Soil aggregate mediates the impacts of land uses on organic carbon, total nitrogen, and microbial activity in a Karst ecosystem

Glomalin related soil protein, soil aggregate stability and soil aggregate-associated organic carbon under agroforestry practices in southern Ethiopia

Land degradation in Ethiopia is escalating due to high population density and the shift from tree-based agricultural systems, like agroforestry practices (AFP), to monoculture farming. These land use changes, compounded by climate change, threaten biodiversity and soil resources. Key soil health parameters, such as glomalin, soil aggregation, and aggregate stability, are negatively impacted by such practices. Agroforestry is proposed as a sustainable alternative to address these challenges. This study aimed to evaluate the effects of AFPs on soil glomalin, soil aggregate stability (mean weight diameter, MWD), and the relationship between soil aggregates and soil organic carbon (SOC). Undisturbed soil samples were collected from 0 to 30 cm and 30-60 cm depths in four land use types: home garden (HAFP), cropland (ClAFP), woodlot (WlAFP), and trees on soil and water conservation-based agroforestry (TSWAFP). Results showed significantly higher glomalin-related soil protein (GRSP) in HAFP and WlAFP compared to ClAFP and TSWAFP (p < 0.05). HAFP also exhibited the highest soil aggregate stability (SAS) and MWD, followed by WlAFP. These findings suggest that agroforestry practices can significantly enhance soil health, ecosystem stability, and long-term sustainability, contributing to land restoration efforts.

Impacts of abandoned sloping farmland on soil aggregates and aggregate-associated organic carbon in karst rocky desertification areas

Vegetation restoration after the abandonment of sloping farmland can effectively promote the sequestration of soil organic carbon (SOC), with soil aggregates playing a pivotal role. However, in abandoned farmlands in karst regions with varying degrees of rocky desertification, the relationship between soil aggregates, aggregate-associated organic carbon (AAOC), and total SOC content remains unclear. Taking abandoned sloping farmlands (5 years, 10 years, and 15 years) with different levels of rocky desertification (no rocky desertification, potential rocky desertification, slight rocky desertification, and moderate rocky desertification) in a typical karst area as research objects, this study investigated the dynamic characteristics of the particle size distribution of soil aggregates, total SOC, and AAOC. The results indicated that total SOC content in the 0-20 cm soil layer increased after abandonment in all levels of rocky desertification, peaking after 15 years. The abandoned sloping farmland with moderate desertification showed the best recovery effect. Post-abandonment vegetation restoration increased the content of 5-10 mm soil aggregates, but decreased those of 1-2 mm and < 0.25 mm aggregates. Particularly for 5-10 mm aggregates, the contribution of AAOC to total SOC significantly increased over time. Moreover, a strong correlation was observed between >1 mm aggregates and total SOC (p < 0.05). The increase in total SOC was primarily driven by the growth of AAOC in 5-10 mm aggregates. In general, vegetation restoration is an effective approach for enhancing total SOC content in abandoned sloping farmland with varying degrees of rocky desertification.

Soil variables driven by host plant and growth season affect soil microbial composition and metabolism in extremely arid desert ecosystems

Assessment changes of soil microbial community structure and function is important in understanding the response to desert ecosystem management. In present study, variations of soil microbial community and edaphic factors associated with five desert shrubs were determined in Anxi extremely arid desert in Northwest China in growing (summer), deciduous (autumn), and snowfall (winter) seasons. For that, the microbial composition and catabolic metabolism were examined using methods of phospholipid fatty acid (PLFA) and Biolog EcoPlate, respectively. Regardless of plant species and seasonal patterns, the microbial community was mostly dominated by gram-negative bacteria (GN); and the carbohydrates, amino acids and polymers were the main carbon sources for desert microbial metabolism. Microbial biomass and metabolic levels were significantly higher in both summer and winter than those of autumn. There was no correlation between soil microbial community and carbon utilization in winter; but GN was positively correlated with metabolism of amines carbon sources in summer, while fungal community presented the strongest correlation with suites of carbon sources' metabolic levels in autumn, indicating the uncoupled relationship between microbial community and function in desert ecosystems. Desert shrubs significantly influenced the composition of soil microbial community, whereas the variation of microbial catabolic metabolism was most attributed to seasonality. Nevertheless, the effects of both plant species (21.3 %) and climate variation (84.9 %) interacted with soil properties, indicating the seasonality of soil nutrients predominately determined the changes in composition and metabolism of desert microbes. Both the comprehensive seasonal level and the intra-seasonal paired correlation analysis proved that phosphorus was the key factor in determining microbial community composition, while ammonia and nitrate nitrogen were more correlated to microbial functional metabolism. Additionally, soil moisture and organic carbon in desert environment also induced the shifts in ratio of fungi and bacterial communities. We conclude that the seasonal patterns of soil microbial community and metabolic function in extremely arid desert are predictable, and mainly influenced by specific soil factors driven by desert shrubs and climate factors. These findings will provide a basis for evaluating the management of soil resources and microbial function in desert environments.

Difference in total N and its aggregate-associated N following cropland restoration in a karst region, Southwest China

Cropland conversion has been cited as one of the most effective measures for increasing soil nitrogen pool in karst degraded regions. However, it is still unclear how N associated with aggregate patterns and their contribution to net soil N accumulation after cropland conversion. The experiment included four treatments with one control and three restoration strategies, that is, maize-soybean rotation cultivation (the control), sugarcane, mulberry, and forage grass cultivation. Soil samples were selected to determine the soil aggregate amount and its associated N content and stock across 0-30 cm soil layer. Macro-aggregate (> 2 mm) was the predominant aggregate fraction in all cropland use types and had the largest N stock. Forage grass cultivation substantially increased N stocks in bulk soil and aggregate fractions. The N contents and stocks associated with aggregate were shown to be positively correlated with bulk soil N stocks. Furthermore, the increase in N stock in forage grass soil was largely caused by an increase in N stock within macro-aggregates (> 2 mm), which is further attributed to the increased N content within macro-aggregates. Overall, forage grass cultivation replaced maize-soybean cultivation which was proposed as an ecological restoration model to improve soil N sequestration capacity due to its function in increasing the N stock of aggregate in the karst degraded region of Southwest China.

Effects of Different Land Use Types and Soil Depths on Soil Mineral Elements, Soil Enzyme Activity, and Fungal Community in Karst Area of Southwest China

The current research was aimed to study the effects of different land use types (LUT) and soil depth (SD) on soil enzyme activity, metal content, and soil fungi in the karst area. Soil samples with depths of 0–20 cm and 20–40 cm were collected from different land types, including grassland, forest, Zanthoxylum planispinum land, Hylocereus spp. land and Zea mays land. The metal content and enzyme activity of the samples were determined, and the soil fungi were sequenced. The results showed that LUT had a significant effect on the contents of soil K, Mg, Fe, Cu and Cr; LUT and SD significantly affected the activities of invertase, urease, alkaline phosphatase and catalase. In addition, Shannon and Chao1 index of soil fungal community was affected by different land use types and soil depths. Ascomycota, Basidiomycota and Mortierellomycota were the dominant phyla at 0–20 cm and 20–40 cm soil depths in five different land types. Land use led to significant changes in soil fungal structure, while soil depth had no significant effect on soil fungal structure, probably because the small-scale environmental changes in karst areas were not the dominant factor in changing the structure of fungal communities. Additionally, metal element content and enzyme activity were related to different soil fungal communities. In conclusion, soil mineral elements content, enzyme activity, and soil fungal community in the karst area were strongly affected by land use types and soil depths. This study provides a theoretical basis for rational land use and ecological restoration in karst areas.

Effects of Different Land Use Types and Soil Depth on Soil Nutrients and Soil Bacterial Communities in a Karst Area, Southwest China

To reveal the effect of the interactions between soil depth and different land use types on soil nutrients and soil bacterial communities in a karst area, fifty soil samples from five different karst land use types in Huajiang town, Guizhou province, Southwest China were collected, and the soil bacteria were analyzed using high-throughput absolute quantification sequencing. Our results showed that land use types (LUT) and soil depth (SD) significantly influenced the content of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NN), ammonium nitrogen (AN) and available soil phosphorus (AP), and pH; further, the interaction of LUT and SD also significantly influenced SOC, NN, NA, AP, and pH. In addition, LUT clearly impacted the Chao1 and Shannon indexes, but, SD and LUT * SD markedly affect Chao1 and Shannon index, respectively. All the soil bacterial communities were significantly different in the five different five land use types according to PERMANOVA. Importantly, Acidobacteria and Proteobacteria were the predominant phyla at soil depths of 0–20 cm and 20–40 cm among all the LUTs. At 0–20 cm, TN, AN, and SOC exerted a strong positive influence on Acidobacteria, but NN exerted a strong negative influence on Acidobacteria; at 20–40 cm soil, TN and AN exerted a strong positive influence on Acidobacteria; TP exerted no marked influence on any of the phyla at these two soil depths. At 0–20 cm of soil depth, we also found that Chao1 index changes were closely related to the TN, SOC, AN, and NN; similarly, Shannon index changes were significantly correlated to the AN, TN, and SOC; the PCoA was clearly related to the TN, SOC, and AN. Interestingly, at soil depth of 20–40 cm, Chao 1 was markedly related to the TN and pH; Shannon was markedly correlated with the SOC, TP, AN, and AP; and the PCoA was significantly correlated with the TN and pH. Our findings imply that soil nutrients and soil bacteria communities are strongly influenced by land use types and soil depth in karst areas.

Effect of land-use and land-cover change on mangrove soil carbon fraction and metal pollution risk in Zhangjiang Estuary, China

Land-use and land-cover change (LULCC) is the main cause of mangrove deforestation and degradation. However, the effect of LULCC on mangrove soil organic carbon (SOC) fractions and metal pollution risks, and the difference between the effects of those two soil evolutions are largely unknown. Here, we collected soil samples from natural systems (mangroves and mudflat), land-cover changes (Spartina alterniflora invasion), and anthropogenic land-use changes (cropland and culture pond) in Zhangjiang Estuary. We determined the soil aggregate fractions (macro-aggregate, micro-aggregate, and silt-clay fraction) and the associated carbon, and heavy metal dynamics. Our findings suggested that LULCC did not remarkably affect SOC contents, but changed the soil aggregate structures. LULCC significantly increased aggregate-associated carbon fractions, especially macro-aggregate carbon fraction. The large proportion of silt-clay fraction in natural systems was corresponding to a high percentage of mineral organic carbon, indicating that LULCC decreases the mangrove SOC stability. Land-cover change promoted the accumulation of SOC, nitrogen, and heavy metals compared with uninvaded mudflat. The heavy metal contents in mangrove soil were highest among all studied soils, expect for Cd, which suggested that mangrove soil had high metal accumulation. However, land-use changes could stimulate the mobility and dynamics of metals enriched in mangrove soils; these changes, especially in cropland, will also cause a large amount of exogenous Cd being exported into the adjacent aquatic environment. Thus, mangrove shifts metal pollutant from sink to source when affected by land-use changes. The contamination index demonstrated that heavy metals have posed ecological risks, especially for Cd in cropland. Compared with mangrove, land-use change was dominated by single-element pollution, but land-cover change showed low multiple-element complex pollution. These findings elucidate the effects of LULCC on mangrove SOC fraction and metal pollution risk, and are of great significance for designing the long-term management and conservation policies for mangrove managers.

The formation of large macroaggregates induces soil organic carbon sequestration in short-term cropland restoration in a typical karst area

Cropland restoration induces litter and root inputs and promotes the development of biological soil crusts (BSCs), which may promote aggregate formation and soil organic carbon (SOC) sequestration. However, litter, roots and BSCs have not been simultaneously considered when assessing soil aggregate and aggregate-associated SOC fraction responses to cropland restoration in subtropical areas. Here, we measured particulate organic carbon (POC) and mineral-associated organic carbon (MOC) in bulk soils and soil aggregates after 15 years of cropland restoration. Soil samples of cropland (CL) and four cropland restoration types (plantation forest [PF], forage grassland [FG], mixed plantation of forest and forage grassland [FF], and abandoned natural grassland [NG]) from depths of 0-30 cm were collected. Cropland restoration significantly increased SOC and POC in bulk soil at the 0-5 cm depth. However, only in FG did SOC significantly increase at depths of 5-15 cm, and POC significantly increased at depths of 5-30 cm. The large macroaggregate (5-10 mm and 2-5 mm) proportions increased significantly at the 0-15 cm depth after cropland restoration, and FG, FF and NG also increased the 5-10 mm aggregate proportions at the 15-30 cm depth. The SOC sequestration in bulk soil with cropland restoration was attributed to increases in the aggregate-associated organic carbon (AAOC) pool in large macroaggregates, which was mainly attributed to the increased aggregate amount rather than the increased AAOC concentration in large macroaggregates. Our results also indicated that an increase in aggregate-associated particulate organic carbon (AAPOC) led to an increase in AAOC. Variation partitioning indicated that the formation of large macroaggregates was controlled by the litter-moss-root interactive effect in this karst area. FG could be a better short-term cropland restoration strategy, increasing large macroaggregates in deeper soil layers better than the other vegetation types and promoting soil carbon sequestration in deeper soil layers.

Microstructure characteristics of aggregates and Cd immobilization performance under a 3-year sepiolite amendment: A field study

Sepiolite is an efficient mineral for the immobilization of Cd in contaminated soils. Here, we conducted a 3-year field experiment to investigate the effect of sepiolite on soil aggregation and porosity, Cd availability, and organic carbon content in the bulk and aggregate soils and Cd accumulation by leafy vegetables. The sepiolite-treated soils showed a 15.4%-53.4% and 5.5%-63.0% reduction in available Cd content in the bulk soil and different particle-size aggregates, respectively. Moreover, the Cd concentrations in the edible parts of Brassica campestris, Lactuca sativa L., and Lactuca sativa var. ramosa Hort. decreased by 5.9%-26.2%, 22.8%-30.1%, and 14.4%-19.1%, respectively, compared with those of the control groups. Treatments with 0.5%-1.5% sepiolite resulted in a significant increase (P < 0.05) in the proportion of 0.25-5.0 mm aggregates, and the increase in the mean weight diameter and geometric mean weight of the soil aggregates indicated that sepiolite treatments enhanced soil aggregate stability. Furthermore, three-dimensional X-ray computed tomography imaging showed that sepiolite treatments resulted in an increase in the total area, average size, and pore perimeter of aggregates, with the maximum values being 1.63-, 1.41-, and 1.401-fold higher than those of the corresponding control groups, respectively. The highest values of soil organic carbon and particulate organic carbon were obtained in 1.5% sepiolite-treated soils and were 2.07- and 1.91-fold higher than those of the control groups, respectively. Additionally, the level of organic carbon functional groups in the bulk soil and different particle-size aggregates generally increased with increasing sepiolite application. Overall, sepiolite, as a soil amendment, not only reduced toxic element bioavailability and uptake by plants but also enhanced soil structure and function.

Cd immobilization and soil quality under Fe-modified biochar in weakly alkaline soil

Cost-effective and environment-friendly implementation techniques are critical to the success of remediation in large-scale cadmium (Cd) contaminated agricultural soil. Field experiments were conducted to investigate the effect of Fe-modified biochar on Cd bioavailability in soils and uptake by maize (Zea mays L.), soil aggregate distribution and stability, and microbial community composition in weakly alkaline Cd-contaminated soil. Results showed that Fe-modified biochar optimized the structure and stability of soil aggregates. Moreover, the content of soil organic carbon increased by 6.59%-20.36% when compared with the control groups. However, DTPA-Cd concentration under the treatment of Fe-modified biochar was suffered by 37.74%-41.65% reduction in contrast with CK, and the significant decrease (P < 0.05) was obtained at 0.5% Fe-modified biochar. Moreover, sequential extraction procedures showed that the acid soluble and reducible states of Cd was converted into oxidizable and residual form. The addition of Fe-modified biochar inhibited Cd accumulation in maize, being 41.31%-76.64% (Zhengdan 958), 38.19%-70.95% (Liyu 86) and 52.30%-59.95% (Sanbei 218) reduction, respectively, in contrast with CK. The activity of catalase, urease and alkaline phosphatase in soil increased gradually with the addition of Fe-modified biochar. The enhancement in the number of soil bacterial OTUs and the values of Shannon, Chao1, ACE index indicated that Fe-modified biochar promoted the richness and diversity of bacterial communities. Therefore, the improvements of soil environment and biological quality indicated that Fe-modified biochar should be an alternative agent on remediation of Cd-contaminated soils.

Variation of soil organic carbon and physical properties in relation to land uses in the Yellow River Delta, China

Soil physical properties and soil organic carbon (SOC) are considered as important factors of soil quality. Arable land, grassland, and forest land coexist in the saline-alkali reclamation area of the Yellow River Delta (YRD), China. Such different land uses strongly influence the services of ecosystem to induce soil degradation and carbon loss. The objective of this study is to evaluate the variation of soil texture, aggregates stability, and soil carbon affected by land uses. For each land use unit, we collected soil samples from five replicated plots from "S" shape soil profiles to the depth of 50 cm (0-5, 5-10, 10-20, 20-30, and 30-50 cm). The results showed that the grassland had the lowest overall sand content of 39.98-59.34% in the top 50 cm soil profile. The content of soil aggregates > 0.25 mm (R_0.25), mean weight diameter and geometric mean diameter were significantly higher in grassland than those of the arable and forest land. R_0.25, aggregate stability in arable land in the top 30 cm were higher than that of forest land, but lower in the soil profile below 20 cm, likely due to different root distribution and agricultural practices. The carbon management index (CMI) was considered as the most effective indicator of soil quality. The overall SOC content and CMI in arable land were almost the lowest among three land use types. In combination with SOC, CMI and soil physical properties, we argued that alfalfa grassland had the advantage to promote soil quality compared with arable land and forest land. This result shed light on the variations of soil properties influenced by land uses and the importance to conduct proper land use for the long-term sustainability of the saline-alkali reclamation region.

The Spatial Distribution Characteristics of Soil Organic Carbon and Its Effects on Topsoil under Different Karst Landforms

Karst landforms are widely distributed in Guizhou Province, and the karst terrain is complex. To investigate the spatial distribution characteristics of soil organic carbon (SOC) in topsoil in different karst landforms, a total of 920 samples were taken from different karst landforms. The study areas, Puding, Xingyi, Guanling, Libo and Yinjiang in Guizhou Province, represent the karst plateau (KP), karst peak-cluster depression (KPCD), karst canyon (KC), karst virgin forest (KVF) and karst trough valley (KTV) landforms, respectively. The characteristics of the SOC contents in areas with different vegetation, land use and soil types under different karst landforms were analyzed. The dimensionality of the factors was reduced via principal component analysis, the relationships among SOC content and different factors were subjected to redundancy analysis, and the effects of the main impact factors on SOC were discussed. The results showed that there was a large discrepancy in the SOC contents in the topsoil layers among different types of karst landforms, the changes in the SOC content in the topsoil layer were highly variable, and the discrepancy in the upper soil layer was higher than that in the lower soil layer. The SOC contents in the 0–50 cm topsoil layers in different karst landforms were between 7.76 and 38.29 g·kg⁻¹, the SOC content gradually decreased with increasing soil depth, and the descending order of the SOC contents in different karst landforms was KTV > KVF > KC > KPCD > KP.

Soil aggregates stability and storage of soil organic carbon respond to cropping systems on Black Soils of Northeast China

Monoculture and improper management may reduce soil fertility and deteriorate soil structure in Black soils (Mollisols) of Northeast China. The experiment was carried out from 2015 to 2016 in Black Soils comprising five cropping systems: continuous corn (CC), soybean-corn rotation (SC), corn-soybean rotation (CS), fallow-corn (FC), and fallow-soybean (FS). Our results showed that CS and FS treatments significantly increased mean weight diameter (MWD) and fractal dimension (D) in mechanical stability aggregates (MSAs), and increased MWD and geometric mean diameter (GMD) in water-stable aggregates (WSAs) compared with CC treatment. These two treatments were also significantly increased water-stable aggregates stability rate (WSAR), but decreased percentage of aggregates destruction (PAD) than CC treatment. Meanwhile, CS and FS treatments exhibited a higher carbon accumulation than CC treatment in bulk soils. Soil organic carbon (SOC) concentration in WSA0.106-0.25,WSA2-5 mm and WSA0.5-1 mm had a dominant effect on aggregate stability. Simutaneously, SOC in WSA>5 mm affected SOC concentration in bulk soils. As a whole, the CS and FS treatments can increase the percentage of macro-aggregates, enhance aggregate stability, as well as increase SOC concentration in bulk soils and all soil aggregate sizes.

Co-regulation of photosynthetic capacity by nitrogen, phosphorus and magnesium in a subtropical Karst forest in China

Leaf photosynthetic capacity is mainly constrained by nitrogen (N) and phosphorus (P). Little attention has been given to the photosynthetic capacity of mature forests with high calcium (Ca) and magnesium (Mg) in the Karst critical zone. We measured light-saturated net photosynthesis (A_sat), photosynthetic capacity (maximum carboxylation rate [V_cmax], and maximum electron transport rate [J_max]) as well as leaf nutrient contents (N, P, Ca, Mg, potassium [K], and sodium [Na]), leaf mass per area (LMA), and leaf thickness (LT) in 63 dominant plants in a mature subtropical forest in the Karst critical zone in southwestern China. Compared with global data, plants showed higher A_sat for a given level of P. V_cmax and J_max were mainly co-regulated by N, P, Mg, and LT. The ratios of V_cmax to N or P, and J_max to N or P were significantly positively related to Mg. We speculate that the photosynthetic capacity of Karst plants can be modified by Mg because Mg can enhance photosynthetic N and P use efficiency.