The role of tephra covers on soil moisture conservation at Haleakala's crater (Maui, Hawai'i)

The influence of tephra covers on soil water was studied in Haleakala (Maui, Hawai'i) during two summers; eight sites with tephra layers and silverswords (Argyroxiphium sandwicense DC.) were sampled at 2415–2755 m. At each site, eight paired-sample sets were obtained in bare soils and under adjacent tephra, at three depths. Tephra were sharply separated from underlying soils and showed prominent vertical stratification. Tephra clast size-distribution was assessed by photosieving and on interstitial-gravel samples; stones included 45.6% cobbles, 29.4% pebbles, and 25% blocks.Moisture content increased with depth in both positions, but soils below tephra had more water at all depths than exposed areas. Surface soils beneath tephra contained 83% more water than bare ground. Soils at 5–10 cm had  106% greater moisture under rocks, but only  70% at 10–15 cm. Differences between plots were statistically significant ( p < 0.001) for surface soils, but less pronounced for subsoils. Soils above 2650 m had greater water content than at lower elevations, and moisture disparity between sample pairs increased with altitude.All soils were coarse, with  20% gravel and  94% sand; most fine material (≤ 0.063 mm) was silt, as clay content was negligible. Organic-matter percentage was low (1.65%). Bulk density and porosity were associated with moisture variation both in tephra-insulated and bare soils; 80% of field moisture was statistically (p < 0.001) accounted for by pore space. Air and soil temperatures were recorded at three sites during  one-week periods prior to moisture sampling. Tephra substantially decreased soil maxima and daily thermal amplitude in underlying soils, but did not noticeably affect nightly minima. Thin (5–6 cm) tephra layers were nearly as effective as thicker (9–15 cm) deposits in depressing soil maxima. Possible water-conservation mechanisms under tephra include: decreased evaporation due to ground shielding and lower maxima; reduced capillary flow; greater infiltration depth; nocturnal dew condensation; and fog interception by blocks.     

Response of silicate weathering to monsoon changes on the Chinese Loess Plateau

Eolian deposits of loess on the Chinese Loess Plateau are investigated to decipher the influence of sequentially changed monsoon climate on silicate weathering process. Detailed hydrochloric acid dissolution experiments are applied to establish a sensitive proxy for silicate weathering. Combined elemental and mineralogical studies show that the minerals which are susceptible to incipient chemical weathering can be totally dissolved in hot hydrochloric acid (80°C, 3mol/L) after 4h, while other stable minerals are nearly unaffected. Thus, the hydrochloric acid dissolvable fractions (ADF) of loess and paleosol are more sensitive than bulk samples in the weathering reaction. Since Mg is tend to be leached out from the ADF while Al is conserved during the incongruent weathering, Mg/Al ratio of the ADF (Mg/Al_ADF) could indicate the weathering intensities of the Mg-bearing minerals, mainly chlorite. The Mg/Al_ADF of the modern soils on the Chinese Loess Plateau is closely correlated to the local precipitation amounts, suggests that the intensity of summer monsoon is a key factor in the weathering of Mg-silicate. The silicate weathering intensity of the loess-paleosol deposits of past 130Kyrs shows strong procession cycle of 23Kyrs coupled by glacial-interglacial variation. The 23Krys cycles are coherent to the variations of the intensity of summer monsoon, while the 100Kyrs glacial-interglacial variation may relates to the changes in winter monsoon. Phased intensification of East Asian summer monsoon, and thus enhanced silicate weathering and atmosphere CO₂ drawdown, in response to the uplift of Tibetan Plateau, may be another mechanism relating the late Cenozoic tectonics to global cooling.     

Photosynthetic light use efficiency of three biomes across an east–west continental-scale transect in Canada

Light use efficiency (LUE) is used widely in scaling and modeling contexts. However, the variation and biophysical controls on LUE remain poorly documented. Networks of eddy covariance (EC) towers offer an opportunity to quantify _g, the ratio of P, gross primary productivity, to Q_a, absorbed photosynthetically active radiation (PAR), across climate zones and vegetation types. Using data from the Fluxnet Canada Research Network (n = 24 sites) in 2004, we examined the relationship between daily and yearly _g, driving variables, and site characteristics on a site-specific and plant functional type (PFT) basis using tree regression and linear regression. Data were available for three biomes: grassland, forest, and wetland. Yearly _g values ranged from 0.1 to 3.6 g C MJ⁻¹ Q_a overall. Daily _g was highest in the grassland (daily median ± interquartile range: 3.68 ± 1.98 g C MJ⁻¹ Q_a), intermediate in the forested biome (0.84 ± 0.82 g C MJ⁻¹ Q_a), and lowest for the wetlands (0.65 ± 0.54 g C MJ⁻¹ Q_a). The most important biophysical controls were light and temperature, to the exclusion of water-related variables: a homogeneity of slopes model explained c. 75% of the variation in daily _g. For a subset of sites with diffuse PAR data, the ratio of diffuse to total PAR, a proxy for cloudiness, was a key predictor. On the yearly time scale, _g was related to leaf area index and mean annual temperature. Aggregating to PFTs did not show functional convergence within any PFT except for the three wetland sites and the Picea mariana toposequence at the daily time step, and when using the Köppen climate classification on a yearly time step. The general lack of conservative daily _g behavior within PFTs suggests that PFT-based parameterizations are inappropriate, especially when applied on shorter temporal scales.     

Measured and modelled rainfall interception loss from an agroforestry system in Kenya

Rainfall interception losses from an agroforestry system in semi-arid Kenya comprising Grevillea robusta and maize were measured over a period of 33 months. These measurements showed that interception was slightly higher (10.2%) under trees with no maize understorey, than in the intercropped treatment (9.8%), and was directly related to the degree of tree canopy cover. Interception estimates using the reformulated version of the Gash analytical model were 4% lower than measured totals, and were strongly dependent on both canopy cover and on monthly variations in the mean rainfall rate ( , mm h⁻¹).     

Tillage effect on C stocks of a clayey Oxisol under a soybean-based crop rotation in the Brazilian Cerrado region

A large area (180 Mha) of central Brazil is occupied by a savanna biome known as the Cerrado. Annual rainfall in this region varies from 1200 to 2000 mm, although there is a long (5 month) dry season with almost no rain. This region is regarded by Brazilians as their agricultural frontier and there is a steady growth in the area dedicated to permanent cropping in the region, which today is estimated to occupy 14 Mha. Owing to the dearth of long-term experiments, the impact of continuous cropping on soil carbon stocks remains unclear. The objective of this study was to evaluate the effects of different tillage systems (zero till (ZT) and conventional tillage (CT)) on the change in soil carbon stocks over a 20-year period of the same crop sequence compared to that under a neighbouring area of native vegetation (NV). Only approximately 10 Mg ha⁻¹ of soil carbon in the 0–100 cm depth interval was lost under continuous ZT. However, under CT systems losses were greater (up to 30 Mg C ha⁻¹) when the mouldboard plough was used and/or tillage was performed twice a year. We did not have access to instrumentation to accurately assess soil charcoal but the C/N data and peroxide and dichromate oxidative techniques suggested that 40% of soil C was in this form. The ¹³C natural abundance of soil profiles indicated that residues of crops (maize) and the spontaneous annual fallow of Brachiaria spp. resulted in integration of significant C₄ residues to a depth of at least 40 cm. It would appear that zero tillage, which is already widely adopted in the Cerrado region of Brazil, will have only a small negative long-term impact on soil C stocks, but ploughing, especially more than once a year, will lead to considerably larger soil C losses.     

Alteration of secondary minerals along a time series in young alkaline soils derived from carbonatic wastes of soda production

Industrial activities result in increasing amounts of technical substrates being deposited in landfills. These substrates are subject to weathering and pedogenic processes. We studied the chemical and mineralogical transformations on naturally weathered waste deposits of soda industry. Four sites differing in age (15, 19, 57, and 70 years) and derived from carbonatic slurry (mainly CaO^.H₂O, CaCO₃, NaCl) were selected. The formed soils, calcareous spolic Regosols, are weakly to strongly alkaline with pH values ranging from 8 to 12. Within 15 years, the substrate's initial pH of 12 drops rapidly in the topsoil due to the reaction of dissolved Ca either with CO₂ from the atmosphere or evolved by microbial respiration and finally stabilizes at around 8.1. All soils show high electrical conductivity, up to 12.3 mS cm^− 1 at the youngest site. The electrical conductivity strongly decreases within 70 years of weathering due to leaching processes and the formation of less soluble secondary minerals. The content of organic C in the studied soils ranges from 2.4 to 70.8 g kg^− 1 and stocks increase with site age. Soil structure and soil color change distinctly. The binding of CO₂ results in large amounts of carbonate, increasing with time. Seventy years after deposition, calcite [CaCO₃] dominates the topsoil (0–30 cm depth), comprising about 80% of the soil material. The mineral composition was characterized by X-ray diffraction. Besides calcite, we found different quantities and different distributions of the less common minerals ettringite [Ca₆(Al(OH)₆)₂(SO₄)₃  26H₂O], thaumasite [Ca₆(Si(OH)₆)₂(CO₃)₂(SO₄)₂  24H₂O], hydrocalumite [Ca₂Al(OH)₇  2H₂O] and hydrotalcite [Mg₆Al₂(CO₃)(OH)₁₆  4H₂O]. Formation and alteration of these minerals are basically influenced by changes in the soil pH. With progressing weathering neither thaumasite nor ettringite are stable due to the non-favorable soil reaction (pH  8.1). In contrast, hydrocalumite and hydrotalcite exist in all investigated soils. They are stable also under weakly alkaline conditions and thus may exist in all carbonatic soils. Results indicate a surprisingly rapid soil development driven by the highly dynamic formation and alteration of minerals in carbonatic substrates under alkaline conditions.     

Evaluation of land surface radiation balance derived from moderate resolution imaging spectroradiometer (MODIS) over complex terrain and heterogeneous landscape on clear sky days

Many studies on land surface radiation balances have relied on geostationary satellites. These satellites have provided data with high temporal resolution (less than 3 h); however, the spatial resolution was too coarse (20–250 km scale) to investigate local-scale land surface radiation balances. Moderate resolution imaging spectroradiometer (MODIS) – onboard both the Terra and Aqua satellites – yields a tradeoff with regard to this problem by providing higher spatial resolution (1 km scale) and sensing all over the earth nearly twice a day during daytime; this provides a potential tool for the periodical monitoring of the land surface energy balance. The reliability of MODIS-derived estimates is, however, affected by the presence of multiple error sources, such as those related to heterogeneous land cover and complex topography. In this study, we have used atmospheric (5 and 10 km scale) and land (1 km scale) products obtained from both the Terra and Aqua MODIS devices as inputs in order to estimate the radiation components (1 km scale) under clear daytime conditions over a heterogeneous farmland area and a rugged deciduous forest in the Korea Flux Network (KoFlux). The reliability of these estimates and the associated errors were evaluated by comparing against field measurements taken for 41 and 26 clear days with regard to the farmland and forest sites, respectively. Solar radiation was successfully retrieved with a root mean square error (RMSE) of 20 W m⁻² for both the Terra and Aqua devices over the flat farmland site, whereas the rugged forest site exhibited corresponding values of 40 and 65 W m⁻² RMSE values with consistent positive biases (presumably caused by topographic effects). The RMSE values of the downward longwave radiation were 20 W m⁻² for both the Terra and Aqua devices for both these sites. The sensitivities of the upward components of the shortwave and longwave radiations varied with the RMSE values to the scale of the spatial heterogeneity of both the sites. Consequently, the RMSE values of the net radiation ranged from 33 to 61 W m⁻² for both the devices at both the sites. Our results suggest that the scales of the patch mosaics within the landscapes need to be quantified for proper retrieval of the MODIS-derived radiation products. More extensive validation efforts are required to identify and account for major error sources across diverse land surface conditions.     

Direction-dependent behaviour of hydraulic and mechanical properties in structured soils under conventional and conservation tillage

The development of soil structure units with defined forms and dimensions (e.g. platy by soil compaction or prismatic up to subangular-blocky by swelling–shrinkage processes) can lead to direction-dependent behaviour of mechanical and hydraulic properties. However, little research has investigated direction-dependent behaviour directly. Undisturbed samples were collected at different horizons and orientations (vertical and horizontal) of Stagnic Luvisols derived from glacial till (Weichselian moraine region in Northern Germany). A direct shear test determined the cohesion (c) and the angle of internal friction (φ). The water retention curve (WRC), the saturated hydraulic conductivity (k_s) and the air permeability (k_a) were also measured. The air-filled porosity (_a) was determined and pore continuity indices (N) and blocked porosities (_b) were derived from the relationship between k_a and _a.Although the pore volume as a scalar is isotrop, the saturated hydraulic conductivity and air permeability can be anisotropic. In the seedbed (SB) and plough pan (PP) of conventionally managed soils the effective porosity is non-direction-dependent, however, differences in k_s as a function of sampling direction can reach one order of magnitude in PP (k_sh > k_sv). The shear strength parameters do not present a significant anisotropy, although, a pronounced spatial orientation of soil aggregates (e.g. induced by soil compaction in a plough pan) lead to direction-dependent shear strength (by σ_n: 10 kPa, σ_tv: 12 kPa and σ_th: 19 kPa). This behaviour was especially observed in pore continuity indices (e.g. vertical and horizontal oriented aggregates observed in Bvg and PP presented _bv < _bh and _bv > _bh, respectively) showing that the identification of soil structure can be used as the first parameter to estimate if hydraulic properties present a direction-dependent behaviour at the scale of the soil horizon, which is relevant in modelling transport processes.     

Dynamics of upward and downward N2O and CO2 fluxes in ploughed or no-tilled soils in relation to water-filled pore space, compaction and crop presence

Sharp peaks in nitrous oxide (N₂O) fluxes under no-tillage in wet conditions appear to be related to near surface soil and crop cover conditions. Here we explored some of the factors influencing tillage effects on short-term variations in gas flux so that we could learn about the mechanisms involved. Field investigations revealed that a cumulative emission of 13 kg N₂O–N ha⁻¹ over a 12-week period was possible under no-tillage for spring barley. We investigated how reducing crop cover and changing the structural arrangement of the water-filled pore space (WFPS) by short-term laboratory compaction influenced N₂O and carbon dioxide (CO₂) fluxes in upward and downward directions in core samples from tilled and untilled soil. Increasing the downward flux of N₂O within a soil profile by changing soil or moisture conditions may increase the likelihood of its further reduction to N₂ or dissolution. We took undisturbed cores from 3 to 8 cm depth, equilibrated them to −1 or −6 kPa matric potential, incubated them and measured N₂O and CO₂ fluxes from the upper and lower surfaces in a purpose-designed apparatus before and after compaction in an uniaxial tester. We also measured WFPS, air permeability, bulk density and air-filled porosity before and after compaction. Spring barley was tested in 1999 and winter barley in 2000.Fluxes of N₂O were from 1.5 to 35 times higher from no-tilled than ploughed even where the soil was of similar bulk density. Reduction of the crop cover increased CO₂ flux and could reduce N₂O flux. The effects of structural changes induced by laboratory compaction on the fluxes of N₂O and CO₂ were not influenced greatly by the tillage and crop cover treatments. Fluxes from the upper surfaces of cores (corresponding to 3 cm soil depth, upwards direction) could be up to 100 times greater (N₂O) or 8 times (CO₂) than from the lower surfaces (8 cm depth, downwards direction). These differences between surfaces were greatest when N₂O fluxes were very high in no-tilled soil (4.2 mg N₂O–N m⁻² h⁻¹) as occurred when WFPS exceeded 80% or became blocked with water, an effect that was increased by our compaction treatment. In general N₂O fluxes increased with WFPS. The production and emission of N₂O were strongly influenced by the soil physical environment, the magnitude of the water-filled pore space and continuity of the air-filled pore space in particular, produced in no-till versus plough cultivation.     

Using an unplanned experiment to evaluate the effects of hatcheries and environmental variation on threatened populations of wild salmon

Efforts to conserve depleted populations of Pacific salmon (Oncorhynchus spp.) often rely on hatchery programs to offset losses of fish from natural and anthropogenic causes, but their use has been contentious. We examined the impact of a large-scale reduction in hatchery stocking on 15 populations of wild coho salmon along the coast of Oregon (USA). Our analyses highlight four critical factors influencing the productivity of these populations: (1) negative density-dependent effects of hatchery-origin spawners were 5 times greater than those of wild spawners; (2) the productivity of wild salmon decreased as releases of hatchery juveniles increased; (3) salmon production was positively related to an index of freshwater habitat quality; and (4) ocean conditions strongly affect productivity at large spatial scales, potentially masking more localized drivers. These results suggest that hatchery programs’ unintended negative effects on wild salmon populations, and their role in salmon recovery, should be considered in the context of other ecological drivers.     

Reduced effective population size in an overexploited population of the Nile crocodile (Crocodylus niloticus)

Unchecked exploitation of wildlife resources is one of the major factors influencing species persistence throughout the world today. A significant consequence of exploitation is the increasing rate at which genetic diversity is lost as populations decline. Recent studies suggest that life history traits affecting population growth, particularly in long-lived species, may act to moderate the impact of population decline on genetic variation and lead to remnant populations that appear genetically diverse despite having passed through substantial demographic bottlenecks. In this study we show that the retention of genetic variation in a partially recovered population of Nile crocodile is deceptive, as it masks the reality of a significant decline in the population’s effective size (N_e). Repeated episodes of unchecked hunting in the mid to late 20th century have today led to a five-fold decrease in the population’s N_e. Using current census data we estimate the contemporary N_e/N ratio as 0.05 and, in light of quotas that permit the ongoing removal of adults, simulated the likely effects of genetic drift on extant levels of variation. Results indicate that even if the current effective size is maintained, both allelic diversity and heterozygosity will decline. Our findings have complex implications for long-lived species; an emphasis on the retention of genetic variation alone, whilst disregarding the effects of population decline on effective size, may ultimately obscure the continued decline and extinction of exploited populations.     

Eight years of carbon dioxide exchange above a mixed forest at Borden, Ontario

This paper summarizes results from 8 years (1996–2003) of eddy covariance-based ecosystem CO₂ exchange measurements at the Borden Forest Research Station (44°19′N, 79°56′W). The site represents a mid-latitude, 100-year-old, mixed deciduous and coniferous forest dominated by red maple, aspen and white pine. The years 1996 and 1997 were relatively cold, had a late spring and received below average photosynthetic photon flux density (PPFD). This contrasts with an early spring, warmer soil and air temperatures during 1998–1999, and with distinctly wet year of 2000 and dry years of 2001–2003. The combination of early spring, warmer air and soil temperature and relatively high level of PPFD was associated with higher net ecosystem productivity (NEP) that peaked during 1999. Photosynthetic capacity was reduced and NEP showed a mid-growing season depression during the dry years of 2001–2003. Annual average ecosystem respiration (R) determined from a light response model was 30% less than R derived from a logistic respiration equation, relating night time CO₂ flux and soil temperature. However these independently determined R values were well correlated indicating that the site is unaffected by fetch and spatial heterogeneity problems. Based on the combined 8 years of growing season daytime data, an air temperature of 20–25 °C and a vapor pressure deficit (VPD) of 1.3 kPa were found to be the optimal conditions for CO₂ uptake by the canopy. Over the 1996–2003 period, the forest sequestered carbon at an average rate of 140 ± 111 gC m⁻² y⁻¹. The corresponding gross ecosystem photosynthesis (GEP) and R over this period were 1116 ± 93 gC m⁻² y⁻¹ and 976 ± 68 gC m⁻² y⁻¹, respectively. The annual carbon sequestration ranged from 19 gC m⁻² in 1996 to 281 gC m⁻² in 1999. However, these estimates were sensitive to frictional velocity threshold () used for screening data associated with poor turbulent mixing at night. Increasing from 0.2 m s⁻¹ (based on the inflection point in the nighttime CO₂ flux vs. u_* relationship) to 0.35 m s⁻¹ (determined using a selection algorithm based on change-point detection) modified the 8-year mean NEP estimate from 140 ± 111 gC m⁻² y⁻¹ to 65 ± 120 gC m⁻² y⁻¹. Both approaches show that the Borden forest was a low to moderate sink of carbon over the 8-year period.     

Incorporation of spectral data into multivariate geostatistical models to map soil phosphorus variability in a Florida wetland

Hybrid geostatistical prediction methods incorporate (i) spatially-explicit soil observations and exhaustive grids of ancillary environmental variables (e.g. derived from remote sensing), (ii) spatial autocorrelation, (iii) spatial covariation, and/or (iv) combinations of the above. In numerous studies of terrestrial soils it has been shown that hybrid geostatistical methods outperform univariate spatial and regression (aspatial) methods. However, hybrid methods have rarely been employed to predict soil properties in wetlands. In this study we used spectral data and derived indices from two remote sensors (Landsat ETM+ and ASTER), with different spatial resolutions, from different seasons, but with similar spectral range, ancillary environmental data, as well as floc and soil total phosphorus (TP) observations from 111 sites. The specific objective of our study was to evaluate the performance of aspatial methods (multivariate regressions — REG), univariate spatial (Ordinary Kriging — OK) and hybrid/multivariate geostatistical methods (Regression Kriging — RK and Co-kriging — CK) in predicting the spatial variability and distribution of floc and soil TP in a subtropical wetland, WCA-2A, in the Florida Everglades. Measured floc TP ranged from 194 to 1865 mg kg^− 1 with a median of 751 mg kg^− 1 and standard deviation (SD) of 381 mg kg^− 1. According to cross-validation, predictions for floc TP based on the root mean square prediction error (RMSE) were best in the following order: RK_quadratic (134.9) > RK_multivariate (201.1) > OK (206.1) > CK (212.1) > REG_multivariate (218.3) > REG_quadratic (220.3) > REG_linear (264.4); and based on the mean prediction error (ME) followed the order RK_multivariate (0.9)  RK_quadratic (1.1) > CK (− 6.7) > REG_multivariate (18.2) > REG_linear (25.1) > OK (− 27.3) > REG_quadratic (27.3). The Normalized Difference Vegetation Index (NDVI)-green derived from Landsat ETM+ showed the largest predictive power for floc TP. Measured soil TP ranged from 155 to 1702 mg kg^− 1 with a median of 433 mg kg^− 1 and standard deviation of 316 mg kg^− 1. Predictions for soil TP based on RMSE were best in the following order: RK_ASTER (200.1) > CK_ASTER (238.2)  CK_ETM (239.0) > OK (258.0) > RK_ETM (279.2) > REG_ASTER (281.8) > REG_ETM (356.1); and based on ME followed the order: CK_ASTER (0.1)  CK_ETM (0.2) > RK_ASTER (− 5.2) > RK_ETM (− 31.5) > OK (− 41.8) > REG_ASTER (94.4) > REG_ETM (133.7). The NDVI showed the largest predictive power for soil TP. This comparative study in a subtropical wetland demonstrated the benefits of incorporating remote sensing data into floc and soil TP prediction models. Overall, hybrid geostatistical methods (CK and RK) performed better than regressions and spatial univariate models (OK) in the prediction of floc and soil TP. Depending on the strength of the spatial covariance between primary and secondary variables (CK) and the ability of the regression model in RK to explain the variability of a target variable (e.g., floc or soil TP), either CK or RK performed best. Our findings in this wetland confirmed results from earlier studies on terrestrial soils indicating the superior performance of hybrid geostatistical methods in predicting soil properties.     

Relationships among dust outbreaks, vegetation cover, and surface soil water content on the Loess Plateau of China, 1999–2000

We analyzed relationships among dust outbreaks, Normalized Difference Vegetation Indices (NDVI), and surface soil water content (0 to 2 cm depth) on the Loess Plateau, a significant dust source area of East Asia. World Surface Data for wind speed and current weather, coarse-resolution data for NDVI, and a three-layer soil model for surface soil water content were used. The threshold NDVI for preventing dust outbreaks was about 0.2 when the wind speed ranged from 7 to 8 m s^− 1. This threshold NDVI corresponds to a vegetation cover of 18%. The threshold ratio of surface soil water content to the field capacity (θ_r) was about 0.2. Conditions facilitating dust outbreaks on the Loess Plateau are when NDVI is less than 0.2 with wind speed  7 m s^− 1 and θ_r < 0.2, and when NDVI is greater than 0.2 with wind speed  9 m s^− 1 and θ_r < 0.2.     

Effects of wetland construction on nutrient, SOM and salt content in semi-arid zones degraded by intensive agricultural use

The aim of this study was to determine whether experimental-created wetlands affect soils by altering the levels of soil organic matter (SOM), inorganic nutrients contributed by fertilization and soil salinity/sodicity. We compared the properties of soils in permanently and intermittently flooded experimental wetlands artificially created in conventionally tilled agricultural areas 7 years after abandonment. To assess the long-term effects of wetlands, 18 wetlands created spontaneously by flooding with water from irrigated fields were compared to nearby croplands. Experimental-created wetlands did not exhibit a significant increase in SOM, but soil salinity was reduced, and inorganic nutrients were significantly lower than they were in the reduced in regard to reference croplands. In the soils of the observational natural wetlands that formed after irrigation, SOM increased significantly, and the levels of inorganic nutrients and salinity were reduced compared to agricultural soils. In the Ebro river Basin, 7 years of restoration might not be sufficient time for a significant accumulation of SOM but, after a prolonged period (40 years), SOM increased. In experimental-created wetlands, inorganic nutrients were slowly converted into SOM, but a portion of the NO₃-N was denitrified, and some of the P was adsorbed by soil particles. Guidelines are provided for the creation of wetlands in existing and new agricultural developments in semi-arid regions.     

Autoclaving kills soil microbes yet soil enzymes remain active

Biocidal treatment of soil is used to remove or inhibit soil microbial activity, and thus provides insight into the relationship between soil biology and soil processes. Chemical (soil pH, phosphodiesterase, protease) and biological (substrate induced respiration) characteristics of three contrasting soils from tropical savanna ecosystems in north Queensland, Australia were measured in field fresh samples and following autoclaving (121 °C/103 kPa for 30 min on two consecutive days). Autoclaving treatment killed the active soil microbial biomass and significantly decreased protease activity (90%) in all three soils. Phosphodiesterase activity in kaolinitic soils also significantly decreased by 78% and 92%. However, autoclave treatment of smectitic soil only decreased phosphodiesterase activity by 4% only. This study demonstrates phosphodiesterase can remain stable in extreme conditions. This might be a characteristic vital to the cycling of phosphorus in shrink–swell clays in Australian tropical savanna ecosystems.     

Controls on latent heat flux and energy partitioning at a peat bog in eastern Canada

Surface and atmospheric controls on latent heat flux (Q_E) and energy partitioning were assessed during three growing seasons at the Mer Bleue peat bog. The surface consisted of a sparse canopy (maximum leaf area index 1.3) of low, mostly evergreen shrubs over moss-covered hummocks and hollows. Available energy was partitioned mostly to Q_E (Bowen ratio often less than 0.5) throughout the growing seasons over an extensive range of water table fluctuation (as much as 50 cm). Q_E was often at or below the equilibrium rate due to surface (low moss water content, strong vascular stomatal control) and/or atmospheric (low vapour pressure deficit (D_a)) factors. Turbulent energy fluxes varied with net radiation and the magnitude of the fluctuations were affected by D_a and moss water content. It is suggested that a change in source partitioning for Q_E led to a change in Q_E − D_a dynamics. Early in the growing season the moss was wet and the vascular canopy was replacing leaves, thus Q_E increased as D_a increased because moss, which reacts passively to D_a, contributed strongly to Q_E. Later in the growing season as water table declined and the evaporation load reduced moss and fibric peat water contents, moss contributed less strongly to Q_E and vascular contribution became more important. Also, stomatal control became more influential in reducing bulk surface conductance for water vapour and Q_E in response to increasing D_a.     

Estimation and evaluation of dynamic properties as indicators of changes on soil structure in sugarcane fields of Sao Paulo State – Brazil

The indiscriminate management and use of soils without moisture control has changed the structure of it due to the increment of the traffic by agricultural machines through the years, causing in consequence, a soil compaction and yield reduction in the areas of intensive traffic. The purpose of this work was to estimate and to evaluate the performance of preconsolidation pressure of the soil and shear stress as indicators of changes on soil structure in fields cropped with sugarcane, as well as the impact of management processes in an Eutrorthox soil structure located in Sao Paulo State. The experimental field was located in Piracicaba's rural area (Sao Paulo State, Brazil) and has been cropped with sugarcane, in the second harvest cycle. The soil was classified by EMBRAPA [EMBRAPA, 1999. Centro Nacional de Pesquisa de Solos. Sistema Brasileiro de Classificação de Solos, Embrapa, Brasília, 412 pp.] as an Eutrorthox. Undisturbed samples were collected and georeferenced in a grid of 60 m × 60 m from two depths: 0–0.10 m (superficial layer – SL) and in the layer of greatest mechanical resistance (LGMR), previously identified by cone index (CI). The investigated variables were pressure preconsolidation (σ_p), apparent cohesion (c) and internal friction angle (). The conclusions from the results were that the SLSC was predicted satisfactorily from σ_p as a function of soil moisture; thus, decisions about machinery size and loading (contact pressures) can be taken. Apparent cohesion (c), internal friction angle () and the Coulomb equation were significantly altered by traffic intensity. The σ_p, c and maps were shown to be important tools to localize and visualize soil compaction and mechanical resistance zones. They constitute a valuable resource to evaluate the traffic impact in areas cropped with sugarcane in State of Sao Paulo, Brazil.     

Transpiration and canopy conductance at two slope positions in a Japanese cedar forest watershed

Plant–soil system patterns and processes along a slope are among the greatest causes of uncertainty in estimating watershed-scale transpiration (E). Tree-to-tree and radial variations in xylem sap flux density (F_d), in addition to tree biometrics, were measured over a 2-year period (2005–2006) in two slope stand positions. The areas of interest consisted of an upper slope plot (UP) and a lower slope plot (LP) in a Japanese cedar (Cryptomeria japonica D. Don) forest watershed and the environmental controls of stand E for each plot were compared. Canopy stand E (E_C) and canopy stomatal conductance (G_C) in the UP were less than those in the LP during the growing season, while those in the UP were greater than those in the LP over winter. In addition, mean stand F_d (J_S) in the UP was greater than that in the LP over winter, but J_S values were similar in the UP and LP except in the winter, which allows us to extrapolate watershed-scale E based on J_S estimated from F_d measurements of a partial stand in the watershed. However, this relationship contains a bias and differed between 2005 and 2006. Although there were significant differences in soil moisture conditions between the UP and LP in both years, a systematic relationship between the similarity in J_S and soil moisture conditions was not found. The bias was due to a tendency for J_S in the LP to be greater than that in the UP in 2006. This tendency was amplified because J_S in the LP was greater than that in the UP around an atmospheric humidity deficit (D) of 1–1.5 kPa and frequencies of this D range were higher in 2006 than in 2005. The greater J_S in the LP at D  1–1.5 kPa could be explained by the difference in the response of G_C to D between the UP and LP. Our results suggest this to be the cause of the similarity in J_S values for the UP and LP and for the occasional abortion of its similarity. However, even when the bias or the occasional deviation is disregarded, the error in estimating stand E from a partial stand is so small that it is comparable to an F_d measurement error. For example, the error when using only the LP was 6.6% for stand E.     

Weathering rates of fine pebbles at the soil surface in Kärkevagge, Swedish Lapland

In 1994, nylon mesh bags containing 6.2-mm diameter fragments of either freshly crushed dolomite or granite were placed on meadow, solifluction meadow, birch, heath and alpine tundra vegetation that cover surfaces within Kärkevagge, a glaciated arctic–alpine valley in Swedish Lapland. Each vegetation category contained three replicate sites comprising one bag of each rock type. In 1995, the study was extended to Dryas heath and willow (two sites only) surfaces. Percentage annual weight loss was used as a metric of chemical weathering rates. One site in each of the 1994 categories was measured in 1995. All sites were measured in 1999. Two dolomite and granite standards stored in the laboratory and subjected to identical cleaning and weighing regimes to the field samples, exhibited no more than a 0.24% total weight loss after 5 years. The percentage weight loss in the subgroup examined after 1 year (dolomite 0.54%/year; granite 0.41%/year) significantly exceeded the average annual loss for the subsequent 4 years in all cases (dolomite 0.40%/year; granite 0.07%/year). Ten sites with a 5-year record had a mean annual loss of 0.326±0.115%/year (two standard errors) for dolomite and 0.121±0.020%/year for granite. Assuming an exponential decay model, a 0.326%/year rate of loss produces a ‘half-life’ of approximately 212 years and a 0.121%/year loss rate, a half-life of approximately 570 years. Kruskal–Wallis one-way ANOVA on rank-ordered data determined significant differences in dolomite pebble mass loss between the Dryas heath and willow plus meadow microenvironments, as well as between the birch forest and the willow plus meadow microenvironments. The underpinning factor behind these differences appears to be moisture differences. A similar test failed to determine statistically significant differences for the granite pebbles. Dolomite appears to be a viable medium for determining mass loss rates over quite modest time periods providing that the number of samples is sufficiently large.