Accurate estimation of reference evapotranspiration (ET0) is very important in hydrological cycle research, and is essential in agricultural water management and allocation. The application of the ...standard model (FAO-56 Penman-Monteith) to estimate ET0 is restricted due to the absence of required meteorological data. Although many machine learning algorithms have been applied in modeling ET0 with fewer meteorological variables, most of the models are trained and tested using data from the same station, their performances outside the training station are not evaluated. This study aims to investigate generalization ability of the random forest (RF) algorithm in modeling ET0 with different input combinations (refer to different circumstances in missing data), and compares this algorithm with the gene-expression programming (GEP) method using the data from 24 weather stations in a karst region of southwest China. The ET0 estimated by the FAO-56 Penman-Monteith model was used as a reference to evaluate the derived RF-based and GEP-based models, and the coefficient of determination (R2), Nash-Sutcliffe coefficiency of efficiency (NSCE), root of mean squared error (RMSE), and percent bias (PBIAS) were used as evaluation criteria. The results revealed that the derived RF-based generalization ET0 models are successfully applied in modeling ET0 with complete and incomplete meteorological variables (R2, NSCE, RMSE and PBIAS ranged from 0.637 to 0.987, 0.626 to 0.986, 0.107 to 0.563 mm day−1, and −2.916% to 1.571%, respectively), and seven RF-based models corresponding to different incomplete data circumstances are proposed. The GEP-based generalization ET0 models are also proposed, and they produced promising results (R2, NSCE, RMSE and PBIAS ranged from 0.639 to 0.944, 0.636 to 0.942, 0.222 to 0.555 mm day−1, and −1.98% to 0.248%, respectively). Although the RF-based ET0 models performed slightly better than the GEP-based models, the GEP approach has the ability to give explicit expressions between the dependent and independent variables, which is more convenient for irrigators with minimal computer skills. Therefore, we recommend applying the RF-based models in water balance research, and the GEP-based models in agricultural irrigation practice. Moreover, the models performance decreased with periods due to climate change impact on ET0. At last, both of the two methods have the ability to assess the importance of predictors, the order of the importance of meteorological variables on ET0 in Guangxi is: sunshine duration, air temperature, relative humidity, and wind speed.
•Agriculture in karst hills is more likely to cause more soil and nutrient losses.•Karst structure might lead to fast nutrient loss and low fertilizer use efficiency.•Results suggest nutrient losses ...controlled by landform and climate condition.•Carbonate weathering and carbon cycle are strongly impacted by agriculture in karst regions.•Beneficial management practices should be considered for sustainable development.
The karst region in southwestern China is a typical region with fragile ecological environments, and the coordination of regional agricultural development and environmental protection faces enormous challenges. Based on the soil and hydrological characteristics of karst in southwestern China, this study summarized water-related environmental issues caused by agricultural activities in this karst region. Agriculture in the karst hills is more likely to cause soil and nutrient losses due to the fast hydrological flow through special karst structures with high permeability. Thus, this review emphasized the impacts of agricultural development on the riverine biogeochemical cycles of elements based on previous studies. Meanwhile, the carbon cycle is also strongly impacted by agricultural activities in this karst region due to enhanced carbonate weathering by nitric acid from the nitrification of ammonium. This weathering mechanism represents a net source of atmospheric CO2 and might impact regional and global carbon cycling in the Anthropocene. Based on the results summarized in this study, we advocate that in the future, better management of agricultural land, improvement of fertilizer use efficiency, and boosting of nutrient recycling rate should be taken into account for reducing nutrient losses and water quality deterioration. Targeted management of local agricultural practices along with guidance from scientific research results is needed to be devoted to sustainable development of agriculture and economies while protecting water environment.
•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.
Evaluating the bioavailability of Cd to rice (Oryza sativa L.) was essential in the karst region, Southwestern China, where the soils have previously been shown to be anomalously enriched in Cd ...through geogenic processes. In this research, we examined the bioavailability of Cd to rice samples collected from 278 sites in Guangxi province, where rice is the most widely cultivated cereal crop that is responsible for the largest human dietary exposure to Cd. Both soil chemical extraction and soil-plant transfer modelling approaches were used to predict the bioavailability to rice. Some of the soil types were highly enriched in Cd, but their bioavailability was low, since the soil carbonates raised soil pH and remarkably reduced Cd bioavailability. In contrast, acidic soils (Ca was largely leached) with relatively low total Cd, the grown rice plants accumulated higher Cd in their grains. Results from CaCl2 extraction experiments provided good predictions for Cd in rice grain grown in soils of different types. Stepwise multiple regressions revealed soil pH and soil Ca content were the dominant factors that control the transfer of Cd from soil to rice. An extended Freundlich-type model and a polynomial surface model provided good prediction for Cd in rice grains. The diffusive gradients in thin films (DGT) technique gave the best estimation of soil Cd bioavailability, whereas water-extracted soil solution Cd provided relatively poor fits. Regional soil threshold that derived using the models, can avoid exceedance of Cd in rice and thereby enable local agricultural practitioners or authorities to develop appropriate management for croplands with high Cd background.
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•The bioavailability of Cd in soils with high geochemical background was characterized as highly soil-type dependent.•Soil Ca, pH, and total Cd are the controlling factors of Cd bioavailability.•The DGT technique was a better approach to assess Cd bioavailability to rice.•Derived regional soil threshold can prevent exceedance of Cd in rice.
Using various approaches to characterize soil behaviors and the bioavailability of Cd to rice grown in high Cd background soils derived under unique karst geologies.
The geographical distributions of Cd and several other metals (Cu, Ni, Pb, Zn, and Cr) were characterized in 308 terra rossa samples across the Guangxi karst region. We found significant enrichments ...of heavy metals in the saprolites and terra rossa developed in 30 profiles, which is mainly caused by the weathering of Cd-enriched carbonate rocks, while the subsequent pedogenic processes were the dominant factor of the enrichments for Cu, Ni, Pb, Zn, and Cr. Sequential extraction analysis indicated that geogenic Cd and Pb in terra rossa mostly distributed in the residual fractions and exhibited low mobility, whereas the amorphous Fe/Mn oxide fraction was the principal Cd-bearing phase in Cambisols. The good correlation of Fe, Al, and Ti in related bedrocks, saprolites, and terra rossa suggested that in-situ pedogenetic processes provided most of the parent materials for terra rossa. The residual accumulation during the special pedogenesis in the karst region, caused elevated Cd and Pb concentrations with increasing weathering intensity, which was indicated by chemical index of alternation (CIA). In addition, results of Pb isotopic fingerprinting confirmed that terra rossa mainly derived from insoluble residues of underlying carbonate rocks. The allochthonous input of Pb also occurred during pedogenesis, whereas the transport and deposition of non-carbonate materials (clasolite/granite derived soils) was only a minor source to Pb input in terra rossa and the anthropogenic impact seemed to be negligible.
•Geographical and soil type dependent heavy metal distributions in terra rossa were characterized.•Sequential extraction results showed that the majority of Cd and Pb was bound in the residual phase.•Residual accumulation caused elevated Cd and Pb concentrations with increasing weathering maturity.•Naturally allochthonous inputs made relatively minor contribution to metal enrichments.
Understanding the impacts of climate change and human activities on vegetation is of great significance to the sustainable development of terrestrial ecosystems. However, most studies focused on the ...overall impact over a period and rarely examined the time-lag effect of vegetation's response to climatic factors when exploring the driving mechanisms of vegetation dynamics. In this study, we identified key areas driven by either positive or negative human activities and climate change. Taking the three karst provinces of southwest China as the case study area, a Leaf Area Index (LAI)-climate model was constructed by quantifying the time-lag effect. Then the associated residual threshold was calculated to identify the vegetation change areas dominated by human activities and climate change. The results showed that, during the implementation period of ecological restoration projects from 1999 to 2015, positive impact areas of human activities were mainly distributed among the implementation areas of ecological restoration projects, accounting for 5.61% of the total area. For another, the negative impact areas were mainly distributed across the mountainous area of Yunnan Province, accounting for 1.30% of the total area. Karst landform had the greatest influence on the areas dominated by positive human activities, whereas both topography and karst landform significantly affected the areas dominated by negative human activities. Urban development level had the greatest impact on the areas dominated by climate change. The outcomes of this study provided scientific supports for the sustainable development of ecological restoration projects in China's karst region.
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•The time-lag effect was considered to establish the LAI-climate model.•Residual threshold was used to separate the impacts of climate and human activities.•LAI dynamics changed significantly during the two stages of 1983-1998 and 1999-2015.•Dominant impact areas of climate change and human activities were identified.•Karst landform significantly affected the distribution of human positive impact area.
Due to the characteristics of the lithology and the complexity of the geomorphology, the karst areas of Southwest China are experiencing severe soil erosion. Knowledge of the effects of lithology and ...geomorphology on sediment yield are therefore essential for developing measures to effectively control sediment delivery in this region. The objective of this study was to investigate the relative importance of lithological and geomorphological factors for sediment yield in 40 selected karst mountainous catchments in Southwest China. Because of the high co-dependence of these variables, a partial least squares regression (PLSR) model was used to explore the quantitative relationships between annual sediment yield and 30 lithological and geomorphological factors. Results showed that the lithology and geomorphology were critically important to annual sediment yield and could explain 65.9% of the total variance in sediment yield. The dominant lithological and geomorphological factors controlling annual sediment yield were karst coverage, drainage density, basin relief, plane curvature, elevation, maximum elevation, average slope, dolomite coverage, relative relief, and catchment area. This importance or these contributions of lithological and geomorphological factors to annual sediment yield provides a good reference for selecting key factors when developing soil erosion models. This study yields a greater understanding of the influences of lithology and geomorphology on sediment yield, and is helpful for better catchment management in karst regions.
•Sediment yield response to various lithological and geomorphological variables is studied.•A catchment with large karst coverage generates low sediment yield.•Drainage density, relief properties, and catchment area greatly affect sediment yield.
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•Based on the circuit theory, we constructed Hechi’s ecological security pattern.•Maximum eco-economic benefits are achieved when 7000 as the resistance threshold.•The ecological ...pattern of “three axis, five belts, six zones, multiple cores” is proposed.•The concept of “Mountain, Water, Forest, Field, Lake and Grass” guides restoration.•The research results can guide ecosystem protection management of karst areas.
Under the influence of global ecosystem changes, the coordinated development of all natural elements has become a top priority in maintaining ecological security. In this context, China has put forward the concept of “mountain, water, forest, field, lake and grass as a community of life”, with a systemic thinking and holistic concept to promote spatial protection and restoration of the national territory. This study uses the logical line of “Diagnosing the all-natural elements base - Guarding the natural security boundaries- Integrated conservation and restoration of natural elements” to comprehensively construct a spatial ecological security pattern in the typical karst area of Hechi, China. We extract ecological sources based on ecosystem service functions and ecological sensitivity. And then considering the effects of both habitat quality and anthropogenic activities to construct ecological resistance surface. The key ecological elements such as ecological corridors, pinch points and barriers are identified according to circuit theory, and the ecological conservation and restoration pattern of mountain, water, forest, field, lake and grass is optimized on the basis of the ecological security pattern, identifies priority areas for protection and restoration, delineates ecological control zones and proposes zoning strategies for ecological control. The study shows that there were 22 ecological sources in Hechi, covering an area of 4886.40 km2, accounting for 14.59% of the study area, mainly distributed in areas with higher habitat quality, such as the west and south; Using 7000 as the cumulative resistance threshold, 34 ecological corridors were identified with an area of 1873.19 km2, radiating in a spider web pattern across the whole area; 32 pinch points, mostly concentrated in the northeastern region, and identified barriers covering an area of 1,966.91 km2, which is the focus of future restoration. And constructed the ecological conservation and restoration pattern of “three axis, five belts, six zones and multiple cores”. The results of the study can provide scientific guidance for the protection of ecological security in the ecologically fragile areas of karst and the comprehensive restoration and management of regional ecosystems, which are of great significance for the sustainable development of global ecosystems.
Soil nitrogen (N) availability is crucial in governing the dynamics of ecosystem structure and function. However, the variation pattern of soil N availability during post-agricultural succession has ...been poorly understood for karst regions. Here, soil N availability variables, including concentrations of soil total N (TN), dissolved inorganic N (DIN=ammonium (NH4+)+nitrate (NO3−)), dissolved organic N (DON), rates of net N mineralization and nitrification at different successional stages, i.e. cropland, grassland, shrubland and secondary forest were investigated at a regional scale, in order to get a general pattern of soil N availability dynamics following agricultural abandonment in the karst region of southwest China. Concentrations of soil TN, soil NO3−, DIN, net N mineralization and nitrification rates increased from grassland to forest. Soil NO3− concentration in secondary forest (37.71±5.01mgNkg−1) reached the level in cropland (31.01±2.93mgNkg−1). Soil NO3−:NH4+ ratio in grassland was not significantly different from 1, but increased directly and reached 13.02±1.16 in secondary forest, equivalent to the level in cropland (15.15±1.98). The dynamics of soil DIN:DON ratio followed the same pattern as soil NO3−:NH4+ ratio. Our study suggests that soil N availability increases and soil N cycling becomes progressively open following agricultural abandonment in the karst region, southwest China.
•Soil N availability increases along post-agricultural succession in karst regions.•N cycle becomes more open as succession proceeds.•Ca and Mg are important contributors to the increase of soil N availability.
Karst region in Guangxi Province, Southwest China.
Both soil properties and rainfall characteristics have a critical influence on splash erosion and aggregate stability. However, the impact of ...electrolytes on splash erosion has received less attention. In this study, soils from 4 typical land use types (cropland, orchard, artificial forest and secondary forest) in karst region were utilized as the research objects. NaCl, KCl, and MgCl2 solutions of different concentrations were used to determine the stability of the aggregates via fast wetting (FW) treatment of Le Bissonnais (LB) method and the characteristics of splash erosion via simulated rainfall experiments.
Results revealed that both the land use types and electrolyte significantly influenced the soil aggregate stability and splash characteristics. Specifically, the change in land use from cropland to orchard, artificial forest and secondary forest significantly increased the aggregate stability, thus decreasing soil erodibility. The aggregate stability decreased and the total splash rate increased as the electrolyte concentration increased. The total splash erosion rate of the 4 soils was positively correlated with the <0.5 mm particle size released by aggregate fragmentation under simulated rainfall conditions. The results can provide a theoretical basis for soil erosion control and soil quality improvement in the karst region of Southwest China.
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•The stability of aggregates decreases with the rise of electrolyte concentration.•Particle content that greatest affect aggregate stability varies from distinct soils.•Total splash rate and electrolyte concentration have a power function relation.•Total splash rate positively correlate with <0.50 mm content by aggregate crushing.
