VARIATION IN SOIL ORGANIC CARBON AND NITROGEN STOCKS ALONG A TOPOSEQUENCE IN A TRADITIONAL MEDITERRANEAN OLIVE GROVE

The impact of the topographical position on soil properties was evaluated in an olive grove with traditional tillage. Three topographical positions: summit, backslope and toeslope were chosen for evaluation. The soil samples were taken from four soil sections of 0·25 m (0–1 m). The soil organic carbon (SOC) and N content increased along the downslope direction (5·5, 6·5 and 7·1 g C kg⁻¹ and 0·3, 0·8 and 0·9 g N kg⁻¹ in the surface layer in the summit, backslope and toeslope respectively) as well as SOC and N stocks, considering the two first soil sections. In addition, there was movement of the most erodible textural fraction (silt). However, the total SOC stock (refer to 1 m of depth) did not vary with respect to the topographical position, but the total N stock (refer to 1 m of depth) varied significantly. These increases were due to erosion processes that occur along the toposequence, leading to organic matter transfers from the summit to the toeslope. All the stratification ratios calculated were lower than 2, indicating the low quality of the soils. Therefore, alternative management techniques that avoid soil erosion must be considered in olive grove in order to increase the soil quality and fertility. Copyright © 2014 John Wiley & Sons, Ltd.     

Variation in soil chemical properties along toposequences in an arid region of the Levant

Detailed characterization of soils and their variation along different topography positions has not been investigated in depth for Mediterranean arid regions. There is a need to accurately understand the variation and the spatial distribution of soil properties within dry region of the Levant. Such understanding is required to optimize the use and management of scarce land and water resources. The objective of this study was to examine the effect of hillslope characteristics on the variation of selected soil chemical properties in an arid Mediterranean climate. At each of five selected transects four sites were chosen to represent four different topographic positions: summit, shoulder, backslope and toeslope. A soil profile was examined at each site and a representative sample from each horizon was withdrawn for chemical analyses. The analyses indicated that generally, the carbonate contents of the surface horizons decreased from higher to lower positions of the toposequence, the carbonate content increased with depth for profiles occupying the lower positions. This suggests more intense leaching within soil at lower positions. The effect of steepness and curvature on controlling the variation of soil properties was obvious at the summit and shoulder positions. Leaching process seems to hinder the effect of steepness and curvature for soils at lower positions. Lower positions receive runoff water and organic matter from upper positions, which complicate the relationship between landform shape and organic matter content. Continuous tillage resulted in lower organic matter contents for soils at lower positions. Higher pH values were reported for soils down along the transect due to the movement of soil material from upper hillslope positions. Electrical conductivity, exchangeable calcium and potassium contents decreased downslope due to higher moisture accumulation. However, no obvious relationship was found between the variation of pH or EC in one hand and the variation of steepness and curvature on the other. Potassium content was variable due to its greater mobility. The analyses indicated that variation in the soil CEC is governed by two factors: the leaching pattern, which is controlled by hillslope position, and the accumulation of Eolian carbonates at the soil surface. The distribution of iron oxides and types of clay minerals indicated more weathering in a descending direction and with soil depth, which is attributed to higher availability of soil moisture along the same direction. These relationships suggested systematic variation of chemical properties along toposequences in this arid environment.     

Slope gradient and shape effects on soil profiles in the northern mountainous forests of Iran

In order to evaluate the variability of the soil profiles at two shapes (concave and convex) and five positions (summit, shoulder, back slope, footslope and toeslope) of a slope, a study of a virgin area was made in a Beech stand of mountain forests, northern Iran. Across the slope positions, the soil profiles demonstrated significant changes due to topography for two shape slopes. The solum depth of the convex slope was higher than the concave one in all five positions, and it decreased from the summit to shoulder and increased from the mid to lower slope positions for both convex and concave slopes. The thin solum at the upper positions and concave slope demonstrated that pedogenetic development is least at upper slope positions and concave slope where leaching and biomass productivity are less than at lower slopes and concave slope. A large decrease in the thickness of O and A horizons from the summit to back slope was noted for both concave and convex slopes, but it increased from back slope toward down slope for both of them. The average thickness of B horizons increased from summit to down slopes in the case of the concave slope, but in the case of convex slope it decreased from summit to shoulder and afterwards it increased to the down slope. The thicknesses of the different horizons varied in part in the different positions and shape slopes because they had different plant species cover and soil features, which were related to topography.     

Simulation and 137Cs tracer show tillage erosion translocating soil organic carbon,phosphorus, and potassium

Research on how tillage‐induced soil redistribution affects soil properties is limited for complex slopes in nonmechanized agricultural areas. The objectives of this study are (1) to examine the vertical redistribution of soil organic C (SOC), extractable P and K induced by tillage on a complex slope, (2) to assess the effects of tillage erosion on soil profile properties, and (3) to elucidate the variations in soil properties induced by both vertical mixture and downslope transport of soil within the landscape. Simulated tillage was conducted in the Yangtze Three Gorges Reservoir Area, China. The ¹³⁷Cs data showed that intense tillage caused the soil vertical mixture and downslope transport. The redistribution of ¹³⁷Cs and soil constituents varied with the number of tillage passes and location in relation to curvature. SOC was completely depleted with the disappearance of soil profiles at the summit position, while SOC concentrations decreased by 26% for the till layer and increased by 29% for the sublayer at the toeslope position for the 15‐tillage operation, as compared with those of pretillage. The vertical redistribution of extractable P and K followed a similar pattern to that of SOC. The gap and variation in soil constituents between the till layer and sublayer declined after tillage, suggesting that the mixing effect of tillage attenuates the variability of soil properties in the vertical direction. Net loss and gain of soil constituents occurred at the summit and toeslope positions, respectively, suggesting that the downslope transport of soil induced by tillage accentuates the variability of soil properties in the lateral direction.     

Association of soil organic carbon with physically separated soil fractions in different land uses of Costa Rica

Abstract

Yams (Dioscorea spp.) are known to be very demanding in terms of soil fertility. The influence of deleterious soil fertility on the growth of yams, however, has not yet been studied. A field study was carried out in Côte d'Ivoire, in a fertile forest site and in a low fertility savanna site, submitted to identical climatic conditions, to determine how soil fertility affects Leaf Area Index, Leaf Area Ratio, Crop Growth Rate, Tuber Bulking Rate, Radiation Interception, Radiation Use Efficiency and final tuber yields in D. alata and D. rotundata, grown in the presence of adequate fertilizers. While germination and establishment of both species were not affected by the site, growth parameters of D. alata were superior to those of D. rotundata. This resulted in higher yields of D. alata, when compared to D. rotundata. Only the radiation use efficiency was higher in D. rotundata than in D. alata. Both species reached higher Crop growth rate and Tuber bulking rate and produced higher fresh tuber yields at the forest site than at the savanna site, reflecting the known demand of yams for high fertility soil. Correlation analyses showed significant relationships between the mean Leaf Area Index measured over the entire growth period, Tuber Growth Rate calculated between tuber initiation and harvest, and fresh tuber yields, suggesting that the development of leaf area is determining the growth rate of the tuber which itself determines the fresh tuber yield. The decrease in D. alata tuber yields at the savanna site was explained by a decrease in Leaf Area Index, while the decrease in D. rotundata tuber yields at this site was explained by a decrease in Radiation Use Efficiency. Both effects might have been the consequence of a suboptimal nitrogen and potassium nutrition at the savanna site.     

Watershed-scale assessment of soil quality in the loess hills of southwest Iowa

Soil quality is a concept that integrates soil biological, chemical and physical factors into a framework for soil resource evaluation. Conventional tillage practices can result in a loss of soil organic matter and decreased soil quality. The potential for soil quality degradation with tillage may vary depending upon landscape position and the spatial distribution of critical soil properties. Information on how to accurately integrate soil spatial information across fields, landscapes and watersheds is lacking in the literature. The primary objective of this study was to evaluate the long-term effect of conventional and ridge-tillage on soil quality in three small watersheds at the Deep Loess Research Station near the town of Treynor in southwest Iowa. Soil types included Monona silt loams in summit positions, Ida or Dow silt loams in backslope positions, and Napier or Kennebec silt loams in footslope positions. We removed surface soil cores from transects placed along topographic gradients in each watershed and quantified total soil organic C (SOC), total soil N (TN), particulate organic matter C (POM-C) and N (POM-N), microbial biomass C (MB-C), N mineralization potential (PMIN-N), nitrate N, extractable P and K, pH, water-stable macroaggregates (WSA), and bulk density (BD). We used terrain analysis methods to group the data into landform element classes to evaluate the effect of topographic position on soil quality. Results indicate that soil quality is higher under long-term ridge-tillage compared with conventional tillage. Soil quality differences were consistently documented among the three watersheds by: (1) quantification of soil indicator variables, (2) calculation of soil quality index values, and (3) comparison of indicator variable and index results with independent assessments of soil function endpoints (i.e. sediment loss, water partitioning at the soil surface, and crop yield). Soil quality differences under ridge-till were found specifically for the backslope and shoulder landform elements, suggesting that soil quality increases on these landform elements are responsible for higher watershed-scale soil quality in the ridge-tilled watershed.     

Mineral composition of tobacco leaf samples part I: As affected by soil fertility,variety, and leaf position

Abstract

Three tobacco varieties were grown under four soil fertility conditions. Leaf samples were taken at bloom stage from upper and lower positions and analyzed for eleven elements. Results show how leaf position and variety affect leaf mineral composition with variable soil conditions. Therefore, variety and leaf position should be clearly defined before leaf nutrient levels can have a suitable degree of diagnostic value. However, variety differences were of less importance because nutrient concentration differences were generally related to appropriate changes in yield measurements.     

Below-ground organic carbon and decomposition potential in a field-forest glacial-outwash landscape

Relationships of soil properties to land use and topography may vary among locales because of differences in other controlling factors. We evaluated relationships of below-ground C, defined as soil organic C plus fine-root C, and decomposition potential to site characteristics at Cedar Creek Natural History Area in east-central Minnesota, USA. The landscape, formed in glacial outwash sand, has a complex spatial pattern of grasslands and forests resulting from interactions among cultivation, agricultural abandonment, topography, and fire. Below-ground C was higher in mature forests than in either adjacent abandoned agricultural fields or uncultivated prairie for both the O Horizon (0.57 vs. 0.13 kg m^-2) and the underlying 0–10 cm of soil (2.1 vs. 1.0 kg m^-2) but was similar at 10–30 cm (2.0 vs. 2.0 kg m^-2) and 30–50 cm (1.4 vs. 1.5 kg m^-2). The higher C in surface soil under forests contrasted with published observations for finer textured substrates. Below-ground C was constant across forest summit, shoulder, and backslope positions, and increased at the toeslope position. Average fine-root (<2 mm diameter) C at the depth of 0–50 cm was 0.2 kg m^-2 and represented 4% of below-ground C. In contrast to an expected trend of monotonically increasing decomposition with increased temperature, cellulose decomposition during a 60-day field incubation increased with temperature on cool forest slopes but decreased with temperature in warm fields. Nutrient availability, water availability, and microbial biomass may confound this relationship. The results indicate diverse controls on decomposition in this field-forest landscape.     

Applying the CSM-CERES-Wheat model for rainfed wheat with specified soil characteristic in undulating area in Iran

Models can provide researchers with an opportunity to understand the multidimensional relationships among factors affecting crop growth. All such factors, including field condition, must be tested using models. The main objective of this study was to apply the Cropping System Model (CSM)-Crop-Environment Resource Synthesis (CERES)-Wheat model to uneven areas for simulating rainfed wheat. First, the model was calibrated at a field cultivated with wheat variety Sardari. The model was then applied to different points at a ‘hummocky’ evaluation site. At this site, 69 sample points from 5 different slope positions (summit, shoulder, backslope, footslope, and toeslope) were specified and physical and chemical soil properties as well as final grain and total aboveground wheat biomass at each of 69 points were measured. The overall results showed that the normalized root-mean-square error varied between 15% and 34% for the top weight; this was considered to be good enough for undulating areas. The model efficiency (EF) also ranged from 0.35 to 0.26 both in simulation of final grain and in top weight. It can be concluded that CSM-CERES-Wheat model can be effectively applied for yield prediction in hills. It should be noted that use of the CSM-CERES-Wheat model is not recommended for estimation of grain yield in individual slope positions unless more detailed water and fertilizer data are also available.     

Relationships between vegetation and soil and topography in a dry warm river valley,SW China

Understanding of the relationships between vegetation and soil and topography would be very important for ecosystem restoration and management efforts in the dry valleys of Himalayan region but how to clarify the complicated relationships and figure out key factors for practical purpose is a challenge. The main objective of this research was to propose a four-staged procedure by combining several multivariate statistical techniques to detect the relationships between vegetation and soil and topography, and thereby identify the key factors for the degraded ecosystem restoration and management. Forty-three plots (5 m × 5 m) were selected for the field survey of the vegetation, soil and topography variables in the dry warm river valley of the upper Minjiang River, Sichuan Province, China. Cluster analysis (CA) demonstrated that high plant diversity, cover and height were associated with good soil quality and favorable topographic positions with lower solar incident radiation, runoff and soil erosion potential. Correlation analysis (simple correlation analysis and canonical correlation analysis) and multiple linear stepwise regression analysis affirmed that plant diversity was mainly correlated with soil water content, and soil water content was mainly determined by soil texture (clay content). Soil clay content alone could explain about 70% of the total variance. Identifying the favorable topographic position and the distribution pattern of soil texture and its controlling mechanisms is thus very important for restoration practices. In the process of ecosystem restoration, we should promote the co-evolution of vegetation and soil, and follow the natural succession sequence. Some relevant conservation polices are also needed to reduce human disturbance on ecosystem.     

长期施肥下我国水稻土基础地力变化趋势

【目的】水田具有较好的水热稳定性,且基础地力对水稻产量的贡献大,所以水稻土基础地力高低是保证水稻高产稳产的重要指标。研究长期不同施肥下我国不同区域水稻土基础地力的变化态势及其特征不仅为水稻土的农业可持续生产提供理论依据,还为评价和建立长期合理的施肥模式提供数据支持。【方法】以不施肥处理(CK)的水稻产量表征其基础地力,以不施肥产量与常规施肥(施氮、磷、钾肥,NPK)产量的比值作为基础地力贡献率,分析了我国7个水稻连作或稻麦轮作农田生态系统下长期施肥稻田的基础地力和基础地力贡献率随时间的变化趋势和特征,比较了不同地点及不同轮作方式的差异探讨了基础地力贡献率与水稻产量及其相关指标的关系。【结果】长期不施肥水稻产量随时间的变化总体上较稳定,平均变化速率仅为-0.3 kg/(hm~2·a)。不同水稻土类型间差异大红壤水稻土基础地力易呈下降趋势而紫色水稻土较稳定。不同轮作方式上,晚稻和单季稻的不施肥产量随时间呈上升趋势,分别增加1.6和29.2 kg/(hm~2·a),而早稻不施肥产量呈逐年下降趋势,平均下降20.0kg/(hm~2·a)。同一区域的试验点,白沙(BS)和望城(WangC)不施肥的水稻产量逐年下降速率最大,下降值达47~82kg/(hm~2·a);而南昌(NC)的早晚稻、武昌(WuC)和遂宁(SN)的单季稻均呈逐年上升的趋势,上升速率达44~81 kg/(hm~2·a)。水稻多年(20年)平均基础地力贡献率较高(各地变化幅度42%~68%,平均值为59.7%)其中早稻、晚稻和单季稻分别为55.1%、67.1%和54.9%。基础地力贡献率与不施肥的产量可持续性指数(SYI)间呈显著的正相关关系(r=0.655,n=13)。7个试验点的多年施肥处理水稻平均产量为5100 kg/hm~2,显著高于不施肥产量(2911 kg/hm~2),多年施肥措施对水稻产量的贡献率为40.3%。【结论】长期不施肥晚稻和单季稻产量稳定或有上升趋势而早稻产量不稳定,部分呈显著下降趋势,其变幅大小与水稻土类型有关,其中红壤水稻土的早稻产量易呈下降趋势而紫色水稻土的最稳定;施肥条件下各试验点水稻产量随时间变化较稳定。从长期角度而言,提高南方水稻土基础地力是提升其产量可持续性的有效途径之一。     

Impact of agricultural land use on physicochemical properties of soils derived from sedimentary rocks in Malaysia

ABSTRACT

In Malaysia, soils derived from sedimentary rocks are extensively used for agricultural purposes with oil palm and rubber being the main dwellers. In order to understand the environmental impact of these perennial crops planting, the variability of physicochemical properties of 25 representative soils derived from sedimentary rocks under different ecosystems (agriculture land and natural forest) at six study sites spread across Malaysia was examined. Among the soil physicochemical properties, total soil organic carbon, total nitrogen, and fertility level were found to be generally higher in the forest ecosystems followed by rubber plantation and finally oil palm plantations. Likewise, projection of principal component analysis showed an associative relationship between soil physicochemical properties and microhabitats. Finally, this study showed that soils from different agricultural and natural sites, but derived from similar sedimentary rocks, had distinctive weathering conditions and soil properties. Therefore, site-specific field management according to soil type, soil management techniques as well as fertilizer strategies are required to maximize crop production and to sustain ecosystem services. The output of this study will enable farmers to improve their crop yield via the selection of suitable crops cultivation based on soil characteristics.     

Soil redistribution and organic carbon accumulation under long‐term (29 years) upslope tillage systems

Few studies have demonstrated soil redistribution under upslope tillage (UT) rather than downslope tillage (DT) and its impact on soil organic carbon (SOC) redistribution in long‐term agricultural practices in hillslope landscapes. We selected two neighbouring sites from the Sichuan Basin, China, one under DT and the other under UT, to determine the pattern of soil and SOC redistribution under a long‐term UT practice. DT caused soil loss at upper slope positions and soil accumulation at lower slope positions. However, UT resulted in soil accumulation at upper slope positions and soil loss at lower slope positions. The total erosion rate decreased by 60.5% after 29 years of UT compared with DT. Having the same direction of soil movement by tillage and water exaggerated total soil loss, whereas having the two movements in the contrasting direction of soil for the two reduced it. SOC stocks at positions from summit to downslope were much larger (33.8%) and at toe‐slope positions were only slightly greater (4.5%) in the UT soils than comparable values for the DT site. The accumulation rate of SOC at the UT site increased by 0.26 Mg/ha/year compared with that at the DT site. It is suggested that soil movement by water and tillage erosion occurred in the same direction accelerates the depletion of SOC pools, whereas the opposite direction of soil movement for the two can increase SOC accumulation. Our results suggest that UT has significant impacts on soil redistribution processes and SOC accumulation on steeply sloping land.     

Short-range variation in a Wisconsin soilscape (USA)

Here we report on the variation of a soilscape in south central Wisconsin, USA. The variation in soil properties and soil features results in four soil order (Entisols, Inceptisols, Alfisols and Mollisols). Observations were made along a 200 m transect in a field that was cultivated since 1870. Slopes ranged from 7.5% on the back slope to 0% in the lower part. The soilscape had a total relief difference of 7.0 m. The soils were studied by 41 soil pits (60 cm), 6 soil pits (125 cm), 15 soil augers (100 cm), and ground-penetrating radar imagery. The summit and shoulder consist of coarse glacial outwash (loamy sands) over limestone whereas the lower part is lacustrine sediments over coarse outwash (loams, silty loams). The A-horizon thickness ranged from 14 to 52 cm with thick A horizons at the toeslope that also had the lowest soil pH. The soil organic carbon (SOC) contents of the A horizons ranged from 11.6 to 46.9 g C kg^–1, and the higher contents are in the lower part of the soilscape. SOC stocks (0–20 cm depth) ranged from 50 to 70 Mg C ha^–1 on the summit and backslope, but were 80 to 95 Mg C ha^–1 in the flat part of the soilscape. The lowest soybean yields (1.6 Mg ha^–1) were found at the summit and the highest yield (6.3 Mg ha^–1) at the lower end of the backslope. Soybean yields were correlated to the thickness of the A horizon, and every 10 cm increase in A horizon thickness yielded an extra 0.6 Mg soybeans ha^–1. Analysis of spherical magnetic particles was used to estimate soil erosion rates that were highest on the backslope (16.2 Mg ha^–1 yr^–1) and rates of soil deposition in the lowest part of the soilscape was 18.8 Mg ha^P1 yr^–1. It seems that there is no net soil and SOC loss within this soilscape. All in all, we found 4 soil taxonomic orders within 200 m. The variation in this soilscape was substantial and probably enhanced by 140 years of cultivation.     

The impact of mulch type on soil organic carbon and nitrogen pools in a sloping site

Three mulch treatments were tested for their ability to control erosion on a sloping site. Additionally, the choice of mulch can also enhance revegetation success and improve soil organic matter input. This study aimed to investigate the effects of three mulching treatments, hydro-seeding, granite mulch and forest mulch, on soil C and N pools at different positions on highly erodible slope with approximately 30 % gradient. Soil moisture, total C (TC), total N (TN), hot water-extractable organic C (HWEOC), hot water-extractable total N (HWETN), microbial biomass C and N (MBC and MBN), inorganic N and potentially mineralisable N were measured. All variables were significantly higher in soils amended with forest mulch than those with hydro-seeding and granite mulch, for the same slope positions. Soil moisture was significantly higher in the lower slope position than middle and upper slope positions in hydro-seeding and granite mulch treatments, whereas no slope effect was observed on soil moisture under the forest mulch application. In the forest mulch treatment, the upper slope position had higher soil TC, TN, HWEOC, HWETN, MBC, MBN, NO₃ ^?-N and total inorganic N than the middle and lower slope positions. Five years following mulch application, forest mulch still exerted a significant influence on soil fertility compared to the other treatments and the influence on soil moisture suggests that this treatment would be the most effective in the control of water-driven soil erosion on this steep site.     

The effect of environmental variables on soil characteristics at different scales in the transition zone of the Loess Plateau in China

As a result of human disturbance and topographic variability, land use mosaics are characteristic of the transition zone of the Loess Plateau in Shaanxi Province, China. Soils are particularly sensitive to change when land degradation processes are dominant. An understanding of the influences of environmental variables is required to inform land management and agricultural production. In this study the relationships between land type, topography and soil properties were analysed for Hengshan County at two different scales using detrended canonical correspondence analysis. The results show that variations in soil properties are strongly influenced by topography, land use and vegetation. Slope, elevation and aspect are also of importance at the county and small catchment scales of analysis. Though land use type proved to be of lesser importance than topographic factors, ANOVA analysis showed that there were significant differences in soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), available nitrogen, available potassium and clay %. Areas under vegetables had significantly higher SOM and TN content at the catchment scale. The results will enhance our ability to predict spatial and temporal changes in soils. In addition, it is shown that soil fertility could be improved by land reform and management in the hilly‐gully area.     

Cotton lint yield variability in a heterogeneous soil at a landscape scale

Landscape variability associated with topographic features affects the spatial pattern of soil water and N redistribution, and thus N uptake and crop yield. A landscape-scale study was conducted in a center pivot irrigated field on the southern High Plains of Texas in 1999 to assess soil water, soil NO₃-N, cotton (Gossypium hirsutum L.) lint yield, and N uptake variability in the landscape, and to determine the spatial correlation between these landscape variables using a state-space approach. The treatments were irrigation at 50 and 75% cotton potential evapotranspiration (ET). Neutron access tubes were placed at a 15-m interval along a 710 m (50% ET) and 820 m (75% ET) transect across the field. Soil NO₃-N in early spring was autocorrelated at a distance varying between 60 and 80 m. Measured soil volumetric water content (WC), total N uptake, and lint yield were generally higher on lower landscape positions. Cotton lint yield was significantly correlated to soil WC (r=0.76), soil NO₃-N (r=0.35), and site elevation (r=−0.54). Differences of site elevation between local neighboring points explained the soil water, NO₃-N and lint yield variability at the micro-scale level in the landscape. Soil WC, cotton lint yield, N uptake, and clay content were crosscorrelated with site elevation across a lag distance of ±30–40 m. The state-space analysis showed that cotton lint yield was positively weighted on soil WC availability and negatively weighted on site elevation. Cotton lint yield state-space models give insights on the association of soil physical and chemical properties, lint yield, and landscape processes, and have the potential to improve water and N management at the landscape-scale.     

中国南方潴育性水稻土产量演变及其肥力驱动因素分析

系统分析了我国广西桂林、钦州及玉林3个潴育性水稻土长期常规施肥(F)和不施肥(CK)处理下,双季稻产量及土壤肥力随时间的变化趋势,并借助通径分析探明主要肥力因素对各点水稻产量的驱动差异和关联。结果显示,不施肥条件下,桂林点的基础地力较低,其产量随时间呈显著下降趋势(各季产量均值666~846 kg hm-2),而玉林和钦州点基础地力相对较高,其产量在试验期间相对稳定(各季产量均值3 500~4 577 kg hm-2)。常规施肥下,桂林点产量显著提高(较CK增产522%),且随时间呈显著上升趋势,另两试验点较CK增产20%~67%。常规施肥下,土壤肥力随时间呈稳定或不同程度的上升趋势,其中土壤有效磷初始值低于10 mg kg-1的桂林和钦州点上升幅度最大且均达到显著水平,而初始值为50 mg kg-1的玉林点变化不大。土壤有机质与全氮变化在各地不同,气温偏低的桂林点有机质呈上升趋势且全氮含量较高(3 g kg-1),气温偏高的玉林和钦州点有机质呈下降趋势且全氮含量较低(1~2.5 g kg-1),表明有机质的累积和释放上存在差异。双季稻产量的主要肥力驱动因子各地有差异,桂北地区桂林点土壤有效磷为首要因素,桂南地区玉林和钦州点土壤有机质及氮含量为主要的肥力因素。因此,依据区域特征采取有针对性措施是持续、高效培肥土壤的保证。     

Field calibration of an impedance soil water probe for the shallow seedbed across field topography

Impedance soil water probes enable frequent and non‐destructive determination of soil water status in situations where gravimetric soil sampling is too demanding of time and sampling space. The ThetaProbe is an impedance soil water probe requiring calibration for local soil conditions, because measurement accuracy can be affected by properties of the soil. Often, only a single calibration is performed for an experimental site. An experiment investigating the seedbed to 75‐mm depth across a field topography with variable soil properties was examined to determine which soil properties affected the calibration of the ThetaProbe, and if soil‐specific calibration was required to derive suitable estimates of the water status in the experiment. Experimental factors examined included hillslope aspect, hillslope position, crop residue and soil depth. Soil properties, other than volumetric water content, significantly affecting the probe measurements were bulk density, electrical conductivity and temperature. The probe underestimated soil water at very low water contents, and overestimated soil water at contents greater than 11 m³ m^?3, compared with gravimetric measurements. A single calibration, not corrected for hillslope position at a water content of 20 m³ m^?3, overestimated water content by 0.02 m³ m^?3 in the summit hillslope position and underestimated water content by 0.04 m³ m^?3 in the toeslope position. A single calibration, not corrected for soil depth at a water content of 20 m³ m^?3, overestimated water content by 0.02 m³ m^?3 in the 0‐ to 25‐mm soil layer and underestimated water content by 0.03 m³ m^?3 in the 50‐ to 75‐mm layer. The complexity of microsites in a shallow seedbed requires soil‐specific calibration in field experiments containing heterogeneous soil properties.     

Effects of topography on soil organic carbon stocks in grasslands of a semiarid alpine region,northwestern China

Purpose

Soil organic carbon (SOC) in mountainous regions is characterized by strong topography-induced heterogeneity, which may contribute to large uncertainties in regional SOC stock estimation. However, the quantitative effects of topography on SOC stocks in semiarid alpine grasslands are currently not well understood. Therefore, the purpose of this research study is to determine the role of topography in shaping the spatial patterns of SOC stocks.

Materials and methods

Soils from the summit, shoulder, backslope, footslope, and toeslope positions along nine toposequences within three elevation-dependent grassland types (i.e., montane desert steppe at ~?2450 m, montane steppe at ~?2900 m, and subalpine meadow at ~?3350 m) are sampled at four depths (0–10, 10–20, 20–40, and 40–60 cm). SOC content, bulk density, soil texture, soil water content, and grassland biomass are determined. The general linear model (GLM) is employed to quantify the effects of topography on the SOC stocks. Ordinary least squares regressions are performed to explore the underlying relationships between SOC stocks and the other edaphic factors.

Results and discussion

In accordance with the present results, the SOC stocks at 0–60 cm show an increasing trend in respect to the elevation zone, with the highest stock being approximately 37.70 g m^?2 in the subalpine meadow, about 2.07 and 3.41 times larger than that in the montane steppe and montane desert steppe, respectively. Along the toposequences, it is revealed the SOC stocks are maximal at toeslope, reaching to 14.98, 31.76, and 49.52 kg m^?2, which are also significantly larger than those at the shoulder by a factor of 1.38, 2.31, and 1.44, in montane desert steppe, montane steppe, and subalpine meadow, respectively. Topography totally is seen to explain about 84% of the overall variation in SOC stocks, of which 70.61 and 9.74% are attributed to elevation zone and slope position, while the slope aspect and slope gradient are seen to plausibly explain only about 1.84 and 0.01%, respectively.

Conclusions

The elevation zone and the slope position are seen to markedly shape the spatial patterns of the SOC stocks, and thus, they may be considered as key indicating factors in constructing the optimal SOC estimation model in such semiarid alpine grasslands.