Glomalin-related Soil Protein Content Research Articles

Microbial necromass and glycoproteins for determining soil carbon formation under arbuscular mycorrhiza symbiosis

Arbuscular mycorrhizal fungi (AMF) form symbioses with most terrestrial plants and critically modulate soil organic carbon (C) dynamics. Whether AMF promote soil C storage and stability is, however, largely unknown. Since microbial necromass C (MNC) and glomalin-related soil protein (GRSP) are stable microbial-derived C in soils, we therefore evaluated how AMF symbiosis alters both soil C pools and their contributions to soil organic C (SOC) under nitrogen fertilization, based on a 16-weeks mesocosm experiment using a mutant tomato with highly reduced AMF symbiosis. Results showed that SOC content is 4.5 % higher following AMF symbiosis. Additionally, the content of MNC and total GRSP were 47.5 % and 22.3 % higher under AMF symbiosis than at AMF absence, respectively. The accumulations of GRSP and microbial necromass in soil were closely associated with mineral-associated organic C and the abundance of AMF. The increased soil living microbial biomass under AMF symbiosis was mainly derived from AMF biomass, and fungal necromass C significantly contributed to SOC accumulation, as evidenced by the higher fungal:bacterial necromass C ratio under AMF symbiosis. On the contrary, bacterial necromass was degraded to compensate for the increased microbial nutrient demand because of the aggravated nutrient limitation under AMF symbiosis, leading to a decrease in bacterial necromass. Redundancy analysis showing that bacterial necromass was negatively correlated with soil C:N ratio supported this argument. Moreover, the relative change rate of total GRSP was consistently greater in nitrogen-limited soil than that of microbial necromass. Our findings suggested GRSP accumulates faster and contributes more to SOC pools under AMF symbiosis than microbial necromass. The positive correlation between the contributions of GRSP and MNC to SOC further provided valuable information in terms of enhancing our understanding of mechanisms underlying the maintenance of SOC stocks through microbial-derived C.

Straw management and fertilization improve soil aggregate stability by inducing biological binding agents and specific keystone genera

Improving soil aggregate stability through proper fertilization management is essential for crop growth. However, how different fertilization regimes affect soil aggregate stability driven by microorganisms and biological binding agents remains largely unexplored. For this purpose, we conducted a 9-year field experiment involving two soil textures (i.e., loam and clay soils) and five fertilization regimes (i.e., no fertilizer or crop straw return (CK), crop straw return (S), chemical fertilization (N), crop straw return with chemical fertilizer (SN), and crop straw return with chemical fertilizer and 20% nitrogen substituted with chicken manure (SNM)). Mean weight diameter (MWD) of soil aggregates, glomalin-related soil protein (GRSP) content, and microbial community composition were investigated. The SN and SNM treatments had similar MWD, which was 70.08–76.45% higher than the CK, and the S and N treatments had similar MWD, which was 20.35–34.76% higher than the CK, indicating better effects of SN and SNM on the improvement of the soil aggregate stability than the S or N treatment. High easily extractable GRSP and total GRSP contents contributed to the improved MWD. Meanwhile, some specific keystone genera identified from the keystone modules (co-occurring groups of soil biota including arbuscular mycorrhizal fungi, bacteria, and nonmycorrhizal fungi) were recognized as the important factors regulating the MWD, though the selected specific genera in the loam soil differed from the clay soil. Altogether, our results highlighted the fundamental role of the GRSP and selected specific keystone microbial taxa in improving the soil aggregate stability and provided a list of microbial taxa linking to MWD in different soils, which could be specifically targeted in specific soil.

Glomalin-Related Soil Protein Plays Different Roles in Soil Organic Carbon Pool Maintaining among Different Grassland Types

Glomalin-related soil protein (GRSP) is an important component of soil organic carbon (SOC), which can promote long-term SOC sequestration. However, GRSP distribution characteristics and its contribution to the SOC pool among different grassland types remain poorly understood. Therefore, six grassland types (alpine meadow, mountain meadow, temperate meadow steppe, temperate steppe, temperate desert steppe, and temperate desert) were chosen to evaluate the contribution of GRSP to the SOC pool and the factors that influence GRSP accumulation in the Irtysh River Basin in China. The results revealed that GRSP (EE-GRSP, T-GRSP) accumulated more in the 0–10 cm soil layer than in the 10–20 cm soil layer (p < 0.05). GRSP content was higher in alpine grasslands (15.69 mg·g−1) than in desert grasslands (5.45 mg·g−1). However, their contribution to the SOC pool exhibited an opposite trend, whereas GRSP-C/SOC even accounted for 11.88% in the desert grasslands. The redundancy analysis (RDA) showed that SOC was the top important positive regulator for GRSP accumulation both in the two layers (explanatory rate > 80%). Besides the SOC factor, the two soil layers had different factors in regulating GRSP accumulation. Changes in GRSP content in the 0–10 cm soil layer were more strongly associated with mean annual temperature (MAT), sand content, soil water content (SWC), and silt content. In contrast, in the 10–20 cm soil layer, GRSP content was more influenced by SWC, electrical conductivity (EC), and pH (p < 0.05). Additionally, the main factor in the GRSP content variation was the interaction between climate and soil in the two soil layers (explanatory rate > 80%). Our findings underscore the critical role of GRSP in facilitating SOC sequestration within desert grasslands and elucidate the primary factors driving GRSP distribution across varying soil depths.

Impact of Silver Nanoparticles on Arbuscular Mycorrhizal Fungi and Glomalin-Related Soil Proteins in the Rhizosphere of Maize Seedlings

Glomalin-related soil protein (GRSP), an important arbuscular mycorrhizal (AM) fungal by-product, plays a key role in preserving or sequestrating soil organic carbon (C). Silver nanoparticles (AgNPs) have become an emerging contaminant and their impacts on soil ecosystems attract increasing concerns. The dynamics of AM fungi and GRSP could therefore form the basis for an in-depth exploration of the influences of AgNPs on soil ecosystems. This study investigated the effects of AgNPs on mycorrhizal growth and AM fungal communities, as well as the GRSP contents in maize (Zea mays L.) soils, with a pot experiment. The contributions of GRSP to soil organic C and the correlations of GRSP with soil organic C were also evaluated. The results indicated that AgNPs decreased the mycorrhizal colonization, AM fungal biomass, and diversity indices, and strongly shifted the community composition of AM fungi with a reduction in Acaulosporaceae and an enrichment in Glomeraceae. Additionally, AgNPs also decreased the soil’s easily extractable (EE) GRSP and total (T) GRSP contents, resulting in lower contributions of EE-GRSP-C and T-GRSP-C to the soil organic C. Linkage analyses revealed that AM fungal abundances have positive correlations with EE- and T-GRSP, and EE- and T-GRSP also positively correlated with soil organic C, indicating that the negative effects of AgNPs on AM fungal abundances and communities were extended to AM-fungal-associated C processes. Altogether, our study found that AgNPs decreased the AM fungal abundances shaped AM fungal communities, and reduced the soil GRSP content, which might subsequently be unfavorable for soil C storage.

Afforestation enhances glomalin-related soil protein content but decreases its contribution to soil organic carbon in a subtropical karst area

Afforestation on degraded croplands has been proposed as an effective measure to promote ecosystem functions including soil organic carbon (SOC) sequestration. Glomalin-related soil protein (GRSP) plays a crucial role in promoting the accumulation and stability of SOC. Nevertheless, mechanisms underlying the effects of afforestation on GRSP accumulation have not been well elucidated. In the present study, 14 pairs of maize fields and plantation forests were selected using a paired-site approach in a karst region of southwest China. By measuring soil GRSP and a variety of soil biotic and abiotic variables, the pattern of and controls on GRSP accumulation in response to afforestation were explored. The average content of total GRSP (T-GRSP) and its contribution to SOC in the maize field were 5.22 ± 0.29 mg g−1 and 42.33 ± 2.25%, and those in the plantation forest were 6.59 ± 0.32 mg g−1 and 25.77 ± 1.17%, respectively. T-GRSP content was increased by 26.4% on average, but its contribution to SOC was decreased by 39.1% following afforestation. T-GRSP content decreased as soil depth increased regardless of afforestation or not. Afforestation increased T-GRSP indirectly via its positive effects on arbuscular mycorrhizal fungi biomass, which was stimulated by afforestation through elevating fine root biomass or increasing the availability of labile C and N. The suppressed contribution of T-GRSP to SOC following afforestation was due to the relatively higher increase in other SOC components than T-GRSP and the significant increase of soil C:N ratio. Our study reveals the mechanisms underlying the effects of afforestation on T-GRSP accumulation, and is conducive to improving the mechanistic understanding of microbial control on SOC sequestration following afforestation.

Long-term N addition reduced the diversity of arbuscular mycorrhizal fungi and understory herbs of a Korean pine plantation in northern China

With the development of agriculture and industry, the increase in nitrogen (N) deposition has caused widespread concern among scientists. Although emission reduction policies have slowed N releases in Europe and North America, the threat to biodiversity cannot be ignored. Arbuscular mycorrhizal (AM) fungi play an important role in the establishment and maintenance of plant communities in forest ecosystems, and both their distribution and diversity have vital ecological functions. Therefore, we analyzed the effects of long-term N addition on AM fungi and understory herbaceous plants in a Korean pine plantation in northern China. The soil properties, community structure, and diversity of AM fungi and understory herbaceous plants were detected at different concentrations of NH4NO3 (0, 20, 40, 80 kg N ha−1 year−1) after 7 years. The results showed that long-term N deposition decreased soil pH, increased soil ammonium content, and caused significant fluctuations in P elements. N deposition improved the stability of soil aggregates by increasing the content of glomalin-related soil protein (GRSP) and changed the AM fungal community composition. The Glomus genus was more adaptable to the acidic soil treated with the highest N concentration. The species of AM fungi, understory herbaceous plants, and the biomass of fine roots were decreased under long-term N deposition. The fine root biomass was reduced by 78.6% in the highest N concentration treatment. In summary, we concluded that long-term N deposition could alter soil pH, the distribution of N, P elements, and the soil aggregate fractions, and reduce AM fungal and understory herb diversity. The importance of AM fungi in maintaining forest ecosystem diversity was verified under long-term N deposition.

Evaluation of the relation between soil biomass of arbuscular mycorrhizal fungi and glomalin-related soil protein in conservation agriculture

Arbuscular mycorrhizal fungi (AMF) are indicators of soil health and are associated with various soil benefits, primarily linked to glomalin accumulation from hyphal turnover. However, the direct connection between glomalin-related soil protein (GRSP) and AMF has been questioned. The study aimed to investigate the correlation between different fractions of GRSP and fatty acid fractions in soil, as well as the impact of conservation agriculture practices on AMF biomass and GRSP content. Findings revealed a positive correlation between easily extractable (EE) GRSP and phospholipid fatty acid (PLFA) 16:1ω5, while no significant correlations were found for difficultly extractable (DE) or total GRSP fractions. These results highlight the complexity of GRSP dynamics and the need for further research on different fractions and their relation to AMF biomass. Additionally, the study demonstrated that mechanical soil management had a greater impact on AMF hyphal biomass and EE-GRSP compared to residue management. Direct seeding, a reduced tillage approach, led to higher hyphal biomass and EE-GRSP, indicating AMF sensitivity to tillage intensity. This suggests that tillage practices exert a stronger influence on AMF abundance and GRSP content than residue management.

Dynamics of glomalin-related soil protein and soil aggregates during secondary succession in the temperate forest

Despite the key role of glomalin-related soil protein (GRSP) in soil organic C (SOC) and soil aggregate stability, the influence of secondary successional processes on GRSP and the interrelationship of GRSP on SOC and its soil aggregate stability is unclear. Based on this, four stages of the secondary succession (including 20-, 32–, 47-, and 61-year-old stands) and the primary broad-leaved Korean pine (Pinus koraiensis) mixed forest were selected. The results showed that total GRSP (T-GRSP), easily extractable GRSP (EE-GRSP), SOC, mean weight diameter (MWD), and geometric mean diameter (GMD) all tended to increase with the secondary succession at 0–30 cm soil layer. The secondary succession contributed to the accumulation of GRSP content, so much so that after 61 years, both EE-GRSP and T-GRSP reached the level of the primary forest. EE-GRSP, T-GRSP, SOC, MWD, GMD, and GRSP/SOC significantly decreased with the deepening of soil depth in all successional stages. In addition, during the 20- and 32 year-old stands of secondary succession, the proportion of GRSP in the SOC was relatively high. With the progress of secondary succession, the content of macroaggregates (>0.25 mm) in the soil increased significantly and was not significantly different from the primary forest. The correlation analysis showed that T-GRSP was more correlated with SOC than EE-GRSP, while EE-GRSP was more correlated with MWD than T-GRSP. Additionally, there was also a significant positive correlation between MWD and SOC at 0–20 cm soil layer. Our results elucidate that GRSP may stabilize SOC pools by direct contribution to SOC in the early secondary succession, and the stabilizing effect on SOC is more pronounced in the later stages by forming macroaggregates. This enables us to understand the ecological functions played by GRSP in soil C sequestration and the enhancement of soil stability during the secondary succession.

Enhancing soil ecological security through phytomanagement of tailings in erosion-prone areas

Revegetation is effective in improving soil quality in ecologically fragile areas. However, little is known about the impact of diverse phytomanagement strategies of tailings on soil quality and ecological security in erosion-prone areas. We investigated the water stability, soil aggregate nutrients, and the risk of heavy metal contamination of abandoned tailings under phytomanagement and in adjacent bare land on the Loess Plateau. The results showed that phytomanagement significantly enhanced soil aggregate stability, as demonstrated by higher contents of soil organic carbon (SOC), glomalin-related soil protein (GRSP), aromatic-C, and alkene-C in macro-aggregates. The pollution load index (PLI) and ecological risk index (RI) of soil heavy metals were lower in shrub/herbaceous mixed forests than in natural grasslands and planted forests. The risk of heavy metal contamination was higher in macro-aggregates (>0.25 mm) than in micro-aggregates (<0.25 mm) and was significantly and positively correlated with the SOC and GRSP contents of the aggregates. Our study demonstrates that soil aggregate quality is closely related to the fate of heavy metals. Diversified tailing revegetation measures can improve soil quality and ensure ecological security.

Glomalin-related soil proteins respond negatively to fertilization and fungicide application in China's arid grassland

Glomalin-related soil protein (GRSP) are a glycoprotein mainly produced by arbuscular mycorrhizal (AM) fungi. GRSP are deposited in the soil after being released from AM fungal hyphae, and they are believed to improve soil health and carbon (C) storage. However, it is unclear how fertilizer and mycorrhiza suppression affect the content of easily extractable (EE-GRSP) and total glomalin-related protein (T-GRSP) in arid grassland soil. We conducted a 3-yr in situ study to determine the main and interactive effects of nitrogen (N), phosphorus (P), and fungicide (benomyl) addition on EE-GRSP and T-GRSP content in the soil of desert steppe in Northwest China. To further explore the mechanisms influencing GRSP content, the plant community, soil fertility and the AM fungal traits were also identified. Following 3 years of fertilizer and fungicide application, P addition had a negative influence on EE-GRSP content (−12.50%), whereas N addition had no significant effect. Fungicide application reduced EE-GRSP (−18.47%) and T-GRSP content (−18.36%) regardless of N and P addition. By altering the extraradical hyphal length of AM fungi, P and fungicide application decreased the EE-GRSP and T-GRSP content without affecting the vegetation or soil characteristics. Overall, this study provides insights into the dynamics of GRSP in response to soil nutrient enrichment and fungicide application in a resource-limited grassland ecosystem.

Long-term manuring facilitates glomalin-related soil proteins accumulation by chemical composition shifts and macro-aggregation formation

Glomalin-related soil proteins (GRSP), derived from arbuscular mycorrhizal fungi (AMF), contributes significantly to soil stability and carbon sequestration. However, the responses of GRSP accumulation and associated AMF community and diversity to long-term fertilization regimes remain unclear. Here, we investigated the dynamics of GRSP contents, AMF biomass and diversity based on a 29-year fertilization experiment (including control, mineral fertilization, manuring and straw returning treatments). Results showed that GRSP contents increased over years across fertilization treatments. Compared with no fertilization, long-term manuring and straw returning significantly increased bulk soil GRSP by 100% and 80%, respectively, and altered the chemical composition of GRSP by increasing the recalcitrant (aromatic) C proportion. The proportion of aromatic C in GRSP was positively correlated with AMF biomass and diversity (Shannon and Chao1), indicating that the chemical composition of GRSP could be regulated by AMF community and diversity. Moreover, manuring facilitated the formation of macro-aggregates (>250 µm), thus increasing the physical protection of GRSP. The structural equation modeling further demonstrated that GRSP content was positively regulated by soil macro-aggregates, AMF biomass and diversity and their linkage with GRSP chemical composition. Collectively, long-term manuring could facilitate GRSP accumulation by shifts in AMF-mediated GRSP chemical composition (aromatic C) along with enhanced protection of macro-aggregates. This study highlights a feasible way forward for soil quality improvement and carbon sequestration for sustainable agriculture.

Nitrogen fertilizer and landscape position affect soil aggregate size distribution, and intra-aggregate carbon and nitrogen under switchgrass in a marginal cropland

AbstractDedicated bioenergy crops such as switchgrass (Panicum virgatum L.) can be grown on marginally productive lands and positively influence soil properties. However, nitrogen management, and landscape can alter soil structural attributes under bioenergy crop production. This study investigated the impacts of long-term nitrogen fertilization (0-N, 0 kg N/ha; 56-N, 56 kg N/ha and 112-N, 112 kg N/ha) and landscape positions (shoulder and footslope) on soil organic carbon (SOC) and structural attributes under switchgrass production. The 112-N rate enhanced the proportion of 2–4 mm water-stable aggregates by 49%, aggregate associated carbon in 2–4 mm and &gt;4 mm aggregates by 16 and 24%, respectively, aggregate associated nitrogen in &gt;4 mm aggregates by 33% and reduced soil bulk density by 19% compared to the 0-N rate. Footslope position increased the proportion of 2–4 mm water-stable aggregates by 26% and lowered bulk density by 8% compared to the shoulder position. Results showed a significant N-rate × landscape position interaction on SOC and glomalin related soil protein content in bulk soil. Overall, this study showed that nitrogen application to switchgrass planted at footslope on a marginally yielding cropland improved soil structure and physical conditions.

Effects of Glomalin-Related Soil Protein Driven by Root on Forest Soil Aggregate Stability and Carbon Sequestration during Urbanization in Nanchang, China.

Glomalin-related soil protein (GRSP) is a hydrophobic protein released by arbuscular mycorrhizal fungi. It is an important component of the soil carbon pool, and it improves the soil aggregate structure; however, it remains unclear whether GRSP can enhance soil carbon sequestration and improve soil quality during rapid urbanization. The built-up area in Nanchang, China was the study area, and the proportion of impervious surface area was the parameter of urbanization intensity. A total of 184 plots (400 m2) were set up to collect soil samples (0-20 cm) for analysis. Aggregates of five particle sizes were sieved, and the percentage amounts of soil organic carbon (SOC) and GRSP for them were determined. The results showed that the easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP) contents of the four aggregates of <2 mm were 22-46% higher in low urbanization areas than those in high urbanization areas (p < 0.05), indicating that the higher urbanization intensity was associated with the lower GRSP content of different aggregates. The GRSP was significantly positively correlated with SOC (p < 0.05). Moreover, the contribution of GRSP to the SOC pool in the <0.25 mm aggregate was significantly higher than that in other aggregates. In addition, the EE-GRSP content was significantly positively correlated with mean weight diameter (MWD) and geometric mean diameter (GMD) in the four aggregates of <2 mm, whereas it was negatively correlated with fractal dimension (D) in the >2 mm, 1-2 mm and <0.053 mm aggregates. The T-GRSP content showed significant correlations only with MWD, GMD, and D in the 1-2 mm aggregate. This study revealed that increasing urbanization intensity can significantly reduce the GRSP content of different sized aggregates. Moreover, the GRSP content significantly promoted SOC sequestration, and the EE-GRSP content more significantly promoted soil aggregate stability than that of the T-GRSP. These findings provide new ideas for exploring the improvement of soil quality during the process of urbanization.

Spatial change in glomalin-related soil protein and its relationships with soil enzyme activities and microbial community structures during urbanization in Nanchang, China

Rapid urbanization brings about drastic changes in land use and soil quality. Glomalin-related soil protein (GRSP), a sensitive indicator of soil quality, is possibly secreted by arbuscular mycorrhizal fungi (AMF). Besides, soil microorganisms and enzymatic activities play an irreplaceable role in improving soil quality and regulating soil nutrient circulation. However, little data are available on the interactions between GRSP, microorganisms, and enzymatic activities during urbanization. Here, we selected a built-up area (505 km2) in Nanchang, China, and collected soil samples from 184 sites from different urbanized areas. Soil microorganisms, soil physicochemical properties, soil enzymatic activities, vegetation characteristics, and land-use configurations were used to decouple the variations in total GRSP (TG) and easily extractable GRSP (EEG) during urbanization. Results showed that TG and EEG contents significantly decreased by 16.2 % and 19.4 % from low to heavy urbanization areas, respectively. Moreover, N-acetylglucosamine glycosidase (NAG), acid phosphatase (AP), and leucine aminopeptidase (LAP) activities, and the biomass of gram-positive bacteria (G+) and actinomycetes were significantly larger in lightly urbanized than in heavily urbanized areas (p < 0.05). EEG and TG positively correlated with β-1,4-glucosidase (BG), NAG, LAP, AP, G+, gram-negative bacteria, actinomycetes, and total soil microbial biomass (p < 0.05). Variance partitioning analysis showed that soil enzymatic activities are pivotal in GRSP production. However, no direct evidence shows that AMF secrete GRSP. Furthermore, structural equation modeling found that urbanization could diminish soil enzymatic activities by reducing vegetation cover and increasing the proportion of impervious surface area, indirectly causing a decrease in GRSP content. In conclusion, urbanization could indirectly reduce GRSP content by affecting land-use configurations and soil enzymatic activities. In the future, we shall consider improving soil quality by increasing GRSP content during urbanization.

Soil glomalin-related protein affects aggregate N2O fluxes by modulating denitrifier communities in a fertilized soil

Agricultural ecosystems contribute significantly to atmospheric emissions of soil nitrous oxide (N2O), which exacerbate environmental pollution and contribute to global warming. Glomalin-related soil protein (GRSP) stabilizes soil aggregates and enhances soil carbon and nitrogen storage in agricultural ecosystems. However, the underlying mechanisms and relative importance of GRSP on N2O fluxes within soil aggregate fraction remain largely unclear. We examined the GRSP content, denitrifying bacterial community composition, and potential N2O fluxes across three aggregate-size fractions (2000–250 μm, 250–53 μm, and <53 μm) under a long-term fertilization agricultural ecosystem, subjected to mineral fertilizer or manure and their combination. Our findings indicated that various fertilization treatments have no discernible impact on the size distribution of soil aggregates, paving the way to further research into the impact of soil aggregates on GRSP content, the denitrifying bacterial community composition, and potential N2O fluxes. GRSP content increased with the increase in soil aggregate size. Potential N2O fluxes (including gross N2O production and N2O reduction and net N2O production) among aggregates were highest in microaggregates (250–53 μm), followed by macroaggregates (2000–250 μm) and lowest in silt + clay (<53 μm) fractions. Potential N2O fluxes had a positive response to soil aggregate GRSP fractions. The non-metric multidimensional scaling analysis revealed that soil aggregate size could drive the denitrifying functional microbial community composition, and deterministic processes play more critical roles than stochasticity processes in driving denitrifying functional composition under soil aggregate fractions. Procrustes analysis revealed a significant correlation between denitrifying microbial community, soil aggregate GRSP fractions, and potential N2O fluxes. Our study suggests that soil aggregate GRSP fractions influence potential nitrous oxide fluxes by affecting denitrifying microbial functional composition within soil aggregate.

Short-term cultivation limiting soil aggregate stability and macronutrient accumulation associated with glomalin-related soil protein in Eucalyptus urophylla × Eucalyptus grandis plantations

The relationship between glomalin-related soil protein (GRSP) and soil aggregation has been a hot topic of research for its close link to soil stability and quality. However, the short-term cultivation of Eucalyptus poses serious threats to soil stability and nutrient stocks, and the effects of GRSP on soil aggregate stability and macronutrient accumulation remain unclear. The aim is to clarify the potential mechanisms affecting soil aggregate stability and macronutrient accumulation in short-term Eucalyptus plantations. Five Eucalyptus urophylla × Eucalyptus grandis plantations with different cultivation periods (1–5 years) in this study were investigated, and a native evergreen broadleaf forest (0 year) was selected as control. The mean weight diameter index increased in the first 3 years and then significantly decreased during 5 years cultivation of Eucalyptus. Soil organic carbon (SOC) and total nitrogen also decreased after planting Eucalyptus for 3 years, but variation in total phosphorus was not obvious. The relative abundance of Glomeraceae and Claroideoglomeraceae decreased in the 5-year-old Eucalyptus plantations and was positively correlated with GRSP content. In pathway modeling, nutrient-acquisition enzyme activities positively affected GRSP and macronutrient content. Total GRSP (T-GRSP) had higher total effects than easily extractable GRSP on soil aggregate stability, and positively correlated with SOC in macroaggregates. Both T-GRSP and SOC had positive and direct effects on soil aggregate stability. Variance partitioning analysis further explained the contribution of GRSP and SOC to aggregate stability, particularly in >2 and 2–0.25 mm macroaggregates. Our results suggested that GRSP was directly associated with SOC content and soil aggregate stability, and was a potential key factor affecting soil aggregate stability in Eucalyptus plantations. Improving T-GRSP and SOC are efficient approaches for preventing the gradual deterioration of soil aggregate stability. Short-term cultivation should be carefully used in Eucalyptus plantations, and a new cultivation period is needed.

Drip Irrigation at a Soil Water Suction of 30 kPa Helps AMF and GRSP to Enhance Greenhouse Macro-Aggregates

Drip irrigation is fundamental in water-saving agricultural greenhouses, especially in tomato (Solanum lycopersicum L.) greenhouses. However, a long-term drip irrigation has been observed to be associated with soil degradation, concerning both soil aggregate structure and soil microbial community. To evaluate how drip irrigation scheduling influences the soil structure and arbuscular mycorrhizal fungi (AMF), a long-term irrigation experiment was carried out in a tomato greenhouse in 2011, using an irrigation program with dripping water starting when the soil reached a suction of 20 kPa (D20), 30 kPa (D30) and 40 kPa (D40). In 2017, we tested the AMF community and soil aggregate composition by soil wet sieving. Aggregates of 0.25–1 mm represented the main class of aggregates (32.4%–43.1%) in this experiment. At D30, we measured the highest mean weight diameter (MWD) and soil organic carbon (SOC) and glomalin-related soil protein (GRSP) levels. Thus, D30 promoted soil aggregate stability in the greenhouse. According to the high-throughput sequencing results of AMF, Glomus at D30 was the main factor leading to a high soil aggregate stability, because its OTU relative abundance was significantly higher than those of Ambispora and Paraglomus. Through redundancy analysis, the GRSP concentration was positively correlated with the SOC and total N (TN) levels and with the presence of the genera Ambispora, Glomus and Paraglomus. This evidenced that AMF and SOC helped to increase GRSP concentration and aggregate stability. Therefore, initiating irrigation when the soil reaches a water suction of 30 kPa could promote soil aggregate stability by favoring AMF abundance.

Effects of straw incorporation and straw-burning on aggregate stability and soil organic carbon in a clay soil of Bangladesh

The clay soil of Bangladesh is typically low in soil organic carbon (SOC) and generally has poor soil aggregate stability (measured as mean weight diameter, MWD). The addition of organic amendments in the field has the potential to increase soil organic matter (SOM) and MWD. However, the influence of organic amendments on aggregation in the clay soils of Bangladesh is largely unknown. A short-term (105 days) field study has been conducted to evaluate the effects of straw incorporation and straw-burning treatments on MWD and SOC in clay soil under eggplant (Solanum melongena L.). This study consisted of three treatments: (i) control (no amendments), (ii) rice straw (straw applied at 21 t ha−1), (iii) rice straw-burning (ash retained after burning at 7.4 t ha−1). Soil samples were collected after plant harvest, and then MWD, SOC, glomalin-related soil protein (GRSP), microbial biomass carbon (MBC), Fe-oxides, and available soil nutrients (e.g., N, P, K, S, Na, Ca, and Mg) were determined. Our results showed that MWD in straw addition treatment was on average 2 times higher, whereas SOC was 1.3 times higher compared to the control and straw-burning treatments, respectively (P < 0.05). Conversely, the straw-burning treatment reduced the MWD by 19% and the SOC by 20% compared to the control treatments (P < 0.05). Significant improvements in the available K, S, and Mg content of soil were found in the straw incorporation treatment (P < 0.05). We did not find any significant influence of the treatments on the MBC and GRSP content of the soil. Soil organic carbon had a significant correlation with MWD (R2 = 0.80**, P < 0.01), while GRSP and Fe-oxides did not show any relationship with MWD (P > 0.05). Our study demonstrates that the incorporation of rice straw has the potential to increase soil aggregation and SOC stock, at least for the short term in the clay soils of Bangladesh.

Decades of reforestation significantly change microbial necromass, glomalin, and their contributions to soil organic carbon

Microbial necromass carbon (MNC) and glomalin-related soil protein (GRSP) are stable microbe-derived sources of carbon (C) in soils. MNC constitutes a considerable portion of soil organic carbon (SOC). GRSP is beneficial for soil aggregation, quality improvement, and C storage. However, the impact of land use transitions from croplands to forests on MNC, GRSP, and their contributions to SOC are not fully understood. To illuminate these unclear dynamics, soil was collected at depths of 0–10 cm from 30 pairs of adjacent corn fields and woodlands in southwest China. These woodlands, mainly consisting of fir, pine, and rubber, were converted from croplands approximately two decades ago. Soil properties, SOC, MNC, and GRSP content were determined. We found that SOC content increased significantly by 23.24% following reforestation. Additionally, the MNC and total GRSP contents were 46.01% and 20.48% higher, respectively, in woodland soils than in adjacent cropland soils. The contribution of MNC to the SOC pools also significantly increased and their quantities were proportional to the increasing contribution of GRSP to SOC. In woodlands, soil NH4+-N content is the major factor regulating the accumulation of MNC. For cropland systems, the easily extractable GRSP showed a strong positive correlation with MNC accumulation. Overall, our findings show that reforestation from croplands is conducive to soil C sequestration. The positive relationship between the contributions of GRSP and MNC to SOC provides valuable information in terms of enhancing our understanding of mechanisms underlying the maintenance of soil C stocks through microbe-derived C.

Contribution of Arbuscular Mycorrhizal Fungal Communities to Soil Carbon Accumulation during the Development of Cunninghamia lanceolata Plantations

Arbuscular mycorrhizal (AM) fungi can establish mutual association with most land plants, and impact a series of important ecological processes, including plant productivity, ecological succession and soil carbon (C) accumulation. Understanding the AM fungal diversity and community assembly, and their associated soil C sequestration, could be a crucial interest for the forest ecologist. In this study, the AM fungal abundances and community structure as well as glomalin-related soil protein (GRSP) concentrations were investigated in typical development stages (young, middle and mature) of Cunninghamia lanceolate plantations, which are widely distributed species in subtropical regions. The mycorrhizal colonization, spore density, AM fungal biomass and diversity were higher in mature than younger stands. The development of C. lanceolata also increased soil GRSP concentrations, and enhanced their C contribution to soil organic C. Soil difficulty extractable (DE) GRSP demonstrated a greater C contribution to soil organic C relative to easily extractable (EE) GRSP. Linkage analyses found that AM fungal biomass demonstrated a positive correlation with GRSP concentrations, and soil organic C positively related to DE-GRSP and total (T) GRSP. Soil AM fungal community structure differed dramatically across all studied C. lanceolata plantations with a decrease in Gigasporaceae and increase in Acaulosporaceae. Soil AM fungal community assembly was more phylogenetic clustering than expected by chance and primarily shaped by deterministic processes, with a non-shift during the development of C. lanceolata. Collectively, C. lanceolata development shaped the AM fungal communities and enhanced their biomass and GRSP contents, which might, in turn, partially contribute to soil C accumulation.