Soil Microbial Biomass Content Research Articles

Mixing of pine and arbuscular mycorrhizal tree species changed soil organic carbon storage by affecting soil microbial characteristics

Pure and mixed pine forests are found all over the world. The mycorrhizal type affects soil microbial activity and carbon sequestration capacity in pure forests. However, the effects of mycorrhizal type on microbial characteristics and carbon sequestration capacity in pine mixed forests remain untested. Further, making it difficult to predict carbon storage of the conversion from pure pine forests to mixed forests at larger scales. Herein, a meta-analysis showed that the contents of soil microbial biomass, mineral-associated organic carbon, and soil organic carbon in pine mixed forests with introduced arbuscular mycorrhizal tree species (PMAM) increased by 26.41%, 58.55%, and 27.41%, respectively, compared to pure pine forests, whereas those of pine mixed forests without arbuscular mycorrhizal tree species (PMEcM) remained unchanged. Furthermore, the effect size of microbial biomass, mineral-associated organic carbon and organic carbon contents in subsoil of PMAM are 56.48%, 78.49% and 43.05%, respectively, which are higher than those in topsoil. The improvement of carbon sinks throughout the PMAM soil profile is positively correlated with increases in microbial biomass and mineral-associated organic carbon in subsoil, according to regression analysis and structural equation modelling. In summary, these results highlight that the positive effects of introducing arbuscular mycorrhizal tree species rather than ectomycorrhizal tree species into pure pine forests on soil microbial biomass and carbon sequestration. The positive link between microbial biomass, mineral-associated organic carbon, and soil organic carbon suggests an underlying mechanism for how soil microorganisms store carbon in pine mixed forests. Nevertheless, our findings also imply that the soil carbon pool of PMAM may be vulnerable under climate change. Based on the above findings, we propose that incorporating mycorrhizal type of tree species and soil thickness into mixed forests management and biodiversity conservation.

Tillage with Crop Residue Returning Management Increases Soil Microbial Biomass Turnover in the Double-Cropping Rice Fields of Southern China

The variety of soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN) content, and the flux turnover rate of SMBC and SMBN for 0–10 cm and 10–20 cm layers in a paddy field in southern China with different tillage practices were studied. The tillage experiment included conventional tillage and crop residue returning (CT), rotary tillage and crop residue returning (RT), no–tillage and crop residue returning (NT), and rotary tillage with all crop residues removed from the paddy field as a control (RTO). The result showed that the SMBC and SMBN contents at 0–10 cm and 10–20 cm layers in the paddy field with CT, RT, and NT treatments were significantly increased. This result indicates that the flux turnover rate of SMBC and SMBN for 0–10 cm and 10–20 cm layers in the paddy field with CT treatment were increased by 65.49%, 39.61%, and 114.91%, 119.35%, compared with the RTO treatment, respectively. SMBC and SMBN contents and the flux turnover rate of SMBC and SMBN for the 0–10 cm layer were higher than that of the 10–20 cm layer in paddy fields under the same tillage condition. Therefore, applying rotary tillage or conventional tillage and crop residue returning produced beneficial management for increasing soil microbial biomass content and its turnover under a double–cropping rice system in southern China.

Metagenomic insights into inhibition of soil microbial carbon metabolism by phosphorus limitation during vegetation succession.

There is growing awareness of the need for regenerative practices in the fight against biodiversity loss and climate change. Yet, we lack a mechanistic understanding of how microbial community composition and functioning are likely to change alongside transition from high-density tillage to large-scale vegetation restoration. Here, we investigated the functional dynamics of microbial communities following a complete vegetation successional chronosequence in a subtropical zone, Southwestern China, using shotgun metagenomics approaches. The contents of total soil phosphorus (P), available P, litter P, and microbial biomass P decreased significantly during vegetation succession, indicating that P is the most critical limiting nutrient. The abundance of genes related to P-uptake and transport, inorganic P-solubilization, organic P-mineralization, and P-starvation response regulation significantly increased with successional time, indicating an increased microbial "mining" for P under P limitation. Multi-analysis demonstrated microbial P limitation strongly inhibits carbon (C) catabolism potential, resulting in a significant decrease in carbohydrate-active enzyme family gene abundances. Nevertheless, over successional time, microorganisms increased investment in genes involved in degradation-resistant compounds (lignin and its aromatic compounds) to acquire P resources in the litter. Our study provides functional gene-level insights into how P limitation during vegetation succession in subtropical regions inhibits soil microbial C metabolic processes, thereby advancing our understanding of belowground C cycling and microbial metabolic feedback during forest restoration.

Soil microbial community structure, function and network along a mangrove forest restoration chronosequence

Mangrove forests have high ecological, social and economic values, but due to environmental changes and human activities, natural mangrove forests have experienced serious degradations and reductions in distribution area worldwide. In the coastal zones of southern China, an introduced mangrove species, Sonneratia apetala, has been extensively used for mangrove restoration because of its rapid growth and strong environmental adaptability. However, little is known about how soil microorganisms vary with the restoration stages of the afforested mangrove forests. Here, we examined the changes in soil physicochemical properties and microbial biomass, community structure and function, and network in three afforested S. apetala forests with restoration time of 7, 12, and 18years and compared them with a bare flat and a 60-year-old natural Kandelia obovata forest in a mangrove nature reserve. Our results showed that the contents of soil salinity, organic carbon, total nitrogen, ammonium nitrogen, and microbial biomass increased, while soil pH and bacterial alpha diversity decreased with afforestation age. Soil microbial community structure was significantly affected by soil salinity, organic carbon, pH, total nitrogen, ammonium nitrogen, available phosphorus, and available kalium, and susceptibility to environmental factors was more pronounced in bacterial than fungal community structure. The relative abundances of aerobic chemoheterotrophy were significantly higher in 12- and 18-year-old S. apetala than in K. obovata forest, while that of sulfate-reducing bacteria showed a decreasing trend with afforestation age. The abundance of dung saprotroph was significantly higher in 12- and 18-year-old S. apetala forests than in the natural forest. With the increasing afforestation age, the modularity of microbial networks increased, while stability and robustness decreased. Our results suggest that planting S. apetala contributes to improving soil fertility and microbial biomass but may make soil microbial networks more vulnerable.

Effects of straw and plastic film mulching on microbial functional genes involved in soil nitrogen cycling.

Microorganisms regulate soil nitrogen (N) cycling in cropping systems. However, how soil microbial functional genes involved in soil N cycling respond to mulching practices is not well known. We collected soil samples from a spring maize field mulched with crop straw (SM) and plastic film (FM) for 10-year and with no mulching (CK) in the Loess Plateau. Microbial functional genes involved in soil N cycling were quantified using metagenomic sequencing. We collected soil samples from a spring maize field mulched with crop straw (SM) and plastic film (FM) for 10-year and with no mulching (CK) in the Loess Plateau. Microbial functional genes involved in soil N cycling were quantified using metagenomic sequencing. Compared to that in CK, the total abundance of genes involved in soil N cycling increased in SM but had no significant changes in FM. Specifically, SM increased the abundances of functional genes that involved in dissimilatory nitrate reduction to ammonium (nirB, napA, and nrfA), while FM decreased the abundances of functional genes that involved in ammonification (ureC and ureA) in comparison with CK. Other genes involved in assimilatory nitrate reduction, denitrification, and ammonia assimilation, however, were not significantly changed with mulching practices. The nirB and napA were derived from Proteobacteria (mainly Sorangium), and the ureC was derived from Actinobacteria (mainly Streptomyces). Mental test showed that the abundance of functional genes that involved in dissimilatory nitrate reduction was positively correlated with the contents of soil microbial biomass N, potential N mineralization, particulate organic N, and C fractions, while ammonification related gene abundance was positively correlated with soil pH, microbial biomass C and N, and mineral N contents. Overall, this study showed that SM could improve soil N availability and promote the soil N cycling by increasing the abundance of functional genes that involved in DNRA, while FM reduced the abundance of functional genes that involved in ammonification and inhibited soil N cycling.

Dynamic changes of soil microorganisms in rotation farmland at the western foot of the Greater Khingan range.

Crop rotation and other tillage systems can affect soil microbial communities and functions. Few studies have reported the response of soil spatial microbial communities to rotation under drought stress. Therefore, the purpose of our study was to explore the dynamic changes of the soil space microbial community under different drought stress-rotation patterns. In this study, two water treatments were set up, control W1 (mass water content 25%-28%), and drought W2 (mass water content 9%-12%). Four crop rotation patterns were set in each water content, spring wheat continuous (R1), spring wheat-potato (R2), spring wheat-potato-rape (R3) and spring wheat-rape (R4), for a total of eight treatments (W1R1, W1R2, W1R3, W1R4, W2R1, W2R2, W2R3, W2R4). Endosphere, rhizosphere and bulk soil of spring wheat in each treatment were collected, and root space microbial community data were generated. The soil microbial community changed under different treatments and their relationship with soil factors were analyzed using a co-occurrence network, mantel test, and other methods. The results revealed that the alpha diversity of microorganisms in the rhizosphere and bulk soil did not differ significantly, but it was significantly greater than in the endosphere. The bacteria community structure was more stable, fungi alpha-diversity significant changes (p < 0.05), that were more sensitive to the response of various treatments than bacteria. The co-occurrence network between fungal species was stable under rotation patterns (R2, R3, R4), while the community stability was poor under continuous cropping pattern (R1), and interactions were strengthened. Soil organic matter (SOM), microbial biomass carbon (MBC), and pH value were the most important factors dominating the bacteria community structural changed in the endosphere, rhizosphere, and bulk soil. The dominant factor that affected the fungal community structural changed in the endosphere, rhizosphere, and bulk soil was SOM. Therefore, we conclude that soil microbial community changes under the drought stress-rotation patterns are mainly influenced by soil SOM and microbial biomass content.

Soil microbial residue characteristics in Pinus massoniana lamb. Plantations

A large amount of stable soil organic matter (SOM) is derived from microbial necromass, which can be assessed by quantifying amino sugar biomarkers. Pinus massoniana Lamb. Plantations are widely distributed in China and play a vital role in forest carbon sequestration. However, the patterns of soil microbial residue remain poorly understood. In this study, amino sugars were used to characterize patterns of soil microbial residues at three soil depths (0-10, 10-20, and 20-30cm) in P. massoniana plantations of different ages (young, middle-aged, near-mature, mature, and over-mature; denoted as YG, MD, NM, MT, and OM, respectively). In the topsoil (0-10cm), the total nitrogen (TN) content of the OM forest was the highest, whereas the soil organic carbon (SOC) content of the MT forest was the highest. Consistent with changes in SOC and TN, total microbial residue content decreased with increasing soil depth. However, the total microbial residues C to SOC contribution increased considerably with increasing depth, suggesting that more SOC was derived from microbial residues in the subsoil than that from the topsoil. The fungal residue C to SOC contribution was higher than that of bacterial residue C. Total amino sugar content in the topsoil increased with increasing age, and MT and OM had a significantly higher content than that of other forests. At all soil depths, SOC and TN content predominantly determined microbial necromass, whereas soil microbial biomass content predominantly determined microbial necromass in the topsoil; soil pH predominantly determined microbial necromass in the 10-20cm soil layer; and soil pH and Ca2+ content were the primary factors in the soil layer below 20cm. The study provides valuable insights into controls of microbial-derived organic C could be applied in Earth system studies for predicting SOC dynamics in forests.

Effects of no-tillage and different stover mulching amounts on soil microbial community and microbial residue in the Mollisols of China.

To investigate the effects of no-tillage and different amounts of stover mulch on soil microbial community composition and their residues, we set up a field experiment of different amounts of stover mulch under no-tillage on the long-term maize conservation tillage station located in the Mollisols area of Northeast China (built in 2007), including without stover mulch (NT0), 1/3 stover mulch (NT1/3), 2/3 stover mulch (NT2/3) and full stover mulch (NT3/3), and the conservation tillage (plowing without stover mulch, CT) as control. We analyzed phospholipid fatty acid, amino sugar biomarker and soil physicochemical properties at different soil layers (0-5 cm, 5-10 cm, 10-20 cm). The results showed that compared to CT, no-tillage without stover mulch (NT0) did not affect soil organic carbon (SOC), total nitrogen (TN), dissolved organic carbon and nitrogen (DOC, DON), water content, microbial community and their residue. The main effects of no-tillage and stover mulch were found in the topsoil. Specifically, the NT1/3, NT2/3 and NT3/3 significantly increased SOC content by 27.2%, 34.1% and 35.6%, respectively, phospholipid fatty acid content was significantly increased under NT2/3 and NT3/3 by 39.2% and 65.0%, respectively, and NT3/3 significantly increased the content of microbial residue-amino sugar by 47.2% in the depth of 0-5 cm compared with CT. The variations in soil properties and microbial community induced by no-tillage and different amounts stover mulch decreased with soil depth, with almost no difference in the 5-20 cm soil layer. SOC, TN, DOC, DON, and water content were the main factors influencing the composition of the microbial community and the accumulation of microbial residue. Microbial biomass was positively correlated with microbial residue, particularly fungal residue. In conclusion, all stover mulch treatments promoted SOC accumulation to different degrees. When there is sufficient stover, it is advisable to opt for no-tillage with full stover mulch, as it is most conducive to the increases of soil microbial biomass, microbial residue and SOC. In case when the amount of stover is inadequate, however, no-tillage with 2/3 stover mulch can still improve soil microbial biomass and SOC content. This study would provide practical guidance for stover management in conservation tillage and sustainable agricultural development in the Mollisols area of Northeast China.

Distribution Characteristics and Influencing Factors of Soil Biological Indicators in Typical Farmland Soils

Soil biodiversity drives soil-based ecosystem services and is an important indicator of soil health. To understand the responses of important soil biological indicators to different farmland use contexts, 72 fields in three agricultural regions of China were used as research objects. The distribution characteristics and the factors influencing six indicators (carbon and nitrogen contents of soil microbial biomass (MBC, MBN, respectively), soil respiration (SR), soil catalase activity (CAT), soil acid phosphomonoesterase activity (APA), and soil earthworms) were investigated using field monitoring and indoor analysis. The MBC, SR, CAT, and APA indicators showed significant differences among the regions (p &lt; 0.05). Correlation and redundancy analyses indicated that the important factors affecting MBC, MBN, and soil respiration were cation exchange capacity, total N, organic matter, hydrolytic N, and soil bulk density, whereas the important factors affecting APA and earthworms were total phosphorus, available phosphorus, and available potassium. None of these factors had a significant effect on CAT. Climatic conditions, soil types, and farmland practices all have complex impacts on soil biodiversity. The results showed that attention should be paid to improving the physical conditions of the soil and to increasing soil fertility levels when establishing sustainable farm management patterns.

Secondary vegetation succession on the Loess Plateau altered the interaction between arbuscular mycorrhizal fungi and nitrogen-fixing bacteria

Secondary vegetation succession on the Loess Plateau altered the interaction between arbuscular mycorrhizal fungi and nitrogen-fixing bacteria

Effects of Different Long-Term Fertilizer Management Systems on Soil Microbial Biomass Turnover in a Double-Cropping Rice Field in Southern China

Soil microbial biomass content is usually regarded as an early indicator of changes in soil quality and soil fertility in paddy fields. Soil microbial biomass turnover is mainly influenced by the application of different fertilizer management systems. However, there is still a need to further investigate the effects of different long-term fertilizer management systems on soil microbial biomass turnover in paddy fields under the double-cropping rice (Oryza sativa L.) system. Therefore, the effects of different long-term (36 years) fertilizer practices on soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) contents, and the flux turnover rates of SMBC and SMBN at the 0–10 cm and 10–20 cm layers in a double-cropping rice field in southern China were investigated in the present paper. The field experiment included four different fertilizer treatments: MF, RF, OM, and CK. The results showed that SMBC and SMBN contents at the 0–10 cm and 10–20 cm soil layers with RF and OM treatments were increased compared with the MF and CK treatments. Compared with the CK treatment, SMBC contents at the 0–10 cm and 10–20 cm soil layers with RF and OM treatments increased by 35.72% and 50.28%, and 32.29% and 42.77%, respectively. SMBN contents at the 0–10 cm and 10–20 cm soil layers with RF and OM treatments increased by 15.52% and 22.70%, and 16.32% and 21.49%, respectively. The fluxes of SMBC and SMBN at the 0–10 cm and 10–20 cm soil layers with RF and OM treatments were significantly higher than those of the CK treatment. This result indicated that the flux turnover rates of SMBC and SMBN at the 0–10 cm and 10–20 cm soil layers with the MF, RF, and OM treatments were significantly higher than those of the CK treatment. Compared with the CK treatment, the flux turnover rates of SMBC and SMBN at the 0–10 cm and 10–20 cm soil layers with OM treatment increased by 46.10% and 48.59%, and 73.39% and 116.67%, respectively. SMBC and SMBN contents, and the flux turnover rates of SMBC and SMBN at the 0–10 cm layer were higher than those of the 10–20 cm layer under the same fertilizer treatment condition. Early rice and later rice yields with RF and OM treatments were significantly higher than those of the MF and CK treatments. As a result, the combined application of crop residue and organic manure with inorganic fertilizer management is a beneficial practice for increasing soil nutrients and rice yield under the double-cropping rice system in southern Chin.

Fertilizer Potential of Organic-Based Soil Amendments on cv. Sangiovese (V. vinifera L.) Vines: Preliminary Results

The intensification of highly specialized viticulture has led to a dramatic decrease of soil fertility that can be restored by increasing soil organic matter using organic fertilizers. The aim of the present experiment was to evaluate the effect of different organic amendments on vine vegetative growth and nutritional status, soil N availability and microbial biomass, as well as on yield and grape quality. The experiment was carried out in 2020 and 2021, on cv. Sangiovese (Vitis vinifera L.) vines grafted on 110 Richter (V. berlandieri × V. rupestris) planted in February 2019. Plants were fertilized yearly in spring with (1) mineral fertilization (MIN), (2) municipal organic waste compost (MOW), and (3) sewage sludge compost (SS). The application of SS increased nitrate availability in both years, while the supply of organic matter (no matter the source) enhanced soil microbial biomass content. Plant nutritional status was in the optimal range for all treatments, with an increase of N in SS and K in MOW. Fruit yield in 2020 was not influenced by treatments, while in 2021 it was enhanced by MIN and MOW, which also induced a higher berry quality. Plant vegetative growth was stimulated by the application of SS. In conclusion, from these preliminary results we observed a higher N availability as a consequence of SS supply that resulted in a higher plant biomass, but reduced yield and berry quality, supporting the theory that for vineyards, N should be carefully managed to reach an equilibrium between vegetative and reproductive activity.

Abandoned pastures and restored savannas have distinct patterns of plant–soil feedback and nutrient cycling compared with native Brazilian savannas

Abstract Around 40% of the original Brazilian savanna territory is occupied by pastures dominated by fast‐growing exotic C4 grasses, which impact ecosystem nutrient cycling. The restoration of these areas depends on the re‐establishment of soil processes. We assessed how restoration of abandoned pastures through direct seeding of native species and land‐management practices (burning and ploughing) affect soil nutrient cycling dynamics compared with native savannas. We compared the activity of soil enzymes related to carbon, nitrogen (N) and phosphorus (P) cycling as well as soil microbial biomass and soil chemical properties (pH and the concentration of N, P, potassium [K] and soil organic matter) among abandoned pastures, native savanna and restored areas. Abandoned pastures had faster nutrient turnover than native savanna, dominated by slow‐growing native species. This pattern was evident from the overall higher biomass‐specific enzyme activities in abandoned pastures than in native savanna. Compared with native savanna, restored areas had similar levels of soil enzyme activities, but lower microbial biomass and soil organic matter. The low enzyme activity in restored areas was likely related to a reduced soil organic carbon concentration due to practices such as burning and ploughing, rather than the restoration of plant–soil feedback. The lower immobilization of nutrients in microbial biomass and lower retention of nutrients in restored areas, compared with native savanna, is expected to favour the re‐establishment of fast‐growing exotic species. Synthesis and application. Despite reducing the resprouting and germination of exotic grasses and improving the establishment of native grasses in the short term, restoration practices have major impacts on the soil microbial community and soil fertility. The reduction of soil microbial biomass and organic matter content reduces the immobilization of soil nutrients and is expected to favour a fast nutrient turnover in the ecosystem. This may result in the re‐establishment of exotic grasses in the long term. Future efforts should focus on the recovery of soil organic matter content and the establishment of soil microbial communities similar to native ecosystems after the application of land‐management practices. Therefore, the restoration of abandoned pastures should consider a greater focus on restoring soil carbon and nutrient cycling

Effects of organic matter addition on the stoichiometric homeostasis of soil microbes in Pinus tabuliformis forest in Taiyue Mountain, China.

In Taiyue Mountain of Shanxi Province, five types of organic matter, i.e., biochar, maize straw, leaves of Quercus mongolica and Pinus tabuliformis, and sawdust of wood stem, were separately added to the soils of a P. tabuliformis forest. Nutrient content, enzyme activity, and microbial biomass were analyzed to elucidate the characteristics of soil ecoenzymatic stoichiometry and the element homeostasis of soil microorganisms. The results showed that the addition of woody sawdust significantly increased soil nitrogen and phosphorus content by 17.1% and 37.6%, and enhanced carbon, nitrogen, and phosphorus contents of soil microbial biomass by 118.0%, 41.0%, and 176.6%, respectively. The activities of carbon-, nitrogen- and phosphorus-targeting enzymes (i.e., β-1, 4-glucosaminosidase, β-1,4-N-acetylglucosaminosidase, leucine aminopeptidase and acid phosphatase) generally increased with the C:N of the added organic matter (biochar<Q. mongolica leaf< P. tabuliformis leaf < maize straw < woody sawdust). This indicated that ecoenzymatic stoichiometry was controlled by soil nutrient content and micro-bial biomass. The limitation of soil microbial growth by phosphorus was not alleviated after the addition of different organic matters, as indicated by the results of enzymatic ratio and its vector value. Soil microbial biomass carbon and nitrogen contents and their stoichiometry (C:N, C:P, N:P) were homeostatic, whereas the microbial biomass phosphorus content fluctuated slightly. The stabilities of microbial element contents and their proportion were mainly ascribed to the allocation of extracellular enzyme production. The susceptibility of soil microbial biomass phosphorus to the addition of organic matter indicated phosphorus limitation of microbial growth.

Investigation of the effects of the conversion of forests and rangeland to cropland on fertility and soil functions in mountainous semi-arid landscape

Soil fertility and (micro)biological activities are prominent indicators of soil ecological functions interrelated with nutrient cycles and stocks. The responses of soil functions and belowground biota to degradation of natural ecosystems and land use changes have received little attention, especially in semi-arid ecosystems of mountainous areas that are sensitive and fragile habitats. Soils were sampled from two natural vegetation types: forest (Carpinus orientalis - Zelkova carpinifolia) and rangeland (Berberis integerrima - Crataegus melanocarpa), and two cultivated croplands (under Triticum aestivum), located in a mountainous semi-arid region of northern Iran, that were converted from the forests and rangelands thirty years ago. Soil properties, C and N stocks and fractions, and related microbial activities were analysed in 0–10 and 10–20 cm. C stocks in the upper 10 cm soil were affected by land use, while no differences were found in the lower depth. C stocks in the topsoil reached 49, 37, 33 and 29 Mg ha−1 for forest, rangeland, croplands (converted from forest and converted from rangeland), respectively. Soil N stocks under forest (5.2 Mg ha−1 for top 10 cm and 4.8 Mg ha−1 for 10–20 cm depths) were higher compared to the other land use types. Particulate and dissolved organic matters under croplands were almost half of that under forest and rangeland. High soil respiration, microbial biomass, ammonium and nitrate content, as well as N mineralization in 0–10 cm under forest were 2 times larger than in 10–20 cm. Concluding, conversion of forest and rangeland to croplands (converted from forest > converted from rangeland) has decreased the functions of soil due to removal of tree cover, decreasing litter layer, organic matter input and soil moisture and also increase of soil temperature. Consequently, more care should be taken by preserving natural vegetation in mountainous semi-arid ecosystems, and higher organic inputs are necessary to maintain microbial properties and functions.

Differences in organic matter properties and microbial activity between bulk and rhizosphere soil from the top- and subsoils of three forest stands

• More labile SOC in the subsoil compared to the topsoil at two sampling sites. • Higher F 14 C values in the rhizosphere indicating a younger apparent 14 C age. • More abundant and diverse community in the rhizosphere compared to the bulk soil. • Higher activity of extracellular enzymes in rhizosphere compared to the bulk soil. The distribution of soil organic carbon (SOC) in subsoil is much more heterogeneous than in topsoils. This is largely caused by more localized inputs of fresh substrates and nutrients that mainly derive from rhizodeposition creating hotspots of microbial activity. To elucidate the role of rhizosphere soil as an important input path for substrates into deeper soil layers, we investigated differences of organic matter distribution and microbial degradation in the rhizosphere soil and the surrounding bulk topsoil as well as in the subsoil. In addition, we considered the effect of different parent materials on SOC dynamics by analyzing soils developed in pleistocene sand, triassic sandstone and loess all covered with similarly aged beech forest within the same climatic region. Besides basic soil parameters, basal respiration, microbial biomass (C mic ) and 14 C content were determined. Further, extracellular enzymes involved in C-, N- and P-cycling were analyzed. Differences in the microbial community were investigated using a taxon-specific real time qPCR approach. The results revealed significant differences of the microbial community composition between bulk and rhizosphere in top- and subsoil at the investigated sample sites. The mean SOC contents were 322% and 475% higher in the rhizosphere soil in both the top- and subsoil, respectively. The microbial biomass (C mic ) in the rhizosphere was up to 80% higher in the topsoil and on average 200% higher in the subsoil (SOC normalized), compared to the respective bulk soil. The qPCR results for all samples showed a higher diversity and abundance in the rhizosphere compared to the respective bulk samples and a shift in community composition with increasing soil depth. Total activity of the analyzed extracellular enzymes involved in different nutrient cycles differed significantly between bulk and rhizosphere soil and revealed that the microbial community in the rhizosphere subsoil can effectively utilize substrates even under scarce availability. The SOC normalized basal respiration at the Pleistocene sand and Loess sites and K 2 SO 4 -extractable C were higher in the subsoil compared to the topsoil showing more labile C in deeper soil layers with younger 14 C in the rhizosphere. These results show that the rhizosphere at the investigated forest sites is a hotspot for microbial activity, which is more pronounced in the subsoil where the relevance of these hotspots is important for C turnover.

Plant Nitrogen Uptake From Insect Frass Is Affected by the Nitrification Rate as Revealed by Urease and Nitrification Inhibitors

Insect protein production is considered a sustainable alternative to livestock protein which furthermore utilizes waste streams. Its production can have positive but also potentially negative environmental effects, which require evaluation. Frass, the byproduct of insect production, is regarded an efficient organic fertilizer or soil amendment. However, several studies report negative frass effects on plant growth and nitrogen (N) cycling. Therefore, a pot trial was carried out which sought to understand N release from frass and subsequent growth and nutrient uptake of Italian ryegrass. Mealworm frass (MWF) or buffalo worm frass (BFW) was applied at two rates (1.5 and 3% w/w) to a soil-sand mix. To evaluate N release processes, frass was applied alone, with a nitrification inhibitor (NI), a urease inhibitor (UI), or both (NI+UI). Plant N, nutrient uptake and soil inorganic N were measured at the experiment's end. To gauge whether altered N fluxes induced changes in the microbial community, soil microbial biomass, bacterial/archaeal abundances and ergosterol content as a fungal biomarker, were determined. Both frass types and application rates stimulated microbial growth and N mineralization. The 3% rate inhibited seed germination, possibly due to salinity or ammonia toxicity. At the 1.5% rate, both frass types were effective fertilizers. MWF led to higher biomass and nutrient uptake, owing to its higher extractable nutrient concentrations. The 3% rate caused nitrite accumulation in the absence of NI. NI improved plant biomass, nutrient uptake, stimulated archaeal and bacterial abundances and prevented nitrite accumulation. UI reduced N mineralization, showing that a substantial fraction of frass organic N is ureic. UI enhanced fungal contribution to the microbial biomass, revealing the importance of bacteria in frass N mineralization processes when UI is not applied. NI and UI combined, induced greater N release from frass than UI or NI alone. Our study demonstrated the usefulness of NI and UI in studying N release from frass. NI can improve plant N uptake and minimize N losses following frass application, reducing its potentially negative effects. UI can retard N release from frass, allowing its application as a slow-release fertilizer, but should not be used concurrently with NI.

Imbalanced nitrogen–phosphorus input alters soil organic carbon storage and mineralisation in a salt marsh

• Effects of imbalanced N–P input on SOC fractions and mineralisation were tested. • N-P input ratio crossing 15:1 may trigger the loss of SOC in a salt marsh. • Low and medium N–P input ratio increased labile SOC fractions. • Imbalanced N–P input increased the bacterial taxa with high-N adaptability. • N–P input ratio affected SOC mineralisation by altered labile C and microbial diversity. A large imbalance in soil nitrogen (N) and phosphorus (P) inputs induced by anthropogenic activities is anticipated to profoundly influence soil carbon (C) budgets in salt marshes. In this study, we hypothesized that imbalances in the nitrogen–phosphorus (N–P) input would result in the nonlinear response of soil organic carbon (SOC) content, fractions and mineralization to the N–P input ratio. We applied three N–P input ratios (low (5:1), medium (15:1), high (45:1)) in a salt marsh of the Yellow River Delta (YRD) for four years (in which N added increased from 8.67 to 26.01 g N m −2 y −1 and P added decreased from 1.73 to 0.58 g P m −2 y −1 ) and quantified their impacts on SOC fractions and SOC mineralisation. The control treatment did not receive fertilization. The results showed that the N and P input led to overall increases in the availability of soil nutrients (i.e., inorganic N (IN) and available P (AP)), stimulation of plant biomass and changes of microbial community structure (i.e., γ- and δ-Proteobacteria and Acidobacteria ). N and P input increased soil dissolved organic carbon (DOC) and decreased aromatic DOC components through improving N availability and stimulating plant growth. Notably, though, there may be a threshold N–P input ratio between 15:1 and 45:1 that, once crossed, triggers the loss of SOC. Appropriate increase in N availability induced by low and medium N-P input ratios would stimulate the SOC mineralization. However, excessive N-P input ratio would reduce SOC mineralization. Path analysis indicated that N–P input ratios dominantly regulate SOC mineralisation by changing soil DOC and microbial biomass (MBC)contents and microbial community structure. Thus, we speculate that the continuous increase in N input causes a growing N–P imbalance that reduces SOC stocks, despite a reduction in SOC mineralisation.

The Influence of Forest Management and Changed Hydrology on Soil Biochemical Properties in a Central-European Floodplain Forest

Anthropogenic modifications to water regimes are one of the main factors threatening the stability and existence of floodplain forests. This study presents an analysis of topsoil biogeochemistry within three floodplain forest stands with different levels of human alteration. Decreasing contents of soil organic carbon (OC) and microbial biomass were observed along the gradient from natural to plantation forest. High annual variations were observed in soil N contents and in microbial biomass, while comparable spatial variations were observed within the natural forest. High ground-water levels resulted in increased accumulation of available Na+ and SO42− in the natural forest soil, yet the concentrations of ions were at sub-saline levels. The increasing contents of available Mn, SO42− or Cl− had mostly positive effects on soil microbial activity across the sites, though the results indicate the existence of a certain ecological limit for soil microorganisms. Reintroduction of surface-water flooding should be considered in future forest and water management to promote the dilution of ions accumulated in soils and natural deposition of sediments rich in organic matter (OM) at the sites.

川西亚高山天然次生林不同演替阶段土壤-微生物生物量及其化学计量特征

开展不同恢复演替阶段天然次生林土壤-微生物生物量及其化学计量特征关系的研究，可为有效和持续管理川西亚高山次生林提供科学依据。以川西亚高山米亚罗林区20世纪60、70、80年代3种采伐迹地经自然恢复演替形成的次生林（SF60、SF70和SF80）和岷江冷杉（Abies faxoniana）原始林（PF）为研究对象，探讨了表层（0-20 cm）土壤有机碳（C_soil）、全氮（N_soil）、全磷（P_soil）含量及微生物生物量碳（C_mic）、氮（N_mic）、磷（P_mic）含量随自然恢复演替的变化特征，分析了它们的化学计量比与微生物熵（qMB）之间的相互关系。结果表明：（1）随着恢复演替年限的增加，C_soil和N_mic含量显著降低，N_soil和P_soil及C_mic和P_mic含量呈现先升后降的显著变化趋势，且3种次生林的表层土壤碳、氮、磷及其微生物生物量的含量均低于PF。（2）次生林恢复年限对土壤微生物熵C（qMBC）和P（qMBP）没有显著影响，但对土壤微生物熵N（qMBN）存在显著影响。（3）土壤-微生物化学计量不平衡性C_imb：N_imb随自然恢复演替进程呈先降后升的显著变化趋势，C_imb：P_imb呈不显著的降低趋势，N_imb：P_imb呈现显著降低趋势。冗余分析显示，N_imb：P_imb和C_mic：N_mic是影响qMB变化的主导因子，其中N_imb：P_imb解释了qMB变化的62.6%，说明土壤氮磷及其活性组分（N_mic和P_mic）含量变化可能会影响到qMB变化。综上可知，次生林近60 年的自然恢复演替引起了土壤碳氮磷含量的显著变化；天然次生林土壤-微生物生物量碳氮磷化学计量比主要受到氮磷的协同影响，且SF60土壤质量状况较差，为此，对SF60林分可适当增加氮素供给以促进其林木生长，进而提升土壤质量。;The change characteristics of soil and soil microbial biomass contents of the natural secondary forests could significantly affect the soil organic carbon (C_soil), total nitrogen (N_soil), total phosphorus (P_soil), soil microbial biomass carbon (C_mic), microbial biomass nitrogen (N_mic), microbial biomass phosphorus (P_mic) contents, and their stoichiometry. To gain insights into the change characteristics of soil and soil microbial biomass contents and their stoichiometry along different successional stages of natural secondary forest, four forest types were selected in Miyaluo forest area of western Sichuan, China, which compose of three natural secondary forests (SF60, SF70 and SF80 from clear-logging area during the 1960s, 1970s and 1980s, respectively) and one Abies faxoniana primary forest (PF, 160 years old). C_soil, N_soil and P_soil, C_mic, N_mic and P_mic in 0-20 cm soil were measured. The results showed that contents of C_soil and N_mic marginally significantly decreased, whereas the contents of N_soil and P_soil, C_mic and P_mic increased initially but decreased over time late in the course of restorative succession of natural secondary forests. The contents of C_soil, N_soil, P_soil, C_mic, N_mic, and P_mic in the three natural secondary forests were lower than those in the PF, while the contents of C_soil, N_soil and soil microbial biomass had no significant changes between SF80 and SF70. The content of P_soil was significantly higher in SF70 than that in SF80. The soil C:N:P stoichiometry ratio was dominated by the synergy effect interaction of N_soil and P_soil contents. There was no obvious difference among forests of different successional stages for soil microbial nitrogen carbon quotient (qMBC) and phosphorus quotient (qMBP), but did exist for soil microbial nitrogen quotient (qMBN). qMBN was significantly greater in SF60 than that in the other two successional stages and PF. C_imb:N_imb ratio of the stoichiometric imbalances showed first decreased and then increased; C_imb:P_imb and N_imb:P_imb ratio of the stoichiometric imbalances showed a slightly and significantly gradual reduction, respectively. Redundancy analysis showed that soil microbial quotient (qMB) was significantly correlated with N_imb:P_imb and C_mic:N_mic ratio. N_imb:P_imb ratio revealed 62.6% of the variations in qMB, which implied that changes of qMB values were effected by the contents of N_soil, P_soil and their active components (N_mic and P_mic). Furthermore, our results revealed that nitrogen and phosphorus contribution could be an optimal strategy to meet the microbial stoichiometric demands and enhance nutrient availability for natural secondary forest at the early-successional stages (<60 a) in high-altitude sub-alpine forest ecosystems.