Publications

<p><span>Approximately 50–70% of C stored in soil is derived from the roots or root-associated microorganisms, reflecting their importance for soil organic matter (SOM) formation. Microorganisms are the major driver for SOM decomposition and their activities are modified by the input of labile organics such as root exudates and low molecular weight organic substances derived from litter decomposition. Such short-term changes in the turnover of SOM caused by moderate organic additions into the soil were defined as priming effects (PE). Priming effects can influence global carbon (C) storage in soil and lead to climate feedbacks by accelerating the decomposition of organic matter (OM). In natural ecosystems, input of nitrate (NO</span><span><sub>3</sub></span><span><sup>−</sup></span><span>) and ammonium (NH</span><span><sub>4</sub></span><span><sup>+</sup></span><span>) into the soil can be derived from nitrogen (N) deposition and biological N fixation. Although availability N can alter the magnitude and direction of priming, it remains unclear whether additions of NO</span><span><sub>3</sub></span><span><sup>−</sup></span><span> and NH</span><span><sub>4</sub></span><span><sup>+</sup></span><span> have distinct effects on the decomposition of OM. Thus, the aims of this study were to investigate the responses of OM decomposition along a decay continuum (i.e. decreasing decomposition degree) to labile C and N inputs and determine the PE induced by the two N forms. Leaf litter, wood litter, organic soil horizon, and mineral soil, with a broad range of C:N ratios were collected along a decay continuum in a typical subtropical forest and incubated for 38 days with </span><span><sup>13</sup></span><span>C labeled glucose and N (NO</span><span><sub>3</sub></span><span><sup>−</sup></span><span>) additions. Based on the very broad range of C:N ratios in OM in soil and inputs of labile C and N, we demonstrated the OM decomposition within a decay continuum as well as PE intensities and the thresholds for the switch of PE directions. In contrast to NH</span><span><sub>4</sub></span><span><sup>+</sup></span><span> additions, NO</span><span><sub>3</sub></span><span><sup>−</sup></span><span> generally accelerated the decomposition of all OM. Priming of plant litter was dependent on the C:N ratios of the labile inputs. However, leaf litter decomposition was more controlled by N addition than wood litter. Glucose addition greatly increased the priming of OM decomposition, demonstrating energy limitation for microorganisms. Distinct priming patterns were observed between NO</span><span><sub>3</sub></span><span><sup>−</sup></span><span> and NH</span><span><sub>4</sub></span><span><sup>+</sup></span><span> additions, both for the individual OM types and for all four types of OM. The PE induced by labile C and N inputs can increase or reduce C sequestration depending on C:N stoichiometric ratios of labile inputs. Net C losses caused by PE can be observed in organic soil and plant litter with low C and N additions, but all four OM substrates increased C sequestration under high C addition. Minor differences in priming along the continuum were observed where the OM C:N ratio was below 30 when NO</span><span><sub>3</sub></span><span><sup>−</sup></span><span> was added and where the labile C:N ratio was less than 55 when NH</span><span><sub>4</sub></span><span><sup>+</sup></span><span> was added. Thus, </span><span>changes in the composition of deposited N (atmospheric deposition and fertilization) may induce distinct climate feedbacks. Under future climatic conditions by global warming and elevated CO</span><span><sub>2</sub></span><span>, more labile C inputs via root exudates could accelerate litter decomposition. Effects of N however, depend on the N form: NH</span><span><sub>4</sub></span><span><sup>+</sup></span><span> to NO</span><span><sub>3</sub></span><span><sup>−</sup></span><span> due to the energy necessary for microorganisms for NO</span><span><sub>3</sub></span><span><sup>-</sup></span><span> reduction.</span></p>

Silicon (Si), although not considered essential, has beneficial effects on plant growth which are mostly associated with the ability to accumulate amorphous (phytogenic) Si, e.g. , as phytoliths. Phytogenic Si is the most active Si pool in the soil–plant system because of its great surface‐to‐volume ratio, amorphous structure, and high water solubility. Despite the high abundance of Si in terrestrial biogeosystems and its importance, e.g. , for the global C cycle, little is known about Si fluxes between soil and plants and Si pools used by plants. This study aims at elucidating the contribution of various soil Si pools to Si uptake by wheat. As pH affects dissolution of Si pools and Si uptake by plants, the effect of pH (4.5 and 7) was evaluated. Wheat was grown on Si‐free pellets mixed with one of the following Si pools: quartz sand (crystalline), anorthite powder (crystalline), or silica gel (amorphous). Silicon content was measured in aboveground biomass, roots, and soil solution 4 times in intervals of 7 d. At pH 4.5, plants grew best on anorthite, but pH did not significantly affect Si‐uptake rates. Total Si contents in plant biomass were significantly higher in the silica‐gel treatment compared to all other treatments, with up to 26 mg g –1 in aboveground biomass and up to 17 mg g –1 in roots. Thus, Si uptake depends on the conversion of Si into plant‐available silicic acid. This conversion occurs too slowly for crystalline Si phases, therefore Si uptake from treatments with quartz sand and anorthite did not differ from the control. For plants grown on silica gel, real Si‐uptake rates were higher than the theoretical value calculated based on water transpiration. This implies that Si uptake by wheat is driven not only by passive water flux but also by active transporters, depending on Si concentration in the aqueous phase, thus on type of Si pool. These results show that Si uptake by plants as well as plant growth are significantly affected by the type of Si pool and factors controlling its solubility.

Carbon dioxide (CO2) efflux from soil (or soil respiration, SR) is one of the most important yet variable characteristics of soil. When evaluating large areas, CO2 efflux modeling serves as a viable alternative to direct measurements. This research aims to identify site-specific differences and their effects on empirical CO2 efflux modeling. The experimental data from 25 years of field observations were utilized to identify the optimal site- and weather-specific models, parameterized for normal, wet, and dry years, for the forest and grassland ecosystems located on similar Entic Podzols (Arenic) in the same bioclimatic coniferous–deciduous forest zone. The following parameters were considered in the examined models: mean monthly soil or air temperatures (Tsoil and Tair), amount of precipitation during the current (P) and the previous (PP) months, and the storage of soil organic carbon (SOC) in the top 20 cm of soil. The weighted non-linear regression method was employed to estimate the model parameters for the normal, wet, and dry years. To increase the magnitude of the model resolutions, we controlled the slope and intercept of the linear model comparison between the measured and modeled data through the change in R0—CO2 efflux at Tsoil = 0 °C. The mean bias error (MBE), root-mean-square error (RMSE), and determination coefficient (R2) were employed to assess the quality of the model’s performance. The measured Tsoil, Tair, and P, as well as the litter (for forest) or sod (for grassland) horizon (modeled by the Soil SCLmate Statistical Simulator (SCLISS)), and soil temperatures (Tlit_m, Tsoil_m) and moistures (Mlit_m, Msoil_m), were used for SR simulation. For the CO2 efflux in the forest ecosystem with the lower SOC availability for mineralization, the direct Tsoil and Tair measurements in combination with SOC storage provided better parameterization for the empirical TPPC model. For the CO2 efflux in the grassland ecosystem with the high SOC availability for mineralization, the temperature became the governing factor, and the TPPrh model provided better performance over all the considered models. The model’s performance was the best for the wet years, and the worst for the dry years for both ecosystems. For forest ecosystems, the model performance for average precipitation years was equivalent to that in wet years. For grassland ecosystems, however, the model performance was equivalent to that in dry years due to differing exposure and hydrothermal regimes. The wet-year R0 obtained for both forest and grassland ecosystems differed from the normal- and dry-year values. The measured SR values relevant for the R0 estimations distribute along the precipitation range for the forest and along the temperature range for the grassland. The SCLISS-modeled Tlit_m and Mlit_m provide good alternatives to direct atmospheric measurements, and can be used as initial temperature and moisture data for CO2 efflux modeling when direct soil and moisture observations are not available on site.

Understanding the mechanisms of soil organic carbon (SOC) sequestration in forests is vital to ecosystem carbon budgeting and helps gain insight in the functioning and sustainable management of world forests. An explicit knowledge of the mechanisms driving global SOC sequestration in forests is still lacking because of the complex interplays between climate, soil, and forest type in influencing SOC pool size and stability. Based on a synthesis of 1179 observations from 292 studies across global forests, we quantified the relative importance of climate, soil property, and forest type on total SOC content and the specific contents of physical (particulate vs. mineral-associated SOC) and chemical (labile vs. recalcitrant SOC) pools in upper 10 cm mineral soils, as well as SOC stock in the O horizons. The variability in the total SOC content of the mineral soils was better explained by climate (47%-60%) and soil factors (26%-50%) than by NPP (10%-20%). The total SOC content and contents of particulate (POC) and recalcitrant SOC (ROC) of the mineral soils all decreased with increasing mean annual temperature because SOC decomposition overrides the C replenishment under warmer climate. The content of mineral-associated organic carbon (MAOC) was influenced by temperature, which directly affected microbial activity. Additionally, the presence of clay and iron oxides physically protected SOC by forming MAOC. The SOC stock in the O horizons was larger in the temperate zone and Mediterranean regions than in the boreal and sub/tropical zones. Mixed forests had 64% larger SOC pools than either broadleaf or coniferous forests, because of (i) higher productivity and (ii) litter input from different tree species resulting in diversification of molecular composition of SOC and microbial community. While climate, soil, and forest type jointly determine the formation and stability of SOC, climate predominantly controls the global patterns of SOC pools in forest ecosystems.

Biological nitrogen fixation (BNF) is strongly affected by the carbon (C) and nitrogen (N) stoichiometry in soil and depends on the input of organic C. Due to the high metabolic costs of nitrogenase activity, however, the response of BNF to organic C input and its impact on microbial turnover remain unclear. To address this knowledge gap, we combined 15N2 tracing with high-throughput sequencing by adding glucose or glucose plus mineral N fertilizer for a 12-day incubation in three cropland soils. Glucose addition alone strongly changed the BNF activity (0.76–2.51 mg N kg−1 d−1), while BNF was completely absent after mineral N fertilization. This switch-on of BNF by glucose addition supported equally high rates of microbial growth and organic C mineralization compared with the direct mineral N assimilation by microorganisms. Glucose-induced BNF was predominantly catalyzed by Azotobacter-affiliated free-living diazotrophs (>50 % of the total nifH genes), which increased with diverse nondiazotrophs such as Nitrososphaera, Bacillus and Pseudoxanthomonas. Structural equation models (SEMs) and random forest (RF) analyses consistently revealed that the soil C:N ratio and Azotobacter-affiliated diazotrophic abundances were the key factors affecting glucose-induced BNF. Our findings emphasize the importance of free-living diazotrophs for microbial turnover of organic C in soil.

Plant roots interact with rhizosphere microbes to accelerate soil organic matter (SOM) mineralization and promote nutrient acquisition. Root-mediated changes in SOM turnover largely depend on root-C input and soil nutrient availability. Hence, interspecific competition and nutrient uptake dynamics over plant development stages as well as spatiotemporal variability in C input may modify SOM turnover. To investigate the effect of intraspecific competition on SOM mineralization at three growth stages (heading, flowering and ripening), we grew maize (C4 plant) under three planting densities on a C3 soil. 13C-natural abundance and 15N-pool dilution were applied in situ to determine C- and N-mineralization rates. Soil C- and N-mineralization rates were tightly coupled and peaked at maize flowering. However, the C-to-N-mineralization ratio increased with N, indicating that microbes mineralize N-rich components to mine SOM for N. Furthermore, intraspecific competition did not affect root biomass; instead, plants shaped root morphology towards higher specific root length as an efficient strategy competing for nutrient. Hence, root morphologic traits rather than root biomass per se were positively related to C- and N-mineralization. Overall, plant competition for nutrients controlled the intensity and mechanisms of soil C- and N-turnover by the adaptation of root traits and nutrient depletion.

The swift proliferation of forests converted into monoculture plantations has profound impacts on soil nutrients, microbial communities, and many ecological processes and functions. Nematodes are soil microfauna that play a pivotal role in biogeochemical cycling and in soil food web, whereas the response of soil nematode communities and energy flows to forest conversion remains unknown. Here, we assessed the community composition and the energy flows of the nematode food webs as a function of soil chemistry after conversion from natural forests (Forest) to four plantations (8-year-old): Amygdalus persica (Peach), Myrica rubra (Berry), Camellia oleifera (Oil), and Cunninghamia lanceolata (Fir). After forest conversion, soil organic carbon (SOC) and total nitrogen (TN) contents decreased by 65 % and 55 %, respectively. Forest conversion strongly reduced the abundance (particularly large-bodied omnivorous-predatory nematodes), diversity, maturity, and stability of the soil nematode community. The shifts in composition and structure of nematode communities after forest conversion are reflected in changes in the abundance of predominant genera and trophic taxa, especially bacterivorous, fungivorous, and omnivorous-predatory nematodes. Acrobeloides notably increased, whereas Plectus, Prismatolaimus, Tylencholaimus, and Tripyla decreased. Accordingly, the abundances of r-strategy nematodes (cp value = 1–2) increased, but that of the K-strategists (cp value = 3–5) declined. Additionally, the energy flow across the soil nematode food web was reduced by 36 % and flow uniformity declined by 24 % after forest conversion. These changes in nematode diversity and abundance were triggered by diminishing soil C and N contents, thereby affecting the energy flows via the nematode food webs. Thus, forest conversion affects soil biotas and multi-functions from the perspective of nematode food web structure and energy flows, and underlines the interconnections between ecosystem and energy dynamics across multi-trophic levels, which is crucial for sustainable forest management.