Red Soil Region Of China Research Articles

A Remote Sensing Approach to Estimating Cropland Sustainability in the Lateritic Red Soil Region of China

Spatiotemporal assessment and a comprehensive understanding of cropland sustainability are prerequisites for ensuring food security and promoting sustainable development. However, a remote sensing-based approach framework that is suitable for large-scale and high-precision assessment and can reflect the overall sustainability of cropland has not yet been developed. This study considered a typical lateritic red soil region of Guangdong Province, China, as an example. Cropland sustainability was examined from three aspects: natural capacity, management level, and food productivity. Ten typical indicators, including soil organic matter, pH, irrigation guarantee capability, multiple cropping index, and food productivity, among others, were constructed using remote sensing technology and selected to represent these three aspects. Based on the indicator system, we assessed the spatiotemporal patterns of cropland sustainability from 2010 to 2020. The results showed that the natural capacity, management level, and food productivity of cropland had improved over the 10 years. The cropland sustainability score increased from 67.95 to 69.08 over this period. The sustainability scores for 68.64% of cropland were increased and were largely distributed in the eastern and western region of the study area. The croplands with declining sustainability scores were mostly distributed in the central region. The prefecture-level regions differed in cropland sustainability, with Zhongshan, Zhuhai, and Qingyuan cities exhibiting the highest values, and Zhanjiang the lowest. Exploring the underlying mechanisms of cropland sustainability and proposing improvement measures can guide decision-making, cropland protection, and efficient utilization, especially in similar lateritic red soil regions of the world.

Dose effect of pig manure addition on cbbL-harboring bacterial community in a paddy soil

CO2 fixation by autotrophic microbes has a significant effect on C cycle in agricultural filed. Organic fertilization is an eco-friendly strategy for environmental protection and agricultural sustainability via ameliorating soil microbial community. However, the information of the dose effect of organic fertilizer on soil CO2 fixing microbes has been less documented. In this study, the cbbL gene, a key gene in the Calvin-Benson-Bassham cycle, was used to characterize the CO2-fixing microbes under the gradient rates of pig manure (PM) addition (0, 1400, 2800, 5600, 11,200, 22,400, and 44,800 kg ha−1) in a 5-year field experiment with a double-rice cropping system in the red soil region of China. The results showed that Proteobacteria and Actinobacteria accounted for ∼99.4 % of total cbbL-harboring bacteria in average. High PM doses (22,400 and 44,800 kg ha−1), rather than Low PM doses (11,200 kg ha−1 and less), would significantly change the cbbL-harboring bacterial community attributes. Specifically, high PM doses decreased the alpha-diversity of cbbL-harboring bacterial community and the relative abundances of some lineages belonging to Proteobacteria and Actinobacteria, such as Bradyrhizobium Methylibium, and Variovorax, but increased the proportions of some stress-tolerant taxa (i.e. Hydrogenophaga and Methyloferula). Redundancy analysis (RDA) and partial least squares path modeling (PLS-PM) showed that pH, AP, and metals (Cu and Zn) negatively influenced the cbbL-harboring bacterial diversity and components through PM addition. Collectively, our study demonstrated that high PM addition would potentially threaten the soil cbbL-harboring microbes, and a rational application of manure-derived organic fertilizer should be addressed in agriculture for maintaining soil bio-diversity in future.

Response of Soil Bacteria of Dicranopteris dichotoma Populations to Vegetation Restoration in Red Soil Region of China

Response of Soil Bacteria of Dicranopteris dichotoma Populations to Vegetation Restoration in Red Soil Region of China

Combined effects of rainfall types and antecedent soil moisture on runoff generation at a hillslope of red soil region

AbstractThe runoff threshold is one of the important parameters in hillslope hydrology. Information on the combined roles of rainfall types and antecedent soil moisture in the runoff generation threshold is still scarce. Therefore, we linked the runoff generation behaviours of a 150‐m hillslope with rainfall types and antecedent soil moisture conditions during a 4‐year observation period between January 2017 and December 2020 in the red soil region of China. We found that the surface runoff of a hillslope could be modelled with a relatively high accuracy using HYDRUS 1D. Rainfall event types, classified into four types (I, II, III and IV) using a machine learning based k‐means clustering algorithm, significantly affected the modelling accuracy of runoff. The prediction accuracy decreased as the level of rainfall type increased, with decreasing from 0.93 for type I to 0.41 for type IV and RMSE increasing from 0.0082 to 0.025 accordingly. The scope of the runoff threshold, represented by antecedent soil water storage of 0–30 cm and event rainfall amount (ASW+P), ranged between 97 and 139 mm. That is, when ASW+P was lower than the lower limit, no runoff occurred in all of the rainfall events, and runoff would be generated without considering the hydrological conditions if ASW+P was higher than 139 mm. The driving factors of runoff generation were strongly dependent on the classified rainfall types. Hourly maximum rainfall intensity was the main contributor to runoff generation for the relatively slight rainfall events (i.e., types I and II). The rainfall amount, however, played a dominant role for the rainfall types with larger amounts and intensities (i.e., types III and IV). These findings provide insights into the method of determining runoff thresholds as well as the inner mechanisms of runoff generation behaviours at the hillslope scale under regions with similar climatic and hydrological conditions to the present study.Highlights Heavy rainfall events were the main error sources of runoff modelling. The threshold of runoff generation was identified by the combination of antecedent soil moisture and rainfall. Threshold scope of runoff generation ranged between 97 and 139 mm. Rainfall classification is valuable in understanding runoff generation behaviours. Runoff driving factors depended on rainfall types.

Soil acidification and loss of base cations in a subtropical agricultural watershed

Soil acidification along with base cations loss degrades soil quality and is a major environmental problem, especially in agroecosystems with extensive nitrogen (N) fertilization. So far, the rates of proton (H+) production and real soil acidification (loss of base cations) remain unclear in subtropical agricultural watersheds. To assess the current status and future risk of soil acidification in subtropical red soil region of China, a two-year monitoring was conducted in a typical agricultural watershed with upland, paddy fields, and orchards where high N fertilizers are applied (320 kg N ha−1 yr−1). H+ production, neutralization and base cations losses were quantified based on the inputs (rainwater, inflow of water, and fertilizer) and outputs (outflow of water, groundwater drainage, and plant uptake) of major elements (K+, Ca2+, Na+, Mg2+, Al3+, NH4+, NO3−, SO42−, Cl−, and H+). The result showed that total H+ production in the watershed was 5152 molc ha−1 yr−1. N transformation was the most important H+ source (68%), followed by excess plant uptake of cations (25%) and H+ deposition (7%). Base cations exchange and weathering of minerals (3842 molc ha−1 yr−1) dominated H+ neutralization, followed by SO42− adsorption (1081 molc ha−1 yr−1), while H+ and Al3+ leaching amounted to 431 molc ha−1 yr−1, only. These results state clearly that despite significant soil acidification, the acidification of surface waters is minor, implying that soils have buffered substantially the net H+ addition. As a result of soil buffering, there was abundant loss of base cations, whose rate is significantly higher than the previously reported weathering rate of minerals in red soils (3842 vs 230–1080 molc ha−1 yr−1). This suggests that the pool of exchangeable base cations is being depleted in the watershed, increasing the vulnerability of the watershed, and posing a serious threat to future recovery of soils from acidification.

Spatial Variation and Influencing Factors of Trace Elements in Farmland in a Lateritic Red Soil Region of China

Trace elements in farmland soil are important indicators of soil quality and farmland health, and also maintain the nutrient balance and promote the healthy growth of plants. In this study, taking Conghua District of Guangzhou city as the study area, the effects of topography, soil, land use, and other factors on trace elements in soil were investigated, and the spatial variability of boron (B), manganese (Mn), molybdenum (Mo), copper (Cu), and zinc (Zn) in farmland soil in a typical red soil region were mapped using a geographically weighted regression (GWR) method. The pH and land economic index (LEI) were important factors affecting the changes in trace element concentrations in the five soils, and the Cu and Zn concentrations were clearly affected by human factors. In the study area, 86.99% of B measurements were classified as low and very low levels, 50.61% and 49.20% of Mo measurements were also low and very low, 71.79% of Mn measurements were classified as moderate, while 91.02% of Cu and 52.95% of Zn measurements were classified as high. After a cross validation, the GWR Kriging (GWRK) model results of each element were relatively stable, and the order of the fitting coefficient (R2) was Cu > Zn > B> Mn > Mo. This study clarifies the spatial distribution and influencing factors of soil microelements in the studied region. This information can be used to improve the nutrient imbalance, further guide agricultural production, strengthen the management of farmland, and improve the healthy productivity of cultivated land.

Nitrogen Mineralization, Soil Microbial Biomass and Extracellular Enzyme Activities Regulated by Long-Term N Fertilizer Inputs: A Comparison Study from Upland and Paddy Soils in a Red Soil Region of China

A long-term experiment (38 years) was conducted to elucidate the effects of long-term N addition on the net N mineralization in both paddy and upland soils, based on their initial soil N status, with and in connection with soil microbial biomass and N cycling extracellular enzyme activities. Two treatments without N addition CK (No fertilizer) and K (inorganic potassium fertilizer) and two treatments with N addition (inorganic nitrogen fertilizer) and NK (inorganic nitrogen and potassium fertilizer) were placed in incubation for 90 days. Results showed that the total N and soil organic carbon (SOC) contents were higher in the treatments with N application compared to the treatments without N in both paddy and upland soils. The SOC content of paddy soil was increased relative to upland soil by 56.2%, 45.7%, 61.1% and 62.2% without N (CK, K) and with N (N and NK) treatments, respectively. Site-wise, total N concentration in paddy soil was higher by 0.06, 0.10, 0.57 and 0.60 times under the CK, K, N and NK treatments, respectively, compared with upland soil. In paddy soil, soil microbial biomass nitrogen (SMBN) was higher by 39.6%, 2.77%, 29.5% and 31.4%, and microbial biomass carbon (SMBC) was higher by 11.8%, 11.9%, 10.1% and 12.3%, respectively, in CK, K, N and NK treatment, compared with upland soil. Overall, compared to upland soil, the activities of leucine-aminopeptidase (LAP) were increased by 31%, 18%, 20% and 11%, and those of N-acetyl-b-D-glucosaminidase (NAG) were increased by 70%, 21%, 13% and 18% by CK, K, N and NK treatments, respectively, in paddy soil. A significantly linear increase was found in the NO3−-N and NH4+-N concentrations during the 90 days of the incubation period in both soils. NK treatment showed the highest N mineralization potential (No) along with mineralization rate constant, k (NMR) at the end of the incubation. SMBC, SMBN, enzyme activities, NO3−-N and NH4+-N concentrations and the No showed a highly significant (p ≤ 0.05) positive correlation. We concluded that long-term N addition accelerated the net mineralization by increasing soil microbial activities under both soils.

Navel orange fine root nutrient content and rhizosphere effects varied with tree ages and soil depths in a hilly red soil region of China

ABSTRACT Root nutrients are required for the growth and development of roots and shoots, which are closely related to soil quality and fruit productivity. In this study, trees of contrasting stand ages, including young, middle-aged, and old trees, were selected in field sites with common management practices in the navel orange (Citrus sinensis) orchards of Southern China. The fine root N decreased with soil depth and the fine root P was higher of old trees (1.82 g kg-1) than young (1.57 g.kg-1) and middle-aged trees (1.37 g. kg-1). Soil organic C, total N, and P generally increased with plant age in the rhizosphere and bulk soils, while reverse trend was found for sucrase and urease activities. Interestingly, acid phosphatase activity was much higher at a lower depth (202 and 82 g.kg-1 OC d-1 for rhizosphere and bulk soils, respectively) than at an upper depth (46 and 43 g.kg-1 OC d-1 for rhizosphere and bulk soils, respectively). In conclusion, root and soil nutrients generally increased with plant growth. Fine root N decreased while fine root P remained unchanged with soil depth due to the content and rhizosphere effect of index at different soil layers related to N and P, respectively.

The Combination of Lime and Plant Species Effects on Trace Metals (Copper and Cadmium) in Soil Exchangeable Fractions and Runoff in the Red Soil Region of China

The water-soluble heavy metal ions in contaminated soil may enter aquatic ecosystem through runoff, thus causing negative impact on the water environment. In this study, a two-year in situ experiment was carried out to explore an effective way to reduce the runoff erosion and water-soluble copper (Cu) and cadmium (Cd) in a contaminated soil (Cu: 1,148 mg kg−1, Cd: 1.31 mg kg−1) near a large Cu smelter. We evaluated the ability to influence soil properties by four Cu-tolerance plant species (Pennisetum sp., Elsholtzia splendens, Vetiveria zizanioides, Setaria pumila) grown in a contaminated acidic soil amended with lime. The results show that the addition of lime can significantly reduce the exchangeable fraction (EXC) of Cu and Cd in soil (81.1–85.6% and 46.3–55.9%, respectively). Plant species cannot change the fraction distributions of Cu and Cd in the lime-amended soils, but they can reduce the runoff generation by 8.39–77.0%. Although water-soluble Cu concentrations in the runoff were not significantly differed and water-soluble Cd cannot be detected among the four plant species, the combined remediation can significantly reduce 35.9–63.4% of Cu erosion to aquatic ecosystem, following the order: Pennisetum sp. > Elsholtzia splendens > Vetiveria zizanioides > Setaria pumila. The implication of this study would provide valuable insights for contaminated soil management and risk reduction in the Cu and Cd contaminated regions.

Variations in rill morphology on unpaved road surfaces and their controlling factors

AbstractRills play important roles in hillslope soil loss as both sediment sources and concentrated flow routes. The construction of unpaved roads in hilly regions changes the underlying surface conditions and leads to unique rill development. Via a terrestrial laser scanning‐aided field survey of eight typical unpaved road segments in the hilly red soil region of China, road surface rill morphology was quantified in detail based on 0.05 m‐resolution digital elevation models. The results illustrated that the rill network on a road segment was mainly composed of one major rill. The average rill network bifurcation ratio was 5.04, and the ratio was closely correlated with road surface area. Rill width and cross‐sectional area (CSA) generally increased along roads, and ephemeral gullies formed in the lower part of some road segments. Slope length, gradient and surface roughness of the underlying road were closely related to the rill morphology factors. The winding roads exhibited higher rill widths, depths and CSAs in straight or highly curved road segments than in other segments. Intergraded factors composed of slope length and gradient were proposed to establish prediction equations for rill and gully CSA. The critical slope gradient and length for rill initiation were negatively related to each other, and the average critical slope was 7.4%. Similarly, the positions of the peak CSA were determined by a function of slope gradient and contributing road length (L = 46.98/sin[S] − 174.3). These results can help guide the prediction of road surface rill soil loss and the design of conservation treatments.

Spatial variability of soil properties in red soil and its implications for site-specific fertilizer management

Assessing spatial variability and mapping of soil properties constitute important prerequisites for soil and crop management in agricultural areas. To explore the relationship between soil spatial variability and land management, 256 samples were randomly collected at two depths (surface layer 0–20 cm and subsurface layer 20–40 cm) under different land use types and soil parent materials in Yujiang County, Jiangxi Province, a red soil region of China. The pH, soil organic matter (SOM), total nitrogen (TN), cation exchange capacity (CEC), and base saturation (BS) of the soil samples were examined and mapped. The results indicated that soils in Yujiang were acidified, with an average pH of 4.87 (4.03–6.46) in the surface layer and 4.99 (4.03–6.24) in the subsurface layer. SOM and TN were significantly higher in the surface layer (27.6 and 1.50 g kg−1, respectively) than in the subsurface layer (12.1 and 0.70 g kg−1, respectively), while both CEC and BS were low (9.0 and 8.0 cmol kg−1, 29 and 38% for surface and subsurface layers, respectively). Paddy soil had higher pH (mean 4.99) than upland and forest soils, while soil derived from river alluvial deposits (RAD) had higher pH (mean 5.05) than the other three parent materials in both layers. Geostatistical analysis revealed that the best fit models were exponential for pH and TN, and spherical for BS in both layers, while spherical and Gaussian were the best fitted for SOM and CEC in the surface and subsurface layers. Spatial dependency varied from weak to strong for the different soil properties in both soil layers. The maps produced by selecting the best predictive variables showed that SOM, TN, and CEC had moderate levels in most parts of the study area. This study highlights the importance of site-specific agricultural management and suggests guidelines for appropriate land management decisions.

Effect of Groundcovers on Reducing Soil Erosion and Non-Point Source Pollution in Citrus Orchards on Red Soil Under Frequent Heavy Rainfall

Periods of consecutive days with heavy rain of high intensity are common in the red soil region of China, increasing unpredictable risks of soil erosion and non-point source pollution on sloping orchards. Grass cover, as a type of vegetation management, is useful for controlling soil erosion and pollution. However, the potential of different kinds of groundcover plants in combating soil erosion and non-point source pollution remains unclear under the rainfall conditions in this region of China. This study included 7 d of simulated rainfall applied to a set of six treatments: Bare soil control, natural grass, and four groundcover treatments, Trifolium repens, T. repens, and Lolium perenne, Vicia sativa and Festuca elata, Medicago polymorpha, and Cynodon dactylon. The effects of the treatments on runoff volume, and soil, nitrogen, and phosphorus losses were evaluated. The results indicated that greater soil erosion and non-point source pollution occurred over the first 3d of daily 1-h simulated rainfall events. Also, the beneficial effects of the groundcover plants were greater earlier in the 7-d period of daily heavy rain, particularly in reducing runoff and nitrogen loss on the second and third day. Compared with bare soil, all the groundcovers showed a reduction effect in varying degrees, among which T. repens treatment was more effective. T. repens treatment showed an overall reduction in runoff and soil loss by 25.5% and 91.5%, respectively, and total nitrogen, nitrate nitrogen, ammonia nitrogen and total phosphorus loss by 25.5%, 74.6%, 90.7%, and 81.8%, respectively. These findings indicated that single planting of perennial pasture T. repens with short stems is an effective management option to limit soil erosion and non-point source pollution in sloping citrus orchards of southern China.

The Effects of Tillage Practices on Interrill Erosion in the Red Soil Region of China

HighlightsConservation practices (straw mulch, manure, peanut-orange intercrop) reduced soil and water loss vs. tillage.Reductions in soil loss were greatest under high rainfall intensity or low soil moisture.Measured interrill erodibility was relatively high in dry run tests and decreased with the final infiltration rate.ABSTRACT.Tillage practices have been adopted extensively to control runoff and soil loss from croplands. To quantify the effects of tillage practices on erosion processes in interrill areas, microplot rainfall simulation experiments were conducted in peanut fields with five tillage practices in the red soil region of China. The results showed that conservation tillage practices, including straw mulch cover (PC), manure fertilizer (PM), and peanut-orange intercropping (PO), could significantly conserve water and reduce soil loss rates by more than 50% compared with traditional furrow (PF) and peanut-radish rotation (PR) treatments. The variations in the hydrologic and soil loss indexes among the treatments were larger in the dry run tests than in the wet run tests. Accordingly, the efficiencies of the conservation practices in reducing soil loss were reflected mainly under a high rainfall intensity or low soil moisture in this study. Additionally, the runoff generation indexes were significantly correlated with the soil loss rate in all five treatments. The measured interrill erodibility was highest in the PR treatment (944,124 kg s-1 m-4) and lowest in the PO and PM treatments (571,797 and 584,238 kg s-1 m-4, respectively), as the organic matter content and aggregate stability were relatively high in the two conservation treatments. Influenced by the dynamics of loose materials and possible surface sealing, the interrill erodibility was relatively high and decreased with the final infiltration rate when measured under the dry run conditions. These results could guide cropland soil management and improve process-based erosion models. Keywords: Interrill erodibility, Interrill erosion, Rainfall simulation, Red soil region, Tillage practices.

Role of groundcover management in controlling soil erosion under extreme rainfall in citrus orchards of southern China

Extreme rainfall is becoming more frequent and intense due to climate change and increasing human activity. Changes in rainfall will increase the risks and uncertainty of water erosion from orchards. Groundcover management is an important factor affecting hydrological and erosive processes of orchards. However, the roles of different groundcover managements for controlling soil water erosion caused by extreme rainfall are not fully understood. In this study, four groundcover managements (i.e., control bare land, orchard with no cover, orchard with grass cover and orchard with cover crops) were used to analyze runoff and erosion characteristics related to extreme and ordinary rainfalls in the red soil region of China. Based on the rainfall-runoff data measured on runoff plots, 356 natural rainfall events from 2001 to 2012 were analyzed, and rainfall depths and maximum 30-min rainfall intensities were used to select extreme rainfall events, according to the criteria of the World Meteorological Organization for extreme rainfall. There were 25 extreme rainfall events with a probability of 7.0% during the study period. Extreme rainfall events played the destructive role in inducing soil loss. The average extreme runoff coefficient and soil loss amount were 2.8 and 11.1 times higher than the values from ordinary rainfall, respectively. The contribution of extreme rainfall to sediment yield was 44.2%, which far higher than those to runoff generation (15.8%). Moreover, the runoff coefficients and erosion amounts varied greatly among the different groundcover managements under extreme rainfall, and were ranked in the following order: control bare land > orchard with no cover > orchard with cover crops > orchard with grass cover. However, the highest percentage of soil loss amount caused by extreme rainfalls were 59.0% in orchard with cover crops, and the lowest in orchard with grass cover (26.0%). Therefore, extreme rainfall should cause more attention in improving the groundcover management strategy for orchard soil erosion control, and groundcover like grass cover was extremely needed even after interplanting crops for the serious soil loss.

Comparison of carbon sequestration efficiency in soil aggregates between upland and paddy soils in a red soil region of China

There is limited information on carbon sequestration efficiency (CSE) of soil aggregates in upland and paddy soils under long-term fertilization regimes. In a red soil region of southern China, an upland soil experiment started in 1986 and a paddy soil experiment commenced in 1981. These experiments were conducted using different fertilization treatments. After 30 years, soil organic carbon (SOC) content and stock of different aggregate components were analyzed. The results showed that the SOC contents and stocks in upland soil were lower than in paddy soil. In both upland and paddy soils, the SOC contents and stocks of all aggregate components in NPKM (combined treatment with chemical nitrogen (N), phosphorus (P), potassium (K) fertilizers and manure) were the highest among all treatments. Compared with CK (no fertilizer), SOC content of all aggregate components in NPKM was increased by 13.21−63.11% and 19.13−73.33% in upland and paddy soils, respectively. Meanwhile, the change rates in SOC stock of all aggregate components in upland soil were lower than in paddy soil, although the change rate of SOC stock of all aggregate components in NPKM was higher than in other treatments. Furthermore, a linear equation could fit the relationships between carbon (C) input and change rate of SOC stock (P<0.05). Results indicated that the sum of CSE from all aggregate components in upland soil (16.02%) was higher than that of paddy soil (15.12%) in the same climatic condition and from the same parent material. However, the CSEs from all aggregates were higher than that of bulk soil, although the result from bulk soil also showed that the CSE of upland soil was higher than that of paddy soil.

Vegetation coverage change and its response to topography in a typical karst region: the Lianjiang River Basin in Southwest China

Karst areas are usually ecologically fragile complex terrains, where the topography considerably affects vegetation coverage. Although the relationship between vegetation and topography has been studied extensively, this relationship is not well understood in the karst areas. In this study, vegetation coverage was estimated in a typical karst basin for the years 1995, 2005, and 2016, using remote sensing data and the dimidiate pixel model. The spatial–temporal variation in vegetation coverage and its response to topography were quantitatively analyzed. Overall, the results showed that the vegetation coverage in this region increased from 1995 to 2016. The area under extremely high vegetation coverage increased from 26.6 to 56.0%, while vegetation coverage levels decreased in all other areas. Moreover, vegetation coverage varied with topography. It first increased with elevation but then continuously decreased, and the decrease in vegetation coverage was the most obvious on the steeper slopes. Vegetation coverage was higher with sunny aspects than with shady ones. The effects of ecological restoration were apparent in the Lianjiang River Basin from 1995 to 2016, especially on the sunny aspects at elevations between 650 and 1100 m and with a slope steepness < 30°. The results will help us identify the influencing factors and build models to predict soil loss from forests with little understory vegetation in the red soil region of China. They also improve our understanding of the effect of topography on vegetation coverage in the karst areas. Accordingly, appropriate vegetation restoration measures can be taken for varied terrains.

Effects of soil properties, topography and landform on the understory biomass of a pine forest in a subtropical hilly region

Soil conservation will remain an essential subject for forest in slope land. However, the interactions between soil, vegetation and complex topography are not fully understood. To address this concern, field studies were performed in a typical small watershed in the subtropical hilly red soil region of China. This site covers an area of approximately 3.2 ha, and the mean slope steepness of the land is 15°. Soil properties (including available and total N, P, K, soil organic carbon (SOC), CEC, soil texture), understory vegetation biomass and diversity index on different slopes (different slope steepness, slope aspects, and slope positions) were measured. Meanwhile, interrelationships between understory vegetation biomass and soil properties, slope steepness and altitude were evaluated via redundancy analysis. The study results showed that the soil nutrients and SOC contents were greater in the down-slope and north and west slopes. And the soil properties displayed high spatial heterogeneity and had semi-variance structure, which were mainly derived from random factors. Due to obvious erosion characteristics and the negative correlations between soil properties and slope positions, we suspected that soil erosion was the primary random factor. In addition, the mean total understory vegetation biomass in the pine forest was 283.58 g m−2 (ranging from 66.60 to 573.79), and higher vegetation biomass was found in down-slope and north and west slopes. Further analyses indicated that topography (slope steepness and altitude) and soil properties combined contributed to 58.7% of the variations in the understory biomass, and they individually had a contribution of 17.3% and 41.4%, respectively. Moreover, altitude (height of slope positions) alone explained 15.9% of the variation of the vegetation biomass. This study indicated that soil properties, which were highly affected by slope aspect and slope position, were the most important factor in influencing understory vegetation biomass in subtropical pine forest. Slope position also had a tremendous direct influence on soil vegetation. Controlling understory soil erosion is essential for conservation of pine forest land in subtropical China.

Influence of Sampling Point Discretization on the Regional Variability of Soil Organic Carbon in the Red Soil Region, China

Research on the regional variability of soil organic carbon (SOC) has focused mostly on the influence of the number of soil sampling points and interpolation methods. Little attention has typically been paid to the influence of sampling point discretization. Based on dense soil sampling points in the red soil area of Southern China, we obtained four sample discretization levels by a resampling operation. Then, regional SOC distributions were obtained at four levels by two interpolation methods: ordinary Kriging (OK) and Kriging combined with land use information (LuK). To evaluate the influence of sample discretization on revealing SOC variability, we compared the interpolation accuracies at four discretization levels with uniformly distributed validation points. The results demonstrated that the spatial distribution patterns of SOC were roughly similar, but the contour details in some local areas were different at the various discretization levels. Moreover, the predicted mean absolute errors (MAE) and root mean square errors (RMSE) of the two Kriging methods all rose with an increase in discretization. From the lowest to the largest discretization level, the MAEs of OK and LuK rose from 4.47 and 3.02 g kg−1 to 5.46 and 3.54 g kg−1, and the RMSEs rose from 5.13 and 3.95 g kg−1 to 5.76 and 4.76 g kg−1, respectively. Though the trend of prediction errors varied with discretization levels, the interpolation accuracies of the two Kriging methods were both influenced by the sample discretization level. Furthermore, the spatial interpolation uncertainty of OK was more sensitive to the discretization level than that of the LuK method. Therefore, when the spatial distribution of SOC is predicted using Kriging methods based on the same sample quantity, the more uniformly distributed sampling points are, the more accurate the spatial prediction accuracy of SOC will be, and vice versa. The results of this study can act as a useful reference for evaluating the uncertainty of SOC spatial interpolation and making a soil sampling scheme in the red soil region of China.

Influence of the Selection of Interpolation Method on Revealing Soil Organic Carbon Variability in the Red Soil Region, China

At present, various interpolation methods have been used for obtaining the spatial variability of soil organic carbon (SOC) in some regions. However, systematic research on accuracy comparison and the influence of method selection on revealing SOC variability is still lacking for the hilly red soil region of China. In this study, the spatial distributions of SOC were interpolated via three kriging methods (ordinary kriging (OK), kriging combined with soil type information (KSt), and with land-use information (KLu)) and three polygon-based methods (linking sample points data to the sampling grid (PGr), to soil type polygon (PSt), and to land-use polygon (PLu)) in Yujiang County, Jiangxi Province, China. To illustrate the influence of method selection on revealing SOC variability, the prediction efficiencies of these methods were compared with soil validation samples. The results showed that the KLu and PLu had high prediction accuracy, followed immediately by the KSt and PSt, while the OK and PGr had very low accuracies. Meanwhile, the SOC distribution contours obtained by KLu, PLu, KSt, and PSt could better reflect the real conditions of the study area, as they were consistent with the variation of land-use or soil types. However, the contours from the OK and PGr had poor performance with sketchy borderlines and regular homogeneous grid cells, respectively. Therefore, great differences exist in the prediction efficiency of different methods, and the interpolation method selection has an important impact on revealing SOC variability in the hilly red soil region of China. The results can provide a useful reference for the efficient SOC prediction and simulation in similar soil regions.

The influence of tillage practices on soil detachment in the red soil region of China

Intensive cropland soil erosion can be caused by disturbance activities. However, conservation measures have been reported to increase soil resistance and decrease erosion. Hydraulic flume experiments were conducted to measure the detachment rate of disturbed soil samples from a peanut field with five tillage practices. Hydraulic variables were adopted to estimate the soil detachment rate and to quantify the rill erodibility parameter. The disturbed soil detachment rates are substantially greater than those for comparable undisturbed soil experiments. The average disturbed soil detachment rates for the 5 different tillage treatments, from lowest to highest, are as follows: straw mulch (PC) < manure (PM) < peanut and orange intercropping (PO) < peanut and radish crop rotation (PR) < conventional downslope furrows (PF). The conservation measures (PC, PM and PO) have a limited effect on increasing the disturbed soil resistance under low flow energy conditions. Thus, the average rill erodibility is lower, and the critical shear stress is higher for the PC, PM and PO treatments than those of the PR and PF treatments. With a slope gradient steeper than 17.6%, there is no significant difference among the detachment rates of all treatments. A linear function of both flow rate and slope gradient can be used to estimate the disturbed soil detachment rate. Both stream power and flow shear stress are more effective than the unit stream power in predicting soil detachment rates. The stream power is the most effective predictor among the three hydraulic parameters studied. Instead of the power functions, the linear functions of the hydraulic parameters are suggested to quantify the disturbed soil detachment rates.