Top Slope Positions Research Articles

Spatiotemporal heterogeneity of soil available nitrogen during freeze–thaw cycles in a watershed: A 3‐year investigation

AbstractThe spatial pattern of soil available nitrogen (AN) is the key reference for farmland fertilization however, it is still unclear how freeze–thaw cycles (FTC) altered the spatiotemporal heterogeneity of AN at the watershed scale. In this study, the geostatistical, traditional analysis and structural equation models were used to detect how topography, land management, and soil properties influence the spatiotemporal heterogeneity of AN in both before‐ and after FTCs of Mollisols of a watershed in Northeast China for three consecutive years. The results showed that AN decreased in >88% of areas of the watershed after FTCs, and the mean value significantly decreased by >7.6% (>10.4 mg kg−1) in 0–20 cm soil depth during 2016–2017 years and 2017–2018 years. The AN distribution, especially in the surface soil layer, tended to be aggregated after FTCs. The AN decreased in the areas near the areas of top slope position, while it tended to increase in areas of downslope position covered by forest although the total N decreased here after FTCs. The AN‐change (ANbefore FTCs minus ANafter FTCs) influenced by land use types was greater than slope steepness. The slope aspect as the key factor combined with slope steepness, slope position, and previous crops influenced the spatiotemporal heterogeneity of AN‐change after FTCs. The higher content of AN before FTCs accelerated more N loss during FTCs, and this process was mainly influenced by soil pH, nitrogen (N), and phosphorus. The N loss (5.7%) did not significantly alter the AN‐change after FTCs in the watershed. Generally, the spatial heterogeneity of AN‐change was mainly influenced by topography, land management, and soil properties (before FTCs) during FTCs in the watershed. In order to sustain a high level of AN, the mediation of N transformation and both vertical‐ and horizontal migration, caused by the dynamics of soil water potential and snowmelt erosion should be considered during FTCs.

Key factors determining soil organic carbon changes after freeze-thaw cycles in a watershed located in northeast China

The transformation and migration process of soil organic carbon (SOC) could be changed during freeze-thaw cycles (FTCs) and may further affect the SOC distribution in the watershed. In this study, both field investigation and lab incubation combined with geostatistics were used to clarify how environmental factors influence the SOC heterogeneity in Mollisol after FTCs, from a watershed in northeast China. The results showed that after FTCs, SOC decreased in 68.5% of the total watershed area at 0–20 cm soil depth, and the mean value decreased by 7.4%. Spatial autocorrelation (Moran's І) decreased in the 0–5 cm and 10–20 cm soil depths after FTCs (P < 0.01), but did not change in 5–10 cm and 0–20 cm soil depths. SOC increased at the top slope positions, the watershed outlet, and the upper slope position of the intersection area between farmland and forestland. The SOC decrease was 25 times and 14 times greater at the 0–5 cm and 0–20 cm soil depths respectively in the forestland than in the farmland. The SOC decrease was significantly higher on the 6–8° slopes than on the 0–2° slopes in 0–20 cm soil depth. SOC increased in most areas of cross-slope tillage (CST), but decreased in most areas of downslope tillage. The increase of SOC (positive change) decreased with increasing soil depth under soybeans field, while the decrease of SOC (negative change) increased with increasing soil depth under corn. Topographical factors alone, and topographical factors combined with land use types all influenced the SOC change in this watershed. High levels of ferrous minerals tended to reduce the rate of SOC after FTCs. SOC change is positively correlated to soil bulk density, FTC frequency and soil moisture after FTCs at the soil depth of 0–30 cm. Equations based on soil properties before FTCs, topographical factors, sediment transport index, runoff intensity index, and tillage method can be used for coarsely predicting SOC distribution after FTCs (45% < R2 < 78%, P < 0.01). Generally, SOC dynamics were mainly determined by topography, land use, and to tillage methods that possibly attributes to soil and water loss during FTCs. Both erosion caused by snowmelt runoff and vertical migration of SOC could be the key factors that changed the SOC spatial pattern. CST was beneficial to conserve SOC during FTCs, while forestland could reduce SOC loss by reducing snowmelt erosion.

The effect of tillage management on microbial functions in a maize crop at different slope positions

Determining whether agricultural soils act as sinks or sources of greenhouse gases (GHGs) requires the quantification of variations in the pools and fluxes of soil organic carbon (SOC) and nutrients, e.g. nitrogen (N), as well as the associated soil microbial responses. In this study, soil was collected from experimental plots of maize (Zea mays) under conventional, minimum or strip tillage treatments established in a sloping field (~10%) of loamy soil in SW England, UK, where maize had been cultivated conventionally for 12 years. Topsoil (0–10 cm) cores were collected from the top, mid, bottom and foot slope positions to investigate soil C, N and microbial properties. The impact of conventional management on potential GHG emissions was assessed by incubating soils collected from the top, mid and bottom slope positions from the conventional treatment. Contents of SOC and total N were greatest at the top slope position and soil mineral N (NO3−-N and NH4+-N) concentrations were greater at the bottom and foot slope positions in all treatments. Biomarker phospholipid fatty acids (PLFA) for Gram positive bacteria and fungi were relatively 13C-enriched at each slope position regardless of treatment, indicating preferential utilization of organic matter from maize (C4) rather than SOC (C3). Around 70% of carbon incorporated into total PLFA was derived from C3-SOC at the slope foot, indicating that more SOC older than 12 years was being mineralized at the depositional position. Effluxes of N2O and CO2, and total GHG emission were greatest from the incubated soils sampled from the bottom slope position, suggesting that conditions in depositional positions of regularly ploughed sloping arable fields may have increased the potential for mineralization and denitrification. We conclude that the C sink potential of the depositional positions of slopes may be diminished by coincidental GHG emissions.

泥炭沼泽湿地关键元素地球化学特征及其对碳排放的影响机制

硫、铁是泥炭沼泽湿地（泥炭地）中重要的生源要素，其参与下的生物地球化学过程对泥炭地碳循环意义重大。选取德国中部两处典型的雨养型泥炭地高海拔样点（TBP）和低海拔样点（TSP），通过原位采集泥炭剖面孔隙水和可溶性气体等，研究了硫、铁元素等地球化学变化规律，结合DOC、甲烷（CH₄）和二氧化碳（CO₂）浓度分布，探讨其对泥炭地碳排放的影响。研究结果表明：（1） TBP中总还原无机硫（TRIS）浓度随深度先增后减，且上部0-87 cm平均浓度远高于87 cm深度以下，上部硫酸盐还原作用强烈。结合上部亚铁、硫化氢（H₂S）浓度分布，得知该范围内H₂S主要是通过微生物硫酸盐还原作用（BSR）生成，同时H₂S在孔隙水扩散过程中易与亚铁结合为硫化亚铁，进而生成稳定的黄铁矿，这一反应过程在约60 cm处减缓。（2） TBP、TSP两处采样点中DOC与亚铁、硫酸盐均有较强相关性，是由于地下水位的波动影响氧化还原程度以及微生物活性。两处采样点DOC均与亚铁呈显著正相关关系，表明铁氧化物在厌氧环境中被还原溶解产生亚铁，与其结合的有机碳被释放到溶液中从而导致DOC浓度的升高。TBP中DOC与硫酸盐呈显著负相关关系，表明硫酸盐作为电子受体被还原的过程中消耗酸度使pH值升高，增强了其中微生物的活性，DOC浓度由此增加。（3） CH₄与硫酸盐、TRIS浓度在剖面上均呈现相反变化趋势，表明硫酸盐输入的增加以及硫酸盐还原活动均会抑制CH₄生成。CO₂/CH₄均大于4，表明硫酸盐作为替代电子受体会使厌氧条件下碳矿化转向多CO₂和少CH₄生成。此外，亚铁对于CH₄生成一定程度上会起到低促高抑的效果，而对于CO₂的生成的影响较弱。表明硫酸盐对于CH₄和CO₂生成的影响高于亚铁。研究着重探究硫、铁等关键元素地下部生物地球化学过程对碳排放的影响机制，研究结果可为泥炭地碳排放核算提供理论支撑。;Sulfur and iron are important biogenic elements in peatland. Their participation in biogeochemical process is of great significance to carbon cycle of peatland. This study aimed to investigate impact of key elements on carbon emission of ombrotrophic peatlands, at top broad position (TBP) and top slope position (TSP) sites. The concentrations and distributions of carbon as DOC, GHG (particularly CH₄ and CO₂) were estimated through an in-situ collection of pore water and soluble gas in peat profile. Combined with the geochemical characteristics of sulfur (S) and iron (Fe) elements, their impacts on carbon emissions from peatland were discussed. The results showed that (1) the concentrations of total reduced inorganic sulfur (TRIS) in TBP first increased and then decreased with depth. The average concentration of the upper part was much higher than that of deeper part. The sulfate reduction in the upper part was strong. Combined with the concentrations and distributions of ferrous iron and hydrogen sulfide (H₂S) in the corresponding range, it is inferred that H₂S is mainly generated by bacterial sulfate reduction (BSR) in this range. Meanwhile, during the diffusion process of H₂S in pore water, it is easy to combine with ferrous iron into FeS. And then stable FeS₂ was formed. The reaction process slowed down at about 60 cm. (2) The DOC had a strong correlation with ferrous iron and sulfate in TBP and TSP because the fluctuation of groundwater level affects the redox degree and microbial activity. DOC had a significantly positive correlation with ferrous iron in the two sampling sites, indicating that iron oxide was reduced to dissolved ferrous iron in the anaerobic environment, and the organic carbon combined with ferrous oxide was released into pore water, resulting in the increase of DOC concentration. DOC had a negative correlation with sulfate in TBP, indicating that the acidity was consumed in the reduction process of sulfate as electron acceptor, and the pH value increased, which enhanced the activity of microorganism. Thus the concentration of DOC was increased. (3) The concentrations of CH₄ and sulfate, CH₄ and TRIS in the two sampling sites showed opposite trends with depth on the profile, indicating that increasing sulfate input and sulfate reduction would inhibit the formation of CH₄. The CO₂/CH₄ ratios of the two sampling sites were greater than 4, indicating that sulfate as an alternative electron acceptor may shift carbon mineralization under anaerobic conditions to more CO₂ and less CH₄ production. In addition, high concentration of ferrous iron can inhibit the formation of CH₄, and low concentration of ferrous iron can promote the formation of CH₄. But ferrous iron had a weak effect on the formation of CO₂. The results also show that sulfate concentration plays a dominative role over ferrous iron in CH₄ and CO₂ production. In this study, the effects of underground geochemical factors such as sulfur and iron on carbon emissions were also discussed, which provided more rigorous theoretical support for the correction of carbon emissions from peatlands.

Sources and subsurface transport of dissolved reactive phosphorus in a semiarid, no-till catchment with complex topography.

The subsurface transport of dissolved reactive phosphorus (DRP) from artificially drained agricultural fields can impair water quality, especially in no-till fields. The distribution of soil P in the wheat (Triticum aestivum L.)-dominated Palouse region in the inland U.S. Pacific Northwest varies greatly due to its steep and complex topography, and a legacy (∼130 yr) of excessive soil erosion and deposition processes. The primary goal of this research was to better understand the magnitude and temporal dynamics of DRP export from an artificial drain line and the variability of subsurface DRP leaching within a long-term, no-till field. Dissolved reactive P in drain line effluent was monitored across three water years. Large intact soil cores were extracted at contrasting field locations (toe and top slope positions) to measure DRP leachate concentration and relative P sorption. Drain line DRP concentration was predominantly >0.05mg L-1 and often exceeded 0.1mg L-1 during winter and early spring. Mean leachate DRP levels were significantly higher in toe slope cores than in top slope cores (0.11 and 0.02mg L-1 , respectively). Saturated hydraulic conductivity varied widely across cores and was not correlated with leachate DRP concentration. All soil cores exhibited high P sorption potential, even under conditions of preferential flow. These findings suggest that much of the DRP transport in these landscapes is derived from P hotspots located in toe slope positions. Application of soil P fertilizer amounts in variable rates that account for spatial variability in P transport may minimize P enrichment and subsequent leaching in these locations.

Spatial Variability of Preferential Flow and Infiltration Redistribution along a Rocky-Mountain Hillslope, Northern China

Rock fragments in soil strongly increase the complexity of hydrological processes. Spatial variability of preferential flow and infiltration characteristics, especially along a rocky-mountain hillslope are poorly understood. In this study, five rainfall–dye tracer experiments were performed in the rocky Taihang Mountains, northern China, to investigate the spatial variability of preferential flow and infiltration redistribution on different hillslope positions. Tracers were used to distinguish macropore flow and actual water flow patterns, and preferential flow indices and spatial non–uniformity of the infiltration redistribution were calculated using image analysis. Results showed increasing trends in the dye coverage, maximum infiltration depth, and steady infiltration rate with increased hillslope position, with a preferential flow fraction of 0.10, 0.11, 0.15, 0.29, and 0.26 for the bottom–, down–, mid–, upper–, and top–slope positions, respectively. With increased hillslope position, the spatial non–uniformity of the infiltration redistribution gradually increased in orthogonal and parallel directions to the stained section, and was supported by the fractal dimensions. Positive (gravel mass ratio, saturated water content, altitude, hydraulic conductivity and roots) and negative (bulk density and clay content) impacts on preferential flow and infiltration redistribution were quantitatively emphasized. The characteristic and mechanism of infiltration process were further identified along a rocky-mountain hillslope.

Variations in stem radii of Larix principis-rupprechtii to environmental factors at two slope locations in the Liupan Mountains, northwest China

Relationships between stem growth and climatic and edaphic factors, notably air temperatures and soil moisture for different slopes, are not completely understood. Stem radial variations were monitored at the bottom and top slope positions in a Larix principis-rupprechtii plantation during the 2017 and 2018 growing seasons. Total precipitation during the growing season in 2017 and 2018 was 566 mm and 728 mm, respectively. Stem contractions typically occurred after mid-morning followed by swelling in the late afternoon in both plots, reflecting the diurnal cycle of water uptake and loss. Trees at the two locations showed the same growth initiation (mid-May) because of the small differences in air and soil temperatures. There were no significant differences in cumulative stem radial growth between the bottom plot (1.57 ± 0.34 mm) and the top plot (1.55 ± 0.26 mm) in 2018. However, in 2017, the main growth period of the bottom plot ceased 17 days earlier than in the top plot, while cumulative seasonal growth of the bottom plot (1.08 ± 0.25 mm) was significantly less than the top plot (1.54 ± 0.43 mm). Maximum daily stem shrinkage was positively correlated with air and soil temperatures, solar radiation, vapor pressure deficits, and negatively correlated with volumetric soil moisture content. The maximum daily shrinkage reflected transpiration rates as affected by environmental factors. Daily radial stem increment was correlated with precipitation and volumetric soil moisture in both years, but with air temperatures only in 2017. The seasonal growth of L. principis-rupprechtii Mayr thus shows interannual dynamics, while precipitation constitutes a key driving factor.

Mining the Drilosphere: Bacterial Communities and Denitrifier Abundance in a No-Till Wheat Cropping System.

Earthworms play important roles in no-till cropping systems by redistributing crop residue to lower soil horizons, providing macropores for root growth, increasing water infiltration, enhancing soil quality and organic matter, and stimulating nitrogen cycling. The soil impacted by earthworm activity, including burrows, casts, and middens, is termed the drilosphere. The objective of this study was to determine the effect of earthworms on soil microbial community composition in the drilosphere at different landscape slope positions. Soil cores (50 cm depth) were extracted from three landscape locations (top, middle, and bottom slope positions) on a sloping aspect of a no-till wheat farm. Soil was sampled at the bottom of the soil core from inside multiple earthworm (Lumbricus terrestris) channels (drilosphere) and from adjacent bulk soil. Bacterial communities were characterized for 16S rRNA gene diversity using high-throughput sequencing and functional denitrifier gene abundance (nirK, nirS, and nosZ) by quantitative PCR. Bacterial communities were structured primarily by the landscape slope position of the soil core followed by source (bulk versus drilosphere soil), with a significant interaction between core position and source. The families AKIW874, Chitinophagaceae, and Comamonadaceae and the genera Amycolatopsis, Caulobacter, Nocardioides, and Variovorax were more abundant in the drilosphere compared to the bulk soil. Most of the individual bacterial taxa enriched in the drilosphere versus bulk soil were members of Actinobacteria, including Micrococcales, Gaiellaceae, Solirubrobacterales, and Mycobacterium. In general, the greatest differences in communities were observed in comparisons of the top and bottom slope positions in which the bottom slope communities had significantly greater richness, diversity, and denitrifier abundance than the top slope position. Populations of denitrifiers (i.e., ratio of nirK+nirS to 16S rRNA) were more abundant in earthworm-impacted soils and there was a significant impact of L. terrestris on soil community composition which was observed only in the top landscape position. There were significant correlations between the abundance of nirK and nirS and taxa within Proteobacteria, Acidobacteria, Actinobacteria, Verrucomicrobia, and Chloroflexi, suggesting a broad diversity of denitrifying bacteria. Earthworms influence the soil microbial communities, but the impact depends on the slope location in a variable landscape, which likely reflects different soil characteristics.

Impact of host plants, slope position and subsidence on arbuscular mycorrhizal fungal communities in the coal mining area of north-central China

Coal mining results in surface subsidence and make ecological environment more fragile. The dynamics of arbuscular mycorrhizal (AM) fungi response to host plants, slope position and subsidence have been investigated at the Shendong Bulianta coal mining area in north-central China. A total of 57 AM fungal operational taxonomic units (OTUs) were identified and subdivided into five genera. It was noticed that at the top slope position, the AM fungal infection and extraradical hyphal (ERH) density were significantly higher than at the middle and bottom, AM fungal diversity indices showed a significant decrease with the increasing slope position. Significantly different phylogenetic indices were observed among the three host plants. In addition, the AM fungal ERH density and phylogenetic index were observed to be significantly higher in areas without subsidence. Soil available P and pH significantly influenced AM fungal infection and ERH density. The findings highlight that host plants, slope position and subsidence has strong influence on AM fungal community via edaphic variables change. This study provided the groundwork to further clarify AM fungal ecological function, which can potentially help in vegetation restoration and reduced erosion in coal mining areas.

Short‐Term Vegetation Recovery after a Grassland Fire in Lithuania: The Effects of Fire Severity, Slope Position and Aspect

AbstractIn Lithuania, fire is frequently used by farmers as a tool to remove dry grass, improve soil nutrient status and help soil tilling. However, little is known about the ecological impacts of these fires, including vegetation recovery. The objective of this work is to study the impacts of a spring grassland fire on vegetation recuperation on an east‐facing (A) and a west‐facing slope (B), considering fire severity and slope position, 10, 17, 31 and 46 days after the fire. Because of their effects on fire behaviour, aspect, steepness and heterogeneity of topography favoured higher fire severity on slope B than on slope A. Three different slope positions were identified on slope A – flat top, middle slope and flat bottom – and five on slope B – flat top, middle slope, flat middle slope, slope bottom slope and flat bottom. Fire severity was high in top slope positions. The recovery of vegetation was very fast. By the end of the study, vegetation covered nearly 100% of the study area, although fire severity, aspect and slope characteristics had implications on vegetation recovery. Plant cover recovered quickly on slope A. Ten and 17 days after the fire, plant cover was high in the bottom positions, where fire severity was relatively low. The major vegetation recuperation was observed between 17 and 31 days after the fire, in response to high rainfall. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Spatial distribution of soil phosphorus and herbage mass in beef cattle pastures: effects of slope aspect and slope position

Characterizing and assessing spatial distribution of soil phosphorus and herbage mass in relation to landscape properties, land use, or landscape positions is important for understanding how pasture sustainability can be managed and improved properly. Our reason for conducting this study was to determine the effects of different slope aspects and slope positions on spatial distribution of soil phosphorus and herbage mass in subtropical pastures. Soil and forage samples were collected from contiguous south-, north-, east-, and west-facing slopes across different landscape positions (top slope, middle slope, and bottom slope) in 100 ha of bahiagrass (Paspalum notatum)-based pastures from 2003 to 2006 in subtropical region of southeastern USA. Averaged across years, soils on the north-facing slope contained the greatest amount of soil phosphorus (12.4 ± 2.7 mg kg−1) when compared with other slope aspects. Slope aspect may be acting as an important topographic factor influencing local site microclimate mainly because it determines the amount of solar radiation received. The greatest herbage mass (averaged across year) of 2,967 ± 980 kg ha−1 and the highest phosphorus accumulation of 7.7 ± 3.0 kg ha−1 for bahiagrass were from the top slope position. There was a significant (P ≤ 0.05) decrease in the average herbage mass and phosphorus accumulation with decreasing slope (top to middle slope). Between the top slope and the bottom slope, herbage mass declined from 2,967 ± 980 to 1,805 ± 370 kg ha−1 while phosphorus accumulation was reduced by approximately 40% (7.7–4.6 kg ha−1). Results of our study may increase awareness on how the arrangement of food, water, and shelter and their interactions with topographic and landscape features can significantly influence the movement of animals and utilization of different pastures’ resources. While our study supports our hypothesis that slope aspect and slope position could be of relative importance in controlling spatial distribution of soil phosphorus and herbage mass, broad knowledge of cattle movement in pasture situations is as critical to understanding their impact on agro-ecosystems.

137Cs tracing dynamics of soil erosion, organic carbon and nitrogen in sloping farmland converted from original grassland in Tibetan plateau

There is a shortage of research concerning the relationships between land-use change, soil erosion, and soil organic carbon (SOC) and nitrogen (N) dynamics in alpine environments such as those found in the Tibetan plateau. In this paper, typical sloping farmlands converted from grassland 50 years ago in eastern Tibet were selected to determine dynamics of soil erosion, SOC, and total N associated with land-use change. Soil samples were collected from sloping farmland and control fields (grassland). The (137)Cs, SOC, total N contents, and soil particle size fractions were analyzed in these samples. As compared with the control fields, (137)Cs, SOC, and total N inventories in the sloping farmlands decreased by 30%, 27%, and 33%, respectively. Meanwhile variations in the three parameters were enhanced in the sloping farmlands, with coefficients of variation (CVs) of 38%, 23%, and 20%, respectively, for (37)Cs, SOC, and total N. In addition, SOC and total N inventories significantly decreased with increasing soil erosion in the sloping farmland. In a sloping farmland with a steep 24 degrees gradient, the (137)Cs inventory gradually increased along a downslope transect with its lowest value at 0Bqm(-2) in the top-slope position (0m). The soil clay (<0.002mm) content in such an area increased with decreasing elevation (r=-0.95, p=0.001). Significant correlations between (137)Cs and clay (r=0.92, p=0.003), SOC (r=0.96, p=0.001), or total N (r=0.95, p=0.001) were also found in the farmland. These results showed that converting alpine grassland to sloping farmland accelerates soil erosion, losses in SOC and N, and increases the soil's spatial variability. The combined impacts of tillage and water erosion contributed a significant decrease in the soil's organic carbon and N storages. Particularly in steep sloping farmlands, tillage erosion contributed for severe soil loss, but the soil redistribution pattern was dominated by water erosion, not tillage erosion, due to the lack of boundaries across the field patches. It was also found that (137)Cs, SOC, and total N moved along the same pathway within these sloping farmlands, resulting in net C and N losses during soil redistribution. The negative influences of land-use conversion from grassland to farmland in sloping areas in the Tibetan plateau should be addressed.

Influence of slope position, stand type and rhododendron (Rhododendron ponticum) on litter decomposition rates of Oriental beech (Fagus orientalis Lipsky.) and spruce [Picea orientalis (L.) Link

Oriental beech (Fagus orientalis Lipsky.) and Oriental spruce [Picea orientalis (L.) Link] are the two most common tree species in northeast Turkey. Their distribution, stand type and understorey species are known to be influenced by topographical landforms. However, little information is available as to how these changes affect litter decomposition rates of these two species. Here, we investigated the effects of slope positions (top 1,800 m, middle 1,500 m and bottom 1,200 m), stand type (pure and mixed stands) and purple-flowered rhododendron (Rhododendron ponticum) on litter decomposition rates of Oriental beech and spruce for 4 years using the litterbag technique in the field. Among these three factors, stand type had the strongest influence on litter decomposition (P < 0.001, F = 58.8), followed by rhododendron (P < 0.001, F = 46.8) and slope position (P < 0.05, F = 11.6). Litter decomposition was highest under mixed beech/spruce forest, followed by pure beech and spruce forest. Beech and spruce litter decomposed much faster in mixed bags (beech–spruce) than they did separately under each stand type. Purple-flowered rhododendron significantly reduced litter decomposition of Oriental beech and spruce. Beech and spruce litter decomposed much slower at top slope position than at either bottom or middle position. Differential litter decomposition of Oriental beech and spruce was mainly due to adverse conditions in spruce forest and the presence of rhododendron on the ground which was associated with lower soil pH. Higher elevations (top slope position) slowed down litter decomposition by changing environmental conditions, most probably by decreasing temperature as also other factors are different (pH, precipitation) and no detailed investigations were made to differentiate these factors. The adverse conditions for litter decomposition in spruce forest can be effectively counteracted by admixture of beech to spruce monoculture and by using the clear-cutting method for controlling rhododendron.

Effects of tree species and topography on soil chemistry, litter quality, and decomposition in Northeast Turkey

Leaf litters from beech ( Fagus orientalis Lipsky.) and oak ( Quercus robur L.), and needle litters from fir ( Abies nordmanniana Spach.) and pine ( Pinus sylvestris L.) trees were collected from north-facing site and south-facing site and at three slope positions (top, middle and bottom) on each aspect that varied in soil chemical characteristics (soil pH, cation exchange capacity and base saturation). The litters were analysed for initial total carbon, nitrogen, acid detergent fibre, lignin and cellulose concentrations. Nitrogen, acid detergent fibre and lignin concentrations and carbon:nitrogen and lignin:nitrogen ratios varied significantly within and between species according to soil chemical characteristics on aspects and slope positions. Litter decomposition was studied in the field using the litterbag technique. The litters were placed on two aspects and at three slopes on each aspect in October 2001, and were sampled every 6-month for 2 years. The main effects of aspect, species and slope position on decomposition rates were all statistically significant. Oak leaf litter showed highest decomposition rates, followed by pine, fir and beech litter, and the litters placed on north-facing site decomposed faster than those on the south-facing site. The litters placed at the top slope position decomposed slower than at those at either the bottom or middle positions. Initial lignin concentrations explained most of the variation in decomposition rates between species, and within species for the aspects and the slope positions, but the explained variance showed differences between aspects and slope positions. This result illustrates the important point that litter quality may define the potential rates of microbial decomposition but these are significantly influenced by the biotic and abiotic environment in which decomposition takes place.

Removal of Competing Vegetation from Established Loblolly Pine Plantations Increases Growth on Piedmont and Upper Coastal Plain Sites

Abstract A series of paired plots was installed in loblolly pine plantations at 42 locations in Georgia's Piedmont and Alabama's Piedmont and Coastal Plain. One plot of each pair had all competing vegetation eliminated. The other plot was left as an uncontrolled check. Locations were stratified over two age classes (5-9 and 12-16 yr old) and three slope positions (top, midslope, and bottom). Analysis of 33 surviving locations 8 yr after treatment revealed a positive treatment effect for both individual tree (dbh and total height) and stand characteristics (basal area per acre, total volume per acre, and merchantable volume per acre). There was no difference in volume response between age classes. Slope position was not significant for the individual tree variables, but was significant for the stand variables, with midslopes responding most positively followed by bottom and then top slope positions. Over all locations, the average treatment response was approximately ½ cord/ac/yr. Economic analyses indicate that the magnitude of the response will be economical for many stumpage prices, particularly on midslope and bottom slope positions, in plantations where access and species composition make herbicide spraying possible. South J. Appl. For. 20(4):188-192.

Temporal Variation in Aggregate Stability on Conventional and Alternative Farms

Low-input alternative farm management practices have been shown to decrease erosion rates and increase soil organic matter contents relative to conventional management practices in eastern Washington state. Little is known about the effect of such alternative practices on aggregate stability. Temporal variations in aggregate stability were measured at three slope positions on soils from adjacent farms managed for many years using conventional vs. alternative tillage and crop rotations. The conventional farm was first cultivated in 1908, and uses a winter wheat (Triticum aestivum L.)-spring pea (Pisum sativum L.) rotation with summer fallow every sixth year. The alternative farm was first cultivated in 1909, and uses a winter wheat-spring pea-Austrain winter peas [P. sativa ssp. arvense (L.) Poiret]-spring wheat-spring pea-summer fallow rotation with an alfalfa (Medicago sativa L.)-wheatgrass [Elymus trachycaulus (Link) Gould ex Shinn.] mixture every 19th and 20th year. Surface soils (0–15 cm) were collected at foot-, back-, and topslope positions from both farms during October 1987, March 1988, and June 1988. The 0.5- to 1.0-mm sieved aggregate-size fraction from each farm, landscape position, and sampling time was analyzed for aggregate stability using the high-energy moisture characteristic method. Aggregate stability on both farms decreased significantly from October to March in response to precipitation and cycles of winter freezing and thawing. Significant increases in stability occurred from March to June on both farms. These increases were interpreted in terms of recovery of cohesion by aggregates following their disruption during the winter. Differences in stability resulting from management practices on the two farms were not significant, even though organic C contents on the alternative farm were significantly higher than those on the conventional farm.