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Information on the bioaccumulation of selenium (Se) in soil invertebrates (e.g. earthworms) is rather scarce. In the present study, bioaccumulation of Se in two eco-physiologically different earthworms, namely anecic Pheretima guillemi and epigeic Eisenia fetida , was determined after 28 days exposure to a successive doses of Se-spiked soil, specifically 0.5, 5, 50, and 200 μg Se g −1 soil. The results showed that Se concentration in earthworms elevated with increasing exposure levels, and maximums were up to 54.6 and 83.0 μg g −1 dry weight in Pheretima guillemi and Eisenia fetida , respectively, after 4 weeks exposure to 200 μg Se g −1 soil. Exposure to Se caused significant inhibition on earthworm growth, with the fresh weight loss ranging from 8.9% to 80.5%. Bioaccumulation factors (BAFs), empirically-derived and non-steady state, ranged from 0.12 to 4.17 and generally declined at higher exposure levels. Moreover, BAFs of Pheretima guillemi were higher than those of Eisenia fetida in low-dose Se-spiked soils, but the opposite was true in high-dose soils, indicating there is a species-specific response to exposure of Se between different earthworms. Further research is thus needed to reveal the accumulation pattern of Se in a wider range of earthworm species other than Eisenia fetida , which allows a better risk assessment of excessive Se to soil invertebrates and higher order organisms. [ABSTRACT FROM AUTHOR]

Afforestation is regarded as an effective way to sequester soil organic carbon (C) and hence to attenuate global warming. Nevertheless, so far mechanisms underlying the direction and magnitude of soil C pool variation following afforestation are not well understood. Here soil organic C, total nitrogen (N) and total phosphorus (P) pools in surface mineral soil (0–15 cm) were measured in cropland and plantation forest over two lithology types, i.e., limestone and clasolite, using a paired-site approach in a region of southwest China. The major objective was to test whether the effects of afforestation on soil C, N and P pools were affected by lithology. We found that lithology modulated the effect of afforestation on soil C or N pool, but not on soil P pool. Soil C pool was not altered by afforestation over limestone, but was significantly increased over clasolite ( P < 0.001). Instead, soil N pool was significantly decreased by afforestation over limestone ( P < 0.001), but was not altered over clasolite. Soil P pool was consistently decreased by afforestation regardless of lithology type ( P < 0.05). The molar ratios among soil C, N and P were not affected by lithology, but were significantly elevated by afforestation ( P < 0.01), indicating that soil C sequestration would increasingly be limited by soil N or P following afforestation. Our study suggests that afforestation effects on soil C and N pools may be controlled by lithology, and thus provides a new mechanism to explain the divergent responses of soil C and N following afforestation. [ABSTRACT FROM AUTHOR]

Soil carbon (C) sequestration through cropland conversion has been regarded as a major strategy to absorb atmospheric CO 2 and thus mitigate global warming, but much uncertainty still exists in terms of restoration strategies. In this study, soil C and nitrogen (N) were measured 13 years after cropland conversion in a karst area, southwest China. Four restoration strategies were included, i.e., (i) restoration with an economic tree species Toona sinensis (TS), (ii) restoration with Guimu-1 hybrid elephant grass (GG), (iii) restoration with a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG), and iv) spontaneous regeneration (SR). Cropland under maize-soybean rotation (CR) was used as reference and the space-for-time substitution approach was adopted to evaluate soil C and N stock (0–15 cm) change. Soil C stocks in TS and SR were elevated by 40% and 50%, respectively, relative to CR, while those in GG and ZG were not significantly changed. Soil N stocks in GG were not significantly changed, but stocks in TS, ZG and SR were enhanced by 130%, 81% and 117%, respectively, relative to CR. Significant correlation was found between soil C and N stock changes with rate of relative N stock change greater than that of C stock change. Similar to soil N stock, nitrate concentration, net nitrification rate and δ 15 N values in TS, ZG and SR were greater than those in GG or CR. Stepwise multiple linear regression indicated that exchangeable calcium was the soil variable mainly responsible for the dynamics of both soil C and N. Our results indicate that plantation with economic tree species and spontaneous regeneration are likely equally effective in soil C sequestration. [ABSTRACT FROM AUTHOR]

Abstract: Initial soil pH has been reported to have a great impact on the decomposition of added organic materials and hence to determine the direction and magnitude of subsequent soil pH changes. However, most previous investigations have been conducted on different soil types differing in initial pH and other soil properties. Here, we investigated the effects of initial soil pH on the subsequent soil pH and N mineralization changes caused by addition of crop residues to two soils (a Paleudalf and a Plinthudult) with different pHs induced by treatment with direct electric current. This produced pH gradients of 6.50–3.20 and 6.74–3.81, respectively. Three typical field crop residues differing in C/N ratio, i.e. rice straw C/N = 42, canola residue C/N = 36 and Chinese milk vetch C/N = 14 (vetch), were incubated with the soils for 102 days. With both soils, total CO2 fluxes differed between the type of added crop residues, with vetch > canola residue > rice straw, and decreased with decreasing initial soil pH. The incorporation of crop residues into the two soils at all pHs increased soil pH except for the Paleudalf at pH 3.81 and 4.25 amended with rice straw and canola residue, where pH slightly decreased. As expected, vetch incorporation caused a greater pH increase than the two non-legume residues (rice straw and canola residue), but this effect was transient in soils of higher initial pH. The pH declined rapidly with time in soils of initial pH ≥ 4.40 treated with vetch, while addition of rice straw and canola residue maintained stable pHs. Irrespective of crop residue addition, soil pH continuously increased or remained steady over the 102-day incubation period in the highly acidic soils (pH ≤ 4.25). While accumulated in highly acidic soils, accumulated in higher pH soils. Contrasting effects on nitrification and subsequent pH changes occurred between soil pH ≤ 4.25 and pH ≥ 4.40. However, no clear relationship between net N mineralization and soil pH was found. This study demonstrated that low soil pH greatly inhibited nitrification, while net N mineralization was generally less affected. This, in turn, affected the direction and extent of soil pH changes after addition of crop residues. [Copyright &y& Elsevier]

• Land use conversion from cropland to woodland tended to increase soil Se content. • Land use effect on soil Se bioavailability varied depending on the underlying geology. • SOC and pH were key factors controlling Se bioavailability in soils over limestone and clasolite, respectively. Land use effects on the biogeochemical cycling of the essential trace element selenium (Se) and underlying mechanisms are not well understood yet. Here, total Se (Se total) and its potential bioavailability, as represented by phosphate extractability (Se phosphate), were evaluated in top soils (0–15 cm) of cropland and woodland over two contrasting geological parent materials (i.e. limestone versus clasolite) in a Se-enriched region of southwest China. After about twenty years of conversion of land use from cropland to woodland, Se total in soils significantly (P < 0.05) and marginally significantly (P < 0.1) increased over limestone and clasolite, respectively. In terms of Se phosphate , it was not significantly changed after land use change from cropland to woodland in soil over limestone, whereas remarkably increased in soil over clasolite (P < 0.01). Furthermore, a significant positive relationship between Se phosphate and Se total was found in soil over clasolite, but not in soil over limestone. Land use conversion from cropland to woodland caused a decrease in the proportion of Se phosphate in Se total over limestone, while the opposite was found over clasolite. In comparison, the proportion of Se phosphate in Se total was greater in cropland over limestone than that in cropland over clasolite, indicating that limestone derived soils may have an advantage of producing Se-rich food over clasolite derived soils. SOC content and quality (i.e. C:N ratio) crucially determined Se accumulation and bioavailability in soil over limestone, while pH played a vital role in soil over clasolite. Taken together, our results provide strong evidence that land use effects on Se biogeochemistry can be substantially modulated by the underlying geology, and have important practical implications for effective utilization and management of Se-enriched soil resources in other parts of the world. [ABSTRACT FROM AUTHOR]

• Land use/land cover change affects total soil Se content and its bioavailability. • Soil organic carbon was the overriding edaphic factor controlling soil Se status. • Climate variables strongly influenced soil Se status at the regional scale. • Soil Se retention increased after cropland abandonment mainly due to SOC accumulation. Selenium (Se) is an essential micronutrient for human health, and its abundance and potential bioavailability in the soil are of increasing concern worldwide. To date, how total soil Se and its bioavailability would respond to human disturbance or future environmental change is not yet clear, and associated controlling factors remain incompletely understood. Here, we collected soil samples (0–15 cm) from different land use/land cover types, including active cropland, grassland, shrubland, and secondary forest, in a Se-enriched area of Guangxi, southwest China. Total Se concentration and its potential bioavailability, as estimated by phosphate extractability, were investigated. Total soil Se concentration (Se total) for all samples ranged from 220 to 1820 μg kg−1, with an arithmetic average value of 676 ± 24 μg kg−1 (Mean ± SE, the same below). The concentration of phosphate extractable Se (Se phosphate) varied between 1 and 257 μg kg−1, with an arithmetic mean value of 79 ± 5 μg kg−1, accounting for on average 13 ± 1% of the Se total. Among the four land use/land cover types, Se total and Se phosphate were generally more enriched in the secondary forest than those in the grassland and cropland. The content of soil organic carbon (SOC) was the overriding edaphic factor controlling the abundance and potential bioavailability of Se in topsoils. In addition, climatic variables such as mean annual precipitation and mean annual temperature were also key factors affecting the abundance and potential bioavailability of soil Se. Our results suggest that changes in land use/land cover types may deeply influence Se biogeochemistry likely via alterations in soil properties, particularly SOC content. [ABSTRACT FROM AUTHOR]

Free-living biological N 2 fixation (BNF) in soil, litter and moss is an important pathway for external nitrogen (N) inputs to unmanaged terrestrial ecosystems. However, how free-living BNF responses to temperature, moisture and their interaction have not been well understood, especially in the subtropical regions. Here we investigated the responses of BNF in soil, litter and moss to temperature (15, 25 and 35 °C) and moisture (low, intermediate and high levels) over a one-week period under laboratory conditions. The samples were collected in a subtropical karst forest, southwest China. We found that the responses of BNF in moss, litter and soil to temperature and moisture were divergent. Litter and soil BNF rates increased with temperature under the three moisture levels. However, moss BNF rates were decreased, not changed or increased by warming under low, intermediate and high moisture levels, respectively. At each temperature, the BNF rates increased with moisture for moss and litter. However, the patterns of moisture responses were different at the three temperatures for soil BNF, which was not changed and increased by moisture at 15 °C and 25 °C, respectively; and was greatest under intermediate moisture, but lowest under low moisture at 35 °C. The temperature sensitivity of BNF was generally highest for litter, intermediate for soil and lowest for moss. In contrast, the moisture sensitivity of BNF was generally highest for moss, intermediate for soil, and lowest for litter. Our findings suggest that different temperature or moisture sensitivities of BNF for the three substrates should be integrated into terrestrial ecosystem models in order to better predict N inputs via BNF under climate change. [ABSTRACT FROM AUTHOR]

Abstract Knowledge about resource limitation to soil microbes is crucial for understanding ecosystem functions and processes, and for predicting ecosystem responses to global changes as well. Karst ecosystems are widespread in the world, and play a key role in regulating the global climate, however, the patterns of and mechanisms underlying microbial resource limitation in karst ecosystems remain poorly known. Here we investigated the microbial resource limitation in a karst region, by selecting four main land-use types, i.e. cropland, grassland, shrubland and secondary forest, in areas underlain by two lithology types, i.e. dolomite and limestone, in southwest China. Ecoenzymatic stoichiometry was used as an indicator of microbial resource limitation. Overall, soil microbes in karst ecosystems were more limited by carbon and phosphorus, rather than by nitrogen. Further analyses revealed that the patterns of carbon and phosphorus limitation were different among land-use or lithology types. Microbial carbon limitation was greatest in cropland and forest but lowest in grassland, and was greater under dolomite than under limestone. Microbial phosphorus limitation decreased from secondary forest to cropland under dolomite areas, but showed no difference among ecosystem types under limestone areas, indicating that lithology controls the pattern of microbial phosphorus limitation along the post-agriculture succession. Our study describes a general pattern of microbial resource limitation in karst ecosystems, and we suggest that lithology may provide a new mechanism for explaining the variations of microbial resource limitation along the post-agriculture succession in different regions. Graphical abstract Unlabelled Image Highlights • Soil microbial resource limitation was investigated in karst ecosystems. • Soil microbes were limited by carbon and phosphorus, rather than by nitrogen. • Lithology controls the pattern of microbial P limitation along the succession. [ABSTRACT FROM AUTHOR]

Abstract Soil arylsulfatase (ARS) is an important enzyme that controls the acquisition of organic sulfur and thus the soil sulfur cycling. However, compared to those enzymes that involved in the acquisition of organic carbon, nitrogen, and phosphorus, ARS activity and its regulations have rarely been investigated. Here, a regional-scale investigation on soil ARS activity was conducted in southwest China to determine the major controls on soil ARS activity. Results showed that soil ARS activity increased from cropland through to forest following post-agriculture succession, and was significantly lower in clasolite than in dolomite or limestone. Among 14 studied soil variables, five variables (i.e. soil pH, exchangeable calcium, organic carbon, clay and silt) showed significant relationships with ARS activity, and these relationships were non-linear. Path analysis further showed that the activity of soil ARS was most controlled by soil pH and soil texture, but soil calcium and organic carbon only had indirect or spurious effects on ARS activity. Differing from most previous studies that used linear analysis, our results highlight that non-linear analysis is a better way for exploring the controlling factors of soil ARS activity, and we suggest that at the regional scale, soil pH and soil texture may be the most sensitive indicators of soil ARS activity. Highlights • ARS activity and its regulations were investigated at a regional scale. • We found non-linear relationships between soil properties and ARS activity. • Soil pH and texture are the most sensitive indictors of soil ARS activity. [ABSTRACT FROM AUTHOR]

It has been suggested that elevated nitrogen (N) deposition may increase soil N mineralization in N-limited ecosystems, but the underlying mechanisms have been not adequately explored. Soil N-acquisition enzymes play important roles on organic N mineralization. Thus, their responses to N deposition will be crucial for explaining the above phenomenon. Here, we conducted a meta-analysis from 64 studies to synthesize the responses of soil N-acquisition enzyme activities to N addition. Results showed that N addition significantly increased activities of N -acetylglucosaminidase and urease by 5.5% and 11.6%, respectively. However, N addition had negative or negligible effects on activities of protein-depolymerization enzymes, with no response for non-specific protease and leucine aminopeptidase but a significant decrease of 33.0% for glycine aminopeptidase. Because protein comprises more than 60% of the N in plant and microbial cells, and the protein depolymerization is an important rate-limiting step of organic N mineralization, the suppressed protein depolymerization indicates either that the changes to microbial activity may be not a dominant mechanism for the increased N mineralization in N-limited ecosystems with N addition, or that the increased N mineralization may be overvalued in the previous studies. [ABSTRACT FROM AUTHOR]