Search

Copyright of Journal of Applied Ecology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Understanding the dynamics and availability of soil nitrogen (N) affected by the conversion of cropping patterns is critical for environmental sustainability, especially in tropical soils with low fertility and high N loss. In this study, the 15N tracing technology combined with the 15N tracer model was used to explore the dynamic change of soil N transformation in long‐estabished areca nut (Areca catechu) plantations. Areca nut plantations with different ages (2, 5, 10, 14, and 17 years) and paddy fields in the tropical region of China were studied. The results demonstrated that the gross N mineralization rate (M) of areca nut plantation soil was much lower than that of paddy soil. The NH4+ immobilization (INH4) rate was also significantly reduced in areca nut plantations. Besides, the ONH4 (autotrophic nitrification) in long‐term areca nut planted soil decreased significantly with decreasing ammonia‐oxidizing archaea (AOA) and ammonia‐oxidizing bacteria (AOB) abundance. Inorganic N supply (INS) capacity of areca nut planted soil was much lower than the paddy soil, indicating declined N supply in long‐term areca nut plantations soil. The decline in soil gross N transformations rate and INS capacity of areca nut plantations soil was significantly correlated to reduced levels of TN and soil pH. Thus, agricultural practices that increase the soil pH (e.g., biochar or lime application) and the soil organic matter content (e.g., organic fertilizers) could improve soil INS capacity. [ABSTRACT FROM AUTHOR]

Soil microbial residues play an important role in the formation and stabilization of soil organic matter and can be quantitatively characterized by amino sugars. However, the response of soil microbial residues to agricultural cultivation in karst areas remains unclear. In this study, we collected soil samples from natural reserved land as well as five plantation forests dominated by Citrus trees cultivated for 0, 1, 5, 15, 30 years to examine the effects of agricultural cultivation on the content of microbial residues (amino sugar analysis). Results showed that: (1) Soil Amino Sugars (ASs) contents were significantly reduced after agricultural cultivation along with the sharp decrease in soil organic carbon (SOC). After 30 years of cultivation, the contents of total ASs, glucosamine (GluN), galactosamine (GalN), and muramic acid (MurA) in cultivated soils decreased by 58.22%, 55.30%, 27.11%, respectively, compared with 0 yr.; (2) Microbial residual carbon contribution to SOC increased from 34.11% to 81.33% after 30 years of cultivation, including fungal residual carbon (FRC) (25.79% to 48.6%) and bacterial residual carbon (BRC) (8.32% to 32.72%); (3) Soil GluN/MurA values tended to decrease with increasing cultivation years. The results highlight the significant effect of cultivation years on amino sugar accumulation. It indicates that the years of reclamation in karst areas have different impacts on the organic fractions derived from various microbial communities in the soil organic matter pool, and the microbial residues indicated by amino sugar are of great significance for the interception of soil organic matter. [ABSTRACT FROM AUTHOR]

To investigate the distribution characteristics and hazard levels of eight heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) in karst soil with a high geological background of heavy metals, 32 and 40 surface soil samples were collected from limestone and clastic rock areas, respectively, in the northern part of Mashan County, Guangxi Province, a typical mountainous county dominated by primary industries in China. Geostatistical methods, Pearson's correlation analysis, the geo-accumulation index, and the potential ecological hazard index were applied to explore the influencing factors of those heavy metals and evaluate their potential contamination risks. The results show that (1) the levels of the eight heavy metal elements in the surface soils of karst areas exceeded the background values of soil for Mashan County, the background value of soil (layer A) in China, and abundance value of upper crust. According to the soil pollution risk screening values specified in the Soil Environmental Quality: Risk Control Standard for Contamination of Agricultural Land, the proportions of heavy metals in the soils of karst areas were ranked as Cd (100%) > As (90.6%) > Cr (84.4%) > Zn (68.8%) > Ni (37.5%). Meanwhile, the heavy metals in the soils of non-karst areas did not exceed the overall values for Mashan County, and Ni, Pb, and Zn did not exceed the overall national soil values. One-quarter of Cd in non-karst samples exceeded the risk-threshold screening value. There was a high degree of variation and a significant difference in the contents of heavy metal elements between karst and non-karst areas. (2) The element combinations of As-Cd-Cu-Hg-Ni-Pb-Zn and Cr in karst areas were characterized by the influence of carbonate rock parent material. The non-karst areas were characterized by Ni-Cu-Pb-Zn, As-Cr-Hg, and Cd assemblages, which were mainly influenced by the mixture of laterite parent materials, sand shale parent materials, and basic-rock residual materials, and that may be affected by element migration caused by soil erosion and anthropogenic activities. (3) Analysis of the geo-accumulation index showed that karst areas were generally found to be at the clean to light pollution level, except for in the areas whose samples exhibited medium/high pollution levels for Cd and Cr, with the Cd pollution being the more serious of the two. Small amounts of Cd and Cu were present in the non-karst areas at a light contamination level, while other elements were at the level of no pollution. (4) The results of the potential ecological risk index showed that Cd and Hg were the main ecologically hazardous heavy metal elements in the soils of the study areas. The potential ecological risk level in karst areas was much higher than in non-karst areas, especially for Cd, and was mainly influenced by the carbonate rock parent material. [ABSTRACT FROM AUTHOR]

Enhanced nitrogen (N) deposition has shown significant impacts on forest greenhouse gas emissions. Previous studies have suggested that Chinese forests may exhibit stronger N2O sources and dampened CH4 sinks under aggravated N saturation. To gain a common understanding of the N effects on forest N2O and CH4 fluxes, many have conducted global-scale meta-analyses. However, such effects have not been quantified particularly for China. Here, we present a meta-study of the N input effects on soil N2O emission and CH4 uptake in Chinese forests across climatic zones. The results suggest that enhanced N inputs significantly increase soil N2O emission (+115.8%) and decrease CH4 uptake (−13.4%). The mean effects were stronger for N2O emission and weaker for CH4 uptake in China compared with other global sites, despite being statistically insignificant. Subtropical forest soils have the highest emission factor (2.5%) and may respond rapidly to N inputs; in relatively N-limited temperate forests, N2O and CH4 fluxes are less sensitive to N inputs. Factors including forest type, N form and rate, as well as soil pH, may also govern the responses of N2O and CH4 fluxes. Our findings pinpoint the important role of Southern Chinese forests in the regional N2O and CH4 budgets. [ABSTRACT FROM AUTHOR]

Determining the availability and supply capacity of soil inorganic nitrogen (N) can effectively guide the appropriate application of N fertilizers during crop cultivation. However, the mechanism underlying soil inorganic N production remains unknown for cash crops in karst regions. In this study, the rates of organic N mineralization to ammonium (NH4+) and NH4+ nitrification to nitrate (NO3−) were determined using a 15N tracing technique to evaluate the supply capacity of inorganic N in soils from woodland and pitaya plantations with different cultivation years (3, 9, and 15 years) in the subtropical karst region of China. The conversion of woodland to pitaya plantations significantly decreased the content of soil organic carbon (SOC), total N, calcium (Ca), and magnesium (Mg), along with the soil pH and cation-exchange capacity (CEC), but significantly increased the content of available potassium, available phosphorus, iron, and aluminum, in a more pronounced fashion with the increasing length of pitaya cultivation. The conversion of woodland to pitaya plantations has not significantly changed soil NH4+ and NO3− content, but this land use has resulted in divergent effects on mineralization and nitrification rates. Compared to woodland (5.49 mg N kg−1 d−1), pitaya cultivation significantly reduced the mineralization rate to 0.62–2.38 mg N kg−1 d−1. Conversely, the nitrification rate significantly increased from 4.71 mg N kg−1 d−1 in soil under woodland to 9.32 mg N kg−1 d−1 in soil under 3-year pitaya cultivation, but this rate decreased to 1.74 mg N kg−1 d−1 under 15-year cultivation. Furthermore, the mean residence time of inorganic N was significantly higher in long-term than in short-term pitaya plantations, indicating the decline in inorganic N turnover with the increasing length of pitaya cultivation. Taken together, long-term pitaya cultivation could significantly decrease the supply capacity and turnover of inorganic N in soil. The Ca, Mg, SOC, and total N content, as well as CEC, were significantly and positively related to the mineralization rate, but negatively related to the mean residence time of NH4+ and NO3−, suggesting that the incorporation of organic matter can accelerate the soil inorganic N supply and turnover for long-term pitaya plantation in subtropical regions. [ABSTRACT FROM AUTHOR]

Greenhouse cultivation with high nitrogen inputs is extensively used in the vegetable production in China, leading to severe soil degradation. To effectively improve the quality of degraded soils, reductive soil disinfestation (RSD) has been adopted as a reliable remediation method but high N2O emission would occur in nitrate-riched soils during RSD treatment. This study aimed to apply biochar amendment to reduce N2O emission during the RSD remediation of degraded vegetable soils. The remediation effect and N2O emission were investigated during the RSD process followed by tomato cultivation in pot culture experiment. Results showed that RSD treatments with and without biochar amendment both significantly increased soil pH from 4.0 to 6.1 and decreased NO3− content in degraded soil from 180 to less than 20 mg kg−1. These RSD treatments also promoted the plant growth and vegetable production in the following cultivation. Biochar amendment decreased N2O emission during the RSD treatment stage with a reduction of cumulative emission of 39%. The inhibitive effect of biochar on N2O emission was potentially realized through the increase of soil pH. Biochar amendment may be an effective means to mitigate N2O emission in the RSD application with promoting effects for vegetable growth. [ABSTRACT FROM AUTHOR]

Converting natural forests to managed ecosystems generally increases soil nitrous oxide (N 2 O) emission. However, the pattern and underlying mechanisms of N 2 O emissions after converting tropical forests to managed plantations remain elusive. Hence, a laboratory incubation study was investigated to determine soil N 2 O emissions of four land uses including forest, eucalyptus, rubber, and paddy field plantations in a tropical region of China. The effect of soil carbon (C) and nitrogen (N) fractions on soil N 2 O emissions and related functional genes was also estimated. We found that the conversion of natural forests to managed forests significantly decreased soil N 2 O emissions, but the conversion to paddy field had no effect. Soil N 2 O emissions were controlled by both nitrifying and denitrifying genes in tropical natural forest, but only by nitrifying genes in managed forests and by denitrifying genes in paddy field. Soil total N, extractable nitrate, particulate organic C (POC), and hydrolyzable ammonium N showed positive relationship with soil N 2 O emission. The easily oxidizable organic C (EOC), POC, and light fraction organic C (LFOC) had positive linear correlation with the abundance of AOA– amoA , AOB– amoA , nirK , and nirS genes. The ratios of dissolved organic C, EOC, POC, and LFOC to total N rather than soil C/N ratio control soil N 2 O emissions with a quadratic function relationship, and the local maximum values were 0.16, 0.22, 1.5, and 0.55, respectively. Our results provided a new evidence of the role of soil C and N fractions and their ratios in controlling soil N 2 O emissions and nitrifying and denitrifying genes in tropical soils. [Display omitted] • Converting natural forest to managed plantation decreased soil N 2 O emissions. • The higher particulate organic C and hydrolyzable ammonium N increased soil N 2 O emission. • Soil carbon and nitrogen fraction ratios are more accurate than the total ratios in predicting soil N 2 O emission. • Nitrifying genes dominated in managed forest ecosystems while denitrifying genes dominated in paddy soils. [ABSTRACT FROM AUTHOR]

The variations in soil N cycling after slash-and-burn practice. [Display omitted] • A 15N tracing study was employed to quantify the soil N cycling in the karst area. • Soil gross N transformation rates decrease significantly half-year post-fire. • Slash-and-burn lowers the inorganic N supply capacity in calcareous soils. • Declined soil organic carbon is one of the main causes of the altered N turnover. • Soil microbial community structure rather than population size impacts N turnover. Slash-and-burn is a common practice in the subtropical karst areas of southwest China, as well as in other places around the world, to prepare forest land for agricultural plantations. Understanding the effect of slash-and-burn practice on soil nitrogen (N) supply capacity is critical for the preservation of ecosystem services including long-term plant productivity. This is of particular relevance in mountainous karst regions with thin calcareous soils characterized by high pH, high calcium (Ca), and high soil loss risk. In this study, a 15N tracing approach was employed to quantify the effect of slash-and-burn on gross N -transformation rates and the mineralization-immobilization turnover (MIT) in soils from subtropical karst regions half-year after the burning, right before the agricultural cultivation began. Soil samples were collected from burned and nearby control sites (woodland). Compared to the control sites, the contents of soil organic carbon, total N, and Ca were lower in the burned sites, while no significant difference was observed in soil enzyme activities. The rates of mineralization of organic N to ammonium (NH 4 +), NH 4 + adsorption and release, NH 4 + oxidation to nitrate (NO 3 –) (i.e., autotrophic nitrification, O NH4), and microbial NO 3 – immobilization were significantly lower in burned sites, suggesting that slash-and-burn has a negative effect on MIT in karst regions. The mean residence times of inorganic N were 4.33 ± 0.53 d-1 for NH 4 + and 6.83 ± 1.76 d-1 for NO 3 – in burned soil, which is remarkably higher than the control soil (0.77 ± 0.08 d-1 for NH 4 + and 2.47 ± 0.83 d-1 for NO 3 –), indicating decreased N turnover after slash-and-burn. Abundances of bacteria and fungi, reflected by 16 s rRNA and ITS gene analyses, were not different between burned and control sites, suggesting that microbial community composition and activity, rather than the population size, controls the microbial-modulated N transformations. Owning to the slower NH 4 + and NO 3 – turnover and reduced soil N transformation rates, we conclude slash-and-burn practice significantly lowers soil inorganic N supply capacity, leading to remarkably reduced soil inorganic N content, especially for NO 3 – (from 59.9 ± 8.67 mg N kg−1 to 44.1 ± 1.86 mg N kg−1), which may further restrict the efficiency of agricultural exploitation. Thus, we recommend that the follow-up agricultural activities after slash-and-burn practice need to take this into account in addition to erosion control and nutrient conservation measures. [ABSTRACT FROM AUTHOR]

Purpose: Microorganisms play important ecological roles in NO pool production, e.g., through autotrophic nitrification (the oxidation of NH to NO) and heterotrophic nitrification (the oxidation of organic N to NO). Previous studies have mainly focused on nitrifying microorganisms capable of autotrophic nitrification; however, the contribution of microorganisms to heterotrophic nitrification has not been well investigated. The aim of this study was to identify the contribution of fungi and bacteria to autotrophic nitrification and heterotrophic nitrification in a subtropical coniferous forest soil of China. Materials and methods: A N tracing experiment was conducted in a subtropical forest soil, which was treated with CK (no antibiotics), cycloheximide (fungal inhibitor), and streptomycin (bacterial inhibitor), respectively. Soil was incubated at 25 °C with 60 % water holding capacity (WHC) for 144 h after being labeled with NHNO or NHNO at a 20 atom% N excess. Autotrophic and heterotrophic nitrification rates were calculated using a full process-based N cycle model. Results and discussion: The net nitrification rates were negative in all treatments, ranging from −0.346 mg N kg day in the cycloheximide treatment to −0.179 mg N kg day in the streptomycin treatment. The autotrophic nitrification was almost negligible (only 0.001 mg N kg day) in the soil from subtropical coniferous forest, while heterotrophic nitrification rate was 0.993 mg N kg day. Streptomycin reduced heterotrophic nitrification rate by 22.1 %, while cycloheximide nearly completely inhibited this process. Conclusions: Laboratory results showed that heterotrophic nitrification is a pivotal process producing NO and that fungi rather than bacteria may dominate heterotrophic nitrification in the subtropical coniferous forest soil. [ABSTRACT FROM AUTHOR]

The potential denitrification (PD) rate, NO, NO, and N emission were determined after treatment with 50 mg NO−N kg soil using the acetylene inhibition method, and meanwhile abundance of four denitrifying genes (i.e., narG, nirK, norB, nosZ) was also investigated in subtropical soils of China. Soil samples were collected from conifer forest (C), shrub forest, and farmland. These soils were derived from Quaternary red earth and granite. The PD rate and N gas emissions significantly ( p < 0.05) differed between forest and farmland soils; abundance of denitrifying genes was also significantly affected by the land-use change. Correlation and multiple stepwise regression analyses showed that the PD rate was significantly ( p < 0.05) and positively correlated with soil pH but not with soil organic C and total N contents ( p > 0.05). The norB gene copies in farmland soils were significantly higher than in conifer and shrub forest soils ( p < 0.01). Both norB and nosZ gene copies were linearly correlated with soil pH, and the PD rate and N emission rate were significantly correlated with the abundance of norB ( p < 0.05). Probably, soil pH affected denitrifiers targeted by the norB gene, thus decreasing the reduction of NO and NO. [ABSTRACT FROM AUTHOR]

Abstract: To date, very few studies have been conducted to investigate the characteristics of gross nitrogen (N) transformations in subtropical agricultural soils. In this study, 12 natural woodland and 10 agricultural soils were collected to investigate the effects of land use on soil gross N transformations in the humid subtropical zones in China. The results showed that gross autotrophic nitrification rates (average 0.19 mg N kg−1 d−1) in the woodland soils were significantly lower than those determined in the agricultural soils (average 1.81 mg N kg−1 d−1) (p < 0.01). However, the NO3 − immobilization rates (average 0.10 mg N kg−1 d−1) in the agricultural soils were significantly lower than in the woodland soils (average 0.47 mg N kg−1 d−1) (p < 0.01). On average, 98% of the total NO3 − produced could be immobilized into organic-N in the woodland soils, while, it accounted for only 10% in the agricultural soils. These differences in gross N transformations resulted in the inorganic N being dominated by NH4 + in the woodland soils; however, NO3 − dominated the inorganic N in the agricultural soils. The risk of N leaching and runoff from soil sharply increases after woodland soils are converted to agricultural soils. Application of organic fertilizers with high C/N ratios to agricultural soils in subtropical regions to increase soil organic C content and the C/N ratio is expected to improve NO3 − immobilization capacity and reduce the risk of N leaching and runoff from soil. [Copyright &y& Elsevier]

Sulfur dioxide ( SO2) in the atmosphere has been demonstrated to have many adverse impacts on the environment and human health. In this study, deposition of SO2 ranging from 9.0 to 127.8 mg kg−1 with an average of 35.7 mg S kg−1 was found to substantially stimulate NO and N2 O emissions from soils in the humid subtropical areas of Hainan, Fujian, Jiangxi, and Yunnan provinces of China under field conditions. Laboratory tests indicated that the stimulations were mediated biologically as the effects were not observed in sterilized soils. Acidification of soil resulting from SO2 deposition was not responsible for the stimulated NO and N2 O emissions alone as the stimulation did not occur by acidifying soil with HNO3 treatment. By using the 15 N tracing method, we found that the N2 O emissions stimulated by SO2 deposition were from either denitrification, heterotrophic nitrification or both, but not from autotrophic nitrification. Therefore, atmospheric SO2 deposition would most likely stimulate NO and N2 O emissions in acidic soils in which heterotrophic nitrification dominates NO and N2 O production and waterlogged soils in which denitrification dominates NO and N2 O production. [ABSTRACT FROM AUTHOR]

study was carried out to investigate the potential gross nitrogen (N) transformations in natural secondary coniferous and evergreen broad-leaf forest soils in subtropical China. The simultaneously occurring gross N transformations in soil were quantified by a N tracing study. The results showed that N dynamics were dominated by NH turnover in both soils. The total mineralization (from labile and recalcitrant organic N) in the broad-leaf forest was more than twice the rate in the coniferous forest soil. The total rate of mineral N production (NH + NO) from the large recalcitrant organic N pool was similar in the two forest soils. However, appreciable NO production was only observed in the coniferous forest soil due to heterotrophic nitrification (i.e. direct oxidation of organic N to NO), whereas nitrification in broad-leaf forest was little (or negligible). Thus, a distinct shift occurred from predominantly NH production in the broad-leaf forest soil to a balanced production of NH and NO in the coniferous forest soil. This may be a mechanism to ensure an adequate supply of available mineral N in the coniferous forest soil and most likely reflects differences in microbial community patterns (possibly saprophytic, fungal, activities in coniferous soils). We show for the first time that the high nitrification rate in these soils may be of heterotrophic rather than autotrophic nature. Furthermore, high NO production was only apparent in the coniferous but not in broad-leaf forest soil. This highlights the association of vegetation type with the size and the activity of the SOM pools that ultimately determines whether only NH or also a high NO turnover is present. [ABSTRACT FROM AUTHOR]

A 15 N-tracing study was conducted to investigate the gross N transformation rates involved in the production and consumption of NH 4 + and NO 3 − in karst region soils (a calcareous and a red soil) from southwest China under woodland. The gross mineralization rate of organic N to NH 4 + was much lower in calcareous soil (3.71 mg N kg −1 d −1 ) than in red soil (5.57 mg N kg −1 d −1 ), while the total gross nitrification rate was significantly higher in calcareous soil (5.80 mg N kg −1 d −1 ) than in red soil (0.70 mg N kg −1 d −1 ). Microbial NO 3 − immobilization accounted for 2.9% and 153% of the total nitrification in calcareous soil and red soils respectively. Thus the calcareous soils in the karst region have a low biological N retention capacity and high N losses, which are further aggravated by the high precipitation and fractured soil structure. [ABSTRACT FROM AUTHOR]

The bioavailable contents of heavy metals in karstic soils are a subject of increasing concern since the uptake of heavy metals by plants can pose a severe threat to food safety and public health. However, the bioavailable contents of heavy metals and their effective factors are poorly understood in karst regions. Calcareous soil and red soil developed from carbonate and clastic rocks, respectively, were chosen from a typical karst region (Guilin) of southwestern China, and the total (CT) and available (CA) contents of 11 heavy metals, as well as their influencing factors in soil profiles, were investigated. The results showed that calcareous soil has greater soil organic carbon, total nitrogen, available nitrogen, available potassium, and calcium (Ca) contents than red soil, but lower available phosphorus and C:N. Acid-soluble Ca (Aca) was the dominant fraction in both types of soil. Heavy metals were highly accumulated in calcareous soil, mainly controlled by secondary enrichment in the processing of carbonate rock weathering. For the majority of metals, calcareous soil had higher CT and lower CA than red soil. According to a redundancy analysis (RDA) and Pearson correlation coefficient, the high pH and Ca content in calcareous soils were primary factors influencing both the CT and CA of the metals, especially residual Ca to CT and Aca to CA. Additionally, higher soil cation exchange capacity and clay minerals also probably improved the immobility of heavy metals. [ABSTRACT FROM AUTHOR]

Understanding the changes in soil organic carbon (SOC) storage is important for accurately predicting ecosystem C sequestration and/or potential C losses, but the relevant information, especially for the evolvement of calcareous soil is limited in karst regions. Three calcareous soils with different evolvement intensities were sampled from an evergreen broadleaved forest in the subtropical region of southwest of China to investigate the changes in different SOC fractions and microbial communities. The results showed that: (1) The contents of SOC, dissolved organic carbon (DOC), mineral protected organic carbon (MOC), and recalcitrant organic carbon (ROC) significantly decreased with increasing evolvement intensity of calcareous soil, but pH and the chemical composition of SOC, including Alkyl C, O-alkyl C, Aromatic C, and Carbonyl C, did not significantly change, suggesting that various SOC fractions synergistically decrease with the evolvement of calcareous soil. (2) The evolvement of calcareous soil had a substantial negative effect on total phospholipid fatty acids (PLFA), bacteria (i.e., Gram positive bacteria and Gram negative bacteria), fungi, and actinomycetes, but did not affect the ratio of fungi to bacteria. This result supported the conclusion that various SOC fractions were synchronously loss with the evolvement of calcareous soil. (3) Results from the multivariate statistical analysis showed a significant correlation between SOC fractions (including SOC, DOC, MOC, and ROC) and soil base cations, mainly calcium (Ca), iron (Fe), and aluminum (Al). This strengthens the fact that SOC stability largely depends on the complex relationship between organic matter and mineral composition in soil. Taken together, the reduction of SOC during the evolvement of soil in the karst areas accords with some mechanisms of previous studies (e.g., microbial composition and soil geochemistry), and also has its own unique characteristics (e.g., the relative contribution of carbons to chemical shift regions of CPMAS 13C-NMR spectra and F:B ratio). [ABSTRACT FROM AUTHOR]

Determination of rates of mineralization of organic nitrogen (N) into ammonium-N (NH4+-N) and nitrification of NH4+-N into nitrate-N (NO3−-N) could be used to evaluate inorganic N supply capacity, which, in turn, could guide N fertilizer application practices in crop cultivation systems. However, little information is available on the change of mineralization and nitrification in soils under fruit cultivation systems converted from forestlands in karst regions. In a 15N-tracing study, inorganic N supply capacity in forest soils and three typical fruit crop soils under long-term cultivation was investigated, in addition to factors influencing the supply, in calcareous soils in the karst regions in southwestern China. Long-term fruit crop cultivation decreased soil organic carbon (SOC), total N, and calcium concentrations, cation exchange capacity (CEC), water holding capacity (WHC), pH, and sand content, significantly, but increased clay content. Compared to that of forests, long-term fruit crop cultivation significantly decreased mineralization and nitrification rates to 0.61–1.34 mg N kg−1 d−1 and 1.95–5.07 mg N kg−1 d−1, respectively, from 2.85–6.49 mg N kg−1 d−1 and 8.17–15.5 mg N kg−1 d−1, respectively, but greatly increased the mean residence times of NH4+-N and NO3−-N. The results indicate that long-term fruit crop cultivation could decrease soil inorganic N supply capacity and turnover in karst regions. Both mineralization and nitrification rates were significantly and positively correlated with SOC and total N concentrations, CEC, and WHC, but negatively correlated with clay content, suggesting that decreased soil organic matter and increased clay content were responsible for the decline in mineralization and nitrification rates in soils under long-term cultivation of fruit crops. The results of the present study highlight the importance of rational organic fertilizer application in accelerating soil inorganic N supply and turnover under long-term cultivation of fruit crops in karst regions. [ABSTRACT FROM AUTHOR]

To date, occurrence and stimulation of different nitrification pathways in acidic soils remains unclear. Laboratory incubation experiments, using the acetylene inhibition and N tracing methods, were conducted to study the relative importance of heterotrophic and autotrophic nitrification in two acid soils (arable (AR) and coniferous forest) in subtropical China, and to verify the reliability of the N tracing model. The gross rate of autotrophic nitrification was 2.28 mg kg day, while that of the heterotrophic nitrification (0.01 mg kg day) was negligible in the AR soil. On the contrary, the gross rate of autotrophic nitrification was very low (0.05 mg kg day) and the heterotrophic nitrification (0.98 mg kg day) was the predominant NO production pathway accounting for more than 95 % of the total nitrification in the coniferous forest soil. Our results showed that the N tracing model was reliable when used to study soil N transformation in acid subtropical soils. [ABSTRACT FROM AUTHOR]

In karst landscapes, soil CO2 is a key factor in weathering processes and carbon cycling, where its distribution and migration characteristics directly affect fluxes in carbon source–sink dynamics. We measured the CO2 emission and dissolution rates of carbonate tablets in calcareous soil developed from limestone and red soil developed from clastic rock, in karst and non-karst subtropical forests, in Guilin, southwest China between 2015 and 2018, to analyze their CO2 transfer characteristics and source–sink effects. The results showed similar average soil respiration rates between calcareous soil and red soil, with an average CO2 emission flux of 1305 and 1167 t C km−2 a−1, respectively. Carbonate tablet dissolution rates were bidirectional with increasing depth and were greater in red soil than calcareous soil, averaging 13.88 ± 5.42 and 7.20 ± 2.11 mg cm−2 a−1, respectively. CO2 concentration was bidirectional with increasing soil depth, reaching a maximum at the base of the soil–atmosphere interface (50–60 cm), and the bidirectional gradient was more distinctive in red soil. Change in the carbon isotope value of soil CO2 was also bidirectional in calcareous soils, for which the overall average was 0.87‰ heavier in calcareous than red soil. The carbon sink in calcareous soil in karst regions was estimated to be 11.97 times that of red soil in non-karst regions, whereas its role as a carbon source is just 1.12 times that of red soil, thus indicating the key role of karst soil in the reduction of atmospheric CO2. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) mineralization is closely related to carbon source or sink of terrestrial ecosystem. Understanding SOC mineralization under plum plantation is essential for improving our understanding of SOC responses to land-use change in karst rocky desertification ecosystem. In this study, 2-year, 5-year, and 20-year plum plantations and adjacent abandoned land dominated by herbs were sampled, and a 90-day incubation experiment was conducted to investigate the effect of plum plantations with different ages on SOC mineralization in subtropical China. Results showed that: (1) Plum plantation significantly decreased SOC content compared with abandoned land, but there was no significant difference in SOC content among plum plantations with different ages. Oppositely, the accumulative SOC mineralization (Ct) and potential SOC mineralization (C0) showed different responses to plum plantation ages. (2) The dynamics of the SOC mineralization were a good fit to a first-order kinetic model. Both C0 and Ct in calcareous soil of this study was several- to 10-folds lower than other soils in non-karst regions, indicating that SOC in karst regions has higher stability. (3) Correlation analysis revealed that both Ct and C0 was significantly correlated with soil calcium (Ca), suggesting an important role of Ca in SOC mineralization in karst rocky desertification areas. In conclusion, a Ca-rich geological background controls SOC mineralization in karst rocky desertification areas. [ABSTRACT FROM AUTHOR]

Understanding ecological stoichiometric characteristics of soil nutrient elements is crucial to guide ecological restoration and agricultural cultivation in karst rocky desertification region, but the information about the effect of the geological background on ecological stoichiometric ratios remains unknown. Soils from different landforms, including a basin, slope, and plateau, were sampled to investigate the spatial variance of the ecological stoichiometric characteristics of soil carbon (C), nitrogen (N), and phosphorus (P) under different rocky desertification grades (LRD: light rocky desertification; MRD: moderate rocky desertification; and SRD: severe rocky desertification) in a karst graben basin of Southwest China. Soil C:N ratio was not significantly influenced by rocky desertification grade, which was at a relatively stable level in the same landform, but soil C:P and N:P ratios increased with increasing rocky desertification grade. This change was consistent with increased soil organic carbon (SOC) and total nitrogen (TN) concentrations in the same geomorphic location along with the intensification of rocky desertification, but soil P concentration remained at a relatively stable level, indicating that P may be the limiting macronutrient for plant growth during vegetation restoration in a karst graben area. The soil C:N ratio of slope land was larger than that of the basin and plateau, while the soil C:P ratio and N:P ratio of the slope and plateau were significantly larger than that of the basin. The correlations between pH and C, N, and P stoichiometry decreased significantly when Ca was used as a control variable. In sharp contrast, the correlations between Ca and C, N, and P stoichiometry were highly significant no matter whether pH was used as a control variable, suggesting the important role of Ca in soil C, N, and P stoichiometry in karst graben basins. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reduced plant N limitation with the karst rocky desertification restoration. • The strong linkage between plant N limitation and soil N availability. • Rocky desertification restoration increase inorganic N supply capacity. • Soil structure, organic matter and microbes were drivers of N cycling. Nitrogen (N) limitation of plant growth following vegetation restoration is widespread in global terrestrial ecosystems, especially in karst rocky desertification areas. However, neither the temporal changes in plant N limitation during the restoration of those areas nor the mechanisms underlying N availability are well understood. In this study, several indicators reflecting soil N availability, N transformation rates, and plant communities were investigated in four areas in southwest China differing in their grade of rocky desertification. Our results showed that plant growth was severely N limited in the intense rocky desertification areas. The plant community-level foliar N content, 15N values, and N:P ratio increased significantly as the rocky desertification grade decreased, indicating a decrease in plant N limitation. This was attributed to increased soil N availability, evidenced by the higher soil δ15N values as well as total N and inorganic N contents along the rocky desertification grade. With the decreasing rocky desertification grade, the rates of organic N conversion to ammonium (NH 4 +) and nitrate (NO 3 –), the adsorption of NH 4 + on cation-exchange sites, and the release of adsorbed NH 4 + increased significantly, which could enhance soil inorganic N supply capacity and accelerate NH 4 + turnover to increase N availability. Noticeably, the sharp decrease in the rate of NH 4 + oxidation to NO 3 – with the decrease in the rocky desertification grade led to a shift in inorganic N from NO 3 –-dominated to NH 4 +-dominated. The increased contents of soil organic matter, calcium, iron-aluminum oxides, and sand, the proportion of aggregates > 2 mm, as well as the greater abundances of fungi and bacteria were the primary drivers of the N transformation rates along the rocky desertification grade. Overall, our study highlights the importance of N cycling in controlling N availability and thus in determining plant N limitation in karst rocky desertification areas. The results of the study provide a scientific basis for the ecological restoration of rocky desertification in karst ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: To date, N2O production pathways are poorly understood in the humid subtropical and tropical forest soils. A 15N-tracing experiment was carried out under controlled laboratory conditions to investigate the processes responsible for N2O production in four subtropical acid forest soils (pH<4.5) in China. The results showed that denitrification was the main source of N2O emission in the subtropical acid forest soils, being responsible for 56.1%, 53.5%, 54.4%, and 55.2% of N2O production, in the GC, GS, GB, and TC soils, respectively, under aerobic conditions (40%–52%WFPS). The heterotrophic nitrification (recalcitrant organic N oxidation) accounted for 27.3%–41.8% of N2O production, while the contribution of autotrophic nitrification was little in the studied subtropical acid forest soils. The ratios of N2O–N emission from total nitrification (heterotrophic+autotrophic nitrification) were higher than those in most previous references. The soil with the lowest pH and highest organic-C content (GB) had the highest ratio (1.63%), suggesting that soil pH-organic matter interactions may exist and affect N2O product ratios from nitrification. The ratio of N2O–N emission from heterotrophic nitrification varied from 0.02% to 25.4% due to soil pH and organic matter. Results are valuable in the accurate modeling of N2O production in the subtropical acid forest soils and global budget. [Copyright &y& Elsevier]

Abstract: Reactive forms of nitrogen (Nr) are accumulating at local, regional and global levels largely due to human activities, particularly N-fertilizer production and use as well as fossil fuel combustion. This has resulted in a change in the nitrogen (N) cycle and excess Nr in the environment, which has negative environmental effects. Therefore, characterizing denitrification and the edaphic variables controlling denitrification and its products is the first step in predicting the long-term effects of Nr accumulation. In the present study, six forest soil types in different climatic zones were collected from East China and evaluated for denitrification products following a K15NO3 amendment and subsequent incubation. The results showed that denitrification, indicated by production of nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2), was higher in the studied temperate forest soils than in the studied subtropical and tropical forest soils and was negatively correlated with soil redox potential at the beginning of incubation (r = −0.94, P < 0.01), but not with soil pH. The ratios of NO/total N gas and N2O/total N gas produced during denitrification varied among the soils, and were generally higher in the subtropical and tropical soils. Spearman''s correlation analysis showed that the NO ratio was positively correlated with soil oxidation capacity (OXC) (r = 0.94, P < 0.01) and redox potential at the beginning of incubation (r = 0.86, P < 0.05), but negatively correlated with soil pH (r = −0.83, P < 0.05). The N2O ratio was not significantly correlated with these edaphic variables, but showed a significant correlation to NO ratio (r = 0.83, P < 0.05). These results suggested that the OXC value might be the key factor affecting denitrification rates in soils. One possible explanation for these effects is that large OXC values would result in a higher level soil redox potential, thus suppressing denitrification and enhancing NO and N2O ratios during denitrification. [Copyright &y& Elsevier]

M Norg , organic N mineralization to NH 4 +; I NH4 , NH 4 + immobilization to organic N; O NH4 , NH 4 + oxidation to NO 3 −; DNRA , dissimilatory NO 3 − reduction to NH 4 +; I NO3 , NO 3 − immobilization to recalcitrant organic N; A NH4 , adsorption of NH 4 + on cation-exchange sites. • Natural forest soil has a high inorganic N supply capacity. • Mineralization and nitrification are reduced in degraded rubber plantation soil. • NH 4 + adsorption is increased in degraded rubber plantation soil. • Degraded rubber plantation soil has a low inorganic N supply capacity. • Degraded rubber plantation soil has a low NO 3 − turnover rate. Investigating soil inorganic nitrogen (N) supply and availability can guide soil improvement of rubber (Hevea brasiliensis) plantations in tropical regions, but the mechanisms controlling the inorganic N supply remain unknown. In this study, three natural forests and three degraded rubber plantations located in a tropical karst region of southwestern China were sampled to determine the gross N transformation rates using a 15N tracing method. The soils of the natural forests were characterized by a high inorganic N supply capacity and a high-level nitrate (NO 3 −) production potential, due to the high rates of organic N mineralization to ammonium (NH 4 +) (M Norg) and NH 4 + oxidation to NO 3 − (O NH4) but relatively low rates of immobilization of NH 4 + (I NH4) and NO 3 − (I NO3) to organic N and dissimilatory NO 3 − reduction to NH 4 + (DNRA). In the soils of the degraded rubber plantations, the rates of M Norg , O NH4 , I NO3 , and DNRA were lower but the rates of NH 4 + adsorption on cation-exchange sites (A NH4) increased, resulting in reductions in the inorganic N supply capacity and N availability. In addition, NO 3 − turnover in the soils of the degraded rubber plantations decreased, accompanied by a high mean residence time of NO 3 − and low δ15N values. Soil total N, organic C, phosphorus, and potassium concentrations, water-holding capacity, cation-exchange capacity, and sand content were significantly lower in the soils of the degraded rubber plantations than in those of the natural forests, indicating a decline in soil quality in the former. The significant, positive relationships between these soil properties and the rates of M Norg , O NH4 , I NO3 , and DNRA highlight the importance of the appropriate application of organic N fertilizers as well as phosphorus and potassium fertilizers to stimulate soil N cycling and thereby increase the inorganic N supply. A reduction of the N deficiency in soils used for rubber tree cultivation would alleviate the soil degradation that characterizes many rubber plantations in tropical karst regions. [ABSTRACT FROM AUTHOR]

Fragile karst ecosystems are characterized by complex topographic landscapes associated with high variations in vegetation restoration. Identifying the characteristics and driving factors of nitrogen (N) availability across the topographic gradient is essential to guide vegetation restoration in karst regions. In this study, we collected soil samples and plant leaves along the topographic gradient (ridge, upper slope, middle slope, and foot slope) of convex slopes in the karst fault basin of southwest China, and determined the indicators reflecting soil N availability, N transformation rates, and their controlling factors. Our results showed that foliar N content and δ 15 N value, soil inorganic N content and δ 15 N value, and foliar N:P ratio were substantially lower on the steep hillslopes than on the flat top ridge. Steep slope soils also had a lower enzyme C:N ratio but a higher enzyme N:P ratio than the flat ridge soils. Furthermore, the vector angles calculated by soil extracellular enzyme analysis were below 45 o in all studied soils and decreased significantly with increasing slope, indicating that microbial growth was generally limited by N. These results jointly suggest the declines in soil N availability across the topographic gradient, which are further explained by the changes in soil inherent N transformation processes. As the slope became steeper, soil mineralization and autotrophic nitrification (O NH4 ) rates decreased significantly, while ratio of microbial NH 4 + immobilization to O NH4 and NH 4 + adsorption rate increased significantly, indicating the decrease in soil inorganic N supply capacity. We further found that deteriorated soil structure, decreased soil organic matter and calcium content, altered microbial abundance, and increased ratios of fungi to bacteria and gram-positive bacteria to gram-negative bacteria were the primary drivers of reduced N transformation rates and N availability across the topographic gradient. Overall, this study highlights the critical role of the topography in controlling soil N availability by regulating N transformation processes in karst regions. The topography should be considered an important factor affecting the functions and services of karst ecosystems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)