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.     

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.     

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.     

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.     

Surface energy balance closure by the eddy-covariance method above three boreal forest stands and implications for the measurement of the CO2 flux

Closure of the surface energy balance provides an objective criterion for evaluating eddy-covariance (EC) flux measurements. This study analyses 5 years of EC carbon dioxide, water vapor, and sensible heat flux measurements from three mature boreal forest stands in central Saskatchewan, Canada. The EC sensible and latent heat fluxes, H and λE, underestimated the surface available energy by 11% (aspen), 15% (black spruce), and 14% (jack pine). At all sites, the energy-closure fraction CF responded similarly to the friction velocity u_*, atmospheric stability, and time of day. At night, CF increased from 0.3 at very low-u_* to an asymptotic maximum of 0.9 at u_* above 0.35 m s⁻¹. During unstable-daytime periods, CF varied linearly from 0.7 at low-u_* to 1.0 at high-u_*. The energy imbalance pattern was similar among sites and may be characteristic of the continental, boreal forest.EC measurements of net ecosystem exchange F_NEE have no objective, diagnostic parameter that is equivalent to CF. We therefore derived an analogous F_NEE “closure fraction” CF^NEE by normalizing measured F_NEE against estimates from an empirical model that was tuned to the high-u_* data. CF and CF^NEE responded similarly to u_*, atmospheric stability, and time of day. We discuss two implications for EC flux data post-processing. The results uphold the common practice of rejecting EC measurements during low-u_* periods. They also lend support to the application of energy-closure adjustments to H, λE, and F_NEE.     

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.     

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.     

Influence of soil water on stress–strain behaviour of a compacting soil in semi-arid Kenya

Depending on the top and subsoil textures, semi-arid soils exhibit cohesive and frictional properties that are associated with the relatively high soil strength, bulk density and penetration resistance. The objective of this study was to gain the knowledge of mechanical properties of the compacting chromic luvisols in order to improve the design of tillage tools. Therefore, we applied critical state soil mechanics to study the stress–strain behaviour of the luvisols using triaxial tests under laboratory conditions. Field investigations involved random collection of undisturbed soil samples which were subjected to triaxial testing first by isotropic consolidation and compression and then triaxial shearing. Plots of deviatoric stress against axial strain were made to determine the soil shear strengths at the critical states over different soil water levels and the two soil depths of 0–20 cm for the plough and 20–40 cm for the hard pan layers, respectively. An exponential model used to fit the deviatoric stress–axial strain test data accurately predicted the trends. Soil water significantly influenced the shear strength, cohesion (c′) and internal angle of friction (′) and hence the mechanical behaviour of the luvisols. The regression equations developed showed that c′ and ′ have quadratic relationships with soil water. The very high clay bonding strength in the subsoil (hard pan) layer resulted in high shear strength, bulk density and penetration resistance values for this soil layer. The increase in shear strength with decreasing water content affected the deviatoric stress–axial strain relationships between the upper and lower plastic limits of the sandy soil. Thus, as the soil dried, the soil ceased to behave in the plastic (ductile flow) manner and thus began to break apart and crumble. The crumbling was indicative of brittle failure. The transition stage from an increase to a decrease in c′ and ′ values with soil water occurred in the soil water content range of 6–10%. Knowledge of stress–strain behaviour of compacting soils is of practical significance in the design of appropriate tillage tools for the specific soil type.     

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.     

Relationship between suspended sediment load, channel geometry and land area increment in the Yellow River Delta

The annual river discharges and suspended sediment loads into the Yellow River Delta show a declining tendency with some distinct fluctuations over the last 50 years. The decrease of river discharge and suspended sediment load and the change in the river channel must influence the evolution of the Yellow River Delta. During this period several new river mouths formed via channel switch at the river delta, and the old watercourse was gradually abandoned. Recently, in years with very low annual suspended load, erosion of land area has been recorded for the delta. The aims of this work are (1) to determine what is the critical suspended sediment load needed in order to maintain the land balance of the Yellow River Delta for two periods before and after 1976 (the last time the channel shifted), and (2) to examine the variation in the channel geometry and gradient in response to changes in suspended sediment load and delta area at the river mouth. In order to estimate these critical values, we used statistical method to analyze the relationships between land area increment, and suspended sediment load and channel geometry. In order to examine the variation in the channel geometry, the channel cross-sections of the Q1 and Q6 were compared. The results show that to maintain the land area balance between 1953 and 1973, when the river mouth was the Diaokouhe, the critical annual suspended sediment load entering the delta was 4.21  10⁸ tonnes/a. After the main channel switched to Qingshuigou in 1976, the critical value to maintain the Qingshuigou mouth between 1976 and 1997 was 1.51  10⁸ tonnes/a. To maintain the land area balance for the entire Yellow River Delta between 1976 and 1997 the critical suspended sediment load was 3.18  10⁸ tonnes/a. The annual mean channel thalweg elevation and channel gradient at the river delta increase with increasing land area increment at the Qingshuigou mouth. The critical channel gradient at the channel reach between cross sections Q1 and Q6 is 0.000095. The channel has narrowed during the time period from 1976 to 1997. Also, lateral channel migration has decreased remarkably, resulting in enhanced hydraulic efficiency of the deltaic channel and artificial levees. This channel geometry evolution was influenced by river adjustment and human activities. These results are of importance for the management of the lower Yellow River channel and the delta. Future water diversion or river damming should consider the balance between suspended sediment delivery and delta growth.     

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).     

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.     

Constraining the conditions conducive to dissimilatory nitrate reduction to ammonium in temperate arable soils

Here we offer the first assessment of conditions conducive to dissimilatory nitrate reduction to ammonium (DNRA) in temperate arable soils, through an examination of the potential for this process to occur in a range of soils of contrasting characteristics. NH₄¹⁵NO₃ (6.2 g N m⁻², 25 atom % excess ¹⁵N) was applied, and recovery of ¹⁵N in the pool taken as indicative of occurrence of DNRA. Up to 5% of applied ¹⁵N was recovered in the pool 2 d after addition of N, glucose (44.6 g C m⁻²) and l-cysteine (7.7 g m⁻², 0.9 g N m⁻², 2.3 g C m⁻²). concentrations were positively correlated with soil pH, ratio, bulk density, sand content and concentration, but negatively correlated with soil C and organic N content. Our results demonstrate the potential for DNRA to contribute to N cycling in temperate arable soils, but its detection and significance is likely to depend on the provision of a low molecular weight C source.     

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.     

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.     

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.     

Effects of width, edge and habitat on the abundance and nesting success of scrub–shrub birds in powerline corridors

Concern about declines in scrub–shrub bird populations has resulted in efforts to create and maintain habitat for these species. Vegetation within powerline corridors is managed to prevent contact of vegetation with transmission lines, and comprises approximately 2% of all of habitat for scrub–shrub birds in southern New England. Although previous studies have documented the use of powerline corridors by scrub–shrub birds, important questions remain about the factors affecting the quality of corridors as habitat for these species. We surveyed birds and monitored nests on 15 corridors in western Massachusetts during 2002 and 2003 to determine whether scrub–shrub birds occupy and successfully reproduce in powerline corridors, and to identify the principal factors affecting scrub–shrub abundance and nesting success. We found that corridors were occupied by scrub–shrub birds of high regional conservation priority, however, four of seven focal scrub–shrub bird species were scarce or absent in narrow corridors, and the abundance of these species was highest in corridors of intermediate width. Overall, nest survival was low (0.14) at these sites relative to other types of early successional habitats in the region, however, if we consider only our sites that were wider than the median width (49 m), nest survival in corridors was (0.33), similar to survival rates reported in other studies of scrub–shrub birds. We conclude that powerline corridors provide habitat for early successional birds of conservation concern, with wider corridors (50 m) contributing more to regional conservation of these species.     

Evaluation of non-invasive genetic sampling methods for estimating tiger population size

There is often a conservation need to estimate population abundances of elusive, low-density, wide-ranging carnivore species. Because of logistical constraints, investigators often employ non-invasive ‘captures’ that may involve ‘genetic’ or ‘photographic’ sampling in such cases. Established capture–recapture (CR) methods offer a powerful analytical tool for such data. In this paper, we developed a rigorous combination of captive, laboratory and field-based protocols for identifying individual tigers (Panthera tigris) from fecal DNA. We explored trade-offs between numbers of microsatellite loci used for reliable individual identifications and the need for higher capture rates for robust analyses. Our field surveys of scats were also specifically designed for CR analyses, enabling us to test for population closure, estimate capture probabilities and tiger abundance. Consequently, we could compare genetic capture estimates to results of a ‘photographic capture’ study of tigers at the same site. The estimates using the heterogeneity model (M_h-Jackknife) for fecal DNA survey were [M_t+1 = 26; and ()=66 (12.98)] in close agreement with those from the photographic survey [(M_t+1 = 29; and () = 66 (13.8)]. Our results revealed that designing field surveys of scats explicitly for CR data analyses generate reliable estimates of capture probability and abundance for elusive, low density species such as tigers. The study also highlights the importance of rigorous field survey and laboratory protocols for reliable abundance estimation in contexts where other approaches such as camera-trapping or physical tagging of animals may not be practical options.     

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.     

A general approach to estimate soil water content from thermal inertia

Remote sensing is a promising technique for obtaining information of the earth's surface. Remotely sensed thermal inertia has been suggested for mapping soil water content. However, a general relationship between soil thermal inertia and water content is required to estimate soil water content from remotely sensed thermal inertia. In this study, we propose a new model that relates soil thermal inertia as a function of water content. The model requires readily available soil characteristics such as soil texture and bulk density. Heat pulse measurements of thermal inertia as a function of water content on nine soils of different textures were made to generate a universal Kerstan function. Model validation was performed independently in both laboratory and field, and the retrieved soil water contents from the new model were compared with previous models. Laboratory evaluation on an Iowa silt loam showed that the RMSE of the new model was 0.029 m³ m⁻³, significantly less than [Murray, T., Verhoef, A., 2007. Moving towards a more mechanistic approach in the determination of soil heat flux from remote measurements. I. A universal approach to calculate thermal inertia. Agric. For. Meteorol. 147, 80–87] model (0.109 m³ m⁻³) and [Ma, A.N., Xue, Y., 1990. A study of remote sensing information model of soil moisture. In: Proceedings of the 11th Asian Conference on Remote Sensing. I. November 15-21. International Academic Publishers, Beijing, pp. P-11-1P-11-5.] model (0.105 m³ m⁻³). Similar results were obtained in a field test on a Chinese silt loam: the RMSE of the new model, [Murray, T., Verhoef, A., 2007. Moving towards a more mechanistic approach in the determination of soil heat flux from remote measurements. I. A universal approach to calculate thermal inertia. Agric. For. Meteorol. 147, 80–87] model, and [Ma, A.N., Xue, Y., 1990. A study of remote sensing information model of soil moisture. In: Proceedings of the 11th Asian Conference on Remote Sensing. I. November 15-21. International Academic Publishers, Beijing, pp. P-11-1P-11-5.] model were 0.018, 0.071, and 0.159 m³ m⁻³, respectively. Additionally the model was validated using literature data in which soil thermal properties were estimated from in situ temperature measurements. The mean errors of estimated water content were generally less than 0.02 m³ m⁻³. We concluded that the new model was able to provide accurate water content predictions from soil thermal inertia.