Identification of nitrate leaching hot spots in a large area with contrasting soil texture and management

Identification of nitrate (NO₃) leaching hot spots is important in mitigating environmental effect of NO₃. Once identified, the hot spots can be further analyzed in detail for evaluating appropriate alternative management techniques to reduce impact of nitrate on groundwater. This study was conducted to identify NO₃ leaching hot spots in an approximately 36,000 ha area in Serik plain, which is used intensively for agriculture in the Antalya region of Southern Turkey. Geo-referenced water samples were taken from 161 wells and from the representative soils around the wells during the period from late May to early June of 2009. The data were analyzed by classical statistics and geostatistics. Both soil and groundwater NO₃-N concentrations demonstrated a considerably high variation, with a mean of 10.2 mg kg⁻¹ and 2.1 mg L⁻¹ NO₃-N for soil and groundwater, respectively. The NO₃-N concentrations ranged from 0.01 to 102.5 mg L⁻¹ in well waters and from 1.89 to 106.4 mg kg⁻¹ in soils. Nitrate leaching was spatially dependent in the study area. Six hot spots were identified in the plain, and in general, the hot spots coincided with high water table, high sand content, and irrigated wheat and cotton. The adverse effects of NO₃ can be mitigated by switching the surface and furrow irrigation methods to sprinkler irrigation, which results in a more efficient N and water use. Computer models such as NLEAP can be used to analyze alternative management practices together with soil, aquifer, and climate characteristics to determine a set of management alternatives to mitigate NO₃ effect in these hot spot areas.     

Carbon and nitrogen cycling in a tropical Brazilian soil cropped with sugarcane and irrigated with wastewater

Carbon (C) and nitrogen (N) dynamics in agro-systems can be altered as a consequence of treated sewage effluent (TSE) irrigation. The present study evaluated the effects of TSE irrigation over 16 months on N concentrations in sugarcane (leaves, stalks and juice), total soil carbon (TC), total soil nitrogen (TN), NO₃⁻-N in soil and nitrate (NO₃⁻) and dissolved organic carbon (DOC) in soil solution. The soil was classified as an Oxisol and samplings were carried out during the first productive crop cycle, from February 2005 (before planting) to September 2006 (after sugarcane harvest and 16 months of TSE irrigation). The experiment was arranged in a complete block design with five treatments and four replicates. Irrigated plots received 50% of the recommended mineral N fertilization and 100% (T100), 125% (T125), 150% (T150) and 200% (T200) of crop water demand. No mineral N and irrigation were applied to the control plots. TSE irrigation enhanced sugarcane yield but resulted in total-N inputs (804-1622 kg N ha⁻¹) greater than exported N (463-597 kg N ha⁻¹). Hence, throughout the irrigation period, high NO₃⁻ concentrations (up to 388 mg L⁻¹ at T200) and DOC (up to 142 mg L⁻¹ at T100) were measured in soil solution below the root zone, indicating the potential of groundwater contamination. TSE irrigation did not change soil TC and TN.     

Nitrate leaching in a silage maize field under different irrigation and nitrogen fertilizer rates

Quantification of the interactive effects of nitrogen (N) and water on nitrate (NO₃) loss provides an important insight for more effective N and water management. The goal of this study was to evaluate the effect of different irrigation and nitrogen fertilizer levels on nitrate-nitrogen (NO₃-N) leaching in a silage maize field. The experiment included four irrigation levels (0.7, 0.85, 1.0, and 1.13 of soil moisture depletion, SMD) and three N fertilization levels (0, 142, and 189 kg N ha⁻¹), with three replications. Ceramic suction cups were used to extract soil solution at 30 and 60 cm soil depths for all 36 experimental plots. Soil NO₃-N content of 0-30 and 30-60-cm layers were evaluated at planting and harvest maturity. Total N uptake (NU) by the crop was also determined. Maximum NO₃-N leaching out of the 60-cm soil layer was 8.43 kg N ha⁻¹, for the 142 kg N ha⁻¹ and over irrigation (1.13 SMD) treatment. The minimum and maximum seasonal average NO₃ concentration at the 60 cm depth was 46 and 138 mg l⁻¹, respectively. Based on our findings, it is possible to control NO₃ leaching out of the root zone during the growing season with a proper combination of irrigation and fertilizer management.     

Simulation of transpiration, drainage, N uptake, nitrate leaching, and N uptake concentration in tomato grown in open substrate

Free-drainage or “open” substrate system used for vegetable production in greenhouses is associated with appreciable NO₃⁻ leaching losses and drainage volumes. Simulation models of crop N uptake, N leaching, water use and drainage of crops in these systems will be useful for crop and water resource management, and environmental assessment. This work (i) modified the TOMGRO model to simulate N uptake for tomato grown in greenhouses in SE Spain, (ii) modified the PrHo model to simulate transpiration of tomato grown in substrate and (iii) developed an aggregated model combining TOMGRO and PrHo to calculate N uptake concentrations and drainage NO₃⁻ concentration. The component models simulate NO₃⁻-N leached by subtracting simulated N uptake from measured applied N, and drainage by subtracting simulated transpiration from measured irrigation. Three tomato crops grown sequentially in free-draining rock wool in a plastic greenhouse were used for calibration and validation. Measured daily transpiration was determined by the water balance method from daily measurements of irrigation and drainage. Measured N uptake was determined by N balance, using data of volumes and of concentrations of NO₃⁻ and NH₄⁺ in applied nutrient solution and drainage. Accuracy of the two modified component models and aggregated model was assessed by comparing simulated to measured values using linear regression analysis, comparison of slope and intercept values of regression equations, and root mean squared error (RMSE) values. For the three crops, the modified TOMGRO provided accurate simulations of cumulative crop N uptake, (RMSE = 6.4, 1.9 and 2.6% of total N uptake) and NO₃⁻-N leached (RMSE = 11.0, 10.3, and 6.1% of total NO₃⁻-N leached). The modified PrHo provided accurate simulation of cumulative transpiration (RMSE = 4.3, 1.7 and 2.4% of total transpiration) and cumulative drainage (RMSE = 13.8, 6.9, 7.4% of total drainage). For the four cumulative parameters, slopes and intercepts of the linear regressions were mostly not statistically significant (P < 0.05) from one and zero, respectively, and coefficient of determination (r²) values were 0.96-0.98. Simulated values of total drainage volumes for the three crops were +21, +1 and −13% of measured total drainage volumes. The aggregated TOMGRO-PrHo model generally provided accurate simulation of crop N uptake concentration after 30-40 days of transplanting, with an average RMSE of approximately 2 mmol L⁻¹. Simulated values of average NO₃⁻ concentration in drainage, obtained with the aggregated model, were −7, +18 and +31% of measured values.     

Simulation of bromide and nitrate leaching under heavy rainfall and high-intensity irrigation rates in North China Plain

Heavy rainfall and irrigations during the summer months in the North China Plain may cause losses of nitrogen because of nitrate leaching. The objectives of this study were to characterize the leaching of accumulated N in soil profiles, and to determine the usefulness of Br⁻ as a tracer of surface-applied N fertilizer under heavy rainfall and high irrigation rates. A field experiment with bare plots was conducted near Beijing from 5 July to 6 September 2006. The experiment included three treatments: no irrigation (rainfall only, I0), farmers’ practice irrigation (rainfall plus 100 mm irrigation, I100) and high-intensity irrigation (rainfall plus 500 mm irrigation, I500), with three replicates. Transport of surface-applied Br⁻ and NO₃⁻ (assuming no initial NO₃⁻ in the soil profile) and accumulated NO₃⁻ in soil profiles were all simulated with the HYDRUS-1D model. The model simulation results showed that Br⁻ leached through the soil profile faster than NO₃⁻. When Br⁻ was used as a tracer for surface-applied N fertilizer to estimate nitrate leaching losses, the amount of N leaching may be overestimated by about 10%. Water drainage and nitrate leaching were dramatically increased as the irrigation rate was increased. The amounts of N leaching out of the 2.1-m soil profile under I0, I100 and I500 treatments were 195 ± 84, 392 ± 136 and 612 ± 211 kg N ha⁻¹, equivalent to about 20 ± 5%, 40 ± 6% and 62 ± 7% of the accumulative N in the soil profile, respectively. N was leached more deeply as the irrigation rate increased. The larger amount of initial accumulated N was in soil profile, the higher percentage of N leaching was. N leaching was also simulated in summer under different weather conditions from 1986 to 2006. The results indicated that nitrate leaching in rainy years were significantly higher than those in dry and normal years. Increasing the irrigation times and decreasing the single irrigation rate after fertilizer application should be recommended.     

Nitrate leaching assessment in a long-term experiment under supplementary irrigation in humid Argentina

Applying high rates of nitrogen (N) fertilizer to crops has two major disadvantages: (1) the low N fertilizer use efficiency and (2) the loss of N by leaching, which may cause groundwater nitrate (NO₃⁻) pollution, especially in humid areas.The objectives of this study were to adjust and validate the LEACH-W model simulations with data observed in the field; to quantify nitrate concentrations in the soil solution; to estimate N loss by leaching; and to determine the moments during the year when greatest nitrate transport events occur beyond the rooting profile.A randomized complete block design with four replications was established on a typic Argiudoll. Crop fertilization treatments consisted of three N rates (0, 100, and 200 kg N ha⁻¹) using urea and ammonium nitrate solution (UAN) as the N source. Corn (Zea mays L.) was planted and ceramic soil-water suction samplers were installed to depths of 1, 1.5 and 2 m. Drainage was estimated by the LEACH-W model, which adjusted very well the actual volume of water in the soil profile. Nitrogen losses were statistically analyzed as repeated measure data, using the PROC MIXED procedure.Losses of nitrate-nitrogen (NO₃⁻-N) during the study increased as the rate of N applied increased. At all depths studied, statistically significant higher values were found for 200 N compared to 100 N and 0 N, and for 100 N compared to 0 N (p < 0.001).The greatest NO₃⁻-N losses through leaching occurred during crop growth. Significant differences (p < 0.05) were found between cropping and fallow in the three treatments and depths studied for seasons 4 and 5; these two seasons produced the highest drainage volumes at all depths.     

Identification of irrigation and N management practices that contribute to nitrate leaching loss from an intensive vegetable production system by use of a comprehensive survey

Considerable NO₃⁻ contamination of underlying aquifers is associated with greenhouse-based vegetable production in south-eastern Spain, where 80% of cropping occurs in soil. To identify management factors likely to contribute to NO₃⁻ leaching from soil-based cropping, a survey of irrigation and N management practices was conducted in 53 commercial greenhouses. For each greenhouse: (i) a questionnaire of general irrigation and N management practices was completed, (ii) amounts of N applied in manure were estimated; and for one crop in each greenhouse: (a) irrigation volume was compared with ETc calculated using a mathematical model and (b) total amount of applied fertiliser N was compared with crop N uptake. Total irrigation during the first 6 weeks after transplanting/sowing was generally excessive, being >150 and >200% of modelled ETc in, respectively, 68 and 60% of greenhouses. During the subsequent period, applied irrigation was generally similar to modelled ETc, with only 12% of greenhouses applying >150% of modelled ETc. Large irrigations prior to transplanting/sowing were applied in 92% of greenhouses to leach salts and moisten soil. Volumes applied were >20 and >40 mm in, respectively, 69 and 42% of greenhouses. Chemical soil disinfectants had been recently applied in 43% of greenhouses; associated irrigation volumes were >20 and >40 mm in, respectively, 78 and 48% of greenhouses conducting disinfection. Nitrogen and irrigation management were generally based on experience, with very little use of soil or plant analysis. Large manure applications were made at greenhouse construction in 98% of greenhouse, average manure and N application rates were, respectively, 432 m³ ha⁻¹ and 3046 kg N ha⁻¹. Periodic manure applications were made in 68% of greenhouses, average application rates for farmyard and pelleted manures were, respectively, 157 and 13 m³ ha⁻¹ (in 55 and 13% of greenhouses); the average N rate was 947 kg N ha⁻¹. Manure N was not considered in N fertiliser programs in 74% of greenhouses. On average, 75% of fertiliser N was applied as NO₃⁻. Applied fertiliser N was >1.5 and >2 times crop N uptake in, respectively, 42 and 21% of crops surveyed. The survey identified various management practices likely to contribute to NO₃⁻ leaching loss. Large manure applications and experiential mineral N management practices, based on NO₃⁻ application, are likely to cause accumulation of soil NO₃⁻. Drainage associated with: (i) the combined effect of large irrigations immediately prior to and excessive irrigations for several weeks following transplanting/sowing and (ii) large irrigations for salt leaching and soil disinfection, is likely to leach accumulated NO₃⁻ from the root zone. This study demonstrated that surveys can be very useful diagnostic tools for identifying crop management practices, on commercial farms, that are likely to contribute to appreciable NO₃⁻ leaching.     

Evaluation of the DRAINMOD-N II model for predicting nitrogen losses in a loamy sand under cultivation in south-east Sweden

The DRAINMOD-N II model (version 6.0) was evaluated for a cold region in south-east Sweden. The model was field-tested using four periods between 2002 and 2004 of climate, soil, hydrology and water quality data from three experimental plots, planted to a winter wheat-sugarbeet-barley-barley crop rotation and managed using conventional and controlled drainage. DRAINMOD-N II was calibrated using data from a conventional drainage plot, while data sets from two controlled drainage plots were used for model validation. The model was statistically evaluated by comparing simulated and measured drain flows and nitrate-nitrogen (NO₃-N) losses in subsurface drains. Soil mineral nitrogen (N) content was used to evaluate simulated N dynamics. Observed and predicted NO₃-N losses in subsurface drains were in satisfactory agreement. The mean absolute error (MAE) in predicting NO₃-N drainage losses was 0.16 kg N ha⁻¹ for the calibration plot and 0.21 and 0.30 kg N ha⁻¹ for the two validation plots. For the simulation period, the modelling efficiency (E) was 0.89 for the calibration plot and 0.49 and 0.55 for the validation plots. The overall index of agreement (d) was 0.98 for the calibration plot and 0.79 and 0.80 for the validation plots. These results show that DRAINMOD-N II is applicable for predicting NO₃-N losses from drained soil under cold conditions in south-east Sweden.     

Effects of salinity and fertigation practice on cotton yield and N recovery

The purpose of optimal water and nutrient management is to maximize water and fertilizer use efficiency and crop production, and to minimize groundwater pollution. In this study, field experiments were conducted to investigate the effect of soil salinity and N fertigation strategy on plant growth, N uptake, as well as plant and soil ¹⁵N recovery. The experimental design was a 3 × 3 factorial with three soil salinity levels (2.5, 6.3, and 10.8 dS m⁻¹) and three N fertigation strategies (N applied at the beginning, end, and in the middle of an irrigation cycle). Seed cotton yield, dry matter, N uptake, and plant ¹⁵N recovery significantly increased as soil salinity level increased from 2.5 to 6.3 dS m⁻¹, but they decreased markedly at higher soil salinity of 10.8 dS m⁻¹. Soil ¹⁵N recovery was higher under soil salinity of 10.8 dS m⁻¹ than those under soil salinity of 6.3 dS m⁻¹, but was not significantly different from that under soil salinity of 2.5 dS m⁻¹. The fertigation strategy that nitrogen applied at the beginning of an irrigation cycle had the highest seed cotton yield and plant ¹⁵N recovery, but showed higher potential loss of fertilizer N from the root zone. While the fertigation strategy of applying N at the end of an irrigation cycle tended to avoid potential N loss from the root zone, it had the lowest cotton yield and nitrogen use efficiency. Total ¹⁵N recovery was not significantly affected by soil salinity, fertigation strategy, and their interaction. These results suggest that applying nitrogen at the beginning of an irrigation cycle has an advantage on promoting yield and fertilizer use efficiency, therefore, is an agronomically efficient way to provide cotton with fertilizer N under the given production conditions.     

Simulating soil water flow and nitrogen dynamics in a sunflower field irrigated with reclaimed wastewater

Soil water flow and nitrogen dynamics were simulated in sunflower field during and after the growing period, in Northern Greece. Soil water and nitrogen dynamics were evaluated using a one-dimensional simulation model based on the Galerkin finite element method. We examined the effects of irrigation with reclaimed wastewater and nitrogen fertilizer applications on plant growth, water and nitrogen distribution in the soil profile, water and nitrogen balance components and nitrogen leaching to groundwater. The model simulated the temporal variation of soil water content with reasonable accuracy. However, an over estimation of the measured data was observed during the simulation period. Relatively good agreement was found between the simulated and measured NH₄-N and NO₃-N concentrations over time and depth, whereas fluctuations at greater depths were relatively small. Most of the cumulative nitrate-N leaching (44.7 kg N ha⁻¹) occurred during the winter.     

Soil fertility and nutrient uptake by arecanut (Arecacatechu L.) as affected by level and frequency of fertigation in a laterite soil

Changes in soil fertility status were evaluated for 10 years, from 1996 to 2006 to examine the impact of drip fertigation in a laterite soil and to determine the nutrient uptake pattern of arecanut (Areca catechu L.). Four fertigation levels (25%, 50%, 75% and 100% of recommended fertilizer dose, 100:18:117 g N:P:K palm⁻¹ year⁻¹), three frequencies of fertigation (10, 20 and 30 days) and two controls (control 1: drip irrigation without fertilizer application and control 2: drip irrigation with 100% NPK soil application) were studied. The soil pH increased to 6.0 at the end of experiment in 2006 compared to the pre-experimental soil pH of 5.6 in 1996. In 0-25-cm depth interval, the soil organic carbon (SOC) increased significantly from 1.06% in 1999 to 1.84% in 2006, and in 25-50-cm depth interval, it increased from 0.68% to 1.13%. Temporal variation in available P and K content in arecanut root zone was significant due to drip fertigation. Pooled analysis of data, from 2000 to 2005, revealed significant impact of level and frequency of fertigation and their interaction on available P and K content. At 0-25-cm depth interval, increase in fertigation dose from 50% to 100% NPK did not result in significant increase of Bray’s P content, which remained at par ranging from 5.24 to 5.32 mg kg⁻¹. Fertigation every 30 days resulted in significantly higher available P (5.32 mg kg⁻¹) than fertigation every 10 days (4.49 mg kg⁻¹), while it was at par with fertigation every 20 days (5.09 mg kg⁻¹). The K availability at 0-25-cm depth interval was significantly lower at 25% NPK level (114 mg kg⁻¹) than at 75% (139 mg kg⁻¹) and 100% (137 mg kg⁻¹). With respect to fertigation frequency, the 30-day interval resulted in higher available K of 139 mg kg⁻¹ than 20-day (128 mg kg⁻¹) and 10-day intervals (120 mg kg⁻¹). Availability of P and K at 25-50-cm depth interval followed similar trend as that of 0-25-cm depth interval. The total N uptake (g palm⁻¹ year⁻¹) by leaves, nuts and husk varied between 143 in 0% NPK to 198 in 75% NPK fertigation level. Similarly, the total P uptake (g palm⁻¹ year⁻¹) ranged between 15 for the 0% NPK and 25 for the 75% NPK treatment. The total K uptake (g palm⁻¹ year⁻¹) was 62 for the 75% NPK treatment followed by 56 for the 25%, 56 for the 50%, 54 for the 100% and 46 for the 0% NPK treatments. The nutrient uptake pattern and marginal availability of soil P and K highlight the importance of drip fertigation during post-monsoon season to improve and sustain the yield of arecanut in a laterite soil.     

Treated sewage effluent as a source of water and nitrogen for Tifton 85 bermudagrass

The use of treated sewage effluent in agriculture has been a current practice in several countries. However, in Brazil, there are few studies about this subject. This research work aimed at evaluating the potential utilization of secondary-treated sewage effluent (STSE) as an alternative source of water and nitrogen (N) for Tifton 85 bermudagrass pasture. A field experiment was carried out at Lins, State of São Paulo, Brazil, for 2 years, using a randomized complete block design, with four replications and five treatments, as follows: (i) T1 (control) – irrigation with potable water and addition of mineral-N fertilizer (MNF) – 520 kg N ha⁻¹ year⁻¹; (ii) T2–T5 – irrigation with STSE (31.9 mg total-N L⁻¹) and addition of MNF – 0, 171.6, 343.2 and 520 kg N ha⁻¹ year⁻¹, respectively. Potable water and STSE characteristics were monitored monthly; above ground grass dry matter yield (DM) and crude protein content (CP) were determined bimonthly. Increases in DM and CP were observed for the high MNF rates associated with irrigation with STSE. STSE irrigation can efficiently substitute potable water for irrigation of Tifton 85 bermudagrass pasture and, simultaneously, save 32.2–81.0% of the recommended N rate without loss of grass DM and CP yield.     

Seasonal variation in groundwater levels and quality under intensively drained and grazed pastures in the Montagu catchment, NW Tasmania

Water quality is a significant environmental issue in the Montagu River and its estuary in north-west Tasmania. Groundwater is the major contributor to baseflow for about half of the year. ‘Hump and hollow’ surface drainage is increasingly being used to reduce the effects of seasonal waterlogging on pasture production. However, little is known about the effects of ‘hump and hollow’ structures on watertable levels or intensive grazing on groundwater quality in the catchment. The objectives of this study were to evaluate the impacts of ‘hump and hollow’ drainage by comparing watertable levels in drained and undrained paddocks and to quantify the effects of intensive grazing on groundwater quality underlying pastures.In December 2004, 10 wells and 2 piezometers were installed at depths of 2-6 m at seven sites along two transects across the dairying area of Togari. Water levels were monitored and water samples collected every 2 months were analysed for pH, electrical conductivity, total dissolved solids, ammonium, nitrate, nitrite, total nitrogen, dissolved reactive phosphorous, Ca, Mg, K and Na. Thermotolerant coliforms and Enterococcus were measured when watertable levels were low and high.Watertable levels were within 0.5 m of ground level for over 3 months on undrained sites. ‘Hump and hollow’ surface drainage increased the depth of the unsaturated zone under the ‘humps’ but did not lower the watertable. Watertable levels on the crests of the ‘hump and hollow’ structures rose and fell daily in response to periods of rainfall and drought. Gradients of the groundwater surface, albeit very low, indicated the potential for groundwater flow from the base of the hills to the Montagu River in the centre of the valley.The median nitrate concentration of all samples was 0.018 mg NO₃-N L⁻¹ but one site had nitrate concentrations in excess of that recommended for potable water for a period of 1-2 months. Nitrate concentrations varied seasonally by 20-1000 times with an early winter pulse of nitrate evident both in the groundwater and in the Montagu River. In contrast, the median ammonium concentration in the groundwater was 0.274 mg NH₄-N L⁻¹ which was well above the trigger value for lowland streams. The median concentration of dissolved reactive phosphorus was 0.008 mg P L⁻¹ which was slightly higher than the trigger value. There was some evidence of low levels of faecal bacterial contamination of the shallow aquifers.Transects across the dairying area did not clearly demonstrate increasing concentrations of analytes due to intensive grazing though lower levels of nutrients were generally found at sites adjacent to undisturbed native forest. Variation in water quality parameters along the transects suggested water quality at a site was mostly related to local conditions and hazards.     

Nitrate fate in a Mexican Andosol: Is it affected by preferential flow?

Andosols are the dominant soils in the Valle de Bravo basin, the origin of a significant amount of Mexico City's drinking water. The main land use is agriculture and most of the existing surface water bodies are eutrophic. Nitrogen fertilizer is used extensively. There have been very few studies on nitrate (NO₃⁻) fate in this type of soil and region. Comprehensive laboratory studies were conducted to determine the fate of NO₃⁻ in an Andosol profile from Valle de Bravo, in order to assess the risk of water resources contamination. Nitrate retention was analysed statically (using batch experiments) and dynamically (using intact and packed soil columns) at different soil depths and its competition with Cl⁻ was evaluated. Complementary laboratory experiments were conducted to study water transport through the columns and nitrogen transformations in the soil. In batch and columns, NO₃⁻ adsorption was linear in the range of concentrations studied and higher in the deepest soil layer. Preferential flow pathways were found in the unaltered deeper soil layers, while tillage activity in the top layer destroyed the pore continuity. In spite of the deeper soil layer's greater capacity for NO₃⁻ retention, the presence of preferential flow pathways coupled with high rainfall intensities, makes the NO₃⁻ mobile below the root zone at 1 m depth and increases the risk of groundwater contamination. The results illustrate the complexity of nitrate fate in Andosols and can be used to improve agricultural practices in the central Mexico region.     

Hydrochemical and stable isotopic assessment of nitrate contamination in an alluvial aquifer underneath a riverside agricultural field

Major ions and stable isotopic (δD_water, δ¹⁸O_water, δ¹⁵N_nitrate, δ¹⁸O_nitrate) measurements in concert with hydrochemical modeling were used in order to elucidate the sources and geochemical processes controlling nitrate contamination of shallow alluvial groundwater underneath a riverside agricultural field in the Buyeo area, Korea. Beneath vegetable fields in the sandy soil, the mean nitrate concentration of groundwater was 148.6 mg/L, which is significantly higher than in groundwater (mean 28.8 mg/L) beneath silty soils underneath rice paddy fields. Nitrogen isotope data indicate that synthetic fertilizers are the predominant source of groundwater nitrate in the study area. Denitrification during recharge through rice paddy soils appears to be responsible for the lower nitrate concentrations in groundwater beneath the silty soil zone. The relationship between nitrogen and oxygen isotope data of nitrate also suggests mixing of two different groundwater bodies with nitrates from the silt zone and the sand zone. Geochemical mass balance modeling on hydrochemical data indicates that various agricultural chemicals such as urea, lime, magnesium sulfate and potassium chloride dissolve in vegetable fields of the sandy zone, resulting in significant enrichment of various solutes such as K⁺, Ca²⁺, Mg²⁺, NO₃⁻, SO₄²⁻ and Cl⁻. As a consequence of over-utilization of synthetic nitrogen fertilizers, the sand zone is characterized by very high nitrate concentrations in the groundwater. This study suggests that a reduction of over-fertilization especially on vegetable fields in the riverside sand zone is required to minimize the nitrate contamination of groundwater. This study also shows that combination of geochemical and isotopic techniques with simple mass balance modeling provides information about the causes and processes of nitrate contamination of groundwater underneath a riverside agricultural field. The study also provides sustainable measures to optimize fertilization rate as an important basis of eco-friendly agriculture.     

Simulation of nitrate leaching under irrigated maize on sandy soil in desert oasis in Inner Mongolia, China

Water scarcity and nitrate contamination in groundwater are serious problems in desert oases in Northwest China. Field and ¹⁵N microplot experiments with traditional and improved water and nitrogen management were conducted in a desert oasis in Inner Mongolia Autonomous Region. Water movement, nitrogen transport and crop growth were simulated by the soil-plant system with water and solute transport model (SPWS). The model simulation results, including the water content and nitrate concentration in the soil profile, leaf area index, dry matter weight, crop N uptake and grain yield, were all in good agreement with the field measurements. The water and nitrogen use efficiency of the improved treatment were better than those of the traditional treatment. The water and nitrogen use efficiency under the traditional treatment were 2.0 kg m⁻³ and 21 kg kg⁻¹, respectively, while under the improved treatment, they were 2.2 kg m⁻³ and 26 kg kg⁻¹, respectively. Water drainage accounted for 24-35% of total water input (rainfall and irrigation) for the two treatments. Nitrogen loss by ammonia volatilization and denitrification was less than 5% of the total N input (including the N comes from irrigation). However, 32-61% of total nitrogen input was lost through nitrate leaching, which agreed with the ¹⁵N isotopic result. It is impetrative to improve the water and nitrogen management in the desert oasis.     

Analyzing soil soluble phosphorus transport with root-phosphorus-uptake applying an inverse method

Soil soluble phosphorus (P) transport with root-phosphorus-uptake (RPU) is a critical process for plant growth, cycling of P in soil-plant systems and environment protection. However, modeling soil soluble P transport is extremely challenging because it is difficult to measure the RPU distribution directly, especially in the field. In this study, an inverse method, which was utilized successfully to estimate the root-water-uptake (RWU) rate distribution by Zuo and Zhang (2002) and the source-sink term in the nitrate (NO⁻₃-N) transport equation by Shi et al. (2007), was applied to estimate the RPU rate distribution and analyze soil soluble P transport in the soil-plant systems. A soil column experiment (Exp. 1) and a field experiment (Exp. 2), respectively with winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) growth, were carried out to observe the dynamics of soil water and soluble P. Based on the experimental data in Exp. 1, the average RWU and RPU rate distributions during different irrigation periods were estimated using the inverse method. The relative errors of the total P extracted by wheat between the estimated and measured values during all periods were less than 10%. The estimated RPU rate distribution during the period of 10.5-15.5 days after planting (DAP) was used to optimize the dimensionless RPU factor δ to establish the RPU model (δ = 1.31), which helped to calculate the RPU rate distributions during other periods (from 16.5 to 57.5 DAP) in Exp. 1. The calculated RPU rate distributions were compared well with the estimated profiles by the inverse method, and the root mean squared error between them was less than 0.00005 mg cm⁻³ d⁻¹. Correspondingly, the calculated total P extracted by winter wheat was also comparable with the measured value, with the relative error less than 10%. Similarly, the procedures were employed for summer maize in Exp. 2. The estimated (using the inverse method) and calculated (through the RPU model with δ = 1.38) RPU rate distributions were in good agreement with the root mean squared error as less as 0.000031 mg cm⁻³ d⁻¹. According to the established RPU models (δ = 1.31 and 1.38 for Exps. 1 and 2, respectively), the distributions of soil water content and soluble P concentration were simulated, and compared well with the measured profiles, with the maximum root mean squared error of 0.0088 cm³ cm⁻³ and 0.0066 mg cm⁻³ in Exp. 1, and 0.023 cm³ cm⁻³ and 0.0015 mg cm⁻³ in Exp. 2, respectively. The inverse method should be effective and applicable for estimating the RPU rate distribution, establishing the RPU model and analyzing soil soluble P transport in soil-plant systems, either in laboratory or in the field.     

Yield, water and nitrogen-use response of rice to zeolite and nitrogen fertilization in a semi-arid environment

Water scarcity and soil nitrogen (N) loss are important limitations for agricultural production in semi-arid region especially for rice production. Zeolite (Z) as a soil conditioner can be used to retrain water and nitrogen in near-surface soil layer in lowland rice production system. The objectives of this study were to investigate the effects of different application rates of natural zeolite (clinoptilolite) and nitrogen on rice yield, yield components, soil nitrogen, water use, water productivity in a silty clay soil in 2004 and 2005. Zeolite was only applied in the first year. In order to study the long-term and continuous effect of zeolite on the objectives of the study, no zeolite was applied in the second year and the study was conducted on the same land as the first year. Zeolite and N were applied at rates of 0, 2, 4, and 8 t ha⁻¹ and 0, 20, 40, and 80 kg ha⁻¹, respectively in 2004. In 2005, each plot received the same amount of N as received in 2004. It is concluded that by decreasing N application rates, higher Z application rate is needed to improve grain yield. Highest grain yield was obtained at N application rate of 80 kg ha⁻¹ and Z application rate of 4 t ha⁻¹. Higher grain yield was mostly attributed to lower unfilled grain percentage and higher 1000-grain weight that were a result of higher N application rate and N retention in soil due to Z application. Nitrogen and Z applications resulted in higher grain protein contents and nitrogen recovery efficiency (NRE). Based on these results and due to higher N retention in soil under Z application, improved grain yield quality, nitrogen-use efficiency (NUE), and nitrogen recovery efficiency (NRE) could be obtained at Z application rate of 8 t ha⁻¹ and N application rate of 80 kg ha⁻¹ or more. However, this was not satisfied for NUE. Moreover, it is found that at higher N application rates lower Z application rates are needed to effectively retain soil residual mineral nitrogen. Furthermore, at N application rates of 80 kg ha⁻¹ or more, Z application increased soil water retention and resulted in lower seasonal water use and higher water productivity. In general, it was concluded that the effect of Z application in retaining soil N was also effective in the second year.     

Response of vanilla (Vanilla planifolia A.) intercropped in arecanut to irrigation and nutrition in humid tropics of India

A 5-year field trial to assess the impact of microsprinkler irrigation and nutrition on vanilla grown as intercrop in arecanut plantation was conducted on a laterite soil. Pooled analysis indicated that microsprinkler irrigation at 1.0 Epan resulted in significantly higher green bean yield (842 kg ha⁻¹) than 0.75 Epan (579 kg ha⁻¹). Organic manure application in the form of vermicompost (720 kg ha⁻¹) and FYM (768 kg ha⁻¹) and recommended NPK (718 kg ha⁻¹) produced green bean yield at par with recycling of gliricidia prunings (625 kg ha⁻¹). Irrigation at 1.0 Epan proved superior by registering maximum benefit:cost (B:C) ratio of 2.25 compared to 1.62 at 0.75 Epan. The highest B:C ratio was obtained with recommended NPK (2.27) followed by recycling of gliricidia prunings (2.10), vermicompost (1.87), vermicompost + arecanut husk mulching (1.80) and FYM (1.64). The soil pH increased by 0.4 units in 2008 compared with the pre-experimental soil pH of 5.6 in 2004. Nutrition alone and in combination with irrigation had significant impact on soil pH. Organic manure application increased the soil pH (6.1-6.2) significantly over recommended NPK (5.6) at the end of experiment in 2008. Significant variation in soil organic carbon (SOC) was noticed due to different nutrition treatments. Application of vermicompost and FYM significantly increased the SOC content by 38-54% in 2008 over initial levels in 2004. Bray's P availability was influenced by nutrition and its interaction with irrigation. Application of FYM continuously for 4 years has resulted in significant increase in Bray's P content (41.3 mg kg⁻¹) compared to other nutrition treatments (9.4-17.2 mg kg⁻¹). Irrigation equivalent to 0.75 Epan (223 mg kg⁻¹) increased the K availability significantly over 1.0 Epan (172 mg kg⁻¹). The K availability was significantly higher in recommended NPK (416 mg kg⁻¹) than in other organic treatments (98-223 mg kg⁻¹) at 0-30 cm soil depth. Overall, vanilla responded well to irrigation and nutrition in arecanut-based cropping system with a better economic output and improved soil fertility.     

Evaluation of the VegSyst model with muskmelon to simulate crop growth, nitrogen uptake and evapotranspiration

Like many intensive vegetable production systems, the greenhouse-based system on the south-eastern (SE) Mediterranean coast of Spain is associated with considerable NO₃⁻ contamination of groundwater. Drip irrigation and sophisticated fertigation systems provide the technical capacity for precise nutrient and irrigation management of soil-grown crops which would reduce NO₃⁻ leaching loss. The VegSyst crop simulation model was developed to simulate daily crop biomass production, N uptake and crop evapotranspiration (ET_c). VegSyst is driven by thermal time and consequently is adaptable to different planting dates, different greenhouse cooling practices and differences in greenhouse design. It will be subsequently incorporated into a practical on-farm decision support system to enable growers to more effectively use the advanced technical capacity of this horticultural system for optimal N and irrigation management.VegSyst was calibrated and validated for muskmelon grown in Mediterranean plastic greenhouse in SE Spain using data of four melon crops, two grown in 2005 and two in 2006 using two management strategies of water and N management in each year. VegSyst very accurately simulated crop biomass production and accurately simulated crop N uptake over time. Model performance in simulating dry matter production (DMP) over time was better using a double radiation use efficiency (RUE) approach (5.0 and 3.2 g MJ⁻¹ PAR for vegetative and reproductive growth phases) compared to a single RUE approach (4.3 g MJ⁻¹ PAR). The simulation of ET_c over time, was very accurate in the two 2006 muskmelon crops and somewhat less so in the two 2005 crops. The error in the simulated final values, expressed as a percentage of final measured values was −1 to 6% for DMP, 2-11% for crop N uptake, and −11 to 6% for ET_c. VegSyst provided effective simulation of DMP, N uptake and ET_c for crops with different planting dates. This model can be readily adapted to other crops.