Showing papers by "Yakov Kuzyakov published in 2020"

Dramatic loss of inorganic carbon by nitrogen-induced soil acidification in Chinese croplands.

Abstract: Intensive crop production systems worldwide, particularly in China, rely heavily on nitrogen (N) fertilization, but left more than 50% of fertilizer N in the environment. Nitrogen (over) fertilization and atmospheric N deposition induce soil acidification, which is neutralized by soil inorganic carbon (SIC; carbonates), and carbon dioxide (CO2 ) is released to the atmosphere. For the first time, the loss of SIC stocks in response to N-induced soil acidification was estimated for Chinese croplands from 1980 to 2020 and forecasts were made up to 2100. The SIC stocks in croplands in 1980 were 2.16 Pg C (16.3 Mg C/ha) in the upper 40 cm, 7% (0.15 Pg C; 1.1 Mg C/ha) of which were lost from 1980 to 2020. During these 40 years, 7 million ha of cropland has become carbonate free. Another 37% of the SIC stocks may be lost up to 2100 in China, leaving 30 million ha of cropland (37.8%) without carbonates if N fertilization follows the business-as-usual (BAU) scenario. Compared to the BAU scenario, the reduction in N input by 15%-30% after 2020 (scenarios S1 and S2) will decrease carbonate dissolution by 18%-41%. If N input remains constant as noted in 2020 (S3) or decreases by 1% annually (S4), a reduction of up to 52%-67% in carbonate dissolution is expected compared to the BAU scenario. The presence of CaCO3 in the soil is important for various processes including acidity buffering, aggregate formation and stabilization, organic matter stabilization, microbial and enzyme activities, nutrient cycling and availability, and water permeability and plant productivity. Therefore, optimizing N fertilization and improving N-use efficiency are important for decreasing SIC losses from acidification. N application should be strictly calculated based on crop demand, and any overfertilization should be avoided to prevent environmental problems and soil fertility decline associated with CaCO3 losses.

Carbon and nitrogen recycling from microbial necromass to cope with C:N stoichiometric imbalance by priming

Abstract: The impact of increasing amounts of labile C input on priming effects (PE) on soil organic matter (SOM) mineralization remains unclear, particularly under anoxic conditions and under high C input common in microbial hotspots. PE and their mechanisms were investigated by a 60-day incubation of three flooded paddy soils amended with13C-labeled glucose equivalent to 50–500% of microbial biomass C (MBC). PE (14–55% of unamended soil) peaked at moderate glucose addition rates (i.e., 50–300% of MBC). Glucose addition above 300% of MBC suppressed SOM mineralization but intensified microbial N acquisition, which contradicted the common PE mechanism of accelerating SOM decomposition for N-supply (frequently termed as “N mining”). Particularly at glucose input rate higher than 3 g kg−1 (i.e., 300–500% of MBC), mineral N content dropped on day 2 close to zero (1.1–2.5 mg N kg−1) because of microbial N immobilization. To cope with the N limitation, microorganisms greatly increased N-acetyl glucosaminidase and leucine aminopeptidase activities, while SOM decomposition decreased. Several discrete peaks of glucose-derived CO2 (contributing >80% to total CO2) were observed between days 13–30 under high glucose input (300–500% of MBC), concurrently with CH4 peaks. Such CO2 dynamics was distinct from the common exponential decay pattern, implicating the recycling and mineralization of 13C-enriched microbial necromass driven by glucose addition. Therefore, N recycling from necromass was hypothesized as a major mechanism to alleviate microbial N deficiency without SOM priming under excess labile C input. Compound-specific 13C-PLFA confirmed the redistribution of glucose-derived C among microbial groups, i.e., necromass recycling. Following glucose input, more than 4/5 of total 13C-PLFA was in the gram-negative and some non-specific bacteria, suggesting these microorganisms as r-strategists capable of rapidly utilizing the most labile C. However, their 13C-PLFA content decreased by 70% after 60 days, probably as a result of death of these r-strategists. On the contrary, the 13C-PLFA in gram-positive bacteria, actinomycetes and fungi (K-strategists) was initially minimal but increased by 0.5–5 folds between days 2 and 60. Consequently, the necromass of dead r-strategists provided a high-quality C–N source to the K-strategists. We conclude that under severe C excess, N recycling from necromass is a much more efficient microbial strategy to cover the acute N demand than N acquisition from the recalcitrant SOM.

Organic carbon burial and sources in soils of coastal mudflat and mangrove ecosystems

Abstract: Mangrove organic carbon is primarily stored in soils, which contain more than two-thirds of total mangrove ecosystem carbon stocks. Despite increasing recognition of the critical role of mangrove ecosystems for climate change mitigation, there is limited understanding of soil organic carbon sequestration mechanisms in undisturbed low-latitude mangroves, specifically on organic carbon burial rates and sources. This study assessed soil organic carbon burial rates, sources and stocks across an undisturbed coastal mudflat and mangrove hydrogeomorphological catena (fringe mangrove and interior mangrove) in Bintuni Bay, West Papua Province, Indonesia. 210Pb radionuclide sediment dating, and mixing model of natural stable isotope signatures (δ 13C and δ15N) and C/N ratio were used to estimate organic carbon burial rates and to quantify proportions of allochthonous (i.e., upland terrestrial forest) and autochthonous (i.e., on-site mangrove forest) organic carbon in the top 50 cm of the soil. Burial rates were in the range of 0.21–1.19 Mg C ha−1 yr−1. Compared to the fringe mangroves, organic carbon burial rates in interior mangroves were almost twice as high. Primary productivity of C3 upland forest vegetation and mangroves induced soil organic carbon burial in interior mangroves and this was consistent with the formation of the largest organic carbon stocks (179 ± 82 Mg C ha−1). By contrast, organic carbon stored in the fringe mangrove (68 ± 11 Mg C ha−1) and mudflat (62 ± 10 Mg C ha−1) soils mainly originated from upland forests (allochthonous origin). These findings clearly indicate that carbon sequestered and cycling in mangrove and terrestrial forest ecosystems are closely linked, and at least a part of carbon losses (e.g., erosion) from terrestrial forests is buried in mangrove ecosystems.

Arbuscular mycorrhiza enhances rhizodeposition and reduces the rhizosphere priming effect on the decomposition of soil organic matter

Abstract: Arbuscular mycorrhizal fungi (AMF) represent an important route for plant carbon (C) inputs into the soil. Nonetheless, the C input via AMF as well as its impact on soil organic matter (SOM) stabilization and C sequestration remains largely unknown. A mycorrhizal wild type progenitor (MYC) and its mycorrhiza defective mutant (reduced mycorrhizal colonization: rmc) of tomato were continuously labeled with 13CO2 to trace root C inputs into the soil and quantify rhizosphere priming effects (RPE) as affected by AMF symbiosis and N fertilization. Mycorrhizal abundance and 13C incorporation into shoots, roots, soil and CO2 were measured at 8, 12 and 16 weeks after transplanting. AMF symbiosis decreased the relative C allocation (% of total assimilated C) to roots, in turn increased the net rhizodeposition. Positive RPE was recorded for both MYC and rmc plants, ranging from 16–71% and 25–101% of the unplanted control, respectively. Although net rhizodeposition was higher for MYC than rmc plants 16 weeks after transplanting, the RPE was comparatively lower. This indicated a higher potential for C sequestration by plants colonized with AMF (MYC) because the reduced nutrient availability restricts the activity of free-living decomposers. Although N fertilization decreased the relative C allocation to roots, rhizosphere and bulk soil, it had no effect on the absolute amount of rhizodeposition to the soil. The RPE and N-cycling enzyme activities decreased by N fertilization 8 and 12 weeks after transplanting, suggesting a lower microbial N demand from SOM mining. The positive relationship between enzyme activities involved in C cycling, microbial biomass C and SOM decomposition underlines the microbial activation hypothesis, which explains the RPE. We therefore concluded that AMF symbiosis and N fertilization increase C sequestration in soil not only by increasing root C inputs, but also by lowering native SOM decomposition and RPE.

Nutrient addition reduces carbon sequestration in a Tibetan grassland soil: Disentangling microbial and physical controls

Abstract: Nitrogen (N) and phosphorus (P) availability strongly affects carbon (C) cycling and storage in terrestrial ecosystems. Nutrient addition can increase C inputs into soil via increased above- and belowground plant productivity, but at the same time can accelerate organic matter decomposition in the soil. The mechanisms underlying these effects on soil organic C (SOC) dynamics remain unclear, especially in nutrient-limited alpine ecosystems that have been subjected to increasing N and P availability in recent decades. The aim of this study was to clarify the mechanisms underlying SOC decomposition and stabilization in an alpine grassland soil after four years of N and P additions. The soil aggregate size distribution, microbial community structure (lipid biomarkers), microbial C use efficiency (CUE) and microbial necromass composition (amino sugar biomarkers) were analyzed. Nutrient addition increased dominance of fast-growing bacteria (copiotrophs), while P addition alone intensified the competitive interactions between arbuscular mycorrhizal and saprotrophic fungi. These changes led to decreases in the microbial CUE of glucose by 1.6–3.5% and of vanillin by 8.5%, and therefore, reduced SOC content in the topsoil. The total microbial necromass remained unaffected by nutrient addition, but the contribution of fungal necromass to SOC increased. The increased abundance of arbuscular mycorrhizal fungi and fungal necromass under elevated N availability raised the mass proportion of soil macroaggregates (>250 μm) by 16.5–20.3%. Therefore, fungi were highly involved in macroaggregation following N addition, and so, moderated the SOC losses through enhanced physical protection. Overall, the complex interactions between microbial physiology (CUE), necromass composition (amino sugars) and physical protection (macroaggregation) in mediating SOC dynamics in response to nutrient enrichment were disentangled to better predict the capability of alpine grassland soils to act as a C sink or source under global change.

Contrasting patterns and drivers of soil bacterial and fungal diversity across a mountain gradient

Abstract: Microbial elevational diversity patterns have been extensively studied, but their shaping mechanisms remain to be explored. Here, we examined soil bacterial and fungal diversity and community compositions across a 3.4 km elevational gradient (consists of five elevations) on Mt. Kilimanjaro located in East Africa. Bacteria and fungi had different diversity patterns across this extensive mountain gradient-bacterial diversity had a U shaped pattern while fungal diversity monotonically decreased. Random forest analysis revealed that pH (12.61% importance) was the most important factor affecting bacterial diversity, whereas mean annual temperature (9.84% importance) had the largest impact on fungal diversity, which was consistent with results obtained from mixed-effects model. Meanwhile, the diversity patterns and drivers of those diversity patterns differ among taxonomic groups (phyla/classes) within bacterial or fungal communities. Taken together, our study demonstrated that bacterial and fungal diversity and community composition responded differently to climate and edaphic properties along an extensive mountain gradient, and suggests that the elevational diversity patterns across microbial groups are determined by distinct environmental variables. These findings enhanced our understanding of the formation and maintenance of microbial diversity along elevation, as well as microbial responses to climate change in montane ecosystems.

Rusty sink of rhizodeposits and associated keystone microbiomes

Abstract: Iron hydroxides serve as an efficient ‘rusty sink’ promoting the stabilization of rhizodeposits into soil organic carbon (SOC). Our work aimed to understand the physicochemical and microbial mechanisms promoting rhizodeposit (rhizo-C) stabilization as influenced by goethite (α-FeOOH) or nitrogen (N), using 13C natural abundance methodologies and DNA sequencing, in the rhizosphere of maize (Zea mays L.). The addition of N fertilizer to soil increased the mineralization of both rhizo-C and SOC, while amendment with α-FeOOH decreased rhizo-C derived CO2 and lowered the rhizosphere priming effect by 0.57 and 0.74-fold, respectively, compared to the control soil. This decrease resulted from the co-precipitation of rhizo-C at the reactive α-FeOOH surfaces as Fe-organic matter complexes (FeOM), which was 10-times greater than the co-precipitation on short-range ordered minerals. The highest portion of rhizo-C (67% of the total accumulated in soil) was protected within macroaggregates (>2 mm). Carbon overlapped with α-FeOOH mainly in >2 mm aggregates, as shown by HRTEM-EDS imaging, suggesting that α-FeOOH associated rhizo-C stimulated aggregate formation. Random forest analysis confirmed that the stabilization of rhizo-C was controlled mainly by physiochemical binding within FeOM complexes and macroaggregates. Rhizo-C mineralization was regulated by the keystone microbiome: Paucimonas (β-Proteobacteria) being an r-strategist with rapid growth under soil without nutrient limitation (N treated) and Steroidobacter (Actinobacteria) with branched filaments that can access C and nutrients under oligotrophic conditions (goethite enriched soil). Two-way orthogonal partial least squares analysis revealed that the rhizosphere priming effect was facilitated mainly by the same genera, most likely due to co-metabolism. The genera belonging to Acidimicrobiaceae (Actinobacteria), Cryptococcus and Cystofilobasidium (Basidiomycota) were positively correlated with the accumulation of rhizo-C in the >2 mm aggregate size, which might due to their high affinity towards α-FeOOH and contribution to the development of aggregation via filamentary structures that interact with microaggregates. We suggest that rhizodeposit stabilization in soil was balanced by microbial mineralization and abiotic associations with the “rusty sink” and organisms with branched filaments contributing to the development of aggregation.

Anaerobic oxidation of methane in paddy soil: Role of electron acceptors and fertilization in mitigating CH4 fluxes

Abstract: The anaerobic oxidation of methane (AOM) in marine ecosystems is ubiquitous and largely coupled to sulfate reduction. In contrast, the role of AOM in terrestrial environments and the dominant electron acceptors driving terrestrial AOM needs deeper understanding. Submerged rice paddies with intensive CH4 production have a high potential for AOM, which can be important for greenhouse gas mitigation strategies. Here, we used 13CH4 to quantify the AOM rates in paddy soils under organic (Pig manure, Biochar) and mineral (NPK) fertilization. Alternative-to-oxygen electron acceptors for CH4 oxidation, including Fe3+, NO3−, SO42−, and humic acids, were examined and their potential for CH4 mitigation from rice paddies was assessed by 13CH4 oxidation to 13CO2 under anoxic conditions. During 84 days of anaerobic incubation, the cumulative AOM (13CH4-derived CO2) reached 0.15–1.3 μg C g-1 dry soil depending on fertilization. NO3- was the most effective electron acceptor, yielding an AOM rate of 0.80 ng C g-1 dry soil h-1 under Pig manure. The role of Fe3+ in AOM remained unclear, whereas SO42- inhibited AOM but strongly stimulated the production of unlabeled CO2, indicating intensive sulfate-induced decomposition of organic matter. Humic acids were the second most effective electron acceptor for AOM, but increased methanogenesis by 5–6 times in all fertilization treatments. We demonstrated for the first time that organic electron acceptors (humic acids) are among the key AOM drivers and are crucial in paddy soils. The most pronounced AOM in paddy soils occurred under Pig manure, followed by Control and NPK, while AOM was the lowest under Biochar. We estimate that nitrate (nitrite)-dependent AOM in paddy fields globally consumes ~3.9 Tg C–CH4 yr-1, thereby offsetting the global CH4 emissions by ~10–20%. Thus, from a broader agroecological perspective, the organic and mineral fertilizers control an important CH4 sink under anaerobic conditions in submerged ecosystems. Appropriate adjustments of soil fertilization management strategies would therefore help to decrease the net CH4 flux to the atmosphere and hence the global warming.

Soil Phosphorus Bioavailability and Recycling Increased with Stand Age in Chinese Fir Plantations

Abstract: Phosphorus (P) is a limiting nutrient for plant growth in most forest ecosystems. In response to P deficiency, plants alter root exudates (organic acids, phosphatases, and protons) to increase P bioavailability in soils. However, little is known about how bioavailable P pools (soluble-P, exchangeable-P, hydrolysable-P, and ligand-P extracted by CaCl2, citric acid, enzyme mixture, and HCl solution, respectively) change with stand age, especially for plantation forests. We selected a chronosequence of second-generation Chinese fir [Cunninghamia lanceolata (Lamb.) Hook., Taxodiaceae] plantations with increasing age including 3, 8–11, 16, 20, 25, 29, and 32 years. We measured total P and four bioavailable P pools in organic (O) and mineral horizons, and rhizosphere soil, as well as root exudates in the rhizosphere, litter biomass on the forest floor, and annual P uptake. Soluble-P, exchangeable-P, and ligand-P in the O horizon increased with stand age due to litter accumulation. Exchangeable-P and ligand-P in mineral soil decreased with stand age because of the increasing annual P uptake that depleted inorganic P. Exchangeable-P and ligand-P in the rhizosphere increased with stand age because the decrease in pH and citric acid concentration led to phosphate being more strongly bound to Fe and Al oxyhydroxides. Consequently, the trees’ ability for P mobilization decreased with stand age, but the P recycling within the tree increased. Continuous mineralization of hydrolysable-P by acid phosphatase replenished inorganic P pools, especially in solution. The progressive incorporation of P in the biological cycle with increasing tree age indicates that extending rotation periods might be an appropriate measure to increase P supply.

Organic matter stabilization in aggregates and density fractions in paddy soil depending on long-term fertilization: Tracing of pathways by 13C natural abundance

Abstract: Previous studies on upland soils showed that 13C natural abundance can successfully reveal C stabilization pathways between aggregates and soil organic matter (SOM) density fractions. The direction of C stabilization in paddies can, however, deviate from that in upland soils owing to i) periodic drying–rewetting cycles, with oxygen pulses under oxic conditions, and thus, shifts in microbial processing of organic residues, and ii) intensive organic and mineral fertilization. To trace C stabilization in paddies, soil was sampled from a long-term field experiment under an unfertilized Control and NPK, NPK + straw, and NPK + manure fertilizer regimes. Soil was analyzed for total C, microbial biomass (MB), and dissolved organic C, and separated into three classes based on aggregate size (>250 μm, 53–250 μm, and

Divergence in fungal abundance and community structure between soils under long-term mineral and organic fertilization

Abstract: Fertilization maintains soil fertility and maximizes crop productivity, but changes soil microbial community structure and functions. However, little is known about the effects of long-term mineral and/or organic fertilization on fungal communities which regulate nutrient cycling and crop growth. The responses of fungal abundance and community structure to 29-year mineral and/or organic fertilization were investigated in Ultisol using quantitative polymerase chain reaction (qPCR) and high-throughput sequencing (Illumina Mi-Seq). Compared to the control, mineral N fertilization decreased fungal abundance by 44%, but showed little effect on crop yield and fungal diversity. The NPK fertilization increased crop yield by 4.5 times and fungal diversity by 1.4 times, but had little effect on fungal abundance relative to the control. Manure application increased crop yield by 8.4 times, fungal abundance by 1.8 times and fungal diversity by 1.5 times relative to the control. Application of manure plus NPK increased crop yield by 9.5 times, fungal abundance by 1.9 times and fungal diversity by 1.6 times relative to the control. Long-term fertilization led to dramatic shifts in fungal community composition. Compared to the control, mineral and organic fertilization decreased the relative abundance of Ascomycota and Basidiomycota, respectively. The fungal community composition diverged between soils with mineral and organic fertilization. Changes in soil organic carbon (SOC) and nutrient contents primarily shifted fungal abundance and community structure. Manure application resulted in higher SOC and nutrient contents, and thus affected fungal abundance and community structure more strongly than mineral fertilization. Manure application rather than mineral fertilization increased the relative abundance of certain favorable fungal taxa (e.g., Pseudaleuria and Hypocreales) which suppress crop pathogens. Overall, long-term manure application explained patterns in soil fertility, crop productivity, fungal abundance, diversity and community composition better than mineral fertilization.

Rhizosphere hotspots: Root hairs and warming control microbial efficiency, carbon utilization and energy production

Abstract: Root hairs proliferation and warming strongly influence exudate release, enzyme activities and microbial substrate utilization. However, how the presence of root hairs regulates those processes in the rhizosphere under elevated temperature is poorly known. To clarify these interactions, a wild type maize (with root hairs) and its hairless mutant were grown for 3 weeks at 20 and 30 °C, respectively. We combined zymography (localize hotspots of β-glucosidase) with substrate-induced respiration and microcalorimetry to monitor exudate effects on enzyme kinetics, microbial growth and heat production in the rhizosphere hotspots in response to warming. Root hairs effects were more pronounced at the elevated temperature: i) β-glucosidase activity of the wild type at 30 °C was 21% higher than that of the hairless maize; ii) temperature shifted the microbial growth strategy, whereas root hairs promoted the fraction of growing microbial biomass; iii) Km and the activation energy for β-glucosidase under the hairless mutant was lower than that under wild maize. These results suggest that microorganisms inhabiting hotspots of the wild type synthesized more enzymes to fulfill their higher energy and nutrient demands than those of the hairless mutant. In contrast, at higher temperature the hairless maize produced an enzyme pool with higher efficiencies rather than higher enzyme production, enabling metabolic needs to be met at lower cost. We therefore conclude that root hairs play an important role in regulating enzyme systems and microbial growth to adapt to climate warming.

DNA Stable-Isotope Probing Delineates Carbon Flows from Rice Residues into Soil Microbial Communities Depending on Fertilization.

Abstract: Decomposition of crop residues in soil is mediated by microorganisms whose activities vary with fertilization. The complexity of active microorganisms and their interactions utilizing residues is impossible to disentangle without isotope applications. Thus, 13C-labeled rice residues were employed, and DNA stable-isotope probing (DNA-SIP) combined with high-throughput sequencing was applied to identify microbes active in assimilating residue carbon (C). Manure addition strongly modified microbial community compositions involved in the C flow from rice residues. Relative abundances of the bacterial genus Lysobacter and fungal genus Syncephalis were increased, but abundances of the bacterial genus Streptomyces and fungal genus Trichoderma were decreased in soils receiving mineral fertilizers plus manure (NPKM) compared to levels in soils receiving only mineral fertilizers (NPK). Microbes involved in the flow of residue C formed a more complex network in NPKM than in NPK soils because of the necessity to decompose more diverse organic compounds. The fungal species (Jugulospora rotula and Emericellopsis terricola in NPK and NPKM soils, respectively) were identified as keystone species in the network and may significantly contribute to residue C decomposition. Most of the fungal genera in NPKM soils, especially Chaetomium, Staphylotrichum, Penicillium, and Aspergillus, responded faster to residue addition than those in NPK soils. This is connected with the changes in the composition of the rice residue during degradation and with fungal adaptation (abundance and activity) to continuous manure input. Our findings provide fundamental information about the roles of key microbial groups in residue decomposition and offer important cues on manipulating the soil microbiome for residue utilization and C sequestration in soil.IMPORTANCE Identifying and understanding the active microbial communities and interactions involved in plant residue utilization are key questions to elucidate the transformation of soil organic matter (SOM) in agricultural ecosystems. Microbial community composition responds strongly to management, but little is known about specific microbial groups involved in plant residue utilization and, consequently, microbial functions under different methods of fertilization. We combined DNA stable-isotope (13C) probing and high-throughput sequencing to identify active fungal and bacterial groups degrading residues in soils after 3 years of mineral fertilization with and without manure. Manuring changed the active microbial composition and complexified microbial interactions involved in residue C flow. Most fungal genera, especially Chaetomium, Staphylotrichum, Penicillium, and Aspergillus, responded to residue addition faster in soils that historically had received manure. We generated a valuable library of microorganisms involved in plant residue utilization for future targeted research to exploit specific functions of microbial groups in organic matter utilization and C sequestration.

Decreased rhizodeposition, but increased microbial carbon stabilization with soil depth down to 3.6 m

Abstract: Despite the importance of subsoil carbon (C) deposition by deep-rooted crops in mitigating climate change and maintaining soil health, the quantification of root C input and its microbial utilization and stabilization below 1 m depth remains unexplored. We studied C input by three perennial deep-rooted plants (lucerne, kernza, and rosinweed) grown in a unique 4-m deep RootTower facility. 13C multiple pulse labeling was applied to trace C flows in roots, rhizodeposition, and soil as well as 13C incorporation into microbial groups by phospholipid fatty acids and the long-term stabilization of microbial residues by amino sugars. The ratio of rhizodeposited 13C in the PLFA and amino sugar pools was used to compare the relative microbial stability of rhizodeposited C across depths and plant species. Belowground C allocation between roots, rhizodeposits, and living and dead microorganisms indicated depth dependent plant investment. Rhizodeposition as a fraction of the total belowground C input declined from the topsoil (0–25 cm) to the deepest layer (360 cm), i.e., from 35%, 45%, and 36%–8.0%, 2.5%, and 2.7% for lucerne, kernza, and rosinweed, respectively, where lucerne had greater C input than the other species between 340 and 360 cm. The relative microbial stabilization of rhizodeposits in the subsoil across all species showed a dominance of recently assimilated C in microbial necromass, thus indicating a higher microbial stabilization of rhizodeposited C with depth. In conclusion, we traced photosynthates down to 3.6 m soil depth and showed that even relatively small C amounts allocated to deep soil layers will become microbially stabilized. Thus, deep-rooted crops, in particular lucerne are important for stabilization and storage of C over long time scales in deep soil.

Long‐term active restoration of extremely degraded alpine grassland accelerated turnover and increased stability of soil carbon

Abstract: Soil nutrient contents and organic carbon (C) stability are key indicators for restoration of degraded grassland. However, the effects of long-term active restoration of extremely degraded grassland on soil parameters have been equivocal. The aims of this study were to evaluate the impact of active restoration of degraded alpine grassland on: (a) soil organic matter (SOM) mineralization; and (b) the importance of biotic factors for temperature sensitivity (Q(10)) of SOM mineralization. Soils were sampled from intact, degraded and restored alpine grasslands at altitudes ranging between 3,900 and 4,200 m on the Tibetan Plateau. The samples were incubated at 5, 15 and 25 degrees C, andQ(10)values of SOM mineralization were determined. Structural equation modeling was used to evaluate the importance of vegetation, soil physico-chemical properties and microbial parameters forQ(10)regulation. TheQ(10)of N mineralization was similar among intact, degraded and restored soils (0.84-1.24) and was higher in topsoil (1.09) than in subsoil (0.92). The best predictive factor of CO2-Q(10)for intact grassland was microbial biomass, for degraded grassland was basal microbial respiration, and for restored grassland was soil bulk density. Restoration by planting vegetation decreased theQ(10)of SOM mineralization as soil bulk density, the most important negative predictor, increased in restored grassland. TheQ(10)of SOM mineralization in topsoil was 14% higher than in subsoil because of higher microbial abundance and exo-enzyme activities. The NH(4)(+)content was greatest in intact soil, while NO(3)(-)content was greatest in degraded soil. The SOM mineralization rate decreased with grassland degradation and increased after long-term (>10 years) restoration. In conclusion, extremely degraded grassland needs proper long-term management in active restoration projects, especially for improvement of soil nutrients in a harsh environment.

Effects of drying/rewetting on soil aggregate dynamics and implications for organic matter turnover

Abstract: Drying and rewetting (D/W) of soil have significant impacts on soil organic matter (SOM) turnover. We hypothesised that frequent D/W cycles would release the labile organic matter locked away in soil aggregates, increasing the priming effect (PE) (acceleration or retardation of SOM turnover after fresh substrate addition) due to preferential utilisation by microbes. 13C-labelled lignocellulose was added to the soil, and the effects of 0, 1, or 4 cycles of D/W were evaluated at 5 °C and 25 °C after a 27-day incubation of undisturbed soil cores from a temperate forest (Araucaria araucana). Following the incubation, macroaggregates (> 250 μm), microaggregates (250–53 μm), and silt + clay materials ( 2.0 g cm−3) were determined. D/W cycles caused macroaggregates to increase and a decrease in microaggregates (> 15%) at warm temperatures, and preferential use of the novel particulate organic matter (13C labelled), formerly protected fPOM. CO2 efflux was three times higher at 25 °C than at 5 °C. The D/W cycles at 25 °C had a strong negative impact on cumulative CO2 efflux, which decreased by approximately − 30%, induced by a negative PE of −50 mg C kg−1 soil with 1 D/W cycle and − 100 mg C kg−1 soil with 4 D/W cycles, relative to soil under constant soil moisture receiving 13C-labelled lignocellulose, but no cycles. Increasing the temperature and the number of D/W cycles caused a decrease in substrate use efficiency of particulate lignocellulose. In conclusion, D/W cycles at warm temperatures accelerated OM turnover due to preferential use from fPOM, increasing macroaggregates at the expense of microaggregates. A novel pathway of OM release and PE due to the D/W cycles is discussed.

Synergy effect of peroxidase enzymes and Fenton reactions greatly increase the anaerobic oxidation of soil organic matter.

Abstract: In temperate rainforest soils of southern Chile (38 °S), there are high rates of soil organic carbon (SOC) mineralization under oxygen (O2) limitation. We study the combined effects of Fenton reactions and the activity of two enzymes manganese peroxidase (MnP) and lignin peroxidase (LiP), which was hypothesised potentiate SOC mineralization under anoxic conditions leading to carbon dioxide (CO2) release. Both mechanisms produce free radicals when competing for SOC oxidation in the absence of microorganisms. We quantify the CO2 release by induced Fenton reactions in combination with MnP and LiP under aerobic and anaerobic conditions (20 °C, 36 h) in temperate rainforest soils. CO2 levels released by Fenton reactions and enzyme activity were eight times higher than those released by Fenton reaction and peroxidase enzymes in individual treatment. Approximately 31% of the CO2 released under aerobic soil incubation was found to be abiotic (sterilized), while 69% was biotic (non-sterilized soils), and respective values of 17% and 83% were recorded under anaerobic conditions. The relative fluorescence intensity clearly shows ·OH radicals production from Fenton reactions. In conclusion, levels of MnP and LiP coupled with Fenton reactions strongly increase SOC mineralization under long periods of O2 limitation in temperate rainforest soils.

Temperature sensitivity of decomposition of soil organic matter fractions increases with their turnover time

Abstract: Soil organic carbon (SOC) is an indicator of soil fertility. Global warming accelerates SOC decomposition, consequently, resulting in land degradation. Characterization of the response of SOC decomposition to temperature is important for predicting land development. A simulation model based on temperature sensitivity (Q₁₀) of SOC decomposition has been used to predict SOC response to climate warming. However, uncertain Q₁₀ leads to substantial uncertainties in the predictions. A major uncertainty comes from the interference of rainfall. To minimize this interference, we sampled surface (0–5 cm) soils along an isohyet across a temperature gradient in the Qinghai–Tibetan Plateau. The Q₁₀ of bulk soil and the four soil fractions, such as light fraction (LightF), particulate organic matter (POM), hydrolyzable fraction (HydrolysF), and recalcitrant fraction (RecalcitF), were studied by ¹⁴C dating. Turnover time follows the order: LightF < POM < bulk soil < HydrolysF < RecalcitF. The Q₁₀ follows the order: LightF (1.0) = POM (1.0) < HydrolysF (3.63) < bulk soil (5.93) < RecalcitF (7.46). This indicates that stable fractions are much more sensitive to temperature than labile fractions. We also suggest that protection mechanisms rather than molecular composition regulate SOC turnover. A new concept 'protection sensitivity' of SOC decomposition was proposed. Protection sensitivity relates to protection type and primarily controls Q₁₀ variation. A simulation model based on the Q₁₀ of individual fractions predicted SOC change and land development in the Qinghai–Tibetan Plateau in the next 100 years much effectively as compared to simulations based on one‐pool model (Q₁₀ = 2) or bulk soil (Q₁₀ = 5.93).