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Silicon (Si) speciation and availability in soils is highly important for ecosystem functioning, because Si is a beneficial element for plant growth. Si chemistry is highly complex compared to other elements in soils, because Si reaction rates are relatively slow and dependent on Si species. Consequently, we review the occurrence of different Si species in soil solution and their changes by polymerization, depolymerization, and condensation in relation to important soil processes. We show that an argumentation based on thermodynamic endmembers of Si dependent processes, as currently done, is often difficult, because some reactions such as mineral crystallization require months to years (sometimes even centuries or millennia). Furthermore, we give an overview of Si reactions in soil solution and the predominance of certain solid compounds, which is a neglected but important parameter controlling the availability, reactivity, and function of Si in soils. We further discuss the drivers of soil Si cycling and how humans interfere with these processes. The soil Si cycle is of major importance for ecosystem functioning; therefore, a deeper understanding of drivers of Si cycling (e.g., predominant speciation), human disturbances and the implication for important soil properties (water storage, nutrient availability, and micro aggregate stability) is of fundamental relevance.

Abstract We investigated the effects of substrate (cellulose or starch) and different clay contents on the production of microbial extracellular polymeric substances (EPS) and concomitant development of stable soil aggregates. Soils were incubated with different amounts of montmorillonite (+ 0.1%, + 1%, + 10%) both with and without two substrates of contrasting quality (starch and cellulose). Microbial respiration (CO2), biomass carbon (C), EPS-protein, and EPS-polysaccharide were determined over the experimental period. The diversity and compositional shifts of microbial communities (bacteria/archaea) were analysed by sequencing 16S rRNA gene fragments amplified from soil DNA. Soil aggregate size distribution was determined and geometric mean diameter calculated for aggregate formation. Aggregate stabilities were compared among 1–2-mm size fraction. Starch amendment supported a faster increase than cellulose in both respiration and microbial biomass. Microbial community structure and composition differed depending on the C substrate added. However, clay addition had a more pronounced effect on alpha diversity compared to the addition of starch or cellulose. Substrate addition resulted in an increased EPS concentration only if combined with clay addition. At high clay addition, starch resulted in higher EPS concentrations than cellulose. Where additional substrate was not provided, EPS-protein was only weakly correlated with aggregate formation and stability. The relationship became stronger with addition of substrate. Labile organic C thus clearly plays a role in aggregate formation, but increasing clay content was found to enhance aggregate stability and additionally resulted in the development of distinct microbial communities and increased EPS production.

1. Soil aggregate stability is an important ecosystem property which deteriorates overtime due to agricultural practices. The cessation of cultivation allows the potential recovery of soil aggregate binding agents such as soil micro-organisms and biochemical properties. Consequently, an increase in soil aggregate stability is expected. However, this outcome is difficult to predict because the response of each individual soil component and its contribution to soil stability varies. 2. This study utilized a chronosequence of 12 ex-arable fields in the Bolivian Altiplano, representing six soil ages of abandonment after cessation of potato cultivation, to examine whether soil aggregate stability increases after abandonment and the extent to which changes in soil bacterial and fungal community composition and soil chemical properties are involved in stability recovery. 3. Fields with the longest time since disturbance (15 and 20 years) have a greater proportion of water-stable aggregates than more recently abandoned fields (1 and 3 years) and exhibit larger differences in bacterial and fungal composition. Total N, ${\\mathrm{N}\\mathrm{H}}_{4}^{+}$, C and organic matter also increased with time since the last intensive agricultural practice. 4. Water-stable aggregates were strongly correlated with soil fungal community composition. Analysis of covariance is also consistent with the soil fungal community being an important mediator of the recovery of aggregate stability. 5. Synthesis and applications. Soil aggregate stability increased by 50% over the 20 years following disturbance. This recovery was associated with shifts in soil fungal community composition, as is consistent with fungal mediation of this recovery. Land management strategies focusing on restoration of the soil fungal community may enhance soil aggregate stability, a key aspect for soil conservation, restoration, sustainability of agroecosystems and erosion prevention.

Background Phosphorus (P) fertilizer inputs can increase soil P availability, which improves soil carbon (C) cycling and microbial community structure. However, the potential mechanisms via which P drives soil organic carbon (SOC) and microbial regulation of aggregates formation and stabilization are still unclear. Methods A 10-year field experiment was conducted, including (1) CK, no fertilization; (2) NK, N and K fertilizer addition; (3) NP1K and (4) NP2K, NK with 28 and 56 kg P ha −1 addition, respectively. Results Relative to NK treatment, long-term P fertilizer application significantly increased the proportion of >0.25 mm aggregates and mean weight diameter (MWD), which were increased by 16.4% and 18.0%, respectively. Scanning electron microscopy further confirmed that P addition resulted in better soil structure. Meanwhile, compared with NK treatment, the content of soil exchangeable Ca and SOC (especially stable C=C chemical speciation) was increased with P fertilizer addition, which could form organic-Ca complexes to improve aggregate stability. And compared with NK treatment, the relative abundance of copiotrophic bacteria (i.e., Actinobacteriota ) involved in aggregate formation and stability was increased by 11.3% and 8.4% in NP1K and NP2K, respectively. Additionally, redundancy analysis indicated that the main factor for bacterial diversity was available P (AP). Conclusion Taken together, P fertilizer addition can increase the content of soil exchangeable Ca and SOC (especially C=C) to form organic Ca complexes, while AP improves the microbial community structure, thereby improving the stability of aggregate structure in saline alkali soil.

Soil from the Loess Plateau of China is typically low in organic carbon and generally has poor aggregate stability. Application of organic amendments to these soils could help to increase and sustain soil organic matter levels and thus to enhance soil aggregate stability. A field experiment was carried out to evaluate the effect of the application of wheat straw and wheat straw-derived biochar (pyrolyzed at 350–550 °C) amendments on soil aggregate stability, soil organic carbon (SOC), and enzyme activities in a representative Chinese Loess soil during summer maize and winter wheat growing season from 2013 to 2015. Five treatments were set up as follows: no fertilization (CK), application of inorganic fertilizer (N), wheat straw applied at 8 t ha −1 with inorganic fertilizer (S8), and wheat straw-derived biochar applied at 8 t ha −1 (B8) and 16 t ha −1 (B16) with inorganic fertilizer, respectively. Compared to the N treatment, straw and straw-derived biochar amendments significantly increased SOC (by 33.7–79.6%), microbial biomass carbon (by 18.9–46.5%), and microbial biomass nitrogen (by 8.3–38.2%), while total nitrogen (TN) only increased significantly in the B16 plot (by 24.1%). The 8 t ha −1 straw and biochar applications had no significant effects on soil aggregation, but a significant increase in soil macro-aggregates (>2 mm) (by 105.8%) was observed in the B16 treatment. The concentrations of aggregate-associated SOC increased by 40.4–105.8% in macro-aggregates (>2 mm) under straw and biochar amendments relative to the N treatment. No significant differences in invertase and alkaline phosphatase activity were detected among different treatments. However, urease activity was greater in the biochar treatment than the straw treatment, indicating that biochar amendment improved the transformation of nitrogen in the soil. The carbon pool index and carbon management index were increased with straw and biochar amendments, especially in the B16 treatment. In conclusion, application of carbonized crop residue as biochar, especially at a rate of 16 t ha −1 , could be a potential solution to recover the depleted SOC and enhance the formation of macro-aggregates in Loess Plateau soils of China.

Soil water stress (WS) affects the decomposition of soil organic carbon (SOC) and carbon (C) emissions. Glomalin, released by arbuscular mycorrhizal fungi into soil that has been defined as glomalin-related soil protein (GRSP), is an important pool of SOC, with hydrophobic characteristics. We hypothesized that mycorrhizal fungi have a positive effect on SOC pools under soil WS for C sequestration in GRSP secreted by extraradical mycorrhizal hyphae. A microsystem was used to establish a root chamber (co-existence of roots and extraradical mycorrhizal hyphae) and a hyphal chamber (the presence of extraradical mycorrhizal hyphae) to study changes in plant growth, leaf water potential, soil aggregate stability, SOC, GRSP, C concentrations in GRSP (C GRSP ), and the contribution of C GRSP to SOC after inoculating Rhizophagus intraradices with trifoliate orange ( Poncirus trifoliata ) in the root chamber under adequate water (AW) and WS. Inoculation with R . intraradices alleviated negative effects on leaf water potential and plant growth after 7 weeks of WS. Soil WS decreased SOC and mean weight diameter (MWD), while AMF inoculation led to an increase in SOC and MWD in both chambers, with the most prominent increase in the hyphal chamber under WS. The C concentration in easily extractable GRSP (EE-GRSP) and difficultly extractable GRSP (DE-GRSP) was 7.32 − 12.57 and 24.90 − 32.60 mg C/g GRSP, respectively. WS reduced C GRSP , while AMF mitigated the reduction. Extraradical mycorrhizal hyphae increased GRSP production and C GRSP , along with a more prominent increase in DE-GRSP under WS than under AW. Extraradical mycorrhizal hyphae increased the contribution of C DE-GRSP to SOC only under WS. C EE-GRSP and C DE-GRSP were significantly positively correlated with SOC and MWD. It is concluded that extraradical mycorrhizal hyphae prominently promoted C sequestration of recalcitrant DE-GRSP under soil WS, thus contributing more organic C accumulation and preservation in aggregates and soil C pool.

PurposeStudies predicting the impacts of climate change on erosion have considered numerous variables, such as rainfall erosivity and vegetation cover, but have not considered potential changes in soil erodibility. Erodibility is an intrinsic property of the soil, strongly correlated with the stability of soil aggregates. It is influenced by soil physico-chemical attributes, including the microbiological community. The study aim was to determine how shifts in temperature and moisture conditions, which other studies have shown affect microbiological communities, might affect aggregate stability.MethodsUsing an experimental approach with laboratory microcosms, aggregates from a sandy loam soil and a clay soil were incubated at three temperatures and three moisture conditions in a factorial experimental design. Aggregate stability was quantified using rainfall simulation. Microbiological indicator metrics were measured to evaluate treatment microbiological impacts, including community composition (PLFA), biomass carbon, and respiration.ResultsTemperature and moisture content affected aggregate stability significantly, but differently for the two soil types tested. For the sandy loam soil, aggregate stability decreased significantly with increasing moisture content. For the clay soil, aggregate stability increased significantly with increasing temperature. For both soil textures, temperature and moisture content affected microbiological community composition and respiration. Regression analysis indicated that microbiological properties were significant predictors of aggregate stability.ConclusionOur results emphasise the dynamic nature of soil aggregate stability. Changes in microbiological metrics suggest possible biological mechanisms for aggregate stability changes, which should be investigated further to better understand the potential impacts of climate change on soil erodibility and erosion.

Background and aims It is demonstrated that intercropping improves soil fertility, but its effect on deep soil is still unclear. The major objective of this study was to determine the distribution of arbuscular mycorrhizal fungi (AMF) and soil aggregates and their interrelationship across soil depths in intercropping systems. Methods A three-year positioning experiment based on a two-factor experimental design at two N application levels (N0 and N2) and different cropping systems (maize/soybean intercropping and corresponding monocultures) was started in 2017. Soil samples were collected from 0–15 cm and 15–30 cm for analyzing soil aggregates and from 0–15 cm, 15–30 cm, 30–5 cm, and 45–60 cm for determining the AMF composition. Results It was observed that intercropping improved the macro-aggregate (> 5 mm) content at 0–15 cm and 15–30 cm depths for maize soil and only 0–15 cm depth for soybean soil without N treatment. The application of N decreased the macro-aggregate content in the intercropping soil at 0–15 cm and 15–30 cm depths. Moreover, intercropping significantly improved the AMF diversity of maize and soybean soils across soil depths, while the application of N reduced the AMF diversity of soil across depths. Conclusions The structural equation modeling analysis indicated that the intercropping system influenced the stability of soil aggregates and promoted the formation of large aggregates by altering soil nutrients and the diversity of AMF. The results further revealed the reasons behind soil fertility improvement by adopting crop diversification.

Stable soils provide valuable ecosystem services and mechanical soil stability is enhanced by the presence of arbuscular mycorrhizal fungi (AMF). Soil aggregation, which is the major driver of mechanical soil stability, is often treated as a static phenomenon, even though aggregate turnover is continually ongoing. In fact, some breakdown of macroaggregates is necessary to allow new aggregate formation and inclusion of new organic matter into microaggregates. We determined how aggregate turnover times were affected by AMF by tracking movement of rare earth elements (REE), applied as their immobile oxides, between aggregate size classes, and using X-ray fluorescence microscopy to spatially localize REEs in a sample of aggregates. Here we show that AMF increased large macroaggregate formation and slowed down disintegration of large and small macroaggregates. Microaggregate turnover was increased in the presence of AMF. Internal aggregate organization suggested that although formation of microaggregates by accretion of soil to particulate organic matter is common, it is not the only mechanism in operation.

Background and aims Studies verify that intercropping effects soil nutrient content, enzyme activity, aggregate stability, arbuscular mycorrhizal fungi (AMF) community and glomalin-related soil protein (GRSP) content in Red Soil (Ultisol in the USDA Taxonomy, Acrisol in the WRB Soil Taxonomy) on sloping farmland. However, the comprehensive contribution of soil nutrients, enzyme activity, AMF community and GRSP to the characteristics of water-stable aggregates under different planting patterns of maize and soybean are not fully understood. Methods A long-term field experiment commenced in 2018. Three treatments of maize ( Zea mays L.) monoculture, soybean ( Glycine max L.) monoculture and maize-soybean intercropping were established in an experimental field. The planting patterns, crop varieties and fertilizer rates of each plot were consistent for each of the four years of experiments (2018–2021). Results Results showed that intercropping can improve the concentrations of alkali-hydrolysable nitrogen, available phosphorus and total extractable glomalin-related soil protein, the activities of enzyme (urease, invertase, acid phosphatase and catalase) and the mean weight diameter (MWD) in the rhizosphere soil of maize and soybean. Moreover, results proved that intercropping can potentially increase AMF diversity and macro-aggregates (> 2.0 mm) in the maize rhizosphere and macro-aggregates (0.5-2.0 mm) in the soybean rhizosphere. Conclusion Intercropping of maize and soybean can increase soil aggregate stability in the rhizosphere of the two crops. The easily extractable glomalin-related soil protein was the main factor affecting soil aggregate stability and the formation of > 2.0 mm aggregates in the maize rhizosphere. Soil organic matter was the main factor affecting soil aggregate stability and the formation of 0.5–2.0 mm aggregates in the soybean rhizosphere.