A soil chronosequence in the semi-arid environment of Patagonia (Argentina)

Soil development with time was investigated on beach ridges with ages ranging from about 1380 to 6240 ¹⁴C-years BP at the eastern coast of central Patagonia. The main pedogenic processes are accumulation of organic matter and carbonate leaching and accumulation within the upper part of the soils. Soil formation is strongly influenced by incorporation of eolian sediments into the interstitial spaces between the gravel of which the beach ridges are composed. Different amounts of eolian material in the soils lead to differentiation into Leptosols (containing ≤ 10% fine earth in the upper 75 cm) and Regosols (containing > 10% fine earth). Soil depth functions and chronofunctions of organic carbon, calcium carbonate, pH, Ca:Zr, Mg:Zr, K:Zr, Na:Zr, Fe:Zr, Mn:Zr, and Si:Al (obtained from X-ray fluorescence analysis) were evaluated. To establish soil chronofunctions mean values of the horizon data of 0–10 cm below the desert pavement were used, which were weighted according to the horizon thicknesses. The depth function of pH shows a decrease towards the surface, indicating leaching of bases from the upper centimeters. Chronofunctions of pH show that within 6000 radiocarbon years of soil development pH drops from 7.0 to 6.6 in the Leptosols and from 8.1 to 7.5 in the Regosols. The higher pH of the Regosols is due to input of additional bases from the eolian sediments. Chronofunctions of Ca:Zr and K:Zr indicate progressive leaching of Ca and K in the Regosols, showing close relationships to time (R² = 0.972 and 0.995). Na leaching as indicated by decreasing Na:Zr ratios shows a strong correlation to time only in the Leptosols (R² = 0.999). Both, Leptosols and Regosols show close relationships to time for Fe:Zr (R² = 0.817 and 0.824), Mn:Zr (R² = 0.940 and 0.803), and Si:Al (0.971 and 0.977), indicating enrichment of Fe and Mn and leaching of Si. Leaching of mobile elements takes place on a higher level in the Regosols than in the Leptosols from the beginning of soil formation. Hence, a significant part of the eolian sediments must have been incorporated into the beach ridges very soon after their formation.     

A strand plain soil development sequence in Northern Michigan,USA

A practical problem limiting the theoretical application of a soil chronosequence study lies in the difficulty of identifying an appropriate geomorphic age sequence with more than a few surfaces. A sequence of approximately 75 strand plain beach ridges located along the northern shore of Lake Michigan, USA, overcomes this problem, and was used to explore rates of soil development in the initial stages of podzolization. Well drained pedons were sampled on 24 of the ridges, with an age range of 10 to 5200 years. Chronofunctions were developed using statistical regression techniques. For most forms of extractable Fe and Al, both linear and log–linear functions adequately describe the changes in property with surface age. Dithionite–citrate extractable Fe content, however, shows little correlation with surface age. High r² values (>0.7) for chronofunctions representing most extractable sesquioxide forms suggest that B horizon sesquioxide content increases with surface age, as expected for soils undergoing podzolization. Visual inspection of chronofunction data suggests that scatter increases on surfaces older than 3000 years, possibly because soil spatial variability increases with surface age.Theory suggests that the log–linear form of chronofunctions may be preferable to the linear form for these pedons. In general, equation slopes are steeper for chronofunctions representing only the initial 1650 years of soil development than for those representing the entire 5200-year sequence, suggesting that initially rapid pedogenesis slows over time. Most chronofunctions for pH and weighted profile organic C are not statistically significant, possibly because the temporal resolution of the chronosequence is too coarse to detect valid trends for properties that rapidly approach a steady state.     

A preliminary watershed scale soil quality assessment in north central Iowa, USA

Soil quality assessment has been recognized as an important step toward understanding the long-term effects of conservation practices within agricultural watersheds. Our objective was to assess soil quality within the South Fork watershed of the Iowa River using various indicators and assessment approaches. Soil samples were collected during 2003 and 2004 from 29 areas of 32 ha (80 acres) each along two transects traversing the watershed. Soil pH, Mehlich III extractable P, K, Ca and Mg, electrical conductivity (EC), total organic carbon (TOC), and total N (TN) were measured. The Soil Management Assessment Framework (SMAF) was used to compute a soil quality index (SQI), while soil loss, the soil tillage intensity rating (STIR), N-leaching potential, and soil conditioning index (SCI) were determined for each sampling area using the 2003 version of the Revised Soil Loss Equation (RUSLE2). Overall, there were no soil fertility limitations within the watershed based on an average pH of 6.96 and extractable P and K levels of 36 and 162 mg kg⁻¹, respectively. Soil loss, STIR, N-leaching, and SCI averaged 1.13 Mg ha⁻¹, 68, 3, and 0.4, respectively. The SMAF analysis indicated soils within the watershed were functioning at 87% of their full potential. The lowest indicator score was associated with TOC (0.60) because the average value was only 28.4 g kg⁻¹. The SCI and SQI indices were positively correlated although since it used measured data, the SMAF appears to provide more information about the effects of management practices within the watershed. Soils in upper landscape positions had lower TOC and C:N ratios indicating an increased risks for both erosion and for nitrate leaching. Management of soils on hilltops may be the most effective way to minimize N and P losses within the watershed.     

Mapping soil pH levels across Europe: An analysis of LUCAS topsoil data using random forest kriging (RFK)

Soil pH affects food production, pollution control and ecosystem services. Mapping soil pH levels, therefore, provides policymakers with crucial information for developing sustainable soil use and management policies. In this study, we used the LUCAS 2015 TOPSOIL data to map soil pH at a European level. We used random forest kriging (RFK) to build a predictive model of spatial variability of soil pH, as well as random forest (RF) without co-kriging and boosted regression trees (BRT) modelling techniques. Model accuracy was evaluated using a ten-fold cross-validation procedure. While we found that all methods accurately predicted soil pH, the accuracy of the RFK method was best with regression performance metrics of: R² = 0.81 for pH (H₂O) and pH (CaCl₂); RMSE = 0.59 for pH (H₂O) and RMSE = 0.61 in pH (CaCl₂); MAE = 0.41 for pH (H₂O) and MAE = 0.43 in pH (CaCl₂). Dominant explanatory variables in the RF and BRT modelling were topography and remote sensing variables, respectively. The generated maps broadly depicted similar spatial patterns of soil pH, with an increasing gradient of soil pH from north to south Europe, with the highest values mainly concentrated along the Mediterranean coast. The mapping could provide spatial reference for soil pH assessment and dynamic monitoring.     

Soil CO2 flux in six monospecific forest plantations in Southern Rwanda

Forest soils contain the largest carbon stock of all terrestrial biomes and are probably the most important source of carbon dioxide (CO₂) to atmosphere. Soil CO₂ fluxes from 54 to 72-year-old monospecific stands in Rwanda were quantified from March 2006 to December 2007. The influences of soil temperature, soil water content, soil carbon (C) and nitrogen (N) stocks, soil pH, and stand characteristics on soil CO₂ flux were investigated. The mean annual soil CO₂ flux was highest under Eucalyptus saligna (3.92 μmol m⁻² s⁻¹) and lowest under Entandrophragma excelsum (3.13 μmol m⁻² s⁻¹). The seasonal variation in soil CO₂ flux from all stands followed the same trend and was highest in rainy seasons and lowest in dry seasons. Soil CO₂ flux was mainly correlated to soil water content (R² = 0.36-0.77), stand age (R² = 0.45), soil C stock (R² = 0.33), basal area (R² = 0.21), and soil temperature (R² = 0.06-0.17). The results contribute to the understanding of factors that influence soil CO₂ flux in monocultural plantations grown under the same microclimatic and soil conditions. The results can be used to construct models that predict soil CO₂ emissions in the tropics.     

Carbon and nitrogen distribution in water-stable aggregates under two tillage techniques in Fluvisols of Owerri area, southeastern Nigeria

Organic matter influences soil structure and compactibility by binding soil mineral particles, reducing aggregate wettability, and influencing the mechanical strength of soil aggregates, which is the measure of coherence of inter-particle bonds. This work was carried out to examine how differences in water-stable aggregates influence the distribution of soil organic carbon and soil organic nitrogen under two tillage techniques [minimum tillage (only planting holes were opened) and conventional tillage (raised beds, 30 cm high, prepared manually with traditional hoes)] in soils of a Fluvisol in Owerri, southeastern Nigeria. Three pedons were dug and studied for each of the tillage technique along a soil sequence. Soil organic carbon and soil organic nitrogen distribution in whole soil and in water-stable aggregates under minimum tillage and conventional tillage were determined for the soils. Soil organic carbon contents in water-stable aggregates (WSA) of the pedons varied according to method of tillage. The highest mean values of soil organic carbon were obtained from minimum tillage and in water-stable aggregates 4.75–2.00 mm (16.03 Mg C ha⁻¹), 1.00–0.50 mm (14.06 Mg C ha⁻¹) and water-stable aggregates 2.00–1.00 mm (13.99 Mg C ha⁻¹) whereas under conventional tillage, water-stable aggregates 1.00–0.50 mm with soil organic carbon of 24.6 Mg C ha⁻¹ had the highest soil organic carbon content. Soil organic carbon correlated significantly with mean weight diameter (r = 0.48; P = 0.05; n = 15), water-stable aggregates 4.75–2.00 mm (r = 0.73; P = 0.05; n = 15), water-stable aggregates 2.00–1.00 mm (r = 0.55; P = 0.05, n = 15), water-stable aggregates 1.00–0.50 mm (r = 0.44; P = 0.05; n = 15) whereas no relationship was found between soil organic carbon and water-stable aggregates 0.50–0.25 mm (r = 0.15; P = 0.05; n = 15) and water-stable aggregates <0.25 mm (r = 0.17; P = 0.05; n = 15) in soils under minimum tillage. There was a significant correlation (r = 0.45–0.58; P = 0.05; n = 14) between all water-stable aggregates classes studied and soil organic carbon in soils under conventional tillage. Mean values of soil organic nitrogen were higher in soils under minimum tillage with 4.75–2.00 mm and 2.00–1.00 mm aggregate classes having 1.64 Mg N ha⁻¹ and 1.57 Mg N ha⁻¹ soil organic nitrogen when compared to 1.01 Mg N ha⁻¹ and 1.00 Mg N ha⁻¹ in conventionally tilled soils of the same aggregate classes, respectively. Larger water-stable aggregate classes (4.75–2.00; 2.00–1.00) had slightly more soil organic nitrogen (22–26%) than smaller aggregate classes (1.00–0.50; 0.50–0.25; >0.25) with 14–24% soil organic nitrogen in minimum tilled soils. In soils under conventional tillage, 1.00–0.50 mm, 0.50–0.25 mm and <0.25 mm aggregate classes contributed more soil organic nitrogen (19.66–22.40%) to the soil whereas larger water-stable aggregate classes contributed 19.22% soil organic nitrogen. The proportion of soil organic carbon and total nitrogen retained in soils with higher percentage of water-stable aggregates are less likely to be lost through soil and wind erosion. The higher values of SOC in the whole soil and WSA classes less than 2.00 mm are indications of positive influence of SOC on the stability of these peds.     

Relationships between soil pH and microbial properties in a UK arable soil

Effects of changing pH along a natural continuous gradient of a UK silty-loam soil were investigated. The site was a 200 m soil transect of the Hoosfield acid strip (Rothamsted Research, UK) which has grown continuous barley for more than 100 years. This experiment provides a remarkably uniform soil pH gradient, ranging from about pH 8.3 to 3.7. Soil total and organic C and the ratio: (soil organic C)/(soil total N) decreased due to decreasing plant C inputs as the soil pH declined. As expected, the CaCO₃ concentration was greatest at very high pH values (pH > 7.5). In contrast, extractable Al concentrations increased linearly (R² = 0.94, p < 0.001) from below about pH 5.4, while extractable Mn concentrations were largest at pH 4.4 and decreased at lower pHs. Biomass C and biomass ninhydrin-N were greatest above pH 7. There were statistically significant relationships between soil pH and biomass C (R² = 0.80, p < 0.001), biomass ninhydrin-N (R² = 0.90, p < 0.001), organic C (R² = 0.83, p < 0.001) and total N (R² = 0.83, p < 0.001), confirming the importance of soil organic matter and pH in stimulating microbial biomass growth. Soil CO₂ evolution increased as pH increased (R² = 0.97, p < 0.001). In contrast, the respiratory quotient (qCO₂) had the greatest values at either end of the pH range. This is almost certainly a response to stress caused by the low p. At the highest pH, both abiotic (from CaCO₃) and biotic Co₂ will be involved so the effects of high pH on biomass activity are confounded. Microbial biomass and microbial activity tended to stabilise at pH values between about 5 and 7 because the differences in organic C, total N and Al concentrations within this pH range were small. This work has established clear relationships between microbial biomass and microbial activity over an extremely wide soil pH range and within a single soil type. In contrast, most other studies have used soils of both different pH and soil type to make similar comparisons. In the latter case, the effects of soil pH on microbial properties are confounded with effects of different soil types, vegetation cover and local climatic conditions.     

The application of proximal visible and near-infrared spectroscopy to estimate soil organic matter on the Triffa Plain of Morocco

Soil organic matter (SOM) is a fundamental soil constituent. The estimation of this parameter in the laboratory using the classical method is complex time-consuming and requires the use of chemical reagents. The objectives of this study were to assess the accuracy of two laboratory measurement setups of the VIS-NIR spectroscopy in estimating SOM content and determine the important spectral bands in the SOM estimation model. A total of 115 soil samples were collected from the non-root zone (0–20 cm) of soil in the study area of the Triffa Plain and then analysed for SOM in the laboratory by the Walkley–Black method. The reflectance spectra of soil samples were measured by two protocols, Contact Probe (CP) and Pistol Grip (PG)) of the ASD spectroradiometer (350–2500 nm) in the laboratory. Partial least squares regression (PLSR) was used to develop the prediction models. The results of coefficient of determination (R²) and the root mean square error (RMSE) showed that the pistol grip offers reasonable accuracy with an R² = 0.93 and RMSE = 0.13 compared to the contact probe protocol with an R² = 0.85 and RMSE = 0.19. The near-Infrared range were more accurate than those in the visible range for predicting SOM using the both setups (CP and PG). The significant wavelengths contributing to the prediction of SOM for (PG) setup were at: 424, 597, 1432, 1484, 1830,1920, 2200, 2357 and 2430 nm, while were at 433, 587, 1380, 1431, 1929, 2200 and 2345 nm for (CP) setup.     

The Influence of Soil Properties on the Water Pollution (Nitrate,Phosphate and COD) Following a Single Application of Sewage Sludge to 70 Contrasting Agricultural Soils

The aim was to study the influence of soil properties on the leaching of nitrate, phosphate and organic matter (OM) following the application of sewage sludge to contrasting soils. Seventy agricultural soils from different parts of Spain were amended with sewage sludge (50 t dry weight ha⁻¹), and a controlled column study was developed. After 2, 4 and 6 months of incubation, distilled water, equivalent to an autumn rainfall event of 25 l m⁻² in Mediterranean environments, was applied and leachates collected and analysed: pH, electrical conductivity (EC), chemical oxygen demand (COD), phosphate and nitrate. The mean values of pH in the leachates after 2, 4 and 6 months were similar and close to the neutrality. The highest concentrations for the rest of the parameters analysed were found after 2 months of incubation and diminished for 4 and 6 months, especially COD. Soil pH and texture were the most relevant soil properties controlling the leaching of the analysed parameters. The OM mineralization seemed to be enhanced at high values of soil pH, thus increasing the nitrate and reducing the COD leaching. However, phosphate levels were reduced at high values of soil pH. In addition, leaching was promoted in sandy soils. Other soil properties influenced phosphate leaching being the equivalent calcium carbonate soil content as the most relevant. Soil organic carbon was negatively related to the EC and nitrate concentration in the leachates but resulting in a weak contribution compared with soil pH and texture. Concerns about nitrate pollution have been confirmed.     

中国农耕区土壤有机质含量及其与酸碱度和容重关系

对我国农耕区土壤有机质区域变化及其与酸碱度和容重关系进行系统分析,为耕地地力提升和改善土壤结构提供支撑。基于国家级耕地长期定位监测点913个,统计分析全国及7大区域（东北NE、华北NC、西北NW、长江中游MYR、长三角YRD、华南SC、西南SW）耕层土壤有机质含量、酸碱度及容重变化特征。结果表明,全国农耕区耕层土壤有机质含量平均值为22.4~24.8 g/kg。其中有机质含量中等偏低的监测点位占比达72.5%。不同区域耕层土壤有机质含量差异显著（p<0.05）,MYR耕层土壤有机质含量显著高于其他6个区域。全国农耕区耕层土壤pH和容重平均分别为（6.90±1.20）,（1.30±0.15） g/cm³。不同土壤利用方式对土壤有机质、酸碱度及容重产生影响。水田耕层土壤有机质含量显著高于旱地,旱地耕层土壤pH和容重则显著高于水田。亚当斯方程和指数函数分别推荐拟合土壤容重对有机质含量响应关系（R²=0.09,RMSE=0.17,n=759）,以及土壤pH对土壤有机质含量响应（R²=0.16,RMSE=1.24,n=886）。全国农耕区耕层土壤有机质含量总体中等偏低,呈现出东南向西北依次降低趋势。土壤pH及容重与土壤有机质呈现显著的负相关关系。亚当斯模型及指数方程能较好地拟合土壤容重及pH对有机质的响应关系,可用于非线性插值法补充土壤容重及pH缺失值。     

Influence of temperature and drought on seasonal and interannual variations of soil, bole and ecosystem respiration in a boreal aspen stand

Continuous half-hourly measurements of soil (R_s) and bole respiration (R_b), as well as whole-ecosystem CO₂ exchange, were made with a non steady-state automated chamber system and with the eddy covariance (EC) technique, respectively, in a mature trembling aspen stand between January 2001 and December 2003. Our main objective was to investigate the influence of long-term variations of environmental and biological variables on component-specific and whole-ecosystem respiration (R_e) processes. During the study period, the stand was exposed to severe drought conditions that affected much of the western plains of North America. Over the 3 years, daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ during winter to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-summer. Seasonal variations of R_s were highly correlated with variations of soil temperature (T_s) and water content (θ) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean R_s from 2001 to 2003. Aspen daily mean R_b varied from negligible during winter to a maximum of 2.5 μmol m⁻² bark s⁻¹ (2.2 μmol m⁻² ground s⁻¹) during the growing season. Maximum R_b occurred at the end of the aspen radial growth increment and leaf emergence period during each year. This was 2 months before the peak in bole temperature (T_b) in 2001 and 2003. Nonetheless, R_b was highly correlated with T_b and this variable explained 77, 87 and 62% of the variance in R_b in the respective years. Partitioning of R_b between its maintenance (R_bm) and growth (R_bg) components using the mature tissue method showed that daily mean R_bg occurred at the same time as aspen radial growth increment during each growing season. This method led, however, to systematic over- and underestimations of R_bm and R_bg, respectively, during each year. Annual totals of R_s, R_b and estimated foliage respiration (R_f) from hazelnut and aspen trees were, on average, 829, 159 and 202 g C m⁻² year⁻¹, respectively, over the 3 years. These totals corresponded to 70, 14 and 16%, respectively, of scaled-up respiration estimates of R_e from chamber measurements. Scaled R_e estimates were 25% higher (1190 g C m⁻² year⁻¹) than the annual totals of R_e obtained from EC (949 g C m⁻² year⁻¹). The independent effects of temperature and drought on annual totals of R_e and its components were difficult to separate because the two variables co-varied during the 3 years. However, recalculation of annual totals of R_s to remove the limitations imposed by low θ, suggests that drought played a more important role than temperature in explaining interannual variations of R_s and R_e.     

Use of an instrumented disc coulter for mapping soil mechanical resistance

A soil mechanical resistance sensor with a large-diameter disc coulter was developed to delineate areas of differing soil strength across agricultural fields. The instrumented disc coulter consisted of a 76.2 cm disc with two depth-measuring sensors (rotary potentiometer and ultrasonic proximity sensor) along with a global positioning system (GPS) receiver to georeference operating depth measurements. The consistency and repeatability of the system response were evaluated by making six passes across long-term tillage comparison plots with different degrees of soil disturbance, including: 20 cm plowing, 15 cm disking, 30 cm chiseling, and no-till in several combinations. At the time of testing, standard soil cone penetrometer measurements were taken. The relationship between the average cone index in the 0–30 cm soil profile (CI_0–30 cm) and the disc operating depth was evaluated. In addition, the cumulative energy density of the given depth of penetration defined as specific cone penetration energy (J m⁻² or N cm⁻¹) for each tillage plot was calculated using the cone index profiles. The average measured depth in each tillage plot was compared to the average predicted depth (d_ci) of a fixed specific cone penetration energy (P_ci). Static calibration tests on the depth sensors showed excellent linearity with coefficients of determination (R²) greater than 0.99. The results showed that, on the average, the changes in the depth measured with the rotary potentiometer were 44 and 68% of the changes in the depth measured with the ultrasonic proximity sensor while the disc coulter was passing across, or along, the tillage plots. This difference was primarily due to the sinkage of the tractor wheels. The depth measured with the ultrasonic sensor had significant correlation with both CI_0–30 cm and d_ci. This was partially due to the fact that a significantly high correlation (R² = 0.97) between the CI_0–30 cm and d_ci was observed, which was not expected and originated from the type of soil profiles present. The instrumented disc coulter is a low soil disturbance system and could be used as an inexpensive and simple sensor to obtain information about the mechanical condition of the soil for spot tillage or other management decisions.     

Organic carbon stocks and soil erodibility in Canary Islands Andosols

Soil organic carbon (SOC) plays a key role in the structural stability of soils and in their resistance against erosion. However, and as far as andic soils are concerned, these mechanisms and processes, as well as the influence of the different types of SOC on aggregate stability, are not fully understood. The targets of this paper are: (i) to determine the content and forms of SOC in Andosols under evergreen forest vegetation [laurel (Laurus) and heather (Erica) forest] and (ii) to find out the role of soil organic matter (SOM) in the aggregate stability and in the resistance of Andosols to water erosion. Soil samples have been collected in 80 sites in a 40 km² area under udic soil moisture regime. In them, fulvic and humic acids, Walkley–Black SOC, pyrophosphate-extractable SOC, Fe and Al, potassium sulphate extractable SOC, dissolved SOC, acid oxalate-extractable Fe, Al and Si, USLE K-factor and aggregate stability have been determined. The Andosols over volcanic ash are Aluandic Andosols (non-allophanic Andosols), whereas over basaltic lava flows are Silandic Andosols (allophanic Andosols). The surface (0–30 cm) samples analyzed contain 9.5–30 kg C m^− 2 being significantly higher in allophanic Andosols (p < 0.5). Organic carbon adsorbed onto the mineral fraction (extractable pyrophosphate, C_p) accounts for 35–55% of the total SOC. All samples show a high stability to slaking and raindrop impact, being the first one highly correlated (r = 0.6) with pyrophosphate extractable C (C_p), Fe (Fe_p), and Al (Al_p) in allophanic Andosols, unlike non-allophanic ones. The stability to raindrop impact correlates with pyrophosphate extractable C (C_p) and Fe (Fe_p) in both types of soils (r = 0.3–0.6, p < 0.05). These findings suggest that the high stability to both slaking and water-drop impact is due to the occurrence of allophane–Fe–OC complexes, rather than to the total OC, and the active Fe and Al forms, generated by the weathering of volcanic materials, constitute an essential constituent responsible for C sequestration and resistance to degradation in these soils.     

Specific ion effects on soil aggregate stability and rainfall splash erosion

Soil interparticle forces can pose important effects on soil aggregate stability and rainfall splash erosion. Meanwhile, these interparticle forces are strongly influenced by specific ion effects. In this study, we applied three monovalent cations (Li⁺, Na⁺, and K⁺) with various concentrations to investigate the influence of specific ion effects on aggregate stability and splash erosion via pipette and rainfall simulation methods. The specific ion effects on soil interparticle forces were quantitatively evaluated by introducing cationic non-classical polarization. The results showed that aggregate stability and splash erosion had strong ion specificity. Aggregate breaking strength and splash erosion rate at the same salt concentration followed the sequence as Li⁺ > Na⁺ > K⁺. With decreasing salt concentration, the difference in aggregate breaking strength or splash erosion rate between different cation systems increased initially (1–10^–2 mol L^–1) and later was nearly invariable (10^–2–10^–4 mol L^–1). The experimental results were well quantitatively explained by soil interparticle forces considering cationic non-classical polarization. Furthermore, both aggregate breaking strength and splash erosion rate of three cations revealed a strong positive linear relation with net force subjected to cationic non-classical polarization (R² = 0.81, R² = 0.81). These results demonstrated that different non-classical polarization of cations resulted in different soil interparticle forces, and thus led to differences in aggregate stability and splash erosion. Our study provides valuable information to deeply understand the mechanisms of rainfall splash erosion.     

Soil ridge geometry for green control in French fry potato production on loamy clay soils in The Netherlands

Green potato (Solanum tuberosum L.) tubers are rejected by the processing industry. Therefore their amount should be reduced to minimum. The objective of this study was to assess the reduction of greening of new French fry varieties in experiments conducted from 1998 to 2001 on ridge qualities like geometry and the compression and crumbling degree of the loose soil in the ridge.The experiments were mainly carried out on the experimental husbandry farm Westmaas on a Calcaric Fluvisol having a clay content of ca. 200 g kg⁻¹. French fry potato varieties used were Agria and Fambo, whereas Bintje was used as a reference. Row spacings were 0.75 m (30 in.) and 0.90 m (36 in.). Net ridge size was determined by the internal height (distance between the plate and the top of the ridge) and by the width of the top of the ridge combined with planting near the surface of the plate.For the traditional French fry variety Bintje a reduction in greening was achieved by ridges having a gross size of 0.06–0.07 m². New French fry potato varieties like Agria and Fambo, however, are characterized by higher yields, larger cluster widths and longer tubers than Bintje. These properties favor the greening of tubers and require adjustment of the ridge geometry.Increase of the internal height and of the top width were found to be crucial to reduce greening of tubers to minimum. An increase of the top width of standard 0.75 m ridges from 0.15–0.18 to 0.25 m combined with an internal height of 0.18–0.20 m was realized by the so-called plateau ridges. This ridge type required ca. 0.04 m deeper seedbed than the depth of the seedbed for standard 0.75 m ridges. Without deeper loosening of the seedbed such larger ridges could also be realized by an increase of row spacing to 0.90 m. Net yields of new French fry varieties of plateau and 0.90 m ridges were generally similar or higher than the yields of standard 0.75 m ridges with the same internal height. With 0.90 m ridges over-sizing occurred sometimes.Greening of new varieties of French fry potatoes was reduced to minimum by the new knowledge on ridge geometry: an increase of the internal ridge height to 0.18–0.20 m, a top width to 0.25 m and reduction of cracking by relatively coarse, non-compressed ridges.     

Soil acidification used as a management strategy to reduce nitrate losses from agricultural land

pH is known to be a primary regulator of nutrient cycling in soil. Increasing soil acidity in agricultural systems has the potential to slow down N cycling and reduce N losses from leaching thereby enhancing sustainability and reducing pollution. We conducted a field experiment to investigate the impact of acidity on N leaching in arable and grassland agricultural systems. The results showed that nitrate (NO₃⁻) concentrations in soil water were greater under arable than under grassland. Soil acidification significantly lowered NO₃⁻ concentrations in soil water over winter and spring under grassland, whilst in cereal plots a similar effect was only observed in spring. Our results suggest that soil acidification decreased nitrification causing an accumulation of NH₄⁺ which was not subject to leaching. Dissolved organic nitrogen (DON) concentrations in soil water were significantly greater under arable than grassland. Soil acidification lowered concentrations of DON in soil water, usually to a greater extent in grassland than in arable plots. It was concluded that it may be possible to use careful soil pH management as a tool to control NO₃⁻ leaching without compromising the quality of drainage water, and that this may be more effective on grassland than on arable crops.     

Temporal and spatial variability of soil respiration in a boreal mixedwood forest

Temporal and spatial variability of soil respiration (R_s) was measured and analyzed in a 74-year-old, mixedwood, boreal forest in Ontario, Canada, over a period of 2 years (August 2003–July 2005). The ranges of R_s measured during the two study years were 0.5–6.9 μmol CO₂ m⁻² s⁻¹ for 2003–2004 (Year 1) and 0.4–6.8 μmol CO₂ m⁻² s⁻¹ for 2004–2005 (Year 2). Mean annual R_s for the stand was the same for both years, 2.7 μmol CO₂ m⁻² s⁻¹. Temporal variability of R_s was controlled mainly by soil temperature (T_s), but soil moisture had a confounding effect on T_s. Annual estimates of total soil CO₂ emissions at the site, calculated using a simple empirical R_s–T_s relationship, showed that R_s can account for about 88 ± 27% of total annual ecosystem respiration at the site. The majority of soil CO₂ emissions came from the upper 12 to 20 cm organic LFH (litter–fibric–humic) soil layer. The degree of spatial variability in R_s, along the measured transect, was seasonal and followed the seasonal trend of mean R_s: increasing through the growing season and converging to a minimum in winter (coefficient of variation (CV) ranged from 4 to 74% in Year 1 and 4 to 62% in Year 2). Spatial variability in R_s was found to be negatively related to spatial variability in the C:N ratio of the LHF layer at the site. Spatial variability in R_s was also found to depend on forest tree species composition within the stand. R_s was about 15% higher in a broadleaf deciduous tree patch compared to evergreen coniferous area. However, the difference was not always significant (at 95% CI). In general, R_s in the mixedwood patch, having both deciduous and coniferous species, was dominated by broadleaf trees, reflecting changing physiological controls on R_s with seasons. Our results highlight the importance of discerning soil CO₂ emissions at a variety of spatial and temporal scales. They also suggest including the LFH soil layer and allowing for seasonal variability in CO₂ production within that layer, when modeling soil respiration in forest ecosystems.     

Nitrogen fertilization and cropping system impacts on soil properties and their relationship to crop yield in the central Corn Belt, USA

Evaluating the effects of management practices on soil physical and chemical properties would be valuable to explain field-level variability in crop production. A 23-year-old experiment on a Muscatune soil (fine-silty, mixed, superactive, mesic, Aquic Argiudolls) in Illinois with five N rates [0 (N₀), 70 (N₁), 140 (N₂), 210 (N₃) and 280 (N₄) kg N ha⁻¹] and two cropping systems [continuous corn (Zea mays L.) (CC), and corn–soybean (Glycine max (L.) Merr.) rotation (CS)] was evaluated. Specific objectives were to: (i) evaluate the effects of long-term N fertilization and cropping systems on field level changes in soil physical and chemical properties and crop yield, (ii) identify the most responsive soil physical and chemical properties to N fertilizer and crop management, and (iii) investigate the relationship between the selected soil properties and crop yield. Soil was collected in May 2004 to 30 cm depth and 20 soil physical and chemical properties were measured. The univariate analysis indicated that 14 soil properties were significantly influenced by at least one treatment effect (crops, N or crops × N). Due to multicollinearity among soil properties, principal component analysis (PCA) was used to group correlated properties, resulting in five soil properties such as soil organic carbon stock (OC stock), mean weight diameter (MWD), soil C:N ratio, exchangeable potassium (K⁺) and gravimetric moisture content (ω). Finally, the multiple regression analysis performed between PCA derived soil properties and corn and soybean yields retained all the representative soil properties from PCA except ω as yield predictors for corn (P < 0.001, R² = 0.39) from CC system, whereas none of the soil properties were significantly related to corn and soybean yields from CS system. The soil properties most influenced by long-term N fertilization of continuous corn were successfully identified with PCA and multiple regression. The insignificant relationship between corn and soybean yields from CS system and PCA derived soil properties might be due to the lack of response of soybean to N fertilization. This study shows the integrated use of multivariate and regression analyses in identifying yield determining soil properties by eliminating the multicollinearity among soil properties.     

Leaching of genetically modified Pseudomonas fluorescens through organic soils: Influence of temperature,soil pH,and roots

Summary The effects of soil temperature and bulk soil pH on the vertical translocation of a genetically modified Pseudomonas fluorescens inoculum were studied in reconstituted soil microcosms, in the presence and absence of growing Lolium perenne roots. The inoculated microcosms received one rainfall event per day (5 mm h^-1 for 6 h) for 5 days and the resulting leachate was quantitatively assayed for the presence of the modified pseudomonad. Soil temperature affected the total number of modified pseudomonads detected in the leachate over the 5 days, with significantly lower numbers detected at 25°C compared to 5°C. The bulk soil pH also affected leaching of the inoculum, with significantly greater numbers detected in the effluent at pH 7.5 than at pH 4.5. In the absence of L. perenne, greater numbers of the modified pseudomonads were detected in the pH 7.5 soil after 5 days of leaching compared to soil at pH 4.5. L. perenne roots decreased the number of cells of the inoculum that were leached and detected in the soil after 5 days of leaching. In the soil microcosms used for the pH study the distribution of the inoculum remaining with the soil was altered by L. perenne roots. At each pH value the proportion of cells detected within the soil below the surface 2 cm of the microcosms was greater in the presence of L. perenne roots. The results of this study indicate that soil temperature, bulk soil pH, and the presence of root systems are important factors in determining the extent of inoculum translocation, and should be considered in the design and interpretation of field experiments.     

The ability of the DGT soil phosphorus test to predict pasture response in Australian pasture soils – a preliminary assessment

Diffusive gradients in thin‐films (DGT) technology provides an alternative assessment of available phosphorus (P) for a range of crops, suggesting a preliminary examination of the performance of the new DGT‐P test, compared to existing bicarbonate extractable Olsen and Colwell P tests, for pastures is justified. This study utilized historic data from the Australian National Reactive Phosphate Rock (NRPR) study (1992–1994) that included 25 experimental sites representing a wide range of soil types and climates used for pasture production. Stored (~19 yr) soil samples were analysed for DGT‐P, Olsen P and a single point P buffering index (PBI) and re‐analysed for Colwell P. Results showed the traditional bicarbonate extractable Colwell (r² = 0.45, P < 0.001) and Olsen P (r² = 0.27, P < 0.001) methods predicted relative pasture P response more accurately, compared to the novel DGT‐P test (r² = 0.09, P = 0.03) when all 3 yr of data were examined. We hypothesize that the harsher bicarbonate extraction used for the Olsen and Colwell methods more accurately reflects the ability of perennial pasture roots to access less labile forms of P, in contrast to the DGT‐P test, which does not change the soil pH or dilute the soil and appears unable to fully account for a plants ability to solubilize P. Further studies are needed to compare the capacity of DGT‐P to measure P availability in perennial pasture systems and to better understand the soil chemical differences between pasture and cropping systems.