Publications

Labile C inputs into soils will be partially transformed into soil organic carbon (SOC) through microbial assimilation or physicochemical protection as such mineral-organic interactions and soil aggregation. The C inputs may stimulate the decomposition of native SOC, inducing a phenomenon known as the priming effect. Increasing C inputs may increase SOC content, yet the relative role of these mechanisms in controlling the magnitudes of SOC increase among soils remains unclear. Four soils differing in microbiology and N content were incubated with 13C-labeled glucose at the amounts of 0.5 (G0.5), 1.0 (G1.0), and 2.0 (G2.0) g C kg−1 soil for 48 days. The objectives of this study were 1) to quantify the fates of added glucose-C, the priming effect, and the changes in SOC and mineral N contents after the incubation, and 2) to identify the main mechanisms for SOC increase with the increased amount of C input and the effects of initial soil N availability and microbial composition. 13C isotope probing showed that 19 to 26 % and 37 to 96 % of added glucose were utilized for microbial respiration and growth, respectively, after the incubation in all soils, with the glucose-C use efficiency ranging from 30 to 60 %. The remaining accounted for 4 to 37 % in all cases except for UltisolHN at G1.0 and G2.0 (–23 % and −2%), which was confirmed as the fraction protected in soil or re-utilized by microbes via acid extraction. The soil-protected glucose-C was higher, while glucose-C use efficiency was lower with the increased amount of added glucose only in the soils with low N. Glucose addition induced a positive priming effect, a SOC increase, and a mineral N loss irrespective of soil type and addition amount. Glucose addition increased the cumulative fungal biomass in all cases and the cumulative bacterial biomass only at G2.0 in all soils and at G1.0 in the high-N soils. It increased the final fungal necromass only at G0.5 in MollisolLN and UltisolHN and at G2.0 in MollisolHN and the final bacterial necromass in all cases except for the low-N soils and UltisolHN at G2.0. With the increased amount of added glucose, the SOC content increased linearly and the priming effect and fungal biomass increased non-linearly, with larger magnitudes in the soils with lower N. These results indicated that glucose was quickly protected after addition and remained in the soil until soil N became a limiting factor for microbial utilization. The availability of soil N relative to input C mediated microbial growth and its controls over SOC change via the soil protection and priming effects. The carbon sequestration potential with increased input of dissolved substrates as such glucose e. g. through root exudation was likely larger in soils with lower fertility due to reduced substrate-C use efficiency and increased effect of soil protection despite an increased priming effect. Further studies are needed on more substrate types as they affect substrate-C use efficiency and soil protection mechanisms.

Soil carbon is major driver of climate in the long term because soil can either decrease global warming by carbon sequestration or increase warmin

Associations of iron (hydr)oxides (FeOx) with organic carbon are vital in regulating the stability of soil organic carbon (SOC). Like SOC, FeOx is chemically dynamic in soils, particularly under anaerobic conditions. However, previous research has not clarified how the stability of FeOx (goethite versus ferrihydrite) and the formation pathway of FeOx-OC associations (adsorption versus coprecipitation) affect the stability of FeOx-bound OC and, subsequently, the priming effect (PE) under anaerobic conditions. With an aim to bridge this gap, we incubated paddy soils for 80 d under anaerobic conditions after adding free 13C-glucose, ferrihydrite- or goethite-bound 13C-glucose formed by either adsorption or coprecipitation. Compared with the free glucose addition, the FeOx-bound glucose addition increased 13CO2 production by 8%–21% but reduced 13C–CH4 production by 7%–10%. Ferrihydrite-bound glucose was mineralised more than goethite-bound glucose; this is consistent with its lower crystallinity facilitating reduction and, thus, higher OC bioavailability. Glucose induced a negative priming effect (PE) for CO2 but a positive PE for CH4, whereas FeOx-bound glucose showed the opposite trend. This may be because FeOx-bound glucose provides an energy source and electron acceptor for Fe-reducing bacteria; this promotes the dissimilating reduction of iron and combines with an aggravated microbial P limitation resulting from the FeOx input. The crystallinity of FeOx affected the amount of primed CH4 rather than its formation pathway. In conclusion, the crystallinity of FeOx controls the stability of FeOx-OC associations and the PE of SOC decomposition under anaerobic conditions.

This study investigated the effects of mineral-N addition and intensive mixing (analogous to disturbance by plowing) on decomposition of 14C-labelled maize (Zea mays L.) residue and soil organic matter (SOM). Soils were collected from the upper 5cm of three land use types at Edelweiler, Germany: plow tillage (PT), reduced tillage (RT), and grassland (GL). Soils were incubated for 112 days at 20°C, with or without 14C-labelled maize residue (4gDMkg−1 soil), with or without nitrogen (100mgNkg−1 soil as NH4NO3) and with or without intensive mixing. The effect of mineral-N on maize residue decomposition differed depending on the stage of decomposition and land use type. Nitrogen accelerated residue decomposition rates in the first 5 days in RT and GL soils, but not in PT soil, and decreased residue decomposition rate in all three land use types after 11 days. At the end of the incubation, N suppressed 14CO2 efflux in RT and PT soils, but not in GL soil. Mineral-N did not increase SOM decomposition independently on the land use types. Intensive mixing stimulated decomposition of both plant residue and SOM in all three land use types. However, effects were smaller in GL soil than in RT or PT soil, presumably because stronger soil aggregates in GL would have been less affected by mixing and allowed greater protection of SOM and plant residue against decomposition.

Organic matter input regulates the rate and temperature sensitivity (expressed as Q10) of soil organic matter (SOM) decomposition by changing microbial composition and activities. It remains unclear how the incorporation of litter-made biochar instead of litter affects the Q10 of SOM decomposition. Using a unique combination of two-and three-source partitioning methods (isotopic discrimination between C3/C4 pathways and 14C labeling), we investigated: (1) how maize litter versus litter-made biochar (of C4 origin) addition influenced the Q10 of SOM (C3 origin) under 10°C warming, and (2) how the litter or biochar amendments affected the Q10 of 14C-labeled fresh organic matter (FOM) after long-term incubation. Compared with biochar addition, litter increased the rates and Q10 of mass-specific respiration, SOM and FOM decomposition, as well as the contents of SOM-derived dissolved organic C (DOC) and total phospholipid fatty acids (PLFA). Litter-amended soils have much higher activities (Vmax) of β-glucosidase, N-acetyl-β-glucosaminidase, and leucine aminopeptidase, suggesting larger enzyme pools than in soils with biochar. The Q10 of enzyme Vmax (1.6-2.0) and Km (1.2-1.4) were similar between litter-and biochar-amended soils, and remained stable with warming. However, warming reduced microbial biomass (PLFA) and enzyme activity (Vmax), suggesting decreased enzyme production associated with smaller microbial biomass or faster enzyme turnover at higher temperatures. Reductions in PLFA content and enzyme Vmax due to warming were larger in litter-amended soils (by 31%) than in the control and biochar-amended soils (by 4-11%), implying the active litter-feeding microorganisms have a smaller degree of heat tolerance than the inactive microorganisms under biochar amendments. The reduction in enzyme activity (Vmax) by warming was lower in soils with biochar than in the control soil. Our modeling suggested that the higher Q10 in litter-amended soils was mainly caused by faster C loss under warming, linked to reductions in microbial biomass and growth efficiency, rather than the slightly increased SOM-originated substrate availability (DOC). Overall, using straw-made biochar instead of straw per se as a soil amendment lowers the Q10 of SOM and FOM by making microbial communities and enzyme pools more temperature-tolerant, and consequently reduces SOM losses under warming.

No abstract is provided for this article.

The stability of soil organic matter (SOM) is the trade-off between its bioenergetic profits for microorganisms and the energy barrier for its decomposition. The energy density and activation energy, which determine the SOM stability in topsoil (0–30 cm), belowground biomass and litter in forest and grassland ecosystems, were determined by thermogravimetryand differential scanning calorimetry. Organic matter was fractionated into pools based on their thermal stability — labile, stable, and persistent. The activation energy of thermal decomposition of persistent SOM was 2.8 times higher than for labile SOM. There was more labile SOM per unit of thermally stable SOM in forest soil than in grassland. Therefore, the higher bioenergetic availability of SOM in forest ecosystems suggested a faster carbon turnover compared to grassland soils. Forest soil accumulated 1.5 times more organic carbon than grassland soil. The ratio of energy density to activation energy characterized the potential bioenergetic advantage of organic substrate decomposition by microorganisms, could be used to characterize the SOM quality as an addition to the widely used C:N ratio. The transformation of forest and grassland litter and belowground biomass into SOM was accompanied by a decrease in activation energy and an increase in energy density. These identified processes underline the leading role of progressive decomposition and selective stabilization of microbial metabolites for the SOM formation

Microbial carbon use efficiency (CUE) is a key parameter of initial microbial utilization of organic matter in soil. The responses of CUE to global change factors (GCFs) remain unclear due to their multiple effects and interactions. Here, this study generalized 385 observations obtained using various methods, including 13 C‐/ 14 C‐labeled substrates, 18 O‐labeled water, stoichiometric modeling, and others. The effects of climate change (drought, precipitation, warming), fertilization (nitrogen addition, phosphorus addition, potassium addition, and nitrogen fertilization combined with phosphorus and potassium), land use conversion, and natural restoration, were evaluated along with their 16 associated GCFs on CUE. CUE was insensitive to climate change factors and most fertilization practices, maintaining a mean value of 0.36 under global change scenarios. Farmland conversion to forest and vegetation restoration decreased CUE by 11% and 17%, respectively. Grassland restoration increased CUE by 41%, indicating that grasslands have high potential for soil carbon accrual. Nitrogen fertilization combined with phosphorus and potassium increased CUE by 18% because the combined application of nutrients allows plants to produce organic matter sources with high‐quality and decreases nutrient limitations for microorganisms. Increase in soil pH induced by GCFs leads to higher CUE. The CUE was decoupled from soil organic carbon content under several global change scenarios (e.g., warming, fertilization), suggesting that this relationship is not universally consistent across GCFs. This study provides a new perspective on the responses of CUE to GCFs and deepens our understanding of the global change effects on microbial physiology with consequences for soil carbon cycling.

The decomposition of soil organic matter depends strongly on its availability to microorganisms and their enzymes. The rhizosphere and detritusphere are microbial hot spots due to additional substrate input, leading to high abundance, specific species diversity and functional diversity of microbial communities. However, rhizosphere and detritusphere differ in substrate quality, localization, and duration of input. We hypothesized that the contrasting substrate availability between rhizosphere and detritusphere affects the activity of microorganisms and associated enzymes. Organic carbon (C) from the rhizosphere and detritusphere decreases with soil depth and, consequently, microbial hot spots become rarer and competition for C and nutrients increases. In deeper soil (>40 cm depth) the amount and quality of substrates is expected to decrease and, therefore, the effect of contrasting substrate input to disappear. Plant N uptake is expected to reduce N availability in the rhizosphere of maize compared to the detritusphere and bare fallow. These hypotheses were tested in a factorial field experiment with 1) maize-planted, 2) maize litter-amended, and 3) bare sites. Enzyme kinetic parameters (Vmax, Km, Ka), extractable organic C and microbial biomass C were compared in soil affected by rhizosphere and detritusphere throughout the profile to 70 cm depth, to assess microbial C and nutrient limitations. A decrease in enzyme activity with depth due to resource scarcity and lower substrate quality appeared in planted and litter-amended soil. N limitation in planted soil increased the activity and substrate affinity of proteolytic enzymes to provide for microbial N demand through SOM decomposition. This was in line with lower Vmax ratios (Vmax for C-cycling enzymes divided by Vmax for N-cycling enzymes) in planted relative to litter-amended topsoil. The catalytic efficiency of enzymes decreased 2- to 20-fold from top- (<40 cm) to subsoil (>40 cm), irrespective of the substrate input. Substrate quality in the rhizosphere and detritusphere affected enzyme activities only in the topsoil, whereas a sharp decline of C input with depth led to similar activities in the subsoil. Most of the enzyme indexes reflected shifts in allocation of C and nutrients in the rhizosphere and detritusphere. The presented results underline the role of microorganisms as critical links in the C and nutrient transfers in the rhizosphere and detritusphere.

Biochar (BC) application represents a promising soil management strategy for mitigating CO2 and N2O emissions; however, as concurrent soil pollutants, microplastics may interact with BC. In a 3 month microcosm experiment (25 °C, 60% WHC), we investigated soil C and N dynamics following the addition of polyethylene (PE) microplastics (1 and 5%) to a Calcaric Fluvisol already amended with BC for 1 month. BC alone reduced CO2 and N2O emissions by 11 and 3%, respectively, while PE reduced CO2 and N2O emissions by 11–26 and 4–14%, respectively. The suppression of CO2 emissions by BC and PE microplastics was due to reduced dissolved organic matter (DOM) content as well as increased DOM aromaticity, all of which led to diminished bacterial biomass and β-N-acetyl-glucosaminidase activity. BC decreased N2O emissions by suppressing the nirS and nirK genes while increasing the level of the nifH gene; PE decreased N2O emissions primarily by decreasing the level of the nirK gene. BC alone decreased the microbial necromass carbon content by 35%, primarily due to the suppression of bacterial abundance, thus leading to reduced efficiency in bacterial necromass production. PE had a modest impact, decreasing microbial necromass C by 8–11% in BC-free soil, mainly due to dilution effects. However, in BC-treated soil, PE had a profound influence, as it markedly increased the microbial necromass C by 33–61%. The microbial necromass increased due to the disruption of aggregates, which provided better protection against microbial necromass and a reduction in β-N-acetyl-glucosaminidase activity, which is responsible for necromass mineralization. In summary, the interactive effects of BC and PE microplastics on microbial necromass accumulation, as well as CO2 and N2O emissions, are mainly based on microbial (especially bacterial) necromass and DOM decomposition as well as aggregate destruction. Our findings offer valuable insights for the adaptation and enhancement of soil carbon management strategies in response to the challenge of microplastic contamination.

We compared modifications of soil zymography, a new in situ technique to visualize enzyme activities, based on contact of fluorgenic substrate-saturated me

Priming effects (PE) can greatly influence global carbon (C) storage in soil and lead to climate feedbacks by accelerating the decomposition of organic matter (OM). Although nitrogen (N) availability can alter the magnitude and direction of priming (stoichiometric constrains), it remains unclear whether additions of NO3 − (nitrate) and NH4 + (ammonium) have distinct effects on the decomposition of various 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. Four OM forms, namely 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 labile C (13C labeled glucose) and N (NO3 −) 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 NH4 + additions, NO3 − 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 in soils, demonstrating energy limitation for microorganisms. Distinct priming patterns were observed between NO3 − and NH4 + additions, both for the individual OM types and for all four types of OM. On a basis of C balance between primed C and the remaining added C, PE induced by labile C and N inputs can increase or reduce C sequestration depending on C:N stoichiometric ratios of labile inputs. Our findings provide important insights into the specific role of NO3 − or NH4 + together with labile C inputs (e.g. from root exudation), and thus changes in the composition of deposited N (atmospheric deposition and fertilization) may induce distinct climate feedbacks.