Soil micro‐organisms play a key role in soil biogeochemical cycles, but their growth and activities are often limited by resource availability. Understanding soil processes that are driven by ...micro‐organisms and resource limitation of microbes will help to elucidate controls on soil fertility and improve the ability to predict the responses of an ecosystem to global changes. As a widespread ecosystem type, karst ecosystem develops from limestone or dolomite with unique soil; however, karst ecosystems remain poorly understood regarding their soil microbial processes and microbial resource limitation.
Here, ecoenzymatic stoichiometry was used as an indicator of microbial resource limitation, and to model major microbial processes (i.e. decomposition of soil organic carbon and microbial respiration) in a karst and a non‐karst forest.
Results showed that the modelled decomposition and respiration rates were significantly higher in the karst forest than in the non‐karst forest. In addition, results of ecoenzymatic stoichiometry showed that the karst forest was more carbon‐limited than the non‐karst forest. In contrast, the karst forest was likely saturated with nitrogen, but the non‐karst forest was limited by nitrogen. Both the karst and non‐karst forests were limited by phosphorus, but phosphorus deficiency was more evident in the non‐karst forest than in the karst forest.
These findings highlight the specific profiles of karst ecosystems, and they suggest that the responses of karst ecosystems to global changes should be very different compared to other ecosystems.
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•A meta-analysis was used to address effects of N addition on N-acquisition enzyme.•N addition increased activities of N-acetylglucosaminidase and urease.•N addition had negative or ...negligible effects on protein-deploymerization enzymes.•N addition has minor effects on soil N mineralization by changing microbial activity.
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.
Soil carbon (C) sequestration through cropland conversion has been regarded as a major strategy to absorb atmospheric CO2 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 (0–15cm) 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 δ15N 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.
Biochars produced from swine manure (SM), fruit peels (FP), Phragmites australis (PA) and Brassica rapa (BR) were applied at different rates to a sandy loam soil at 70% moisture. Phospholipid fatty ...acid (PLFA) measurements showed that feedstock type, biochar type and application rate significantly affected the soil microbial communities. PLFAs derived from bacteria, fungi, actinomycetes, G+ve and G−ve bacteria and sulfate reducers were higher with FP biochar at 3% and 1% weight: weight (wt:wt), respectively, followed by SM at 1% and PA biochar at 3%, than in the control soil. The control soil also contained higher concentrations of certain iso:anteiso PLFAs, which are indicative of environmental stress, than did biochar treated soils. Protozoa PLFAs only increased in PA 3% and BR 1% treatments. Redundancy analysis illustrated the relationships between microbial communities and chemical properties within biochar types and addition rates to soil. The analysis indicated that different biochars induced different chemical changes such as increased pH, dissolved organic carbon and total carbon and nitrogen in soil and changed the microbial community structure. These properties may be used as indicators of both soil improvement and C sequestration.
•Microbial community variations in 4 biochars amended soil were studied by PLFAs.•Microbial community changed after biochar induced the change of soil properties.•Only PA 3% and BR 1% treatments were found to increase the PLFAs of protozoa.•Application of some biochars could reduce environmental stress to soil organisms.
•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 (Setotal) and its potential bioavailability, as represented by phosphate extractability (Sephosphate), 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, Setotal in soils significantly (P < 0.05) and marginally significantly (P < 0.1) increased over limestone and clasolite, respectively. In terms of Sephosphate, 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 Sephosphate and Setotal 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 Sephosphate in Setotal over limestone, while the opposite was found over clasolite. In comparison, the proportion of Sephosphate in Setotal 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.
Cropland conversion has a major impact on soil C sequestration. However, it remains unclear about the changes in soil aggregate and their contribution to C accumulation following cropland conversion ...in a karst region, southwest China. In this study, three different cropland use types (sugarcane, mulberry and forage grass cultivation) were selected to replace maize-soybean cultivation. The soil was collected at a depth of 0 to 30 cm for analysis of soil aggregates and their OC content. Results showed that macro-aggregate was the predominant component underlying four cropland use types. Forage grass cultivation remarkably increased the OC stock and aggregate stability (MWD and GMD). OC content and stock associated with aggregate varied with cropland use types and soil depth, but were typically highest in forage grass fields. Macro-aggregates contained higher OC content and stock than other aggregate fractions, along with soil depth underlying four cropland use types. The increases in OC stock in forage grass field was mainly due to increased OC stocks within macro-aggregates, which is further attributed to the increase in OC content within macro-aggregates. Overall, forage grass cultivation replaced maize-soybean cultivation was suggested as an ecological restoration model to enhance soil C sequestration potential, owing to its role in increasing OC stock of aggregation and aggregate stability, in the karst region of southwest China.
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 NH4+ accumulated in highly acidic soils, NO3− 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.
•Initial soil pH controls nitrification rate and determines subsequent pH change.•Contrasting effect on N transformation occurred between pH ≤ 4.25 and pH ≥ 4.40.•Low pH strongly inhibited nitrification but net N mineralization was less affected.
Agricultural abandonment is regarded as a major driver of soil organic carbon (C) dynamics, but the mechanisms underlying the direction and magnitude of soil C dynamics following agricultural ...abandonment are poorly understood. Here dynamics of soil C and N contents during postagricultural succession were investigated in areas underlain by dolomite or limestone by using a space‐for‐time substitution approach in a karst region, southwest China. One hundred twenty‐five sites from cropland, grassland, shrubland, and secondary forest were selected to represent different succession stages. Overall, soil C and N contents were greater (P < 0.05) over limestone than over dolomite mainly due to significantly greater contents of soil C and N in the cropland and grassland underlain by limestone. Both soil C and N contents were lowest in the cropland while highest in the forest. Further analysis indicated that the patterns of soil C and N dynamics differed between the two lithology types. Soil C and N contents increased significantly from cropland to forest over dolomite, while varied insignificantly among succession stages over limestone. Exchangeable calcium explained most of soil C and N variance. We proposed that higher dissolution rate of limestone could replenish the lost calcium so that the calcium levels, and in turn soil C and N contents, were stable from the cropland to the forest. Nevertheless, due to relatively low dissolution rate for dolomite, the calcium level was depleted in the cropland. Following agricultural abandonment, calcium level recovered due to decreased loss, which in turn resulted in recovery of soil C and N.
Key Points
Soil C and N contents are greater over limestone than over dolomite
Soil C and N contents increase after agricultural abandonment over dolomite but not over limestone
Exchangeable Ca is key in controlling soil C and N dynamics following agricultural abandonment
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.
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•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.
PURPOSE: The Escherichia coli (E. coli) O157:H7 survival dynamics in original and pH-modified agricultural soils were investigated to determinate how E. coli O157:H7 survival responded to the pH ...values of different soils, identify the relationships between E. coli O157:H7 survival time (t d ) and soil properties, and assess the potential pathogen contamination after soil pH changed. MATERIALS AND METHODS: The six soil samples were collected from different provinces of China, and 18 pH-modified soil samples were obtained from original soils by treating the original soils with direct electric current. The E. coli O157:H7 cells were inoculated into 24 soils and incubated at soil moisture of −33 kPa and 25 °C. The soils were sampled for determining the numbers of E. coli O157:H7 at given time intervals over the incubation. The effects of soil pH change and other properties on the t d values were analyzed. RESULTS AND DISCUSSION: The t d values in the test soils were between 7.1—24.7 days. Results indicate that soil pH, texture, and free Fe₂O₃ (Fed) were the most important factors impacting the t d values in the test soils. Further, the response of E. coli O157:H7 survival to pH change varied with different soils. In the acidic soils (shorter t d values), the t d values decreased as the pH decreased and Fed increased, while in the neutral or alkaline soils (pH ≥ 6.45, longer t d values), the t d values did not change significantly with pH. CONCLUSIONS: The changes of amorphous and free sesquioxides induced by pH change might strengthen the response of E. coli O157:H7 survival to soil pH. Closer attention should be paid to E. coli O157:H7 long survival in soils and its potential environmental contamination risk.
