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Soil ammonia oxidizers play a critical role in nitrogen cycling and ecological restoration. The composition and structure of soil ammonia oxidizers and their impacting factors were studied in four typical ecosystem soils, tussock (T), shrub (S), secondary forest (SF), and primary forest (PF), during vegetation restoration in the Karst region of Southwest China. The composition and structure of the ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) communities were characterized by sequencing the amoA and arch- amoA genes, respectively. The diversity of soil ammonia oxidizers (except in S) and plant Shannon diversity index gradually increased with vegetation restoration, and the ammonia oxidizer communities differed significantly ( p < 0.001). Amplicons of AOA from the Nitrososphaera cluster dominated all four ecosystem soils. AOB Nitrosospira cluster 3b only appeared in PF and SF soils, while Nitrosospira cluster 3a species were found in all soils. Changes in AOB paralleled the changes in soil ammonium content that occurred with vegetation restoration. Redundancy analysis showed that the distribution of dominant AOB species was linked to pH, soil urease activity, and soil C/N ratio, whereas the distribution of dominant AOA species was mainly influenced by litter nitrogen content and C/N ratio. These results suggested that the composition and structure of the AOB community were more sensitive to changes in vegetation and soil ammonium content, and may be an important indicator of nitrogen availability in Karst ecosystem soils. [ABSTRACT FROM AUTHOR]

Plants associated with symbiotic nitrogen-fixers and soil free-living nitrogen-fixing bacteria are good indicators for detecting the source of nitrogen in natural ecosystems. However, the community composition and diversity of plants associated with symbiotic nitrogen-fixers and soil free-living nitrogen-fixing bacteria in karst shrub ecosystems remain poorly known. The community composition and diversity of soil free-living nitrogen-fixing bacteria and plants, as well as the soil physical–chemical properties were investigated in 21 shrub plots (including different topographies and plant types). The frequency of plants associated with symbiotic nitrogen-fixers was found to be low in the 21 shrub plots. The soil free-living nitrogen-fixing bacterial community structure varied among the 21 shrub soils. Based on a variance partitioning analysis, topography, plant type, and soil pH explained 48.5% of the observed variation in bacterial community structure. Plant type had a predominant effect on community structure, and topography (aspect and ascent) and soil pH had minor effects. A negative correlation between the abundance of the soil free-living nitrogen-fixing bacterial community and the richness index for plants associated with symbiotic nitrogen-fixers was observed. The result of the low frequency of plants associated with symbiotic nitrogen-fixers highlights the importance of sources of fixed nitrogen by soil free-living nitrogen-fixing bacteria in the nitrogen limitation shrub ecosystem of the karst regions. [ABSTRACT FROM AUTHOR]

The article examines the variations of surface soil moisture content in a karst depression area in southwest China during the dry and rainy seasons in 2005. It finds that the variability of surface soil moisture was relatively higher in the dry season than in the rainy season, while on the other hand, the mean soil moisture content was higher during the rainy season, especially after heavy rainfall. It also identifies land use types, bare-rock ratio, and soil organic matter as major factors affecting soil moisture variability in the karst ecosystem.

Particulate organic matter (POM) chemistry and microbial degraders are important determinants of soil carbon (C) mineralization, but the effects of their interactions on C mineralization are largely unknown. Two contrasting soils with different POM chemical composition from karst ecosystems were sampled, sterilized, and cross-inoculated with microorganisms for 124 days to explore the relationships between POM chemical composition, microbial community composition, and soil organic C (SOC) mineralization. Pyrolysis-gas chromatography/mass spectrometry analysis was used to determine POM chemistry, and phospholipid fatty acid (PLFA) analysis was used to characterize the soil microbial community. Microbial cross-inoculation showed that the microbial community composition did not change in response to changes in soil C and that the relatively labile POM components, e.g. , fatty acids and n -alkenes, explained 61.2% of the variation in microbial community composition. Microbial community composition, such as PLFAs cy19:0, i17:0, 10Me16:0, and 18:2ω6,9c, strongly influenced POM chemical composition, explained 94.5% of the variation in POM chemical composition. These results indicated that soil microbial communities could adapt to changes in POM and served as main drivers of POM chemistry alterations. In addition, mineralized soil C (% of SOC) was significantly influenced by microbial community composition, soil source, and their interactions. Redundancy analysis and Mantel tests further revealed that SOC mineralization was strongly affected by POM chemical composition ( e.g. , the content of ketones and p -hydroxyphenyl) and microbial community composition ( e.g. , the content of PLFAs 16:1ω7c and 10Me16:0), and that the influence of microbial community composition on SOC mineralization was highly dependent on POM chemical composition, suggesting that the interactions of POM chemistry and microbial community composition mediate SOC mineralization. These analyses indicate a tight relationship between POM chemistry and microbial community composition, and highlight the importance of their interactions in mediating the persistence of organic matter in karst soils. [ABSTRACT FROM AUTHOR]

Highlights • SOC recovers to levels of primary forest within a shorter period than soil N. • Recuperation time of SOC and soil N had opposite patterns in topsoil and subsoil. • Soil N is capable of sustaining SOC accumulation in the long term in karst regions. Abstract Vegetation restoration following agricultural abandonment is an effective method for soil organic carbon (SOC) sequestration. However, SOC dynamics during post-agriculture succession in different soil layers, and its regulation by soil nitrogen (N), are not well understood. We investigated SOC and soil N dynamics during post-agriculture succession at soil depths of 0–10, 10–20, 20–30, and 30–50 cm in a subtropical karst area in southwest China. The succession sequence included grassland (∼9 years), shrubland (∼25 years), secondary forest (∼52 years), and primary forest, with cropland as a reference. In the 0–50 cm soil profile, the SOC and soil N stocks will recover to the primary forest level in ∼74 years at a rate of 112.35 g C m−2 yr−1 and in ∼100 years at a rate of 12.07 g N m−2 yr−1, respectively. The accumulation rate of SOC and soil N was higher in the topsoil (0–20 cm) than in the subsoil (20–50 cm). The estimated time for SOC stock to recover to the primary forest level in topsoil and subsoil was similar (72 and 76 years, respectively), whereas that for soil N stock require more time in topsoil than in subsoil (106 and 89 years, respectively). The higher rate of relative N stock change than that of SOC suggests that progressive N limitation does not occur in the long term following agricultural abandonment. Our results highlight the importance of soil depth and soil N in determining SOC change for secondary succession. Furthermore, the results suggest that, compared to that of previous studies, which considered only surface soil in the karst regions of southwest China, the recovery period of C and N in soil profile may be longer than that assumed previously. [ABSTRACT FROM AUTHOR]

The addition of exogenous inorganic carbon (CaCO 3 ) and organic carbon has an important influence on soil organic carbon (SOC) mineralization in karst soil, but the microbial mechanisms underlying the SOC priming effect are poorly understood. We conducted a 100-day incubation experiment involving four treatments of the calcareous soil in southwestern China's karst region: control, 14 C-labeled rice straw addition, 14 C-labeled CaCO 3 addition, and a combination of 14 C-labeled rice straw and CaCO 3 . Changes in soil microbial communities were characterized using denaturing gradient gel electrophoresis with polymerase chain reaction (PCR-DGGE) and real-time quantitative PCR (q-PCR). Both 14 C-rice straw and Ca 14 CO 3 addition stimulated SOC mineralization, suggesting that organic and inorganic C affected SOC stability. Addition of straw alone had no significant effect on bacterial diversity; however, when the straw was added in combination with calcium carbonate, it had an inhibitory effect on bacterial and fungal diversity. At the beginning of the experimental period, exogenous additives increased bacterial abundance, although at the end of the 100-day incubation bacterial community abundance had gradually declined. Incubation time, exogenous input, and their interaction significantly affected SOC mineralization (in terms of priming and the cumulative amount of mineralization), microbial biomass carbon (MBC), and microbial community abundance and diversity. Moreover, the key factors influencing SOC mineralization were MBC, bacterial diversity, and soil pH. Overall, these findings support the view that inorganic C is involved in soil C turnover with the participation of soil microbial communities, promoting soil C cycling in the karst region. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AM) fungi are crucial for vegetation regeneration because they play an important role in plant establishment and growth. We investigated the relationships among AM fungal diversity, plant diversity, and soil properties during vegetation restoration in degraded karst ecosystems. Pyrosequencing approach was used to determine the genetic diversity and composition of AM fungal communities in four typical ecosystems of vegetation restoration (tussock (TK), shrub (SB), secondary forest (SF), and primary forest (PF)) in a karst region of southwest China. The diversity, richness, and evenness of plant species were evaluated through field surveys. Basic soil properties were measured. Plant species diversity and soil nutrient contents increased with vegetation restoration from tussock to primary forest, but the diversity of AM fungi followed the order of shrub ≈ secondary forest ≈ primary forest < tussock. The composition of AM fungi and plant communities differed significantly between ecosystems ( p < 0.05). The richness of AM fungi was negatively correlated with both the plant diversity (the indices of plant Shannon–Wiener, evenness and richness) and soil properties (soil available phosphorus (AP), soil organic carbon (SOC) and pH) ( p < 0.05). Redundancy analysis showed that the AM fungal communities closely linked to plant richness, soil organic carbon, soil available phosphorus and pH. These results suggest that the diversity and composition of AM fungi in karst region are influenced by plant communities and soil nutrient conditions. [ABSTRACT FROM AUTHOR]

• Microbial resource limitation is useful to identify effective fertilization strategies. • Soil microbes in karst croplands is limited by C and P, rather than by N. • Karst croplands do not need too much N fertilizer. • Combined inorganic and organic fertilization is better for karst croplands. Fertilization is a key management practice for maintaining or improving soil fertility in agro-ecosystems. Nevertheless, how fertilization strategies impact the status of soil microbial resource limitation is poorly understood. Here, we investigated the effects of long-term (11 years) fertilization on microbial resource limitation in a karst cropland under maize–soybean rotation. Soil microbial resource limitation was assessed using enzymatic stoichiometry. Six fertilization strategies were included, i.e., i) no fertilization (control), ii) inorganic fertilizers only (NPK), iii) inorganic fertilizers plus a low amount of straw (LSNPK), iv) inorganic fertilizers plus a low amount of manure (LMNPK), v) inorganic fertilizers plus a high amount of straw (HSNPK), and vi) inorganic fertilizers plus a high amount of manure (HMNPK). Overall, soil microbes were not limited by nitrogen, but co-limited by carbon and phosphorus across the six fertilization strategies. However, the degrees of microbial resource limitations were different between the control and fertilizer treatments. Application with inorganic fertilizers only aggravated microbial carbon limitation, but combined application of inorganic fertilizers and organic matters did not change the status of carbon limitation relative to the control. None of the fertilizer treatments changed the status of microbial nitrogen limitation. The treatments of NPK, LSNPK and LMNPK alleviated microbial phosphorus limitation, but HSNPK and HMNPK had no significant effects on phosphorus limitation relative to the control. By contrast, the crop production had no significant difference among all fertilizer treatments in the current study. Together, our results indicate that fertilizations can change microbial resource limitation status, which might be a more sensitive indicator to identify effective fertilization strategies relative to the crop production. Here we suggest that karst croplands do not need too much nitrogen fertilizer due to the nitrogen-rich characteristic, and that combined inorganic and organic fertilization strategies are better than single fertilization strategy in karst croplands. [ABSTRACT FROM AUTHOR]