Backslope Positions Research Articles

Topography and canopy cover influence soil organic carbon composition and distribution across a forested hillslope in the discontinuous permafrost zone

AbstractTopography and canopy cover influence ground temperature in warming permafrost landscapes, yet soil temperature heterogeneity introduced by mesotopographic slope positions, microtopographic differences in vegetation cover, and the subsequent impact of contrasting temperature conditions on soil organic carbon (SOC) dynamics are understudied. Buffering of permafrost‐affected soils against warming air temperatures in boreal forests can reflect surface soil characteristics (e.g., thickness of organic material) as well as the degree and type of canopy cover (e.g., open cover vs. closed cover). Both landscape and soil properties interact to determine meso‐ and microscale heterogeneity of ground warming. We sampled a hillslope catena transect in a discontinuous permafrost zone near Fairbanks, Alaska, to test the small‐scale (1 to 3 m) impacts of slope position and cover type on soil organic matter composition. Mineral active layer samples were collected from backslope, low backslope, and footslope positions at depths spanning 19 to 60 cm. We examined soil mineralogical composition, soil moisture, total carbon and nitrogen content, and organic mat thickness in conjunction with an assessment of SOC composition using Fourier‐transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). Soils in the footslope position had a higher relative contribution of lignin‐like compounds, whereas backslope soils had more aliphatic and condensed aromatic compounds as determined using FT‐ICR‐MS. The effect of open versus closed tree canopy cover varied with the slope position. On the backslope, we found higher oxidation of molecules under open cover than closed cover, indicating an effect of warmer soil temperature on decomposition. Little to no effect of the canopy was observed in soils at the footslope position, which we attributed, in part, to the strong impact of soil moisture content in SOC dynamics in the water‐gathering footslope position. The thin organic mat under open cover on the backslope position may have contributed to differences in soil temperature and thus SOC oxidation under open and closed canopies. Here, the thinner organic mat did not appear to buffer the underlying soil against warm season air temperatures and thus increased SOC decomposition as indicated by the higher oxidation of SOC molecules and a lower contribution of simple molecules under open cover than the closed canopy sites. Our findings suggest that the role of canopy cover in SOC dynamics varies as a function of landscape position and soil properties, namely, organic mat thickness and soil moisture. Condition‐specific heterogeneity of SOC composition under open and closed canopy cover highlights the protective effect of canopy cover for soils on backslope positions.

Land use in the Prairie Pothole Region influences the soil bacterial community composition and relative abundance of nitrogen cycling genes

The undulating topography of Prairie Pothole Region of North America creates spatial and temporal variability in soil moisture and nutrient levels, affecting microbial community processes and greenhouse gas emissions. By identifying differences in soil bacterial and archaeal community composition and the abundance of nitrogen cycling genes in permanent cover versus annual crop land over two growing seasons (2017 and 2018), we were able to assess the effects of topography and land use on the functional capacity of the soil microbiome. Permanent grassland cover was associated with higher bacterial diversity in upland positions and lower diversity in low-lying depressions. Bacterial community composition was also significantly different between cultivated and permanent cover at all points along the topographic slope, with the largest effects seen in the footslope and backslope positions. Compared to permanent cover, soil from annual cropland had consistently more abundant nitrifiers, including Nitrospira in the toeslope and backslope, and Nitrososphaeraceae in the shoulder and knoll samples while soils from permanent cover had a greater abundance of several Alphaproteobacteria from Rhodospirillales and Hyphomicrobiaceae across multiple upland positions. Upland soils from annual cropland also had consistently higher abundance of both bacterial and archaeal ammonia oxidizing ( amoA) genes and a higher ratio of nirK: nirS genes compared to those from permanent cover. These differences in microbial community composition were associated with higher N2O and CO2 emissions in upland soils in annual cropland; however, there were no differences in GHG emissions between the two systems in low-lying positions.

Polygenesis and cementation pathways for plinthic and non-plinthic soils on the Upper Coastal Plain, South Carolina

Plinthic soils on the Upper Coastal Plain (UCP) of South Carolina are directly underlain by hard but unconsolidated older Coastal Plain sediment, as are non-plinthic soils on adjacent dissected landforms. The older deposit, called brick material, is integral to plinthite formation via perched soil water hence intensified redox conditions. Currently, few causes are known for horizon matrix cementation of these soils. Six pedons from three hillslope positions were investigated for soil uniformity, selectively dissolved Fe and Si distributions, and micromorphology. Particle-size indices and ground-penetrating radar (GPR) confirmed uniform soils and thick plinthic horizons on summit positions and a discontinuity at shoulder and backslope positions. Contrasts in GPR radar reflections corresponded to the brick material and were thought to be associated with changes in moisture contents, iron mineralogy, pore size, and clay content. Illuvial clay infillings and minute channels confirmed pedogenesis in brick material. In plinthic horizons above the brick material, excess citrate dithionite Fe (Fed) was indicated by over-bulging distributions relative to Fed/clay ratios. Water-soluble silica (Siw) may be recycled in upper sola by capillarity and leaching, move through the discontinuity, and interact with Fe throughout the solum compatible with coarse material encapsulated with Fe-enriched fine material in the brick matrix. Absolute molar contents of oxalate extractable Si (Sio) and Fe (Feo) were low throughout all pedons, however, Sio/(Feo + Sio) ratios ranged from 0.30 to 0.60 in Btvx, Btx, and 2Btx horizons to account for fragic soil properties. Nascent UCP parent material or soils evidently provided adequate constituents for plinthic soil formation. A new theory proposes UCP plinthic soils evolved simultaneously with uniform soils that do not have a water table and have Bt horizons that begin as yellowish or brownish then redden with depth. Ferricrete sourced from nearby extant landscapes may have provided detrital ironstone to the parent material mix.

Soil organic carbon stocks in the high mountain permafrost zone of the semi-arid Central Andes (Cordillera Frontal, Argentina)

This study presents the first detailed soil organic carbon (SOC) inventory for a high mountain permafrost zone in the semi-arid Central Andes of South America. We describe plant cover and soil profiles at 31 sites representing the main land cover and landform types in the Veguitas catchment (Cordillera Frontal, Argentina), which ranges in elevation from c. 3000 to 5500 m. The vegetated area with soil development is largely confined to altitudes of < 3650 m and represents only 8.2% of the total catchment area. Mean SOC 0–100 cm storage for the vegetated portion of the catchment is 3.62 kg C m−2, which is reduced to 0.33 kg C m−2 if we consider negligible SOC stocks in the extensive bare ground and glaciated areas at higher elevations. Hotspots of SOC storage are wet meadow areas, with peat deposits up to 102 cm deep and a maximum observed total SOC storage of 53.07 kg C m−2. These wet meadow areas, however, occupy only 0.11% of the total catchment area and their contribution to mean SOC storage is limited. Among soils at well-drained sites, highest mean SOC 0–100 cm storage is found on backslope positions of moraines that predate the Last Glacial Maximum (6.87 kg C m−2). Only 2% of all SOC stocks in the catchment are found in permafrost terrain and none are located in the permafrost layer itself. The main ecoclimatic control on SOC storage is plant cover, with vegetation limits being sensitive to ambient temperature. Projected increases in temperatures will not remobilize any frozen SOC stocks but will likely result in an upward shift of the upper vegetation belt with soil development creating new areas of phytomass carbon and SOC storage. The area is expected to represent a net C sink and thus a negative feedback on future global warming.

Cover crops and landscape positions mediate corn–soybean production

AbstractEcosystem services and cash crop benefits provided by cover crops (CCs) can be affected by temporal and spatial variability of CC performances as influenced by topographic position of the field. A watershed‐scale study was initiated in 2015 to assess the influence of crop rotations [cereal rye (Secale cereale L.)–soybean [Glycine max (L.) Merr.]–hairy vetch (Vicia villosa Roth.)–corn (Zea mays L.) (CC) and winter‐fallow soybean–winter‐fallow corn (NoCC)] and topography (i.e., shoulder, backslope, and footslope) on corn and soybean productivity in southern Illinois. Cereal rye increased soybean yield by 0.29 Mg ha‐1 at the shoulder position, but it reduced yield by 0.44 Mg ha‐1 at the footslope position when compared with the NoCC treatments. At the footslope position, every 1 Mg ha‐1 increase in cereal rye biomass increased soybean yield by 0.87 Mg ha‐1. Soybean yield was negatively related to the cereal rye biomass at the shoulder and backslope positions. Within the CC rotation, corn yield was greater at the shoulder and backslope positions than at the footslope. Hairy vetch biomass affected corn yield positively within each landscape position. Cover cropping did not improve soybean and corn yield at the footslope position. Site‐specific CC management is critical if the landscape has significant variability in soil characteristics.

Topographic effects on soil microclimate and surface cover crop residue decomposition in rolling cropland

Rolling hill topography is common in agricultural land throughout the Southeastern United States. In areas of complex topography, downslope movement of soil nutrients and water can lead to nutrient losses and soil degradation, as well as spatial yield variability. Additionally, differences in the accumulation of water, slope aspect, and crop biomass production can lead to disparate conditions of soil moisture and soil temperature throughout a given field. Winter cover crops can reduce erosion and mitigate nutrient losses, while also potentially providing a source of nitrogen (N) to the subsequent cash crop as the cover crop residue decomposes. However, given that moisture and temperature at the soil surface are primary controls on residue decomposition, topographic heterogeneity may lead to variable rates of cover crop breakdown and N return to the system. We investigated the effect of landscape position on soil volumetric moisture and temperature at 5 cm depth, and the decomposition and N release rate of two cover crop residues, a sole-seeded cereal rye (Secale cereale L.) cover crop, and a cereal rye/crimson clover (Trifolium incarnatum) mixture over two maize (Zea mays L.) growing seasons in Lexington, KY. We found differences in soil moisture and soil temperature across landscape positions, with backslope positions tending to be the warmest and driest positions, and toeslope positions tending to be wettest. However, we did not observe significant differences in decomposition or residue N release rate among landscape positions or cover crop treatments. Our results emphasize the inherent variability present in abiotic factors in rolling hill cropland but indicate that cover crop residue persistence and N release are relatively consistent among landscape positions.

Spatial and fractal characterization of selected soil nutrients along a catena

Soil nutrient elements influence crop productivity and environmental sustainability and they vary across several slope positions. The objective of the current study was to evaluate the spatial and fractal variability of selected soil macro- and micronutrient elements across five slope positions: summit, shoulder, back, foot, and toe slopes. Triplicate soil samples (0–18 cm) were collected from each slope position from a pasture field planted to tall fescue (Festuca arundinacea syn. schendonorus arundinaceus). Soil nutrient elements analyzed include phosphorus (P), potassium (K), nitrate N (NO3-N), iron (Fe), and manganese (Mn). Additionally, soil pH, bulk density (BD), and soil organic carbon (SOC) were determined. Results show that SOC was 26% higher, while BD was 10% lower at the toe slope compared with the summit. Similarly, NO3-N was 15, 16, 18, and 1% higher at the toe slope position compared with the summit, shoulder, back and foot slopes probably due to nutrient mobility with water flow. Semivariogram analysis showed that the gaussian isotropic model provided be best fit (R2 > 0.97) for all nutrient elements across all slope positions. The range of spatial variability (A0) of nutrient elements was between 5.8 (K at the backslope position) and 71.0 m (P and Fe at the shoulder and back slope positions, respectively). The fractal dimensions of soil nutrient elements across all slope positions ranged from 0.216 to 1.949 suggesting that as the sampling distance exceeds half the A0, soil nutrient elements tend to become self-similar. The variability in soil nutrient elements from the current study can serve as a useful nutrient application recommendation tool for precision agriculture across five slope positions.

Effects of forest floor characteristics on soil labile carbon as varied by topography and vegetation type in the Chinese Loess Plateau

Vegetation type and topographic characteristics are important factors that affect the amount and spatial distribution of the forest floor and the soil carbon cycling. However, the relationships among vegetation type, topographic characteristics, forest floor properties, soil properties and soil labile organic carbon (LOC) fractions remain to be elucidated. Here, we conducted a field observational study to explore the relationships between the soil LOC fractions (microbial biomass carbon (MBC), extractable organic carbon (EOC) and permanganate oxidizable carbon (Pox-C)) and forest floor characteristics under various vegetation types (forest, forest-steppe and steppe) and topographic features (slope position and aspect). Our results showed that the vegetation type and topography are both important factors that significantly influence the forest floor properties (i.e., stock, thickness and coverage). The soil total nitrogen and phosphorus were significantly increased, and the soil pH and bulk density were decreased, with an increase in litter stock, coverage and thickness. Simultaneously, the SOC and soil LOC fractions were increased significantly with the increase in litter stock, coverage and thickness. Over 50% of the variation in the SOC and soil LOC fractions was jointly explained by environmental factors, litter properties and soil properties. Our results demonstrated that the SOC and soil LOC fractions are directly influenced by soil physical and chemical properties (e.g., pH, bulk density and soil total nitrogen) and indirectly affected by external environmental factors. Changes in vegetation and topography lead to increases in the litter stock, coverage, thickness and water content in the backslope position and northern slope, which could create favorable soil nutrient and environmental conditions (e.g., higher soil moisture and lower bulk density) to further enhance SOC and soil LOC fraction sequestration. The forest floor potentially links environmental factors and soil properties and serves as an important factor for soil C cycling.

Identifying impacts of land use change on soil redistribution at different slope positions using magnetic susceptibility

Clear-cutting of Oak forests and intensive cultivation on the hilly regions of the Zagros Mountains has led to plausible threats on the natural ecosystem and catastrophic floods in western Iran. This study was conducted to explore the effects of clear-cutting of natural forests in different slope positions on some soil chemical and physical properties as well as quantification of soil redistribution using magnetic susceptibility. Two adjacent sites, including natural forest and cultivated lands, were selected and a total of six transects at three soil depths (0–10, 10–20, and 20–30 cm) and a total of seventy-two soil samples were examined. Soil properties such as particle size distributions, bulk density (ρb), calcium carbonate equivalent (CCE), soil organic matter (SOM), and magnetic susceptibility at low and high frequencies were measured. Results indicated that clear-cutting and cultivation for 50 years significantly (p < 0.05) increased ρb and CCE and reduced SOM and TN. Moreover, magnetic susceptibility at four slope positions was significantly (p < 0.05) lower compared with the natural forest. Using a simple proportional model to estimate the soil loss or gain and comparing the mean χlf in any given point confirmed a high rate of soil loss in cultivated sites. In the forest soils, low soil loss in shoulder position and high rate of deposition in footslope and backslope positions were obtained. In general, it is revealed that magnetic measures could provide valuable estimates of mid-term soil erosion and sedimentation in the hilly region following land use changes.

Spatial Variability of Rainfed Wheat Production Under the Influence of Topography and Soil Properties in Loess-Derived Soils, Northern Iran

The wheat production variability is not well-understood in hilly region, especially in loess-derived soils of Golestan province in Iran with a sub-humid climate. Topography can greatly influence the production of agricultural crops by affecting soil quality. A study area located in Golestan province was selected in order to assess the spatial variability of wheat production and to develop regression models between the crop, soil properties, and topography attributes. The samples of wheat and soil were randomly taken from 100 points at different hillslope positions (i.e., shoulder, back-, foot-, and toe-slope). The soil physicochemical analysis and the measurement of wheat yield components were conducted. The digital elevation model (DEM; 10 m resolution) was used, and the topographic attributes (i.e., elevation, slope, wetness index, stream power index, curvature, erosivity factor, and watershed specific area) were calculated. The results showed that the greatest total yield and the highest grain yield were estimated to be 14.53 and 4.41 ton ha−1, respectively, in areas with a slope of less than 10%, which were significantly higher than those in the steep areas (slope classes of 10–30% and > 30%). The highest and the lowest total yields, with average values of 15.82 and 5.68 ton ha−1, were observed in the toeslope and shoulder slope positions, respectively. The greatest grain yields were obtained from the foot- and toeslope positions with the average values of 4.61 and 4.66 ton ha−1, respectively. The topographic curvature and wetness index had a significant correlation with the yield of wheat. According to the regression equations, topographic indexes can well justify the spatial variability of wheat yield, indicating the importance of these factors by influencing the distribution of moisture during the process of wheat production in the study region. The enhancements of wheat yield components in the lower slope positions could be attributed to an increase in soil depth and plant available water as well as to the accumulation of further soil organic matter and nutrient elements, including nitrogen and potassium, in such positions as a result of soil redistribution. Moreover, the results illustrated that by using easy accessable, cheap, and none destructive data (DEM derivatives and soil properties); it is possible to predict the production yield of wheat with a reliable estimation. We concluded that for better farming management and productivity in hilly regions, topographic attributes should be considered for plantation. Therefore, this study introduces the most suitable slope positions and topographic attributes for crop production with the least soil degradation. Shoulder and backslope positions are the most unsuitable slopes possibly better for orchards while toeslopes and footslopes could be used for intensive crop production.

Long‐term perennial management and cropping effects on soil microbial biomass for claypan watersheds

AbstractSustainable vegetative management plays a significant role in improving soil quality in degraded agricultural landscapes by enhancing soil microbial biomass. This study investigated the effects of grass buffers (GBs), biomass crops (BCs), grass waterways (GWWs), and agroforestry buffers (ABs) on soil microbial biomass and soil organic C (SOC) compared with continuous corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation (row crop [RC]) on claypan soils. The RC, AB, GB, GWW, and BC treatments were established in 1991, 1997, 1997, 1997, and 2012, respectively, and are located at Greenley Memorial Research Center in Missouri. Soil samples were collected in May 2018 from the 0‐ to 10‐cm depth at summit, backslope, and footslope landscape positions. Within AB treatment, soils were collected from the 50‐cm and 150‐cm tree distance. Total microbial biomass and biomass of gram‐positive bacteria, gram‐negative bacteria, actinomycetes, rhizobia, fungi, arbuscular mycorrhizae, saprophytes, and protozoa were determined by phospholipid fatty acid (PLFA) analysis. Results showed that soil microbial biomass and SOC across all microbial groups were significantly higher (P &lt; .01) under perennial vegetation treatments compared with RC. The footslope position exhibited the highest total microbial biomass compared with the summit and backslope positions. The sampling distance of 50 cm from the tree base demonstrated 16% greater total microbial biomass and 15% higher SOC compared with 150 cm. These findings highlight the influence of landscape on soil biological properties and show that perennial vegetation systems have the potential to increase soil microbial biomass and enhance agricultural sustainability in degraded RC systems.

Freeze-thaw cycles changes soil nitrogen in a Mollisol sloping field in Northeast China

Freeze-thaw-cycles (FTCs) change the dynamics of soil physiochemical and biological processes, and could change the soil nitrogen (N) heterogeneity in sloping fields. In this study, both continuous cropping of corn (CC) and corn-soybean rotation (CS), with-straw-amendment (SA) and with-no-straw-amendment, were considered to investigate how FTCs change soil total N (TN) and available N (alkali-hydrolyzable N, AN) at different slope positions in the Mollisol region of Northeast China. When a large amount of snow fell, after FTCs: (1) TN decreased by 40–79% at all slope positions under CC and CS, mainly influenced by snowmelt-erosion, nitrification-denitrification and N-upward-movement, and (2) AN decreased at both the top slope (51–64%) and back slope (36–81%) positions, mainly influenced by snowmelt-erosion, nitrification-denitrification and N-upward-movement, but increased (2–65%) under CS at the bottom slope position, mainly due to deposition, N-upward-movement, nitrification-denitrification and previous crop. In contrast, when less snow fell, after FTCs: (1) N transformation changed both TN and AN on the slopes, mainly influenced by microorganisms, N-upward-movement, and previous crops, and (2) AN decreased by 20–26% at the bottom slope position under both CC and CS, mainly influenced by nitrification-denitrification and N-upward-movement, but increased by 111–116% at the back slope position under CS. SA did not change the TN distribution on the Mollisol farmland slope after FTCs in a short time period, but it helped sustain a high AN content (4.5–8.6%) in the soil. TN loss was usually severe after FTCs, and should be controlled urgently to increase soil fertility and reduce environmental pollution.

Interplay between soil formation and geomorphic processes along a soil catena in a Mediterranean mountain landscape: an integrated pedological and geophysical approach

Soil spatial distribution often shows high variability because of geo-environmental settings affecting soil formation and geomorphic processes. The paper describes a combined pedological–geophysical approach for investigating the interplay among soil-forming processes, geomorphic dynamics and physico-chemical soil properties in a soil catena in a Mediterranean mountain landscape (southern Italy). Stocks of soil organic carbon (SOC) and total nitrogen (STN) were determined and their interactions with slope position analyzed. Five slope units were defined along the soil catena (summit, shoulder, back, foot, and toe slopes), and in each of them, a soil profile was dug and analyzed. Thickness of soil horizons and contact between soil and parent material were determined by a 2D-electrical resistivity tomography survey. Results showed granitic bedrock is characterized by vary weathering degrees strongly affecting soil type development. Moreover, soil properties variation is related to topographic features and water erosion processes. Soil chemical composition showed high values of SiO2 and Al2O3, probably due to inheritance from underlying granitic parent material. Particularly, Fe2O3 content increased with weathering conditions as shown by chemical index of alteration values. Along foot and toe slopes, reworking and accumulation of colluvial soil lead to the formation of thicker accretionary A horizon. Consequently, summit areas, together with foot and toe slopes had greater SOC and STN stocks than back and shoulder slopes. That because soils are thicker and richer in SOC on summit and valley bottom than in shoulder and back-slope positions. The proposed approach has showed the benefit of an integrated methodology for investigating spatial variations along soil catenas.

Effects of Conservation Practices on Soil Quality Compared with a Corn–Soybean Rotation on a Claypan Soil

Restoration of degraded lands by adoption of recommended conservation management practices can rehabilitate watersheds and lead to improving soil and water quality. The objective was to evaluate the effects of grass buffers (GBs), biomass crops (BCs), grass waterways (GWWs), agroforestry buffers (ABs), landscape positions, and distance from tree base for AB treatment on soil quality compared with row crop (RC) (corn [Zea mays L.]–soybean [Glycine max (L.) Merr.] rotation) on claypan soils. Soil samples were taken from 10‐cm‐depth increments from the soil surface to 30 cm for GB, BC, GWW, and RC with three replicates. Soil samples were collected from summit, backslope, and footslope landscape positions. Samples were taken at 50‐ and 150‐cm distances from the tree base. β‐Glucosidase, β‐glucosaminidase, dehydrogenase, fluorescein diacetate hydrolase (FDA), soil organic carbon (SOC), total nitrogen (TN), active carbon (AC), and water‐stable aggregates (WSA) were measured. Results showed that β‐glucosidase, β‐glucosaminidase, dehydrogenase, FDA, AC, WSA, and TN values were significantly greater (P &lt; 0.01) for the GB, BC, GWW, and AB treatments than for the RC treatment. The first depth (0–10 cm) revealed the highest values for all soil quality parameters relative to second and third depths. The footslope landscape had the highest parameter values compared with summit and backslope positions. The 50‐cm distance of AB treatment had higher values than the 150‐cm distance for all measured parameters. Results showed that perennial vegetation practices enhanced soil quality by improving soil microbial activity and SOC.Core Ideas Permanent vegetative management (trees and grasses) enhanced soil quality. Perennial practices improved microbial activity and increased soil organic carbon. Perennial vegetative practices have agricultural and environmental significance. Establishing perennial practices is an effective approach to enhance soil quality.

Erosion Induced Heterogeneity of Soil Organic Matter in Catenae from the Baltic Sea Catchment

Soil organic matter (SOM) is unevenly distributed in arable fields in undulated landscapes, but the chemical composition resulting from their turnover, transport and deposition processes is insufficiently known. Therefore, we aimed at disclosing the molecular-chemical composition of SOM in four different catenae at shoulderslope, backslope and footslope positions in arable fields in the Baltic Sea catchment, Europe. The backslope positions always had the lowest organic C-contents (Corg) (1.6…11.8 g·kg−1) and C-stocks (3.8…8.5 kg·m−2) compared to the shoulderslopes and footslopes (1.7…17.7 g·Corg·kg−1, 5.4…15 kg·Corg·m−2). In the SOM-poor backslope positions, the organic matter was characterized by relatively high proportions of carbohydrates, phenols + lignin monomers, alkylaromatic compounds, N-compounds and amides, indicating intensive microbial decomposition. By contrast, the footslopes had the largest Corg-contents (9.3…16.5 g·kg−1) and C-stocks (8.9…15 kg·m−2) in the catenae and particular enrichments in lipids, lignin dimers, sterols and free fatty acids. These relatively stabile SOM compound classes are interpreted as leftovers from erosive downslope transport and concurrent microbial decomposition, e.g., they are pronounced at backslope positions, followed by restricted microbial decomposition. This heterogeneous SOM distribution calls for an adapted soil management that reduces erosion and places amendments to field areas, such as the shoulderslope and backslope.

Impacts of nitrogen fertilization rate and landscape position on select soil properties in switchgrass field at four sites in the USA

Switchgrass (Panicum virgatum L.) has been recognized as a potential bioenergy feedstock due to its high yield potential and benefits to the environment. However, little is known about the impacts of nitrogen fertilization and landscape position on soil properties in switchgrass fields at multiple locations in the USA. The objective of this study was to assess the impacts of application of nitrogen fertilization rate (N rate; zero, 0; medium, 56; and high, 112 kg ha−1) and landscape position (shoulder, backslope, and footslope) on soil pH, electrical conductivity (EC), soil organic carbon (SOC), total nitrogen (TN), nitrate-N (NO3−-N), and ammonium-N (NH4+-N). This experiment was established in 2008 at four sites in Oklahoma (OK), South Dakota (SD), New York (NY), and Virginia (VA). Data showed that the N rate significantly increased NO3−-N for the 0–5 and 5–15 cm depths at the SD, NY, and VA sites. Compared with the zero N rate, the high N rate averagely increased NO3−-N by 75.7% for the two depths and three sites. The SOC and TN increased with the increase in N rate at most of the depths. The landscape position significantly reduced soil pH and increased EC, SOC, TN, NO3−-N, and NH4+-N at the footslope compared with the shoulder and backslope positions in most depths at the SD site (no position data at the NY and VA sites). The pH generally increased and the SOC, TN, and NO3−-N reduced with the increase in soil depth. These findings indicate that switchgrass production has the potential to improve or maintain soils in the USA.

Soil organic carbon pool changes in relation to slope position and land-use in Indian lower Himalayas

The increase in the atmospheric abundance of CO2 has raised interest in the potential of soils to sequester carbon (C), which inter alia depends on antecedent C level. While inventories of soil organic carbon (SOC) have been developed for different ecosystems at field or global scales, studies at hill-slope level where SOC redistributes by erosion remain scarce. In addition, quantitative information on the variation in SOC stocks and organic matter quality within hill-slopes would allow to make more realistic estimations of the C sequestration potential of landscapes while suggesting improved land management. The main objective of this study was to evaluate the impact of slope position and land-use (grassland versus cropland) on SOC stocks and organic matter quality (proportion of aggregate associated C and labile pools; water extractable, WEOC; hot water soluble, HWC; potassium permanganate oxidizable, KMnO4-C; microbial biomass, MBC). A total of 108 soil samples from the 0–0.15 m layer were collected from 4 slope positions (forested hilltops, middle slope with bare soils, backslopes under grasslands, cropped bottomlands) at nine catenas with 3 replicates per position. The greatest SOC stocks were found at hilltops and backslopes with respectively 10.1 ± 0.64 and 10.1 ± 0.57 Mg C ha−1. SOC stocks decreased by 17% to 8.4 ± 0.65 Mg C ha−1 in bottomlands and by 58% to 4.2 ± 0.26 Mg C ha−1 at backslopes. The soil organic matter from the hilltops showed the greatest proportion of recalcitrant components (62%) followed by the backslope and bottomland positions with ~42%, and middle slope position with 9%. In addition to showing a greater occurrence of labile C fractions, bare and eroded backslope soils were also depleted in water soluble C (WEOC and HWC), KMnO4-C and microbial biomass C. The results showed that there is considerable potential for C sequestration at middle and backslope position through land rehabilitation by either planting trees or vegetation by grass.

Effects of vegetation and topography features on ecological stoichiometry of soil and soil microbial biomass in the hilly-gully region of the Loess Plateau, China.

The objectives of this study were to explore the effects of vegetation type, topographic features and their combined effects on soil microbial biomass stoichiometry, so as to better understand the interaction of soil, soil microbes and nutrient cycling under different vegetation types in the hilly-gully region of the Loess Plateau. Soils from three vegetation zones (forest zone, forest-steppe and steppe) and five slope positions (south/north backslope, south/north shoulder and summit) were chosen and the effects of vegetation types and topography features on soil and C:N:P ratios in soil microbial biomass were investigated in this study. The results showed that, among the five slope positions, the highest concentrations of soil and soil microbial biomass C, N, P were found at the backslope position and the north-facing slope. The effects of vegetation types on soil and soil microbial biomass C, N and P in the two soil layers were significantly different, whereas the effects of slope aspect and positions were only numerically different. As for different soil layers, the topsoil (0-10 cm) was more affected by slope aspect, while the subsoil (10-20 cm) was more influenced by slop position. While the effects of vegetation type on soil C:N, C:P and N:P and soil microbial biomass C:N, C:P were significant, slope aspect and slope position only influenced soil C:P and N:P. Consequently, on the Loess Plateau, the effects of vegetation type on soil and soil microbial biomass C, N, P were stronger than those of the topographic features. The standardized major axis tests showed that C:N:P stoichiometry in soil microbes was well-constrained, especially in the steppe zone. The soil microbial biomass N:P might be used as a useful tool to assess nutrient limitation of ecosystem processes in terrestrial ecosystems. If combined with plant leaf N:P, they could provide more accurate information to estimate the nutrient limitation of fragile ecosystem in hilly-gully region of the Loess Plateau.

Algorithms vs. surveyors: A comparison of automated landform delineations and surveyed topographic positions from soil mapping in an Alpine environment

Landform delineation has long been used in digital soil mapping to infer soil-relevant information. While its potential as an environmental variable in soil parameter modeling has been investigated for various automated landform delineations, little research has been invested into the relationship between the delineation of landforms by algorithms based on digital terrain models (DTM) and the perception of landforms by the soil surveyor during field work. Five Open Source automated landform classification algorithms and a support vector machine classifier based on single terrain parameters are investigated with regard to their ability to replicate topographic position, at two different scales, as described by surveyors for soil profile sites in the Alpine environment of South Tyrol. We also analyse how the variation of parameters and cell size affects the distribution of the computed landforms. While a clear trend regarding grid cell size and window size can be observed with regard to the difference between macro and meso scale topographic positions, the overall classification accuracy regarding the different topographic position classes was less promising. Although some automated classifications partly resemble the surveyor's classification, a considerable number of issues remain to be investigated in order to explain the lack of reproducibility of surveyor position, some of which are linked to the Alpine environment of the study area. These include the dominance of the backslope position, the objectivity of the surveyor in rugged terrain under forest cover, and the fuzzy nature of classifying topographic position, especially in steep terrain. By applying a forward stepwise feature selection procedure for a model based on single terrain parameters, we show that at macro scale a regional terrain parameter (topographic wetness index) and curvatures at a coarse DTM resolution of 50m are the most influential in distinguishing topographic position, whereas at meso scale it is the topographic position index (TPI) with a search radius of just 70m combined with slope gradient. This study is an important first step towards consolidating topographic perception during field survey and digital terrain analysis, which, at least in Alpine terrain, still requires more investigation.

Simulating soil organic carbon changes across toposequences under dryland agriculture using CQESTR

Soil organic carbon (SOC) and its management under dryland cropping systems are very critical for both crop productivity and environment health. The objective of this study was to evaluate the performance of CQESTR, a process-based C model, in simulating SOC changes across toposequences of selected fields and agriculture management practices along a precipitation gradient in a dryland region of Oregon, USA. Geo-referenced soil samples were collected from summit (SU), shoulder (SH), backslope (BS), footslope (FS), and toeslope (TS) positions during early 1980s and early 2000s. Simulation scenarios were developed based on field management practices, crop rotations, soil properties, and climatic data. CQESTR simulated results were compared with the measured SOC from each landscape position. Significant (P<0.0001) correlations (r=0.93) were found between the measured and the simulated SOC at SU, SH (r=0.91), BS (r=0.83), FS (r=0.89), and TS (r=0.89). The smallest correlation value at BS could be from soil deposition due to erosion. No significant changes in SOC were found between SU, SH, BS, and FS landscape positions; however, TS had the highest SOC (10.8±.8gCkg−1). CQESTR successfully simulated SOC at most of the studied sites and landscape positions, except at TS for a location with high annual deposition of C-rich soil eroded from the upper landscape position. CQESTR could be used to predict SOC changes across toposequence and at the landscape scale level with reasonable accuracy. The results were supported by a linear relation with an r2 of 0.89 and a low mean square deviation (MSD=0.24) between the measured and the simulated SOC.