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BACKGROUND AND AIMS: Our objectives were to evaluate changes in soil aggregate stability along a successional gradient, located in severely eroded Mediterranean gully bed ecosystems and to identify predictors of soil aggregate stability variations among several soil, root traits and plant community characteristics. METHODS: We selected 75 plots in gully beds, representing five successional stages that differ in plant community composition, dominated by herbs, shrubs or trees according to successional stage. In each plot, we measured soil aggregate stability, basic soil characteristics, root traits and plant diversity indices. RESULTS: Soil aggregate stability increased along the successional gradient, being thrice higher in tree-dominated communities as compared to grass-dominated communities. This increase was mainly driven by soil organic carbon (SOC) accumulation. In early successional stages showing low SOC (below 24 g.kg⁻¹ or 12 g.kg⁻¹ in some cases), fine sand content and the percentage of fine roots acted as co-drivers enhancing soil aggregate stability while silt content decreased it. CONCLUSION: Plant succession in severely eroded Mediterranean gully bed ecosystems is accompanied by a strong stabilization of soil aggregates, mainly driven by SOC accumulation and for early successional stages, by soil granulometry and root traits as co-drivers. Stimulating succession thus appears as a promising restoration strategy for severely eroded ecosystems.

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.

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.

Aims The aim of this study was to explore the impact of soil management strategies and the contribution of root traits of plant communities and soil organic carbon (SOC) in explaining soil aggregate stability in vineyards. Methods We measured topsoil aggregate stability, soil properties and root traits of 38 plant communities in an experimental vineyard, previously subjected to different soil management strategies. Then we investigated statistical relations between aggregate stability, root traits and SOC and estimated root trait and SOC contributions to gain insight into aggregate stability. Results Soil management strategies strongly affected soil aggregate stability, with a negative effect of tillage, even after several years of service crop cover. Among the investigated parameters, soil organic carbon was found to contribute the most to aggregate stability. Root mean diameter and root mass density showed positive correlations with aggregate stability, while specific root length showed a negative correlation with aggregate stability. Conclusions Soil aggregate stability is the result of complex interactions between soil management strategies, soil properties and plant root traits. Service crops improve aggregate stability, and a trait-based approach could help to identify service crop ideotypes and expand the pool of species of interest for providing services in agroecosystems in relation with the soil physical quality.

Aims The stability of hillslopes is an essential ecosystem service, especially in alpine regions with soils prone to erosion. One key variable controlling hillslope stability is soil aggregate stability. We aimed at identifying dominant controls of vegetation parameters on aggregate stability and analysed their importance for soil aggregate stability during landscape development. Methods We quantified the aggregate stability coefficient (ASC) and measured plant cover, diversity, root mass and root length, density (RMD, RLD) along two chronosequences with contrasting bedrocks (siliceous, calcareous) in the Swiss Alps. Results We found that ASC developed slower along the calcareous chronosequence. Furthermore, we observed a significant positive effect of vegetation cover and diversity on ASC that was mediated via root density. These relationships developed in a time-depended manner: At young terrain ages, vegetation parameters had a strong effect on aggregate stability compared to older stages. Moreover, RLD was the most powerful predictor of ASC on young terrain, whereas on older moraines RMD became more important. Conclusions We highlight that root density plays a major role in governing ASC for soils differing in moraine ages. The changing importances of RLD and RMD for ASC development suggest different mechanistic linkages between vegetation and hillsope stability during landscape development.

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.

Background and aims Roots and their rhizosphere considerably influence soil structure by regulating soil aggregate formation and stabilization. This study aimed to examine the rhizosphere effects on soil aggregation and explore potential mechanisms along secondary forest successions. Methods Effects of roots and their rhizosphere on soil aggregation in two subtropical secondary forest successions were examined by separating soils into rhizosphere and bulk soils. Soil aggregate mean weight diameter (MWD), soil organic carbon (SOC), soil nutrients, and fine-root traits were simultaneously measured. Results Soil aggregate MWD increased significantly in the bulk soils along secondary forest successions, but did not differ in the rhizosphere soils. Rhizosphere effects on soil aggregate MWD (i.e., root-induced differences between the rhizosphere and bulk soils) were thus significantly higher at the early-successional stage of subtropical forest with low soil fertility than those at the late stages with high fertility. Rhizosphere significantly increased SOC and soil total nitrogen (TN) throughout the entire secondary forest successions, which was nonlinearly correlated with soil aggregate MWD. Principal components regression analysis showed that SOC was the primary abiotic factor and positively correlated with soil aggregate MWD. As for biotic factors, fine-root length density and N concentration were two important root traits having significant effects on soil aggregate stability. An improved conceptual framework was developed to advance our understanding of soil aggregation and rhizosphere effects, highlighting the roles of soil fertility (i.e., SOC and available nutrients), root traits, and forest age in driving soil aggregation. Conclusions Impacts of root-derived organic compounds inputs to rhizosphere on soil aggregation were stronger at the early-successional stage of subtropical forest than those at the late stages. This succession-specific pattern in rhizosphere effects largely resulted from the nonlinear relationships between soil aggregate MWD and SOC concentration with a plateau at high SOC. Incorporating the SOC-dependent rhizosphere effects on biogeochemical cycle into Earth system models might improve the prediction of forest soil C dynamics.

AimsIn order to better understand the changes of soil carbon sequestration capacity in forest after forest mixing, the effects of broadleaf tree invasion on soil aggregate stability and carbon sequestration were studied.MethodsIn northern China, the pure Larix principis-rupprechtii plantations and the Larix principis-rupprechtii plantations invaded by Betula platyphylla at various degrees with the same site conditions were selected (Betula platyphylla had mixed degrees of 0.2 and 0.4). The distribution and stability of soil aggregates were analyzed, and soil organic carbon and active carbon components were determined.ResultsThe distribution of soil macroaggregates (> 0.25 mm) increased with the increase in the mixed degree of Betula platyphylla. The mixture of Betula platyphylla could effectively increased SOC, EOC, DOC and MBC of the original soil and soil aggregates of different diameter classes. The invasion of Betula platyphylla had a positive indirect impact on soil carbon sequestration by affecting the soil physical and chemical properties and the aggregate stability.ConclusionThe invasion of Betula platyphylla had significant positive effects on soil aggregate stability, erosion resistance and soil nutrient status in Larix principis-rupprechtii plantation. Maybe the selection of suitable broadleaf mixed species can improve the soil quality and soil organic carbon sequestration of the Larix principis-rupprechtii plantation in this area.

PurposeSoil aggregates regulate soil water and temperature, soil fertilizer, and leaf gas exchange. In desert steppes, precipitation restricts the growth and development of plants, and it affects the availability of soil carbon and nitrogen, thereby influencing soil aggregate stability. However, studies on precipitation influence on the stability of aggregates are limited.Materials and methodsHere, we conducted a 2-year field experiment in a desert steppe of Siziwang Banner, Inner Mongolia, to test the effect of a changing precipitation gradient “reducing precipitation by 50% (W-50%), natural precipitation (W), increasing precipitation by 50% (W+50%), and increasing precipitation by 100% (W+100%)” on the depth distribution, stability of soil aggregates and aggregate-associated organic carbon content (OC), and total nitrogen (TN) contents. We used a wet sieving method yielding silt and clay (SCA, < 0.053 mm), microaggregates (MIA, 0.053–0.25 mm), small macroaggregates (SMA, 0.25–2 mm), and large macroaggregates (LMA, > 2 mm).Results and discussionOur results indicated that the topsoil (0–30 cm) was dominated by SCA and MIA. Increasing precipitation increased soil aggregate stability and reduced soil erodibility by increasing water-stable aggregates (WSA, > 0.25 mm). In this study, the comprehensive soil aggregate stability score was the highest at W+100%. Although LMA serve as the main carriers of SOC and TN, MIA-associated OC and TN had the highest contribution rate to SOC and TN. This study revealed that bulk soil properties including MBN, BD, MBC, and pH significantly influenced aggregate stability. Additionally, WSA-associated OC were found to be the most crucial contributors to soil aggregate stability.ConclusionsOverall, our study indicates that increasing precipitation is beneficial to WSA accumulation and highlighted the vital role of microbial biomass and WSA-associated OC on maintaining soil aggregate stability under precipitation change.

PurposeThe aim of this study was to investigate the resistance of aggregates to flooding stresses for different soil types and present implications for the restoration of eroded soils.Materials and methodsTwelve field sites for three soil types were selected and separated into four hydrological stress levels at the riparian zones of the Three Gorges Reservoir. Soil samples were collected randomly, followed by lab analysis of soil mechanical composition, soil aggregate and stability, and soil carbon and nitrogen contents in the bulk soil and different sizes of aggregates.Results and discussionClay and silt migrated from the upper water level sites to lower water level sites for Regosols under hydrological stresses; however, the mechanical compositions were not changed for Anthrosols and Luvisols. Total carbon content (TC), total nitrogen content (TN), and carbon and nitrogen ratio (C/N) were highest under strong hydrological stress for all-sized aggregates and bulk soils. Aggregate disintegration under hydrological stresses made organic matter exposed, but the anaerobic environment created by flood avoided organic matter from being decomposed. Most TC and TN in aggregates and bulk soils were negatively correlated with stability. Compared with Anthrosols and Luvisols, Regosols had lower aggregate stability due to its low large macro-aggregate proportions for each stress level. Therefore, much attention should be given to Regosols which has a high potential for erosion. Resistances of aggregates to strong and intermediate hydrological stress were higher for Anthrosols than other tested soils. However, Luvisols had the highest resistance to hydrological stresses because of its higher stability above the elevation of 165 m, due to its highest small macro-aggregate proportion. Therefore, anthropogenic restorations are recommended to stabilize the structure of Anthrosols and Luvisols under weak and strong hydrological stress, respectively.ConclusionsThe operation of the Three Gorges Reservoir forced the riparian ecosystem to undergo periodical flooding stresses. The resistance of soil aggregates to hydrological stresses was lowest for Regosols, which should be concerned urgently to reduce soil losses. Under strong and intermediate hydrological stresses, Anthrosols had greater stability to maintain its original structure. However, the aggregate stability of Luvisols was higher for weak and none hydrological stress levels. Hence, anthropogenic restorations are recommended to take priorities for Anthrosols and Luvisols to reduce soil erosion under weak and strong hydrological stress, respectively.