Energy and water balance measurements for water productivity analysis in irrigated mango trees, Northeast Brazil

Crop water parameters, including actual evapotranspiration, transpiration, soil evaporation, crop coefficients, evaporative fractions, aerodynamic resistances, surface resistances and percolation fluxes were estimated in a commercial mango orchard during two growing seasons in Northeast Brazil. The actual evapotranspiration (E_a) was obtained by the eddy covariance (EC) technique, while for the reference evapotranspiration (E₀); the FAO Penman–Monteith equation was applied. The energy balance closure showed a gap of 12%. For water productivity analysis the E_a was then computed with the Bowen ratio determined from the eddy covariance fluxes. The mean accumulated E_a for the two seasons was 1419 mm year⁻¹, which corresponded to a daily average rate of 3.7 mm day⁻¹. The mean values of the crop coefficients based on evapotranspiration (K_c) and based on transpiration (K_cb) were 0.91 and 0.73, respectively. The single layer K_c was fitted with a degree days function. Twenty percent of evapotranspiration originated from direct soil evaporation. The evaporative fraction was 0.83 on average. The average relative water supply was 1.1, revealing that, in general, irrigation water supply was in good harmony with the crop water requirements. The resulting evapotranspiration deficit was 73–95 mm per season only. The mean aerodynamic resistance (r_a) was 37 s m⁻¹ and the bulk surface resistance (r_s) was 135 s m⁻¹. The mean unit yield was 45 tonne ha⁻¹ being equivalent to a crop water productivity of 3.2 kg m⁻³ when based on E_a with an economic counterpart of US$ 3.27 m⁻³. The drawback of this highly productive use of water resources is an unavoidable percolation flux of approximately 300 mm per growing season that is detrimental to the downstream environment and water users.     

Seasonal and diurnal variation in the deposition velocity of ozone over a spruce forest in Denmark

The flux of O₃ was measured by the eddy-correlation method over Norway spruce in periods when the trees had a very low activity, periods with optimum growth, and periods with water stress. The aerodynamic resistance (r _a ), viscous sub-layer resistance (r _b ) and surface resistance (r _c ) to O₃ were calculated from meteorological parameters and the deposition velocity. The canopy stomatal resistance to O₃ was calculated from measurements of the water vapour flux. The deposition velocities showed a diurnal pattern with night-time values of 3.5 mm s^–1 and day-time values of 7 mm s^–1, when the trees had optimal growth conditions. The surface resistance was highly dominating in day-time and the influence of meteorology low. In night-time the surface resistance to 03 was lower than the canopy stomatal resistance. A low surface resistance was also found in winter-time, when the activity of the trees was low. The surface resistance increased when the trees were subject to water stress. It is concluded that stomatal uptake is an important parameter for the deposition of O₃. However, other processes such as destruction of O₃ at surfaces, reaction with NO emitted from the soil, and reactions with radicals produced from VOC's emitted from the forest, should also be taken into consideration.     

Coupled model of stomatal conductance–photosynthesis–transpiration for paddy rice under water-saving irrigation

To improve the performance of a coupled model based on a Leuning–Ball stomatal conductance (g_s) model for rice under water deficit conditions, leaf temperature rising (ΔT) was incorporated into the Leuning–Ball model and a revised coupled model for simulation of stomatal conductance g_s–net photosynthesis rate (P_n)–transpiration rate (T_r) was developed based on data collected from a rice paddy with nonflooded controlled irrigation in 2003 and 2004. Both a Leuning–Ball and revised Leuning–Ball and coupled model based on both were evaluated with internal conductance (g_ic) determined by different equations. The performance of the Leuning–Ball model was improved under water deficit condition by incorporating ΔT, and the revised Leuning–Ball model performed better than the Leuning–Ball model in the coupled model of stomatal conductance–photosynthesis–transpiration for rice under water deficit conditions. Meanwhile, accuracy in g_ic calculation is essential for simulation of P_n, but not for simulation of T_r. Thus, leaf temperature rising ΔT is suitable as a leaf water status indicator in a simulation of rice leaf gas exchange response to water deficit conditions.     

半干旱黄土地区幼龄侧柏叶蒸腾的数学模型

 通过人工控制水分,形成单株幼龄侧柏的不同土壤水分梯度环境。在自然环境下对侧柏叶片定时、定位进行蒸腾速率及林冠层的光照、空气温度、空气湿度、叶水势和土壤水分等因子的同步观测。蒸腾速率与各个因子的相关分析表明:黄土半干旱地区侧柏蒸腾速率η_t/(μg·cm^-2·s^-1)与光照强度E/(μmol·m^-2·s^-1)、空气饱和差p_v/kPa、叶水势Ψ/kPa、气温t/℃的关系可以分别表示为:η_t=>αE_b,η_t=αp_vb,η_t=αψ^b,η_t=αt²+bt+c;侧柏的蒸腾速率η_t与气孔阻力R_s/(s·cm^-1)和土壤含水量W/％有密切关系,可以分别表示为:η_t=α+bW+cW₂+dW₃,R_s=α+bW+cW₂+dW₃。用气温、空气饱和差、叶水势3个因素建立了半干旱黄土地区幼龄单株侧柏蒸腾速率的非线性指数预测模型:η_t=0.6950_exp（0.03158t-14.2492/p_v+0.7606/Ψ）,经检验获得了满意的数值模拟结果。     

Evapotranspiration and regional probabilities of soil moisture stress in rainfed crops, southern India

The long-term probability of soil moisture stress in rainfed crops was mapped at 0.5° resolution over the Krishna River basin in southern India (258,948 km²). Measurements of actual evapotranspiration (E_a) from 90 lysimeter experiments at four locations in the basin were used to calibrate a non-linear regression model that predicted the combined crop coefficient (K_cK_s) as a function of the ratio of seasonal precipitation (P) to potential evapotranspiration (E_p). Crops included sorghum, pulses (mung bean, chickpea, soybean, pigeonpea) and oilseeds (safflower and sunflower). E_p was calculated with the Penman–Monteith equation using net radiation derived from two methods: (1) a surface radiation budget calculated from satellite imagery (E_pSRB) and (2) empirical equations that use data from meteorological stations (E_pGBE). The model of K_s as a function P/E_p was combined with a gridded time series of precipitation (0.5° resolution, 1901–2000) and maps of E_pSRB to define the probability distributions of P, P/E_p and K_s for sorghum at each 0.5° cell over the basin. Sorghum, a C4 crop, had higher E_a and K_s values than the C3 plants (oilseeds, pulses) when precipitation was low (P < 1 mm d⁻¹) but lower maximum E_a rates (3.3–4.5 mm d⁻¹) compared with C3 crops (oilseeds and pulses, 4.3–4.9 mm d⁻¹). The crop coefficient under adequate soil moisture (K_c) was higher than the FAO-56 crop coefficients by up to 56% for oilseeds and pulses. The seasonal soil moisture coefficient (K_s) for sorghum ranged from 1.0 under high rainfall (July–October) to 0.45 in dry seasons (November–March), showing strong soil moisture controls on E_a. E_pSRB calculated at the lysimeter stations was 4–20% lower than E_pGBE, with the largest difference in the dry season. K_c derived from E_pSRB was only slightly (2–4%) higher than K_c derived from E_pSRB, because the maximum E_a occurred during the monsoon when the differences between E_pSRB and E_pGBE were small. Approximately 20% of the basin area was expected to experience mild or greater soil moisture stress (K_s < 0.80) during the monsoon cropping season 1 year in every 2 years, while 70% of the basin experienced mild or greater stress 1 year in 10. The maps of soil moisture stress provide the basis for estimating the probability of drought and the benefits of supplemental irrigation.     

Influence of temperature and drought on seasonal and interannual variations of soil, bole and ecosystem respiration in a boreal aspen stand

Continuous half-hourly measurements of soil (R_s) and bole respiration (R_b), as well as whole-ecosystem CO₂ exchange, were made with a non steady-state automated chamber system and with the eddy covariance (EC) technique, respectively, in a mature trembling aspen stand between January 2001 and December 2003. Our main objective was to investigate the influence of long-term variations of environmental and biological variables on component-specific and whole-ecosystem respiration (R_e) processes. During the study period, the stand was exposed to severe drought conditions that affected much of the western plains of North America. Over the 3 years, daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ during winter to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-summer. Seasonal variations of R_s were highly correlated with variations of soil temperature (T_s) and water content (θ) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean R_s from 2001 to 2003. Aspen daily mean R_b varied from negligible during winter to a maximum of 2.5 μmol m⁻² bark s⁻¹ (2.2 μmol m⁻² ground s⁻¹) during the growing season. Maximum R_b occurred at the end of the aspen radial growth increment and leaf emergence period during each year. This was 2 months before the peak in bole temperature (T_b) in 2001 and 2003. Nonetheless, R_b was highly correlated with T_b and this variable explained 77, 87 and 62% of the variance in R_b in the respective years. Partitioning of R_b between its maintenance (R_bm) and growth (R_bg) components using the mature tissue method showed that daily mean R_bg occurred at the same time as aspen radial growth increment during each growing season. This method led, however, to systematic over- and underestimations of R_bm and R_bg, respectively, during each year. Annual totals of R_s, R_b and estimated foliage respiration (R_f) from hazelnut and aspen trees were, on average, 829, 159 and 202 g C m⁻² year⁻¹, respectively, over the 3 years. These totals corresponded to 70, 14 and 16%, respectively, of scaled-up respiration estimates of R_e from chamber measurements. Scaled R_e estimates were 25% higher (1190 g C m⁻² year⁻¹) than the annual totals of R_e obtained from EC (949 g C m⁻² year⁻¹). The independent effects of temperature and drought on annual totals of R_e and its components were difficult to separate because the two variables co-varied during the 3 years. However, recalculation of annual totals of R_s to remove the limitations imposed by low θ, suggests that drought played a more important role than temperature in explaining interannual variations of R_s and R_e.     

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.     

The sensitivity of modeled SO2 fluxes and profiles to stomatal and boundary layer resistances

A higher-order closure model for canopy/surface exchange is presented and applied to an oak-hickory forest to model SO₂ deposition for typical summertime conditions. The model is then used as a tool to investigate the sensitivity of modeled fluxes (the deposition velocity) and concentration profiles to the parameters used to compute the leaf boundary layer and stomatal resistances. Both the deposition velocity and concentration profiles show little sensitivity to variations in the leaf boundary-layer resistance (r _b) since it generally comprises only a small fraction of the total resistance to diffusion from the air to the sub-stomatal cavity. The deposition velocity (V _d ) is more sensitive to variations in the minimum stomatal resistance (r _s ,min) and light response coefficient (β) than r _b .It was found that 50% variations in β give a maximum difference of 0.2 cm s^?1 in V _d while 30% variations in r _s , min produce a maximum difference of near 0.3 cm s^?1.     

ECOWAT—A model for ecosystem evapotranspiration estimation

In recent years, the availability of near real-time and forecast standardized reference evapotranspiration (E₀) has increased dramatically. Use of the E₀ information in conjunction with calibration coefficients that adjust for differences between the vegetation and the reference surface provides a method to greatly improve the estimates of actual evapotranspiration (E_a) from landscapes (or ecosystems). Difficulties in estimating evapotranspiration (ET) of well-watered vegetation in an ecosystem depend on local advection and edge effects, wide variations in radiation resulting from undulating terrain, wind blockage or funnelling, and differences in temperature due to spatial variation in radiation, wind, etc. Estimating the ET of an ecosystem that is water stressed is even further complicated because of stomatal closure and reduced transpiration. The Ecosystem Water Program (ECOWAT) was developed to help improve estimates of E_a of ecosystems by accounting for microclimate, vegetation type, plant density, and water stress. The first step in estimating E_a is to calculate E₀ using monthly climate data from one representative weather station in the study area. Then, local microclimate data are used to determine a standardized reference evapotranspiration for the local microclimate (E_m). The ratio K_m = E_m/E₀ is calculated and applied as a microclimate correction factor to estimate E_m. The product of E_m and a vegetation coefficient (K_v = E_v/E_m) is used to estimate the evapotranspiration of the ecosystem vegetation (E_v) under well-watered conditions with a full-canopy cover within the same microclimate. Next, a coefficient for plant density (K_d), which is based on the percentage ground cover, is used to adjust the full-canopy E_v to the evapotranspiration of a sparse canopy from a well-watered ecosystem (E_w). A stress (K_s) coefficient, which varies between 1.0 with no stress to 0.0 with full stress, is determined as a function of available water in the root zone. The predicted actual ecosystem evapotranspiration (E_p) is estimated as E_p = E_w × K_s. In this paper, we present how the ECOWAT model works and how it performs when the predicted actual evapotranspiration (E_p) is compared with measured actual evapotranspiration (E_a) collected in several Mediterranean ecosystems (three in Italy and two in California) over a number of years. The potential use of ECOWAT in integrated fire danger systems is discussed.     

Latent heat flux estimation in clear sky days over Indian agroecosystems using noontime satellite remote sensing data

A simplified evaporative fraction (Λ) based single-source energy balance scheme was tested with moderate resolution (1 km) noontime satellite observations to evaluate clear sky latent heat flux (λE) estimates over diverse agricultural landscapes. This approach uses two-dimensional (2D) scatter between land surface temperature (LST) and albedo to determine Λ. The operational utility of this scheme was demonstrated for estimating regional evapotranspiration and consumptive water use during rabi (November to April) crop growing season to predict pre-harvest wheat yield (error within 15.9% of reported mean) using time series data. The existence of triangular relations between Λ and LAI (leaf area index) or NDVI (normalized difference vegetation index) was found with basal line (hypotenuse) linearly coupled with LAI or NDVI at low level of surface soil wetness. The analysis of diurnal course of in situ Λ proved the validity of constant-Λ hypothesis over pure, uniform, homogeneous crop canopies but showed irregular and wave-like patterns over heterogeneous, mixed crop canopies. The root mean square error (RMSE) of noontime and daytime average λE estimates with respect to in situ λE measurements were also smaller over homogeneous agricultural canopies (41 and 23 W m⁻²) with correlation coefficients (r) 0.94 and 0.96, respectively, from 135 clear sky datasets as compared to RMSE over heterogeneous ones (59 and 28 W m⁻² with r = 0.66 and 0.82, respectively from 22 datasets). The intercomparison with another Λ based approach (LST–NDVI 2D scatter) showed the supremacy of Λ determined from LST–albedo 2D scatter. The efficiency of LST–NDVI scatter was better during the dry down or water limited phases of crop growth only. The uncertainties of λE estimates were attributed to errors in core radiation budget inputs, relative loss of conservativeness of Λ due to canopy heterogeneity, and the inherent limitations of the single-source approach. There is further scope to reduce present λE uncertainties by combining the new findings on Λ (LST–albedo scatter)–NDVI triangular relations, diurnal Λ and two-source radiation budget.     

Transpiration from coppiced poplar and willow measured using sap-flow methods

Transpiration rates from poplar (Beaupré, Populus trichocarpa×deltoides) and willow (Germany, Salix burjatica) clones, grown as short-rotation coppice (three-year-old stems on four-year-old stools) at a site in south-west England, were measured through the summer of 1995. Area-averaged transpiration was estimated by scaling sap-flow rates measured in individual stems to a stand area basis using measurements of leaf area and stem diameter distribution. Sap flow in poplar was measured using the stem heat balance, heat pulse velocity and deuterium tracing techniques; in willow only the stem heat balance method was used. In June and early July the mean daily transpiration from the poplar was 6±0.5 mm day⁻¹, stomatal conductances averaged 0.33 mol m⁻² s⁻¹ for leaves in the upper layer of the canopy and daily latent heat flux often exceeded the daily net radiation flux. Similarly high transpiration was estimated for the willow. The transpiration rates were higher than any reported rates from agricultural or tree crops grown in the UK and arose because of high aerodynamic and stomatal conductances. The high stomatal conductances were maintained even when atmospheric humidity deficits and soil water deficits were large. Much lower rates (1±1 mm day⁻¹) from both clones were recorded in August at the end of a drought period.These results suggest that extensive plantation of poplar or willow short-rotation coppice will result in reduced drainage to stream flow and aquifer recharge.     

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.     

外源抗坏血酸对臭氧胁迫下水稻光合及生长参数的影响

为研究臭氧浓度变化对水稻生长的影响及外源抗坏血酸的防护作用,在田间原位条件下,利用开顶式气室(OTCs)研究了外源抗坏血酸对O3胁迫下水稻光合及生长参数的影响.结果表明:在O3胁迫下,叶片的光合色素含量、气体交换参数、净同化率、相对生长速率及粒/叶面积(cm2)均显著下降;喷施外源抗坏血酸后,叶绿素a含量、叶绿素a/叶绿素b以及水稻叶片的光合速率、蒸腾速率显著升高,而叶绿素b含量和气孔导度变化不显著;外源抗坏血酸对O3胁迫下水稻的净同化率、相对生长速率及粒/叶面积(cm2)影响比较显著,特别是粒/叶面积(cm2)的提高有利于水稻源、库协调发展,为提高水稻产量和改进品质奠定了物质基础.     

Assessing interrill erosion rate from soil aggregate instability index, rainfall intensity and slope angle on cultivated soils in central Greece

In order to develop a new formula for assessing interrill erosion rate by incorporating the soil aggregate instability index, β, erosion plots at seven sites in central Greece were used to measure interrill erosion rate under natural rainfall conditions during a 39-month period. Soils classified as Alfisols, Inceptisols and Entisols with slopes 7–21%, moderately well to excessively drained, clay to loamy textured, were studied. Runoff and total sediment were collected after each ponding rainfall event. The equation E_i=0.628 β S_t^1.3 e^{0.0967I₃₀} was finally proposed (R²=0.939,P<0.001) to describe interrill erosion rate. The term, S_t represents the tangent of the slope angle, and I₃₀ represents the maximum rainfall intensity in 30 min. The addition of the aggregate instability index to improve existing methodologies provide was considered to provide an easy to determine and reliable measure of soil erodibility. Validation with independent data showed that the model predicted interrill erosion well (R²=0.766, P<0.001). Therefore, the proposed model based on the aggregate instability index, β, has the potential to improved methodology for assessing interrill erosion rate.     

Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO₂ efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration (R_s) on soil temperature (T_s) and water content (θ). Daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ in February to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-July. Daily mean T_s at the 2-cm depth was the primary variable accounting for the temporal variation of R_s and no differences between Arrhenius and Q₁₀ response functions were found to describe the seasonal relationship. R_s at 10 °C (R_s10) and the temperature sensitivity of R_s (Q_10Rs) calculated at the seasonal time scale were 3.8 μmol m⁻² s⁻¹ and 3.8, respectively. Temperature normalization of daily mean R_s (R_sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R_s during the growing season. Daily R_sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (θ_fc, 0.30 m³ m⁻³): (1) R_sN was strongly reduced when θ decreased below θ_fc, which reflected a reduction in microbial decomposition, and (2) R_sN slightly decreased when θ increased above θ_fc, which reflected a restriction of CO₂ or O₂ transport in the soil profile.Diurnal variations of half-hourly R_s were usually out of phase with T_s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R_s10 and Q_10Rs parameters calculated from half-hourly nighttime measurements of R_s and T_s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m⁻² s⁻¹ and 5.5 to 1.3, respectively. On average, daily nighttime R_s10 (4.5 μmol m⁻² s⁻¹) and Q_10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R_s10 and Q_10Rs values at low θ. Overall, the use of seasonal R_s10 and Q_10Rs parameters led to an overestimation of daily ranges of half-hourly R_s (ΔR_s) during drought conditions, which supported findings that the short-term temperature sensitivity of R_s was lower during periods of low θ. The use of daily nighttime R_s10 and Q_10Rs parameters greatly helped at simulating ΔR_s during these periods but did not improve the estimation of half-hourly R_s throughout the year as it could not account for the diurnal hysteresis effect.     

Gas exchanges and chlorophyll fluorescence of young coffee plants submitted to water and nitrogen stresses

Abstract

This research aims to evaluate the impact of nitrogen deprivation and water stress on gas exchange and chlorophyll fluorescence in young plants of five cultivars of Arabic coffee. A factorial experiment 5 (cultivars) × 3 (treatments: control without stress, water stress of ?1.5?MPa and stress of N – 0.0?mmol L^?1 N) was carried out in a complete randomized block design with three replicates. Before being submitted to the treatments, the plants were grown in a greenhouse for 240?days, and then transferred to a growth chamber under controlled conditions. Subsequently, after the experimental period of 96?h we measured photosynthetic rate (A), stomatal conductance to water vapor (g_s), transpiratory rate (E), internal and external carbon ratio (Ci/Ca), water use efficiency (A/E), electron transport rate (ETR), actual quantum yield of PS II electron transport (φFSII), and maximum photochemical efficiency of PS II (Fv/Fm). Water stress reduced A, g_s, E, A/E, ETR, φFSII, and Fv/Fm. The nitrogen deficiency reduced ETR, φFSII, and Fv/Fm. Under short-term water stress Catuaí Vermelho maintain the A values due to better stomatal control, reduced water lost by transpiration (E) and better water use efficiency A/E, while Mundo Novo and Acauã show lower damage to Fv/Fm. Short-term nitrogen stress has low impact on A of young plants of Coffea arabica cultivars with adequate N-nutrition.     

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.     

Influence of Boron Nutrition on Membrane Leakage,Chlorophyll Content and Gas Exchange Characteristics in Cotton (Gossypium Hirsutum L.)

Understanding the effect of boron (B) on plant physiology will help to refine the diagnosis of B deficiency and improvement in B fertilizer recommendations for cotton (Gossypium hirsutum L.) growing areas. This study shows the testing of hypotheses “that application of B-fertilizer improves net photosynthetic rate (P_N) and water use efficiency (WUE) for cotton plant on a B-deficient soil [< 0.50 mg B kg^?1 hydrochloric acid (HCl)-extractable] in an arid environment”. Thus, a permanent layout [two-year field experiment (2004 and 2005)] was conducted to study the impact of B fertilizer at 0, 1.0, 1.5, 2.0, 2.5, and 3.0 kg ha^?1 on gas exchange and electrolyte leakage (EL) characteristics of cotton crop (cv. ‘CIM-473’). The soil at experimental site was alkaline (pH 8.1), calcareous [calcium carbonate (CaCO₃ 5.6%)], and silt loam (Typic Haplocambid). Boron use decreased EL of plant membrane (P ≤ 0.05), and increased P_N, transpiration rate (E) and stomatal conductance (g_s), while intercellular concentration of carbon dioxide (CO₂; C_i) significantly decreased (P ≤ 0.05) during both experimental years. There was a positive, but non-significant effect of B concentration on chlorophyll content in plant leaves. Application of 3.0 kg B ha^?1 improved WUE up to 9.7% [4.62 μmol (CO₂) mmol^?1 water (H₂O)] compared to control plants (4.21 [μmol (CO₂) mmol^?1 (H₂O)]. Principal component analysis (PCA) of data indicates positive correlations between leaf B concentration and P_N, E, g_s, and WUE, while a negative relationship existed between leaf B concentration and intercellular CO₂ (C_i). This study showed that addition of B fertilizer in the B-deficient calcareous soil proved beneficial for growth and development for cotton crop by enhancing its WUE and gas exchange characteristics.     

Water use efficiency in a soybean field: influence of plant water stress

Measurements were made in 1980 over a fully-developed soybean (Glycine max (L.) Merrill) canopy at Mead, Nebraska to determine how crop water status influences photosynthesis, evapotranspiration and water use efficiency. Water use efficiency was calculated in terms of the CO₂—water flux ratio (CWFR). Micrometeorological techniques were used to measure the exchange rates of CO₂ and water vapor above the crop canopy. Crop water status was evaluated by reference to volumetric soil moisture (θ_v), stomatal resistance (r_s), and leaf water potential (ψ) measurements.Stomatal resistance (r_s) was independent of ψ when the latter was greater than ?1.1 MPa. r_s increased sharply as ψ dropped below this threshold. Canopy CO₂ exchange (F_c) decreased logarithmically with increasing r_s under strong irradiance. Although F_c was found to be strongly correlated with r_s, the influence of low values of ψ and of high air temperature cannot be discounted since these factors affect the enzymatic reactions associated with photosynthesis. Stomatal closure also reduced evapotranspiration and influenced the partitioning of net radiation.Under strong irradiance the CO₂ water flux ratio (CWFR) decreased with increasing stomatal resistance. This observation is at variance with predictions of certain early ‘resistance’ models, but substantiates predictions of some recent models in which leaf energy balance considerations are incorporated.     

贵州黄壤小白菜生长、品质、光合特性及氮素利用对新型肥料的响应

为探究不同新型肥料对贵州省酸性黄壤小白菜产量、品质、光合特性及肥料利用的影响,同时筛选出适合贵州黄壤施用的新型肥料产品,以贵州酸性黄壤为基础,通过盆栽试验设置对照(CK,不施氮肥)、西洋复合肥(常规施肥)、保水型功能性肥和稳定性缓释肥4个处理,研究了新型肥料对小白菜产量、品质、光合特性以及养分吸收利用的影响。结果表明:施用保水型功能性肥和稳定性缓释肥可显著增加小白菜播种后34 d的生物量,较常规施肥处理相比鲜重分别增加4.16%和22.28%,干重分别增加41.55%和62.35%;施用新型肥料还可以改善小白菜的营养品质,与常规施肥处理相比,保水型功能性肥可显著降低硝酸盐含量18.61%,而还原性糖、V_c和游离氨基酸含量分别增加25.74%、130.95%和16.91%;而稳定性缓释肥则使硝酸盐、还原糖和Vc含量分别提高26.68%、15.35%和50.00%,但是游离氨基酸含量则较常规施肥相比降低14.43%;而且新型肥料还增强了小白菜叶片的光合能力(净光合速率Pn、气孔导度gs、胞间CO_2浓度Ci以及蒸腾速率Tr),其中以稳定性缓释肥处理的小白菜光合能力最佳,且气孔因素是导致净光合速率增加的主要原因。施用新型肥料小白菜对氮素的吸收显著增加,氮肥利用效率显著提高,新型肥料处理的氮肥农学效率(AEN)、偏生产力(PFPN)、生理利用率(PE_N)和表观利用率(REN)平均分别为48.30 kg·kg~(-1)、59.85 kg·kg~(-1)、95.46 kg·kg~(-1)和52.79%,以稳定性缓释肥处理的氮肥利用效率最佳,尤其是氮肥表观利用率达66.66%。此外,相关性分析结果显示,小白菜产量与叶片净光合速率P_n、气孔导度g_s以及蒸腾速率T_r均呈显著正相关关系,说明提高小白菜叶片的气体交换参数P_n、g_s和T_r可以增加小白菜产量;同时小白菜叶片氮含量与氮肥生理利用率和氮肥表观利用率存在极显著相关性,r值分别为-0.937和0.978,表明增加小白菜叶片氮含量可以提高小白菜对氮肥的利用效率。综上所述,新型肥料对贵州酸性黄壤上小白菜的生物增产效应以及光合特性提高等效果显著,可为将来在贵州农业生产中推广应用提供参考和理论依据。