Net Soil Loss Research Articles

Changes in Agricultural Land Use Affecting Future Soil Redistribution Patterns: A Case Study in Southern Tuscany (Italy)

AbstractLand‐use changes (LUCs) can be defined as the result of the direct action of the stakeholders in a particular area and natural or human driving forces. LUCs can influence various processes within the landscape and can have an impact on landscape functions. An analysis of the impact of LUCs on landscape processes can help to focus future rural policies. LUCs in Mediterranean areas particularly affect landscape functions because of their agro‐pedoclimatical characteristics. The aims of this work are as follows: (i) to characterise LUCs in the last 11 years in a typical Mediterranean area, the Trasubbie river basin (southern Tuscany, Italy); (ii) to extrapolate these changes and create spatially explicit LUC scenarios for the near future; and (iii) to simulate how and where the predicted LUCs may affect soil redistribution. We carried out an analysis of LUCs within the study area and used the trends to propose alternative scenarios for 2013. For these years, we spatially allocated land use (using the Conversions of Land Use and its Effects model) and used a landscape process model (landscape process modelling at multi‐dimensions and scales) to assess soil redistribution patterns. Land use in the study area changed almost linearly between 1996 and 2007, with cereals and annual fodder crops decreasing, and vineyards, perennial pastures and land abandonment increasing. Our LUC scenario extrapolates these dynamics to make predictions for 2013. A comparison of LAPSUS results between LUC and baseline scenarios for 2013 showed an increase in terms of net soil loss and total erosion, and a decrease in terms of sediment delivery ratio. Copyright © 2013 John Wiley & Sons, Ltd.

A suspended sediment budget for the agricultural Can Revull catchment (Mallorca, Spain)

Extensive rainfed herbaceous crops are one of the most representative agricultural elements of Mediterranean region, which should be one of the major factors affecting erosion processes. Although land use is commonly seen as resulting in increased sediment yields, the implementation of soil and water conservation practices can have the reverse effect. A sediment budget has been established in Can Revull, a small agricultural catchment (1.03 km2) on the island of Mallorca, by using 137Cs measurements, sediment source fi ngerprinting and more traditional monitoring techniques to quantify the individual components of the budget. A large proportion of the material mobilized from cultivated fi elds without conservation practices (gross erosion of 775 t yr 1, 1,270 t km 2 yr 1) was subsequently deposited either within the fi eld of origin (112 t yr 1, 180 t km 2 yr 1) or at intermediate locations between the source fi eld and the channel network (fi eld-to-channel conveyance loss was 454 t yr 1, 744 t km 2 yr 1). The estimates of sediment accumulation rates on the fl oodplain in the lower reaches of the catchment indicate that the mean sedimentation rate was 0.14 g cm-2 yr-1. This value was extrapolated to the total area of the fl oodplain to estimate a total annual conveyance loss or storage of 445 t yr 1. Monitoring at the catchment outlet over the study period (2004–2008) indicated a mean annual suspended sediment yield of 7 t km-2 yr-1. The sum of the estimates of sediment yield and fl oodplain storage (452 t yr-1) was taken to represent the total annual input of suspended sediment to the channel system. This value was subsequently apportioned using the information provided by the fi ngerprinting investigation, to estimate the mass of sediment reaching the channel network from cultivated fi elds and from eroding channel banks. Thus the annual contribution from channel banks was estimated to be 242 t yr-1. In the case of the contributions from cultivated fi elds, the estimates obtained were, as expected, signifi cantly less than the values of net soil loss from these zones provided by the 137Cs measurements due to conveyance losses associated to fi eld-to-channel conveyance loss. The overall sediment delivery ratios (< 1 %) indicate that approximately 99 % of the sediment mobilized by erosion within the Can Revull catchment is subsequently deposited before reaching the monitoring station. As such, the low sediment outputs from the study catchment should be seen as refl ecting the importance of conveyance losses and storage rather than a lack of sediment mobilization from the catchment surface, although part of the catchment headwaters was modifi ed historically by means of terraces and transverse walls to prevent erosion.

Relationships of 137Cs inventory with magnetic measures of calcareous soils of hilly region in Iran

Erosion is a natural process, but it has been dramatically increased by human activities; and this adversely influences soil productivity and environmental quality. For quantification of soil erosion, several techniques including the use of Cs-137 have been employed. This study was conducted to explore the relationships of Cs-137 inventory with magnetic properties in calcareous soils in western Iran. Ten transects were selected in the hilly region in Chelgerd district of Iran. Soil samples from 0 to 30 and 30–50 cm depths were collected from fifty points to determine Cs-137 inventory, magnetic measures and selected physico-chemical properties (in total there were 100 soil samples). The results showed that simple mass balance model (SMBM) estimated a gross erosion rate of 29.6 t ha−1 yr−1 and a net soil deposition of 21.8 t ha−1 yr−1; hence, a net soil loss of 9.6 t ha−1 yr−1 and a sediment delivery ratio of 31.4%. Simple linear regression and non-linear regression analysis showed that mass magnetic susceptibility (χlf) explained only 33.64% and 45% of variability in Cs-137 in the transects studied. The results of multiple linear regression analysis of 137Cs with magnetic parameters and physico-chemical properties indicated that extractable potassium and χlf explained approximately 61% of the total variability in 137Cs in the area studied. Overall, the results suggest that further research is needed for the use of magnetic characteristics as an alternative technique in place Cs-137 methodology for calcareous soils.

Short‐term soil carbon sink potential of oil palm plantations

AbstractOil palm plantations cover ≈14.6 million ha worldwide and the total area under cultivation is expected to increase during the 21st century . Indonesia and Malaysia together account for 87% of global palm oil production and the combined harvested area in these countries has expanded by 6.5 million ha since 1990. Despite this, soil C cycling in oil palm systems is not well quantified but such information is needed for C budget inventories. We quantified soil C storage (root biomass, soil organic matter (SOM) and microbial biomass) and losses [potential soil respiration (Rs) and soil surface CO2 flux (Fs)] in mineral soils from an oil palm plantation chronosequence (11–34 years since planting) in Selangor, Malaysia. There were no significant effects of plantation age on SOM, microbial biomass, Rs or Fs, implying soil C was in dynamic equilibrium over the chronosequence. However, there was a significant increase in root biomass with plantation age, indicating a short‐term C sink. Across the chronosequence, Rs was driven by soil moisture, soil particle size, root biomass and soil microbial biomass N but not microbial biomass C. This suggests that the nutrient status of the microbial community may be of equal or greater importance for soil CO2 losses than substrate availability and also raises particular concerns regarding the addition of nitrogenous fertilizer, i.e. increased yields will be associated with increased soil CO2 emissions. To fully assess the impact of oil palm plantations on soil C storage, initial soil C losses following land conversion (e.g. from native forest or other previous plantations) must be accounted for. If initial soil C losses are large, our data show that there is no accumulation of stable C in the soil as the plantation matures and hence the conversion to oil palm would probably represent a net loss of soil C.

The dynamics of soil redistribution and the implications for soil organic carbon accounting in agricultural south‐eastern Australia

AbstractAnthropogenically induced change in soil redistribution plays an important role in the soil organic carbon (SOC) budget. Uncertainty of its impact is large because of the dearth of recent soil redistribution estimates concomitant with changing land use and management practices. An Australian national survey used the artificial radionuclide caesium‐137 (137Cs) to estimate net (1950s–1990) soil redistribution. South‐eastern Australia showed a median net soil loss of 9.7 t ha−1 yr−1. We resurveyed the region using the same 137Cs technique and found a median net (1990–2010) soil gain of 3.9 t ha−1 yr−1 with an interquartile range from −1.6 t ha−1 yr−1 to +10.7 t ha−1 yr−1. Despite this variation, soil erosion across the region has declined as a likely consequence of the widespread adoption of soil conservation measures over the last ca 30 years. The implication of omitted soil redistribution dynamics in SOC accounting is to increase uncertainty and diminish its accuracy.

Effect of repeated deforestation on vegetation dynamics for phosphorus‐limited tropical forests

For some predominately phosphorus (P)‐limited ecosystems, vegetation can be sustained under steady state conditions by atmospheric P inputs. The structure of the canopy influences deposition via the ability of the canopy to trap airborne P. This dependence suggests that a positive feedback may exist, which would have important impacts on the process of forest regeneration. One source of disturbance in the tropics is shifting cultivation. Over multiple cycles, studies have shown that shifting cultivation can lead to a large net loss of soil P, with a concomitant decline in forest biomass. In this study a model was developed to assess how shifting cultivation affects vegetation and phosphorus dynamics. This model is applied as a case study to a dry tropical forest system in the Southern Yucatan that is primarily P limited and has experienced shifting cultivation over several decades. Results show that following the second cycle, recovery would only be achieved after 100 years or longer in comparison to ∼30 years after one cycle. Examining the stable states of the system suggests that two stable states exist and that state changes as brought about by repeated disturbance can cause a shift to the other “low vegetation” stable state. Thus, for predominately P‐limited ecosystems undergoing repeated disturbance, the depletion of soil P can significantly affect the long‐term ability of the forest vegetation to recover. Results from this study have widespread implications, as 400 million ha of forest are affected by shifting cultivation.

Laboratory-Scale Evaluation of Anionic Polyacrylamide as an Erosion and Sediment Control Measure on Steep-Sloped Construction Sites

Suspended sediment is the primary pollutant prevalent in runoff waters exiting construction sites with exposed and disturbed land. In this research, the effectiveness of a chemical stabilizer, known as anionic polyacrylamide (PAM), to prevent erosion and promote sedimentation was examined using different application methods (i.e., dry granular vs. semi-dissolved solution). In this study, the effectiveness of Silt Stop 712, a proprietary PAM product, was tested using different application methods at three different rates, i.e., 16.8, 28, and 39.2 kg ha-1 (15, 25, and 35 lbs acre-1), on bare, unseeded, unprotected 1.2 m 0.6 m 0.08 m (4 ft 2 ft 0.25 ft) laboratory-scale soil test plots. Laboratory-scale testing procedures using simulated rainfall were applied to mimic interrill erosion and sediment detachment scenarios similar to a highway embankment with a compacted 3H:1V fill slope. Our results showed that dry PAM treatments applied at the recommended rate of 39.2 kg ha-1 (35 lbs acre-1) were capable of reducing turbidity by 97% and net soil loss by approximately 50% in comparison to the bare soil control. Conversely, semi-dissolved PAM treatments applied at the same rate, i.e., 39.2 kg ha-1 (35 lbs acre-1), followed by a drying period of 48 h prior to the experiment, reduced turbidity by approximately 69% and soil loss by 76%. The lower application rates, i.e., 16.8 and 28 kg ha-1 (15 and 25 lbs acre-1), of semi-dissolved PAM without a drying time were observed to lose effectiveness after about 40 min; however, the higher application rate of 39.2 kg ha-1 (35 lbs acre-1) of semi-dissolved PAM provided consistent turbidity reductions throughout the entire experiment. Based on these results: (1) dry PAM applied directly to the surface at the manufacturers recommended rate was more effective as a sediment control (i.e., turbidity reduction) measure, and (2) semi-dissolved PAM applied directly to the surface at the manufacturers recommended rate and allowed to dry for 48 h prior to rainfall was a more effective erosion control (i.e., soil loss reduction) measure.

Using 137Cs to quantify the sediment delivery ratio in a small watershed

Understanding the sediment delivery ratio (SDR) is important in controlling sediments for the sustainable development of natural resources and in the design of the construction such as dams and reservoirs. The purpose of this investigation is to determine the SDR by the (137)Cs tracing method in a small watershed in the Sichuan Hilly Basin of China. In the study watershed, different land plots are divided according to the land use type, and 97 sampling sites were selected from these plots. The results show that the average net soil loss rates from the forest land and sloping cultivated land are 1759 and 4468t/km(2)a, respectively. No (137)Cs was detectable on the bare rock surfaces and previous work showed that the erosion rate from the bare rock area was 14,260t/km(2)a. In the depositional zone, the sedimentation rates in the Caoto (a kind of cultivated land located at the foot of hills) and paddy field are 3113 and 3562t/km(2)a, respectively. Combining the area of each land use in the small watershed, the SDR of 0.40 is obtained in the past four decades. The (137)Cs technique was shown to provide an effective and rapid means of estimating the SDR within the small watershed.

Homogenization of the Soil Surface Following Fire in Semiarid Grasslands

Semiarid grasslands accumulate soil beneath plant “islands” that are raised above bare interspaces. This fine-scale variation in microtopographic relief is plant-induced and is increased with shrub establishment. Research found that fire-induced water repellency enhanced local-scale soil erosion that reduced variation in microtopographic relief, suggesting that fire may counteract vegetation-driven, fine-scale spatial soil heterogeneity. This article analyzes longer-term measurements (up to 9 yr) of soil microtopography to evaluate the hypothesis that fire in semiarid grasslands results in more homogenous soil microtopographic relief. Changes in soil microtopographic relief were measured prior to and following a total of five fires at three semiarid grasslands within central New Mexico, United States. The fires included three cool-season prescribed fires, a warm-season prescribed fire, and a warm-season wildfire. Four of the five fires resulted in significantly lower soil microtopographic variation that persisted for up to 4 yr. The duration and magnitude of the soil leveling effect was lowest in the grassland with clay-rich soils, indicating a possible soil texture interaction. Although two grasslands had net soil loss following fires, no net erosion occurred at the third grassland, indicating that redistribution of soils can occur without net erosion. These results show that management with prescribed fire reduces biotic-driven variation in soil microtopographic relief in semiarid grasslands that may help limit the transition to shrubland ecosystems in this region.

Calibration of RWEQ in a patchy landscape; a first step towards a regional scale wind erosion model

Abstract Despite the fact that wind erosion seriously affects the sustainable use of land in a large part of the world, validated wind erosion model that predicts windblown mass transport on a regional scale is lacking. The objectives of this research were to modify revised wind erosion equation (RWEQ) to estimate soil loss at a field scale in a way that it could operate at a regional scale, to calibrate the model using ground data collected from a field scale representing different land uses in Khanasser valley, Syria, and to estimate the total sediment fluxes (kg m −1 ) and soil losses (kg m −2 ) for farming fields. We implemented a modified version of RWEQ that represents wind erosion as a transient process, using time steps of 6 h. Beside this a number of adaptations including estimation of mass flux over the field boundaries, and the routing of sediment have been done. Originally, RWEQ was created and calibrated for application at the scale in USA. Due to the adaptations imparted to the original RWEQ and the different environmental condition in Syria of application areas, an intensive calibration process was required before applying the model to estimate the net soil loss from the experimental fields. The results of this test showed that the modified version of RWEQ provided acceptable predictions for the average mass flux from the measurement plot with a linear regression coefficient of R 2 of 0.57 and 0.83 for the ( d ) test for the 20 wind events at the six tested plots.

Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils

Secondary forests and young forest plantations frequently have high rates of tree growth and NPP – higher even than mature forests in similar situations. The nutrients required to sustain this rapid growth are derived from the soil and by external inputs such as rainfall or, in the case of nitrogen, by biological N fixation. In our study of the effects of tree species within replicated experimental plantations in a moist, lowland tropical environment in Costa Rica, high rates of biomass accumulation and productivity were coupled with high rates of N accumulation and cycling. We applied extensive sampling through time within a mass-balance approach to address the question “Where does all the nitrogen come from?”Rates of N uptake by the vegetation in these plantations were extraordinarily high, even by tropical forest standards, reaching 412 kg N ha^-1^ yr^-1^. Rapid decay coupled to tight nitrogen cycling may have provided large amounts of available N for plant uptake, but do not explain the large quantities of N that accrued in the vegetation, up to 1075 kg N/ha over 16 years. Surface soil organic matter stocks in the plantations increased by as much as 320 kgC/ha in surface soils, but soil nitrogen varied differently. Soil N stocks to 1 m depth were depleted by an average of 2119 kg/ha relative to the mature forest. Thus, mineralization of soil organic nitrogen could have supplied the N that accrued in biomass over the 16-yr period, but this apparently occurred without concomitant net loss of soil C. The C:N ratios of soil organic matter (SOM)in the plantations indicated either replacement of SOM with more C-rich detritus or selective removal of N from existing SOM. Regardless, high productivity in these plantations apparently was supported in part by mining of soil nitrogen. Species varied, however. Depletion of soil N stocks was only 219 kg/ha under Vochysia guatemalensis, in which 1075 kgN/ha had accrued. Asymbiotic N fixation is the next most plausible mechanism for supplying plant-available N, and may be enhanced in stands with high availability of recently produced photosynthates.

Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils

AbstractSecondary forests and young forest plantations frequently have high rates of tree growth and NPP – higher even than mature forests in similar situations. The nutrients required to sustain this rapid growth are derived from the soil and by external inputs such as rainfall or, in the case of nitrogen, by biological N fixation. In our study of the effects of tree species within replicated experimental plantations in a moist, lowland tropical environment in Costa Rica, high rates of biomass accumulation and productivity were coupled with high rates of N accumulation and cycling. We applied extensive sampling through time within a mass-balance approach to address the question “Where does all the nitrogen come from?”Rates of N uptake by the vegetation in these plantations were extraordinarily high, even by tropical forest standards, reaching 412 kg N ha^-1^ yr^-1^. Rapid decay coupled to tight nitrogen cycling may have provided large amounts of available N for plant uptake, but do not explain the large quantities of N that accrued in the vegetation, up to 1075 kg N/ha over 16 years. Surface soil organic matter stocks in the plantations increased by as much as 320 kgC/ha in surface soils, but soil nitrogen varied differently. Soil N stocks to 1 m depth were depleted by an average of 2119 kg/ha relative to the mature forest. Thus, mineralization of soil organic nitrogen could have supplied the N that accrued in biomass over the 16-yr period, but this apparently occurred without concomitant net loss of soil C. The C:N ratios of soil organic matter (SOM)in the plantations indicated either replacement of SOM with more C-rich detritus or selective removal of N from existing SOM. Regardless, high productivity in these plantations apparently was supported in part by mining of soil nitrogen. Species varied, however. Depletion of soil N stocks was only 219 kg/ha under Vochysia guatemalensis, in which 1075 kgN/ha had accrued. Asymbiotic N fixation is the next most plausible mechanism for supplying plant-available N, and may be enhanced in stands with high availability of recently produced photosynthates.

Use of beryllium-7 to study the effectiveness of woody trash barriers in reducing sediment delivery to streams after forest clearcutting

Reliable information on the effectiveness of control measures in reducing soil erosion and sediment delivery to streams resulting from clearcutting operations is needed to support the development of best management practices for soil and water conservation in forest areas. To address this need, an investigation of the efficiency of barriers (constructed by piling up woody trash along contours) in reducing soil loss and sediment delivery was undertaken in an area experiencing prolonged highly erosive rainfall events immediately after final forest harvest. The study sites selected were located in south-central Chile and are characterized by a wet autumn and winter and mean annual rainfall of ∼2300 mm. Using a newly developed approach for employing 7Be measurements to quantify soil redistribution over an extended wet season, soil redistribution was documented at two sites located ∼2 km apart, El Monumento (EM) and San Germán (SG), the latter exhibiting less compacted soil. For an autumn period of 82 d in 2006 associated with 989 mm rainfall at EM, the observed net soil loss from two plots bounded at the top and bottom by trash barriers with a 15 and 30 m spacing, respectively, was −0.4 ± 0.1 and −0.75 ± 0.08 kg m −2 and the sediment delivery ratio 34 and 92%. Results obtained for the SG site from three plots with 10, 15 and 30 m spacing, for a 58-d autumn period with 664 mm rainfall in 2008, were −1.0 ± 0.1, −0.7 ± 0.1 and −1.8 ± 0.1 kg m −2 net soil loss, with associated sediment delivery ratios of ∼60, 55 and 88%, respectively. These results suggest that the shorter barrier spacings (∼15 m) are more effective in reducing soil loss. At SG during a 149-d period in 2008, with an extremely high precipitation total of 2089 mm, the net erosion varied only between −5.6 ± 0.4 and −6.5 ± 0.4 kg m −2 and the sediment delivery ratio between 70 and 86% for 10, 15 and 18 m barrier spacings. These results suggest that under conditions of extreme rainfall and unconsolidated soils, the woody trash barriers are unable to retain the large amounts of sediment mobilized upslope and that much of this sediment passes through the barrier, increasing the net soil loss and sediment delivery ratio. Nevertheless, in the absence of the lower woody barrier the soil loss increases. These results emphasize the benefits of installing barriers, and the need to improve their design to maximize trapping efficiency.

Soil redistribution rate and its relationship with soil organic carbon and total nitrogen using 137Cs technique in a cultivated complex hillslope in western Iran

The spatial pattern of soil redistribution rate was investigated using cesium-137 ( 137Cs) within a cultivated complex hillslope in western Iran. The relationship between soil redistribution rate and soil organic carbon and total nitrogen pattern were studied using co-regionalization analysis. Ninety-one soil cores were sampled for 137Cs, total nitrogen, and soil organic carbon measurements. The simplified mass balance model estimated a gross erosion rate of 29.8 t ha −1 yr −1 and a net soil deposition rate of 21.8 t ha −1 yr −1; hence, a net soil loss rate of 8 t ha −1 yr −1. This magnitude of soil erosion rate is higher than the acceptable rate in semiarid regions. Co-regionalization analysis and co-dispersive coefficients among the selected variables showed that only a small fraction of the variability in total nitrogen and soil organic carbon could be explained by soil redistribution and that the remaining might be the result of different management practices by local farmers.

Impact of climate change on soil erosion and the efficiency of soil conservation practices in Austria

SUMMARYThe goal of the present study was to assess the impact of selected soil protection measures on soil erosion and retention of rainwater in a 1·14 km2 watershed used for agriculture in the north-east of Austria. Watershed conditions under conventional tillage (CT), no-till (NT) and under grassland use were simulated using the Water Erosion Prediction Project (WEPP) soil erosion model. The period 1961–90 was used as a reference and results were compared to future Intergovernmental Panel on Climate Change (IPCC) scenarios A1B and A2 (2040–60).The simulations for the NT and grassland options suggested runoff would decrease by 38 and 75%, respectively, under the current climatic conditions. The simulation results suggest that, under future climate scenarios, the effectiveness of the selected soil conservation measures with respect to runoff will be similar, or decreased by 16–53%.The actual average net soil losses in the watershed varied from 2·57 t/ha/yr for conventional soil management systems to 0.01 t/ha/yr for grassland. This corresponds to a maximum average annual loss of about 0·2 mm, which is considered to be the average annual soil formation rate and therefore an acceptable soil loss. The current soil/land use does not exceed this limit, with most of the erosion occurring during spring time. Under future climate scenarios, the simulations suggested that CT would either decrease soil erosion by up to 55% or increase it by up to 56%. Under these conditions, the acceptable limits will partly be exceeded. The simulations of NT suggested this would reduce annual soil loss rates (compared to CT) to 0·2 and 1·4 t/ha, i.e. about the same or slightly higher than for NT under actual conditions. The simulation of conversion to grassland suggested soil erosion was almost completely prevented.The selected soil conservation methods maintain their protective effect on soil resources, independent of the climate scenario. Therefore, with small adaptations, they can also be recommended as sustainable soil/land management systems under future climatic conditions.However, based on the available climate scenarios, climate-induced changes in the frequency and intensity of heavy rainstorms were only considered in a limited way in the present work. As the general future trend indicates a strong increase of rainstorms with high intensity during summer months, the results of the present study may be too optimistic.

Erosional effects on terrestrial resources over the last millennium in Reykjanes, southwest Iceland

The study presents the effect of soil erosion on vegetation, soil accumulation (SA), SA rate (SAR), soil quality, soil mass, and the soil organic carbon (SOC) pool in Brown Andosols and Histosols in a 24-km 2 area in southwest Iceland. Undisturbed prehistoric soils were distinguished from disturbed historic soils using tephrochronology. Soil erosion has been severe during historic time (last 1135 yr), resulting in the increase of the soil mass deposited in soils covered by vegetation by a factor of 7.3–9.2 and net loss of soil in unvegetated areas. The SAR correlated positively with SOC sequestration. SOC is easily transported and, given the extensive accumulation of soil, the net effect of burial and subsequent reduction in decomposition is to increase SOC storage. Nevertheless, the increased accumulation and soil depletion has decreased soil quality, including the SOC, and reduced soil resistance to erosion with the depleted SOC contributing to enrichment of atmospheric CO 2. The initial terrestrial disturbance was triggered by anthropogenic land use during the Medieval Warm Period, followed by volcanic activity approximately three centuries later. The combination of harsh climate during the Little Ice Age and drastic anthropogenic perturbations has led to land degradation at a catastrophic scale.

Urea fertilisation affected soil organic matter dynamics and microbial community structure in pasture soils of Southern Ecuador

In the mountain rainforest region of the South Ecuadorian Andes natural forests have often been converted to pastures by slash-and-burn practice. With advanced pasture age the pasture grasses are increasingly replaced by the tropical bracken leading to the abandonment of the sites. To improve pasture productivity a fertilisation experiment with urea was established. The effects of urea on soil organic matter (SOM) mineralisation and microbial community structure in top soil (0–5 cm depth) of an active and abandoned pasture site have been investigated in laboratory incubation experiments. Either 14C- or 15N-labelled urea (74 mg urea-N kg −1 dw soil) was added to track the fate of 14C into CO 2 or microbial biomass and that of 15N into the KCl-extractable NH 4-N or NO 3-N or microbial biomass pool. The soil microbial community structure was assessed using phospholipid fatty acid analysis (PLFA). In a second experiment two levels of 14C-labelled urea (74 and 110 mg urea-N kg −1 dw soil) were added to soil from 5 to 10 cm depth of the respective sites. Urea fertilisation accelerated the mineralisation of SOC directly after addition up to 17% compared to the non-fertilised control after 14 days of incubation. The larger the amount of N potentially available per unit of microbial biomass N the larger was the positive priming effect. Since in average 80% of the urea-C had been mineralised already 1 day after amendment, the priming effect was strong enough to cause a net loss of soil C. Although the structure of the microbial community was significantly different between sites, urea fertilisation induced the same alteration in microbial community composition: towards a relative lower abundance of PLFA marker characteristic of Gram-positive bacteria and a higher one of those typical of Gram-negative bacteria and fungi. This change was positively correlated with the increase in NH 4, NO 3 and DON availability. In addition to the activation of different microbial groups the abolishment of energy limitation of the microbes seemed to be an important mechanism for the enhanced mineralisation of SOM.

Sediment budgets and source determinations using fallout Cesium-137 in a semiarid rangeland watershed, Arizona, USA

Analysis of soil redistribution and sediment sources in semiarid and arid watersheds provides information for implementing management practices to improve rangeland conditions and reduce sediment loads to streams. The purpose of this research was to develop sediment budgets and identify potential sediment sources using 137Cs and other soil properties in a series of small semiarid subwatersheds on the USDA ARS Walnut Gulch Experimental Watershed near Tombstone, Arizona, USA. Soils were sampled in a grid pattern on two small subwatersheds and along transects associated with soils and geomorphology on six larger subwatersheds. Soil samples were analyzed for 137Cs and selected physical and chemical properties (i.e., bulk density, rocks, particle size, soil organic carbon). Suspended sediment samples collected at measuring flume sites on the Walnut Gulch Experimental Watershed were also analyzed for these properties. Soil redistribution measured using 137Cs inventories for a small shrub-dominated subwatershed and a small grass-dominated subwatershed found eroding areas in these subwatersheds were losing −5.6 and −3.2 t ha −1 yr −1, respectively; however, a sediment budget for each of these subwatersheds, including depositional areas, found net soil loss to be −4.3 t ha −1 yr −1 from the shrub-dominated subwatershed and −0.1 t ha −1 yr −1 from the grass-dominated subwatershed. Generally, the suspended sediment collected at the flumes of the six other subwatersheds was enriched in silt and clay. Using a mixing model to determine sediment source indicated that shrub-dominated subwatersheds were contributing most of the suspended sediment that was measured at the outlet flume of the Walnut Gulch Experimental Watershed. The two methodologies (sediment budgets and sediment source analyses) indicate that shrub-dominated systems provide more suspended sediment to the stream systems. The sediment budget studies also suggest that sediment yields measured at the outlet of a watershed may be a poor indicator of actual soil redistribution rates within these semiarid watersheds. Management of these semiarid rangelands must consider techniques that will protect grass-dominated areas from shrub invasion to improve rangeland conditions.

Comparison of fallout radionuclide (caesium-137) and modelling approaches for the assessment of soil erosion rates for an uncultivated site in south-eastern Australia

Soil erosion rates are quantified using the fallout radionuclide (caesium-137) approach and models (empirical RUSLE and physically based SIBERIA) for a small catchment in south-eastern Australia. Two hillslope transects (under native grass) were sampled for 137Cs activity and soil redistribution rates were determined using empirical and theoretical conversion methods. These soil redistribution rates were compared with RUSLE predictions for the two transects and SIBERIA model predictions for the entire catchment. The net soil loss rates established in this study were also compared with the results of other studies in the region obtained with a range of different methods. Estimates based on 137Cs using an empirical conversion method compared well with published regional rates derived using rainfall-runoff plots, sediment yields and 137Cs, whereas theoretical 137Cs conversion models were found to over-estimate soil redistribution rates. Similarly, the RUSLE model significantly overestimated soil erosion rates in this study as was the case in other studies in the region. The agreement between SIBERIA and 137Cs, and erosion rates obtained elsewhere in the region, provides confidence in SIBERIA for catchment scale erosion assessments. The results of this study demonstrate the limitations associated with using theoretical 137Cs conversion models in environments for which they are not suited. This study also highlights the need for caution when quantifying soil erosion using both field methods and modelling approaches. The results demonstrate that DEM based erosion models are reliable tools for the prediction of soil erosion on the hillslope and catchment scale.

Investigating the effects of afforestation on soil erosion and sediment mobilisation in two small catchments in Southern Italy

Annual soil losses in southern Italy can exceed 100–150 t ha− 1 year− 1. Where erosion on agricultural land is particularly severe, land use change and afforestation are frequently seen as the most appropriate means of reducing erosion risk. However, the overall effectiveness of afforestation in reducing soil erosion remains uncertain, due to the poor development of the forest cover in some areas, leading to significant areas with sparse tree cover, and the erosional impact of forest harvesting, which commonly involves clearcutting. The study reported here addresses this uncertainty and focuses on two small catchments (W2 and W3) located in Calabria, southern Italy, for which measurements of suspended sediment yield are available. Both the catchments originally supported a rangeland vegetation cover and they were planted with eucalyptus trees in 1968. Currently, only catchment W3 supports a continuous forest cover. In catchment W2 the forest cover is discontinuous and there is a significant area of the catchment (ca. 20%) where the tree cover is sparse and the vegetation cover is dominated by natural grasses. Two additional erosion plots were established within catchment W2 in 1991, in order to explore the effect of the density of the tree cover on soil erosion. Information on the sediment yields from the two catchments and the plots for 10 storm events that occurred during the period December 2005–December 2006 and associated information on the 137Cs and excess 210Pb of the sediment, have been used to investigate the effectiveness of afforestation in reducing sediment mobilisation and net soil loss from the catchments involved. The results demonstrate that the areas of greatest soil loss are associated with the slopes where the tree cover is discontinuous, and that forest harvesting by clearcutting causes significant short-term increases in sediment mobilisation and sediment yield. These findings, which are consistent with previous work undertaken within the same area, emphasize the importance of vegetation cover density in influencing rates of soil loss in the study catchments. The study also provided a useful demonstration of the potential for using measurements of the 137Cs and 210Pbex content of sediment, in combination with more traditional sediment monitoring, to investigate sediment sources and to compare the sediment dynamics of catchments subjected to different land management practices.