Influence of water and soil salinity on emergence and early development of potato (Solanum tubersum L.) cultivars and effect of physiological age of seed tubers

Summary Applying saline solutions (EC 3.2 up to 7.0 dS/m) to seed tubers of different cultivars planted in tuff delayed shoot emergence and root and shoot development. Fresh weight of most cultivars was decreased by 60% but cv. Serrana Inta was more tolerant to salinity and fresh weight was decreased by 20%. When planted in 500 cm³ pots containing loessial soil some seed tubers rotted and plant growth was severely inhibited, possibly because soil aeration was impeded. Physiologically ageing seed tubers for 33 days at 6°C decreased sensitivity of most cultivars to moderate salinity whereas ageing for 68 days increased sensitivity to high salinity. Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 2977-E, 1990 series.     

A vegetation reconstruction method to plant <Emphasis Type="Italic">Sedum spectabile</Emphasis> Boreau using drip-irrigation with saline water on a coastal saline soil in region around Bohai Gulf

Soil salinity and saline groundwater are major constraints to the cultivation of crops and landscape plants in coastal regions. With the rapid industrialization and urbanization in these areas, there is an urgent need to improve the landscape to meet the increasing demand of living environments for cities and districts. The aim of this study was to propose a method to plant Sedum spectabile Boreau, a common landscape flower plant, on a very heavy coastal saline soil using drip-irrigation with saline water in region around Bohai Gulf. The salinity levels of irrigation water were 0.8, 3.1, 4.7, 6.3a, nd 7.8 dS/m, respectively. The results showed that the revegetation method, which mainly included ‘ridge planting + saline water drip-irrigation’, was effective in planting Sedum spectabile Boreau for reclaiming coastal saline silt soil. An soil matric potential (SMP) higher above ?5 kPa after transplanted and ?10 kPa after growing season ended, and 6 mm of irrigation water can be used as indicators for Sedum spectabile Boreau drip-irrigation scheduling with 100 % survival rate when irrigated with saline water at <7.8 dS/m in initially saline soils with a gravel–sand layer after tillage. This method is combined with comprehensive utilization of the saline water, agronomic measures, collaborative repair of the soil and plants to offer new views and theoretical support for the protection and development of saline land in coastal regions.     

Simulation for response of crop yield to soil moisture and salinity with artificial neural network

In saline fields, irrigation management often requires understanding crop responses to soil moisture and salt content. Developing models for evaluating the effects of soil moisture and salinity on crop yield is important to the application of irrigation practices in saline soil. Artificial neural network (ANN) and multi-linear regression (MLR) models respectively with 10 (ANN-10, MLR-10) and 6 (ANN-6, MLR-6) input variables, including soil moisture and salinity at crop different growth stages, were developed to simulate the response of sunflower yield to soil moisture and salinity. A connection weight method is used to understand crop sensitivity to soil moisture and salt stress of different growth stages. Compared with MLRs, both ANN models have higher precision with RMSEs of 1.1 and 1.6 t ha⁻¹, REs of 12.0% and 17.3%, and R² of 0.84 and 0.80, for ANN-10 and ANN-6, respectively. The sunflower sensitivity to soil salinity varied with the different soil salinity ranges. For low and medium saline soils, sunflower yield was more sensitive at crop squaring stage, but for high saline soil at seedling stage. High soil moisture content could compensate the yield decrease resulting from salt stress regardless of salt levels at the crop sowing stage. The response of sunflower yield to soil moisture at different stages in saline soils can be understood through the simulated results of ANN-6. Overall, the ANN models are useful for investigating and understanding the relationship between crop yield and soil moisture and salinity at different crop growth stages.     

Equivalent cropping area and whole farm water balance approaches to reduce net recharge to shallow saline groundwater from rice based cropping systems

Sustainability of irrigated agriculture in the semi-arid regions such as the rice growing regions of the southern Australia could be under serious threat due to recharge to saline groundwater leading to secondary salinisation. This paper presents two approaches to reduce net recharge to shallow saline groundwater from ponded rice cropping systems using a soil, water and groundwater management model–SWAGMAN model. First approach applies the concept of growing deep-rooted crop (crop consuming water from soil and shallow water table) over an area equivalent to recharge from a unit area of rice crop to maintain the total water balance. The second approach involves achieving whole farm water balance for average and wet climatic conditions to find out optimum cropping pattern to minimise recharge from rice based system. Results of both approaches are sensitive to water table depth and regional groundwater outflow rates. The first approach was applied to determine an optimal mix of rice/lucerne crop to keep water tables and average soil salinity changes less than 0 m/year and 0.015 dS/m, respectively. The area of lucerne required to match the net groundwater rise caused by 1 ha of rice is 1.27, 2.45 and 3.55 ha for 1, 0.5 and 0.2 ML/ha regional groundwater outflow capacity. Results from second approach revealed that it is possible to control net recharge under above average rainfall using a suitable whole farm cropping mix e.g. 50 ha rice–wheat rotation, 19 ha rice, 25 ha of lucerne and 136 ha of fallow.     

Alkaline phosphatase activity and its relationship to soil properties in a saline–sodic soil reclaimed by cropping wolfberry (Lycium barbarum L.) with drip irrigation

In order to ascertain the alkaline phosphatase (ALP) activity and its relationship with soil properties in saline–sodic soils during reclamation, a study was conducted in a saline–sodic soil reclaimed by cropping wolfberry (Lycium barbarum L.) with drip irrigation, in Ningxia Plain, Northwest China. The soil EC_e, pH and SAR in 0–30 cm were 12.3 dS m^?1, 9.4 and 44.1 (mmol/L)^0.5, respectively. Soil transects with different planting years were intensively sampled, which had a wide gradient of salinity and sodicity. Ranging from 1.1 to 42.4 μg g^?1 h^?1, soil ALP activities increased with the increasing planting years, and showed a large spatial variability within transect. The higher ALP activities were always found beneath the drip emitters. More soil physicochemical properties became related significantly to the ALP activities as the planting years increased, indicating that the ALP activities could be better predicted by other properties after reclamation. Path analyses showed that the negative direct effects of soil pH on ALP activities became clearly dominant as the planting years increased. The positive effects of organic matter on ALP activities exerted indirectly, mainly through pH, total N, and available P. Soil ALP activities decreased exponentially with pH, which varied from 7.38 to 10.00. Our findings demonstrated that soil pH was the limiting factor for improving soil ALP activities in this saline–sodic soil, and after three planting years, soil biological activities and fertility level increased significantly.     

Effect of soil salinity on the wheat and bean root respiration rate at low matric suctions

Water logging and salinity often occur together because rising water table brings salt to the surface. We studied the effects of a range of low soil matric suctions (or nearly paddy condition) (2–33 kPa) and salinity (EC = 0.7–8 dS m^?1 for bean and 2–20 dS m^?1 for wheat) on the root respiration (Rr) in two sandy loam and clay loam soils at greenhouse condition. Results showed that the aeration porosity mainly controls Rr especially at 2 kPa matric suction. As matric suction increases, soil aeration rises and consequently the Rr reaches maximum values (7.9 μmol m^?3 s^?1 for bean and wheat) at 6 and 10 kPa suctions in clay loam and sandy loam soils, respectively. Using a mechanistic soil respiration model reveals that these matric suctions, h, are corresponded to the aeration porosities of 0.18 m³ m^?3 in sandy loam and 0.16 m³ m^?3 in clay loam soils. Bean and wheat Rr remains nearly constant at higher suctions (h > 10 kPa) in sandy loam and decreases slightly in clay loam soil. Gas diffusivity and the root surface area may explain the variation of the Rr between the sandy loam and the clay loam soils. Results showed that the salinity (EC = 6–8 dS m^?1 for bean and EC = 16–20 dS m^?1 for wheat) amplifies the effect of aeration stress at 2 kPa matric suction in both soils. We also observed a strong correlation between root surface area, Rs, and the Rr for all experiments. We concluded that the aeration deficit is not only major factor determining differential plant respiration under adverse stress conditions, and the salinity has a pronounced impact on differences in crop physiological responses.     

The development of a coupled model (PCPF-SWMS) to simulate water flow and pollutant transport in Japanese paddy fields

A new coupled model (PCPF–SWMS) was developed for simulating fate and behavior of pollutant in paddy water and paddy soil. The model coupled the PCPF-1, a lumped model simulating pesticide concentrations in paddy water and 1 cm-surface sediment compartment, and the SWMS-2D, a finite element numerical model solving Richard's and advection-dispersion equations for solute transport in soil compartment. The coupling involved improvements on interactions of the water flow and the concentration the pollutant of at the soil interface between both compartments. The monitoring data collected from experimental plots in Tsukuba, Japan in 1998 and 1999 were used to parameterise and calibrate hydraulic functioning, hydrodynamic and hydrodispersive parameters of the paddy soil. The analysis on the hydraulic functioning of paddy soil revealed that the hard pan layer was the key factor controlling percolation rate and tracer transport. Matric potential and tracer monitoring highlighted the evolution of saturated hydraulic conductivity (K _S) of hard pan layer during the crop season. K _S slightly decreased after puddling by clay clogging and strongly increased after mid term drainage by drying cracks. The model was able to calculate residential time in every soil layers. Residential time of tracer in top saturated layers was evaluated to be less than 40 days. It took 60 days to reach the unsaturated layers below hardpan layer.     

Effect of saline irrigation and water deficit on tuber quality

Summary The effect of salinity and water dificit on the quality of tubers for processing was investigated. Total tuber yield was not affected by the treatments, while the percentage of non-marketable tubers was significantly reduced by high salinity (EC_i=6dS m⁻¹) and by water reduction. Accumulation of dry matter in the tubers was increased by all the treatments, that of proline by salinity only and the content of reducing sugars was increased only by water deficit. The colour of the french fries was similar in tubers from the various treatments.     

Effect of soil salinity on internal browning of potato tuber tissue in two soil types

A study was carried out with potato (Solanum tuberosum; cv. Atlantic) during 2001 and 2002 to determine the effect of soil salinity on internal tuber browning. The effect of varying levels of soil salinity on proline content, polyphenol oxidase enzyme activity, and chlorogenic acid content in potato leaves and tubers was examined. NaCl treatments (2.1, 4.25, 6.38, 8.5 g NaCl L^?1) were applied to the pots, the first 46 days after planting, and four additional treatments were applied, each about 7 days apart. Increasing NaCl concentrations resulted in an increase in browning of tuber tissue and proline content in the tubers. Chlorogenic acid content in the leaves increased up to 6.4 g NaCl L^?1, but then decreased at 8.5 g NaCl L^?1 and in tubers tended to be maximal at the highest saline concentration tested (8.5 g NaCl L^?1). Increasing NaCl concentration resulted in a reduction in yield per plant and average tuber weight, and also increased tuber number. There were major differences in the impact of salinity over the 2-year period, which was probably due to the impact of the growing media; a low organic matter (about 1% OM) silty loam soil and a high organic content (about 90% OM) Muck soil were used in 2001 and 2002, respectively. Tuber browning increased linearly with salinity in 2002, but only markedly increased at 8.5 g NaCl L^?1 in 2001. Sodium and chloride ion concentration was always greater (about two times) at equivalent application rates in the 2002 trial. The high organic matter content soil retained sodium and chloride ions more effectively than the silty loam soil and enhanced the impact of increased salinity concentration on physiological properties of potato plants and particularly on tuber tissue browning and proline accumulation.     

Soil Salinity in South India: Problems and Solutions

SUMMARY

Soil salinity is assuming menacing proportions for production of agricultural and horticultural crops in South India. South India comprises of Andhra Pradesh (AP), Tamil Nadu (TN), Karnataka, Kerala, Goa, and Islands in Bay of Bengal (Andaman and Nicobar) and Arabian Sea (Lakshadweep). It comprises central uplands, Deccan plateau (Karnataka plateau and Telangana plateau of AP), Nilgiri hills of TN, South Sahiailri, Eastern hills (Eastern Ghats, TN, upland) and Coastal Plains. The rainfall ranges from 400 to 500 mm in AP, 450 to 1300 mm in Karnataka, 500 to 1215 mm in TN, 100 to 450 mm in Kerala. Climate is mainly semi-arid in nature. Red soils (Alfisols, Inceptisols, and Entisols) make up about 60–65% and are well drained, blacksoils (Vertisols and Inceptisols) comprise about 20–25% and the rest are coastal sands. Soil salinity problems are encountered in almost all the districts in Karnataka and AP. The area extends to about 0.176 million ha in AP, 0.2 million ha in Karnataka, 0.0427 million ha in TN and about 0.03 million ha in Kerala. Introduction of canal irrigation water and use of underground saline waters accelerated the appearance of salt affected soils. Soil salinity observed in South India ranges from hydrometphic saline soils in Kerala to highly saline (EC 20–30 dS m^?1) in Karnataka and AP. Saline soils were classified in to Natrargids or Solarthrids at subgroup level depending on the occurance of nitric or salic horizon within few centimetres of the surface. Soil salinity has reduced crop yields upto 50% and consequently cropping has been abandoned in many areas. Excess soluble salts can be removed through scrapping the surface salt crust or flushing and leaching or through subsurface drainage depending on the problem. Crops also vary in their ability to tolerate salinity at different stages of growth. In most crops subjected to irrigations with saline waters, germination and early seedling stages are generally the most sensitive and their tolerance increases with age. Salt tolerant varieties and nutrient management of crops in saline soils are important solutions for crop production under saline soils.     

Effect of saline soil on grain quality of rices differing in salinity tolerance

Four varieties of rice, differing in salinity tolerance and grown in saline soil (electrical conductivity 5–6 dS/m) at Sadhoke, Punjab, Pakistan, had lighter grain and higher Na content than control samples. Grains of three out of the four rices grown on saline soils had higher brown rice protein (higher nutritional value), less translucent grain, lower starch and amylose content, and lower K than their control samples, but these differences were not related to salinity tolerance. Alkali spreading value and gel consistency were not affected by culture in saline soil. Cooked rice Instron hardness increased in saline culture in two higher-protein samples of the four rices. Amylograph peak viscosity was suppressed by saline culture.     

Early plastic mulching increases stand establishment and lint yield of cotton in saline fields

Row covering with polyethylene film (plastic mulching) is a common practice for improving emergence, plant growth and yield of cotton in China. This is usually applied after sowing (conventional mulching, CM), but pre-sowing evaporation in spring would cause accumulation of salts and moisture loss in the surface layer of saline soils. Two experiments were conducted in Yellow River delta from 2004 to 2005 and during 2006, respectively to determine if row covering with plastic film 30 d before sowing (early mulching, EM) supports better productivity of cotton than CM in saline fields. In the first experiment, we studied the effects of EM versus CM and no-mulching (NM), on soil microclimate, seedling physiology, cotton yield and earliness. The second experiment was conducted in seven sites to compare cotton yield between the two mulching systems. Results from the first experiment showed that both EM and CM could effectively improve stand establishment, plant growth, earliness and lint yield of cotton relative to NM control. However, compared with CM, EM increased stand establishment rate by 11.4% and plant biomass by 9.9% and lint yield by 7.1%. EM, relative to CM and NM, increased the photosynthesis (Pn) rate 6.8% and decreased malondialdehyde (MDA) concentration 7.4% and Na⁺ level 8%. These improvements were due mainly to delayed accumulation of salts, elevation of soil temperature and reduction of moisture loss with EM. The revenue from EM was higher than that from CM and NM, suggesting the increased yield by EM was enough compensate for additional costs. The multi-site experiment in 2006 showed that the yield advantage of EM over CM was not significant in two sites with lower salinity (ECe = ∼6 dS/m), but substantial (from 9 to 14%) in five sites with higher salinity (ECe = 10–12 dS/m). The overall results suggest that EM is a promising cotton production technique in the saline Yellow River Delta and other cotton-growing areas with similar ecologies.     

Comparison of methanotrophic bacteria, methane oxidation activity, and methane emission in rice fields fertilized with anaerobically digested slurry between a fodder rice and a normal rice variety

Methane oxidation activity (MOA), methanotrophic bacteria (MOB), and CH₄ emission in a paddy field fertilized with anaerobically digested slurry were compared between two varieties: a fodder rice variety Leaf star (LS) and a normal rice variety Kinuhikari (KN). Average MOA and MOB per unit dry root were significantly higher in KN (7.1?μmol?g^?1 CH₄?g^?1 dry root h^?1 and 3.7?×?10⁷ MPN?g^?1 dry root, respectively) than in LS (4.3?μmol?g^?1 CH₄?g^?1 dry root h^?1, 2.1?×?10⁷ MPN?g^?1 dry root), although MOA in the rice root per whole plant was not significantly different since root biomass of LS (1.5?g dry root plant^?1) was significantly higher than that of KN (1.2?g dry root plant^?1). MOA in the soil ranged from 0.031 to 0.087?μmol?g^?1 CH₄?g^?1 dry soil h^?1, but there were no significant differences in both depths (0–5 and 5–15?cm) between the two rice varieties. MOA in the rhizosphere soil was significantly different between the rice varieties at flowering, but not at tillering. Methane emission in the field was lower and MOA and MOB in the rice root were higher in LS than in KN at tillering, but the reverse results were found at flowering and maturity stages. Total CH₄ emission during a growing period was not significantly different between LS (27.4?±?16.9?g CH₄?m^?2) and KN (24.0?±?19.5?g CH₄?m^?2). There was a significant negative relationship between CH₄ emission and rice root MOB (P?<?0.01) or MOA (P?<?0.05) and significant positive relationship was observed between root MOA and MOB (P?<?0.01). This study revealed that choice of rice variety might be an important environmental issue in paddy cultivation since it can influence MOA and MOB in rice root and rhizosphere soil which relate with CH₄ emission.     

Effect of water management on soil respiration and <Emphasis Type="Italic">NEE</Emphasis> of paddy fields in Southeast China

Water management is an important factor in regulating soil respiration and the net ecosystem exchange of CO₂ (NEE) between croplands and atmosphere. However, how water management affects soil respiration and the NEE of paddy fields remains unexplored. Thus, a 2-year field experiment was carried out to study the effects of controlled irrigation (CI) during the rice season on the variation of soil respiration and NEE, with flooding irrigation (FI) as the control. A decrease of irrigation water input by 46.39% did not significantly affect rice yield but significantly increased irrigation water use efficiency by 0.99 kg m^?3. The soil respiration rate of CI paddy fields was larger than that of FI paddy fields except during the ripening stage. Natural drying management during the ripening stage resulted in a significant increase of the soil respiration rate of the FI paddy fields. Variations of NEE with different water managements were opposite to soil respiration rates during the whole rice growth stages. Total CO₂ emission of CI paddy fields through soil respiration (total R _soil) increased by 11.66% compared with FI paddy fields. The increase of total R _soil resulted in the significant decrease of total net CO₂ absorption of CI paddy fields by 11.57% compared with FI paddy fields (p < 0.05). There were inter-annual differences of soil respiration and the NEE of paddy fields. Frequent alternate wetting and drying processes in the CI paddy fields were the main factors influencing soil respiration and NEE. CI management slightly enhanced the rice dry matter amount but accelerated the consumption and decomposition of soil organic carbon and significantly increased soil respiration, which led to the decrease of net CO₂ absorption. CI management and organic carbon input technologies should be combined in applications to achieve sustainable use of water and soil resources in paddy fields.     

Determination of the magnitudes and values for groundwater recharge from Taiwan's paddy field

Flooded paddy fields have many functions, including not only rice production, and ecological and environmental conservation. This work estimates the extent of paddy field infiltration in Taiwan by adopting a one-dimensional Darcy-based soil/water balance model SAWAH (Simulation Algorithm for Water Flow in Aquatic Habitats). A 10 cm thick plow sole layer with a hydraulic conductivity of 0.03 cm/day, coupled with the soil texture and irrigation data obtained from 15 irrigation associations, is used to estimate the volumetric amount of annual infiltration in Taiwan. Simulation results from SAWAH indicate that the plow sole layer controls the movement of infiltrated water, with a rate about 1.8 billion cubic meters annually. The estimated infiltration rate of 1.8 billion m³/yr comprises more than 40% of the annual infiltration recharge to ground water in Taiwan. Additionally, the amount of infiltration recharge to groundwater is equivalent to 20 billion Taiwan dollars NT$ (or 0.65 billion US$) while the yearly rice crop production is 35 billion NT$ (or 1.13 billion US$). It is evident that the infiltration from rice paddy is of great importance to the economy, environment, and water resources conservation in Taiwan.     

Effect of drip irrigation on soil nutrients changes of saline–sodic soils in the Songnen Plain

A field experiment was carried out to research the changes and spatial distributions of soil nutrients in saline–sodic soil for different number of cultivated years under drip irrigation. The distributions of available potassium (AK), available phosphorus (AP), nitrate nitrogen (NO₃ ^?–N), ammonium nitrogen (NH₄ ⁺–N), as well as the amount of total nitrogen (TN), total phosphorus (TP) and organic carbon (OC) in the 0–40 cm soil layers in saline–sodic soils planted with Leymus chinensis for 1, 2, and 3 years were studied. The results showed that the distance from the emitter had an obvious effect on soil nutrients. Drip irrigation had substantial effects on levels of AK, AP, and NO₃ ^?–N. The contents of AK, AP, and NO₃ ^?–N were very high in the area near the emitter in the horizontal direction. In the vertical direction, levels of all of the available and total soil nutrients decreased with increased soil depth. Levels of AK, AP, NO₃ ^?–N, NH₄ ⁺–N, TN, TP, and OC all increased with continued cultivation of crops on saline–sodic soil using drip irrigation. Compared to the nutrients found in soils from the natural L. chinensis grasslands, the contents of AK and TP were higher in the drip-irrigated soils, although the contents of AP, NO₃ ^?–N, and NH₄ ⁺–N were broadly comparable. Given the rate of improvements in nutrient levels, we forecast that the nutrients in drip-irrigated saline–sodic soils should match those of the natural L. chinensis grasslands after 3–6 years of cultivation.     

Response of Sunflowers to Quantities of Irrigation Water,Irrigation Regimes and Salinities in the Water and Soil

Summary

The production of sunflower grains for roasting was investigated in two soil types under different quantities of applied saline and non-saline irrigation water, different irrigation managements, soil salinity due to previous use of saline water or due to a raised water table. It was shown in one experiment, conducted in a loess type soil, that sunflowers extracted water at least to a soil depth of 120 cm, when the available water from the top layers was used up. The crop in this soil consumed all the available soil water from nearly the entire root zone, while in the clay soil limited water was consumed from deep layers, due to the high salinity and lack of aeration.

No decrease in yield was found in the loess soil when 75% of the full amount of water (which was 0.8 of Class A pan evaporation rate) was applied. When only 50% was applied a significant decrease in yield was obtained. In contrast, in the clay soil even 75% of the full amount of water decreased the yield remarkably. Under dry-land conditions approximately 65% of maximum yield was found in the loess soil but only 45% in the clay soil. These differences are all attributed to a shallow active root system in the clay soil. Residual soil salinity from previously use of saline water had no effect on grain production in the loess soil, while saline irrigation water applied during the irrigation season decreased production, but only when water supply was not rate limiting. The combination of saline water and residual soil salinity had a marked effect on the decrease of grain yield under limited irrigation. In both soils a reduction in the amount of water applied per single irrigation and maintaining the entire irrigation period caused a significantly smaller decrease in yield than shortening the irrigation period and applying the full demand.     

The effect of increasing NaCl in irrigation water on growth,gas exchange and yield of tobacco Burley type

Despite the economic importance of tobacco, there is limited field study on the quantitative response of growth and yield to increasing soil salinity. The effects of irrigation with saline water on yield components of field-grown tobacco (Nicotiana tabacum L.) “Burley” type plants were studied over two growing seasons. Growth, dry matter partitioning and gas exchange were measured either in rainfed or fully irrigated plants growing in a clayey–sandy–loam soil. The four fully irrigated treatments received amounts of saline waters at 0.54, 2.5, 5.0 or 10 dS m⁻¹ electrical conductivity (EC_w) equal to crop evapotranspiration. In both years, the electrical conductivity of the saturation phase (EC_e) across the 0.6 m topsoil profile increased with increasing salinity of the irrigation water. Soil moisture was markedly lower in the rainfed treatment than in fully irrigated treatments. Different saline concentrations of irrigation water had virtually no effect on soil moisture. Carbon assimilation rate, stomatal conductance and water use efficiency of the saline treatments were lower than the fully irrigated plants at 0.54 dS m⁻¹ (NW treatment) in 1996, but not in 1997. Transpiration rates were unaffected by salinity in both years. The highest yield was produced by plants irrigated with good quality water. The number of leaves per unit land area was greater for the NW plants, whereas there were no differences between the other four treatments. Salinity decreased plant dry matter and height at harvest, increased dry matter partitioning into leaves and decreased that into stems in both years. Dry matter partitioning to leaves was also greater for the rainfed plants than for the NW plants. Tobacco plants grown under field conditions showed a maximum reduction of relative yield at the highest salinity level of only 31%. The threshold values (0.56 and 0.96 dS m⁻¹) and the EC_e at which a 10% yield reduction was obtained (3.12 and 2.55 dS m⁻¹) calculated from the linear model of response of relative yield to increasing EC_e were typical of moderately sensitive crops. The EC_e values at which 50% yield was reduced (13.34 and 8.91 dS m⁻¹) were indicative of moderate tolerance to salinity.     

Assessment of soil enzyme activities of saline–sodic soil under drip irrigation in the Songnen plain

A field experiment was carried out to research the changes and spatial distributions of soil enzyme activities in saline–sodic soil for a different number of cultivated years under drip irrigation. The distributions of alkaline phosphatase, urease, and sucrase activities within 40 cm in both horizontal and vertical directions of the emitter in saline–sodic soils planted with Leymus chinensis for 1st, 2nd, and 3rd year were studied. A mathematical method was used to determine the relationships between soil enzyme activities and soil environmental factors contain the electrical conductivity of saturated-soil extract, pH value, available nutrient, and organic carbon. Alkaline phosphatase, urease, and sucrase activities all increased with cultivated years in saline–sodic soil under drip irrigation: from 4.5, 1.39 and 19.39 to 20.25, 3.17, and 61.33 μg g^?1 h^?1, respectively, after planting L. chinensis for 3 year. Alkaline phosphatase, urease, and sucrase activities all decreased with increased horizontal and vertical distance from the emitter. After 3 year of drip irrigation, the correlations between soil enzyme activities and soil environment factors had stronger correlations than in the unreclaimed land. After 4–6 years, the soil enzyme activities should attain the level of the natural L. chinensis grassland.     

Effects of temperature and soil moisture on gross nitrification and denitrification rates of a Chinese lowland paddy field soil

Alternate wetting and drying (AWD) irrigation is widely adopted to save water in rice production. AWD practice shifts lowland paddy fields from being continuously anaerobic to being alternately anaerobic and aerobic, thus affecting nitrogen (N) transformations in paddy field soils. Using the barometric process separation technique, a large number of soil cores sampled from lowland paddy field soil profiles were measured for gross nitrification and denitrification rates under different temperature and soil moisture conditions. The gross nitrification and denitrification rates vary with rice growth stages and range between 1.18–30.8 and 0.65–13.54 mg N m^?3 h^?1, respectively. Results indicate that both gross nitrification and denitrification rates increased with the increase in temperature in all three studied soil layers. Gross nitrification rates significantly decrease with increasing soil moisture while denitrification rates increase, and different soil layers demonstrated different rates of variation to the increase in soil moisture. Gross nitrification rates in the cultivated horizon layer decreased more sharply with the increase in soil moisture. High soil water content is favorable to denitrification of all soil layers.