategies with SOM accrual is important to deepen the understanding of soil-microbial interactions.
