Soil Lysimeters Research Articles

The relative influence of land cover, hydrology, and in‐stream processing on the composition of dissolved organic matter in boreal streams

AbstractLow‐order boreal streams are particularly sensitive interfaces where dissolved organic matter (DOM) is transported from soils to inland waters. Disentangling the relative influence of key environmental factors suspected to influence stream water DOM composition is highly relevant to predicting the reactivity and fate of terrestrial DOM entering inland waters. Here we examined changes to DOM composition using absorbance and fluorescence, from 17 boreal streams ranging from first to fourth orders, over 14 months, including the rarely studied winter season, and two snowmelt periods (n = 836). We also analyzed soil pore water samples from three forest soil lysimeters to a depth of 70 cm (n = 60). Of five identified fluorescing parallel factor analysis components, two (C4 and C5) expressed a clear mire wetland or forest signature, providing distinct molecular markers of dominant land cover. In fact, land cover alone explained 49% of the variability in DOM composition. In contrast, seasonal fluctuations in hydrology only contributed to minor shifts (8%) in the composition of stream water DOM, while in‐stream transformations to DOM composition were undetectable. These findings suggest that low‐order boreal streams act as a passive pipe, since in‐stream processing of DOM is restricted by short water residence times (6 h to 2 days). In addition, we demonstrated the sensitivity of optical approaches to distinguish between key terrestrial sources of DOM in the boreal landscape. By distinguishing the proportional leverage of key environmental controls on headwater stream DOM composition, we are better equipped to predict where and when key DOM transformations occur in the aquatic conduit.

Plant Factors for Irrigating Mixed Turfgrass and Shrub Landscapes in a Humid Environment

Landscape water conservation methods and techniques contribute to managing water resources. Use of reference evapotranspiration (ETo) data and landscape coefficients is one method that needs further development. Local ETo data and actual plant water use were used to calculate plant factors (PFs) for three model landscapes composed of mixtures of turfgrass and shrubs. Model landscapes using a sandy loam soil included st. augustinegrass (Stenotaphrum secundatum), privet (Ligustrum japonicum), dwarf burford holly (Ilex cornuta ‘Burfordii Nana’), and dwarf yaupon holly (Ilex vomitoria ‘Nana’) at three ratios of turfgrass to shrub vegetative cover: 80:20, 50:50, and 20:80. Soil was placed into inground lysimeters in a complete randomized block design with soil moisture sensors and a drainage system. Lysimeters were irrigated with a sprinkler system, and water was applied at a rate of 100% replacement of ETo minus precipitation. Lysimeter soil leachate was collected from the drainage system and quantified. After 2 years, the PF of 20:80 and 50:50 turfgrass/shrub combination were greater than the PF of the 80:20 combination. Plant factors for the 80:20, 50:50, and 20:80 turfgrass:shrub combinations were 0.68, 0.97, and 1.01, respectively. There were no seasonal differences in PFs. Total growing season leachate depth over 2 years was 63.4, 30.7, and 12.6 mm for 80:20, 50:50, and 20:80, respectively. Further work on PFs should include other plant combinations, and evaluation in other climatic zones.

Vadose zone oxygen (O2) dynamics during drying and wetting cycles: An artificial recharge laboratory experiment

Summary Vadose zone oxygen dynamics control all subsurface redox reactions and play a decisive role in maintaining groundwater quality. Although drying and wetting events are common in artificial recharge, their effects on subsurface oxygen distribution are poorly documented. We monitored oxygen concentration in the unsaturated zone in a mid-scale (1 m high) laboratory soil lysimeter, which was subjected to short wetting and drying cycles that simulated a highly permeable shallow aquifer recharged by river water. Ten cycles of varying duration were performed for a period of 85 days. Measurements of oxygen in the liquid and the gas phases were recorded every 20 s using non-invasive optical fibers (PreSens). The results provided high-resolution (in time) oxygen concentration maps. The infiltration rate revealed a decreasing trend during wetting cycles associated with biological clogging. Such a decrease with time was accompanied by a depletion of O 2 concentration, occurring within the first few hours of the infiltration. During drying, O 2 concentrations recovered rapidly at all depths owing to air flushing, resulting in a stratified vertical profile consistent with the biological consumption of O 2 along the air infiltration path. Furthermore, drying periods caused a potential recovery of the infiltration capacity while preserving the soil biological activity. Scraping also led to the recovery of the infiltration capacity of the soil but was less effective than drying. Our experiment suggests that the small-scale heterogeneity played a key role in accurately mapping pore-scale O 2 concentrations and should be considered in modeling O 2 fluxes of unsaturated soils under natural or managed recharge conditions.

Long-term fate of the herbicide mefenacet in a rice-grown lysimeter over a period of 6 consecutive years

To show the long-term fate of the herbicide mefenacet in rice paddies, [aniline-14C]mefenacet was applied to a lysimeter of loamy soil with a depth of 1 m, and rice plants were cultivated for 6 consecutive years according to the conventional methods in Korea. Mineralization of [14C]mefenacet to 14CO2 and volatilization from the soil surface were 12.01 and 0.02 %, respectively, of the originally applied amount during the first 23 weeks following application. Throughout the 6-year period, the total proportion of 14C-radioactivity that leached through the lysimeter soil was 0.778 % of the original radioactivity. The total 14C-radioactivity absorbed and translocated by rice plants throughout the 6 years was 2.46 % of the applied 14C. Measurement of the 14C-radioactivity distributed in each part of the rice plant (the straw, the ears without rice grain, the chaff, and the brown rice grain) indicated that the amount of 14C in straw was 25.87 times higher than that in brown rice grain over the 6 years. The 14C-radioactivity remaining in the soil layer after 6 years was 44.58 % of that applied, 91.45 % of which was distributed in the upper 0–20-cm layer. These results strongly indicate that mefenacet moved downward very slowly and more than half of the herbicide applied was released into the air, mainly through mineralization to CO2 in soil during the experimental period.

Integrated field lysimetry and porewater sampling for evaluation of chemical mobility in soils and established vegetation.

Potentially toxic chemicals are routinely applied to land to meet growing demands on waste management and food production, but the fate of these chemicals is often not well understood. Here we demonstrate an integrated field lysimetry and porewater sampling method for evaluating the mobility of chemicals applied to soils and established vegetation. Lysimeters, open columns made of metal or plastic, are driven into bareground or vegetated soils. Porewater samplers, which are commercially available and use vacuum to collect percolating soil water, are installed at predetermined depths within the lysimeters. At prearranged times following chemical application to experimental plots, porewater is collected, and lysimeters, containing soil and vegetation, are exhumed. By analyzing chemical concentrations in the lysimeter soil, vegetation, and porewater, downward leaching rates, soil retention capacities, and plant uptake for the chemical of interest may be quantified. Because field lysimetry and porewater sampling are conducted under natural environmental conditions and with minimal soil disturbance, derived results project real-case scenarios and provide valuable information for chemical management. As chemicals are increasingly applied to land worldwide, the described techniques may be utilized to determine whether applied chemicals pose adverse effects to human health or the environment.

Freezing–thawing effects on phosphorus leaching from catch crops

It is suggested that catch crops be grown to reduce phosphorus (P) losses. However, after exposure to freezing–thawing cycles (FTCs), catch crop material can become a source of P losses to waters in moderately cold climates. This study screened potential P leaching from intact plant material of eight catch crop species: chicory (Cichorium intybus L.), cocksfoot (Dactylis glomerata L.), perennial ryegrass (Lolium perenne L.), red clover (Trifolium pratense L.), phacelia (Phacelia tanacetifolia L.), white mustard (Sinapis alba L.), oilseed radish (Raphanus sativus L. oleiformis) and white radish (R. longipinnatus). The catch crops were grown in six field experiments on clay soils, where soil lysimeters (0.25 m deep) with intact crops were extracted in autumn and used for leaching experiments before and after seven FTCs in the laboratory. The eight catch crops did not reduce P leaching before FTCs. After FTCs, leachate total-P concentrations from ryegrass, oilseed radish and red clover lysimeters were significantly (p = 0.0022) higher than those from the other species and the control without a catch crop. FTCs significantly (p = 0.0064) altered total-P concentration and the proportions of different forms of P. There was a significant increase in total-P concentration in leachate from ryegrass (p = 0.0008) and oilseed radish (p = 0.02). Thus the potential risk of P leaching from ryegrass and oilseed radish material after FTCs must be considered, since they are commonly grown as nitrogen catch crops in the Nordic countries. Moreover, the roots of the tested catch crops contained 7–86 % total-P, which is important when evaluating P leaching risks.

Stabilization of organic matter in soil lysimeters

Using stationary soil lysimeters it was demonstrated that the type of phytocenose determines the processes of stabilization of organic matter and the type of litter reflects the transformation of organic matter. The maximum quantity of large aggregates at the lowest concentrations of carbon is detected in the upper horizons of soils under fallow conditions as compared to those under other communities. The maximum possible loss of carbon, as estimated by the intensity of respiration, is observed in the soils under mixed plantings, while the least was observed for agricultural lands.

Leaching of from stony soils after effluent application.

Irrigation of dairy shed effluent (DSE) onto land is an integral part of New Zealand's farming practice. The use of inappropriate soils can result in contamination of ground waters with microbes and nutrients. A gap in our knowledge is the ability of stony soils to safely treat DSE. Replicates of four stony soils were collected from the Canterbury region of New Zealand as intact soil lysimeters 460 mm in diameter and up to 750 mm deep. The soils had either stones to the surface or 300 to 600 mm fines over stones. To determine breakthrough characteristics, a pulse of DSE (25 mm depth) spiked with bromide (2000 mg L) was applied to the soil cores followed by continuous artificial rainfall, for one pore volume, at 5 mm h. Leachate aliquots were analyzed for , bromide, and NH-N. The lysimeters were then subjected to hoof pugging using a mechanical hoof, and the leaching characteristics of the soil were determined again. breakthrough curves revealed that the potential for to leach through the soils was high for Selwyn very stony soil and low for other soils analyzed. After pugging, leaching of increased in Mackenzie soil with stones to the surface. For most other soil cores, concentrations in soil leachates were low. In soils where stones are close to the surface, especially where the soil matrix is sandy, we anticipate that shallow groundwater is vulnerable to microbial contamination under some land management practices.

Fate and survival of Salmonella Typhimurium and Escherichia coliO157:H7 in repacked soil lysimeters after application of cattle slurry and human urine

Use of cattle slurry as a fertiliser is common practice around the world. Human urine use is not as common, but owing to its fertiliser value this might change in the future. It is essential to minimise the transfer of enteric pathogens through fertilisation, with respect to both animal and public health. Therefore the objective of this research was to study the survival and transport of Salmonella Typhimurium and Escherichia coli O157:H7 in two agricultural soils when applied to soil along with either cattle slurry or human urine over a period of 180 days. Both Salmonella and E. coli O157:H7 were more rapidly reduced when applied together with human urine than when applied with cattle slurry. However, both pathogens persisted in low amounts at 20 and 50 cm depth in both soils throughout the whole study period. No Salmonella or E. coli O157:H7 was detected in the leachate over the 180 day study. The risk of disease transmission is higher when cattle slurry is used as fertiliser compared with human urine. However, the risk of groundwater infiltration would be low as long as water velocity through the soil is moderate. Increased knowledge of pathogen persistence in soil after fertiliser application is a valuable tool for improving risk evaluations and formulating guidelines for the use of cattle and/or human wastes in cropping soils.

Methods of determining the coefficient of water conductivity in soils

The paper is aimed to consider the theory and methods of determining the coefficients of water conductivity and filtration. Soil columns and lysimeters were used for this purpose. It is shown that even a slightly expressed decrease in the moisture content can lead to decreasing the water conductivity in soil. In the range of high values of the moisture potential the coefficient of water conductivity is significantly higher for light-textured soils, on the contrary, within the range of middle and low values it seems higher for loamy soils. The maximum of water conductivity in any soil is close to filtration coefficients, but its minimum tends to zero. The coefficients of water conductivity and filtration become usually close under conditions of descending flow when the moisture is equal to the maximum water capacity or close to it.

Soil amendment with olive mill wastes: Impact on groundwater

Two sets of soil lysimeters were amended with solid and liquid olive mill wastes and the composition of leachate was analysed. Five treatments were carried out using: olive mill wastewater (OMW) at two different rates (80 and 320 m3/ha); OMW pre-treated by catalytical digestion with MnO2; compost obtained by exhausted olive pomace; freshwater as the control. Electric conductivity, pH, potassium, total polyphenols and nitrates were monitored in the leachate as indexes of potential groundwater contamination. The study demonstrated that the impact of all the selected amendments on groundwater was the minimum. OMW was safely applied to soil even at four times the rate allowed by the Italian law, and pre-treatment by catalytical digestion was not necessary to further reduce the impact on groundwater. The application of olive pomace compost was equally safe.

English

In arid and semiarid regions of northern China, there is an increasing interest in using reduced rate of inorganic fertilizer together with water-saving superabsorbent polymer (SAP) for field crop production. Thus, an efficient management of fertilizer and study of metabolic changes in response to SAP application is important for improved production of corn. 24 undisturbed soil lysimeters (35 cm in diameter and 150 cm in depth) were installed in a field lysimeter facility during 2010, to study yield and physiological mechanisms in corn ( Zea mays L.) subjected to application (30 kg ha -1 ) or without application of SAP at different fertilization levels (standard, medium or 75% and low or 50% of conventional fertilization rate). The results show that the corn yield fell by 19.7% under medium and 37.7% under low fertilization; the application of SAP increased it significantly by 80.3%. Although SAP had marginal effect under standard fertilization, plants treated with SAP under reduced fertilization showed a significant decrease in superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR) activities in leaves when compared with control plants. Our results suggest that drought stress as well as fertilizer reduction leads to production of oxygen radicals, which results to oxidative stress in the plant and the application of superabsorbent polymer could conserve soil water and nutrients, making same available for plants to reduce oxidative stress and increase biomass accumulation, especially under reduced fertilization level. Key words : Antioxidant enzymes, lysimeter, corn, drought stress, superabsorbent polymer.

Identifying the water source for subsurface flow with deuterium and oxygen-18 isotopes of soil water collected from tension lysimeters and cores

• Soil water showed different residence time along the profiles. • Piston flow and preferential flow coexisted in the study Entisol. • The mobile soil water was the pre-event water who participated subsurface flow generation. • Rainwater only well mix with mobile soil water in the vadoze zone. • Bulk and lysimeter water showed significant differences in isotopic composition. The conventional identification of soil water with pre-event water limits deep insights into the involvement of stationary and mobile soil water in subsurface hydrological processes. In three tilled sloping field plots at a hilly area of southwestern China dominated by Entisols, soil water collected with a suction lysimeter was distinguished from the total soil water through an analysis of the stable isotopes deuterium and oxygen-18. Differences in the depth profile of soil water before and after storm events were observed and used to examine how rainwater mixes with soil water and to identify the source contribution of different fractions of soil water in subsurface flow generation. Only water in the 0–10 cm soil layer was significantly affected by evaporation and infiltration. Water in the top 5 cm layer of the soil exhibited the lowest residence time because a storm can replace a substantial proportion of the pre-event water. Soil water at the 10–20 cm depth showed the longest residence time, as indicated by its high proportion of pre-event water. The isotopic signatures demonstrated that piston flow and preferential flow coexisted in this soil. High antecedent soil water content and high rain intensity favor the formation of piston flow. The water collected with the suction lysimeter represented the mobile fraction of the pre-event water in the soil, which effectively participates in the generation of subsurface flow. Newly infiltrated rainwater did not well mix with stationary pre-event water in the soil. The use of recent rainfall to represent mobile soil water may provide a practical solution for overcoming the negative effect of the spatial heterogeneity of the isotopic composition of soil water on hydrograph separation results. Bulk soil water and lysimeter water showed significant differences in isotopic composition under low soil water content or in the top soil layer. Stable isotopes in bulk and lysimeter soil water should be monitored synchronously to reveal the sources and pathways of soil water and their contributions to the generation of subsurface flow in the vadose zone.

Estimation of field capacity for aggregated soils using changes of the water retention curve under the effects of compaction

SummaryThe objective of this study is to present a new method for field capacity estimations using the CPV function (changes in pore volume). The method requires water retention data of an aggregated soil before and after a compaction event. The CPV function estimates the threshold of equivalent pore diameter (D‐threshold) between structural and textural porosity using changes in the water retention curve. The D‐threshold corresponds to a water potential associated with field capacity according to the TFC concept (textural field capacity), which assumes that (i) the textural pores define the field capacity conditions and (ii) the D‐threshold and consequently the water potential at field capacity are constant parameters for each soil regardless of its compaction state. Published datasets for different aggregated soils subjected to different compaction processes and data from a lysimeter drainage experiment were used to evaluate the method. The analysis of the drainage experiment was supported by the BUDGETv.6.2 model, which was used to estimate the optimum value of the water content at field capacity (parameter of the model) using observed soil water contents and drainage rates during the drainage experiment (concept of inverse problem). The results for different soils subjected to compaction showed that the estimated water potentials by the CPV method follow the commonly accepted assumptions that govern field capacity conditions, justifying also the assumptions of the TFC concept. The field capacity conditions estimated by the model demonstrated an adequate correspondence to the results of the CPV method, indicating its ability to assess the field capacity conditions of the lysimeter soils.

Nitrate Uptake Rates of Kentucky Bluegrass Genotypes and Their Effect on Nitrate Absorption under Competitive Conditions

ABSTRACTNitrate leaching from turfgrasses continues to be a concern. It is proposed that selecting turfgrass genotypes with higher NO3− absorption abilities could reduce NO3− leaching. This study examined the intraspecific difference in NO3− absorption among Kentucky bluegrass (Poa pratensis L.) genotypes and how such a difference affects N absorption when roots are in competition for soil N. A nutrient solution screening procedure was used to identify Kentucky bluegrass genotypes having high vs. low NO3− uptake capacity. Tillers of 60 Kentucky bluegrass cultivars were rooted and transferred to a continuous flow solution culture system. After establishment, plants were treated to develop moderate N deficiency. Isotopically labeled (15N) KNO3 was introduced at high (1 mM) and low (0.05 mM) concentrations to screen for differences in NO3− uptake. After a brief uptake period, plants were harvested, dried, and analyzed for 15N content to determine N uptake rate. There were significant differences among genotypes for uptake rate at both high and low N concentrations. The 60 genotypes exhibited a wide range of uptake rates, with strong correlation between rates at high and low N. The cultivars Julia and Midnight were selected as representing cultivars with efficient and inefficient NO3− uptake, respectively. Julia had NO3− uptake rates averaging 56% higher than Midnight. A subsequent lysimeter study examined whether higher NO3− uptake capacity would translate into increased N absorption under competitive conditions. Tillers of the two cultivars were planted as a mixed stand in soil or sand column lysimeters. After establishment, 15N‐labeled KNO3 solution at high (2 or 1mM) and low (0.05mM) concentrations was applied to each column. After an uptake period, individual plants of each cultivar were harvested for 15N analysis. Results indicated that Julia absorbed 20 to 50% more NO3− than Midnight at the high N concentration and 25 to 71% more NO3− more than Midnight at the low N concentration. Most differences were statistically significant. This indicates that differences in NO3− absorption by Kentucky bluegrass identified in solution culture translate into differences in absorption of soil N.

Nitrate retention and physiological adjustment of maize to soil amendment with superabsorbent polymers

Abstract Underground water pollution associated with nitrate leaching has become a major concern in areas with intensive cereal production; in areas with dry climates, water scarcity is the main limitation to crop production. We thus hypothesized that use of super absorbent polymers (SAP) may effectively increase nitrogen use efficiency by minimizing leaching and enhancing water and nitrate retention in the soil. Here, we evaluated nitrate movement in soils amended with SAP and determined changes in maize growth based on enzyme activities and physiological parameters. Nitrate retention was studied in six undisturbed soil lysimeters under different fertilizer (standard, medium or 75% and low, or 50% of standard) rates with (30 kg/ha) or without SAP. Maize yield decreased 20% under medium and 38% under low fertilizer rates but SAP application increased yield (P

Assessing Long Term Effects of Bioremediation: Soil Bacterial Communities 14 Years after Polycyclic Aromatic Hydrocarbon Contamination and Introduction of a Genetically Engineered Microorganism

Environmental contamination by organics such as polycyclic aromatic hydrocarbons (PAHs) generally changes native microbial communities. However our understanding of microbial responses has been limited to short term studies (i.e., less than 2-3 years) so long term community responses are not as well understood. In 1996, the genetically engineered microorganism Pseudomonas fluorescens HK44 was released into polycyclic aromatic hydrocarbon (PAH)-contaminated soil in lysimeters to monitor in situ PAH-biodegradation. The objective of this study was to assess the long term impacts of PAH contamination and addition of HK44 on the indigenous soil bacterial community structure. In 2010, 14 years after the lysimeter experiment initiation, lysimeters were unsealed and sampled. Although PAHs were degraded and PAH concentrations fell below detectable levels within approximately the first two years of this experiment, lysimeters that had received PAHs had significantly higher soil organic matter content (1.30 ± 0.23%) than control lysimeters with clean soils (0.81 ± 0.08%). Pyrosequencing of 16S rRNA gene amplicon libraries revealed a distinct bacterial community structure in the lysimeters that had received PAHs. In contrast, there were no discernible differences in soil chemistry or bacterial community structures in lysimeters where HK44 was inoculated compared to those to which HK44 was not inoculated. These results indicate that although the initial perturbations are no longer detectable, the addition of PAHs had long term influences on the bacterial communities, while the introduction of the genetically engineered microorganism HK44 did not.

Surface energy balance and actual evapotranspiration of the transboundary Indus Basin estimated from satellite measurements and the ETLook model

The surface energy fluxes and related evapotranspiration processes across the Indus Basin were estimated for the hydrological year 2007 using satellite measurements. The new ETLook remote sensing model (version 1) infers information on actual Evaporation (E) and actual Transpiration (T) from combined optical and passive microwave sensors, which can observe the land‐surface even under persistent overcast conditions. A two‐layer Penman–Monteith equation was applied for quantifying soil and canopy evaporation. The novelty of the paper is the computation of E and T across a vast area (116.2 million ha) by using public domain microwave data that can be applied under all weather conditions, and for which no advanced input data are required. The average net radiation for the basin was estimated as being 112 Wm−2. The basin average sensible, latent and soil heat fluxes were estimated to be 80, 32, and 0 Wm−2, respectively. The average evapotranspiration (ET) and evaporative fraction were 1.2 mm d−1 and 0.28, respectively. The basin wide ET was 496 ± 16.8 km3 yr−1. Monte Carlo analysis have indicated 3.4% error at 95% confidence interval for a dominant land use class. Results compared well with previously conducted soil moisture, lysimeter and Bowen ratio measurements at field scale (R2 = 0.70; RMSE = 0.45 mm d−1; RE = –11.5% for annual ET). ET results were also compared against earlier remote sensing and modeling studies for various regions and provinces in Pakistan (R2 = 0.76; RMSE = 0.29 mmd−1; RE = 6.5% for annual ET). The water balance for all irrigated areas together as one total system in Pakistan and India (26.02 million ha) show a total ET value that is congruent with the ET value from the ETLook surface energy balance computations. An unpublished validation of the same ETLook model for 23 jurisdictional areas covering the entire Australian continent showed satisfactory results given the quality of the watershed data and the diverging physiographic and climatic conditions (R2 = 0.70; RMSE = 0.31 mmd−1; RE = –2.8% for annual ET). Eight day values of latent heat fluxes in Heibei (China) showed a good resemblance (R2 = 0.92; RMSE = 0.04 mm d−1; RE = 9.5% for annual ET). It is concluded that ETLook is a novel model that can be operationalized further—especially after improving the preprocessing of spaceborne soil moisture data. This preprocessing includes (1) downscaling of topsoil moisture from 25 to 1 km pixels, and (2) translation of topsoil moisture into subsoil moisture values.

Controlled Release Urea Improved Nitrogen Use Efficiency, Activities of Leaf Enzymes, and Rice Yield

The effects of a controlled release urea (CRU) on concentration of N in root zones, activities of N metabolism enzymes in leaves, and N use efficiency of rice (Oryza sativaL.) grown on a silt loam soil in lysimeters were investigated. The entire amount (100–300 kg N ha−1) of the CRU was incorporated into a plug for growing the rice seedlings while the regular urea was split into three applications (the standard practice). Results indicated that the N release of CRU closely matched the N requirements of the rice, and the commingling of the CRU with the rice seeds increased N concentration within the root zone and enhanced the activities of glutamine synthetase (GS), glutamine 2‐oxoglutarate amidotransferase (GOGAT) and nitrate reductase (NR) in leaves. At the same N rate, the apparent N use efficiency of the CRU applied at 300 kg N ha−1was 27.6 and 22.9% higher than that of conventional urea treatment in 2007 and 2008, respectively. Although the CRU at 200 kg N ha−1supplied one‐third less N than the urea at 300 kg N ha−1, the CRU produced 3 to 5.9% more grain than the urea. Placing the CRU with rice seeds without additional fertilizer application during the entire growing season significantly increased N availability in soils and improved rice growth and led to use of less N fertilizers for greater rice grain yield.

Estimation of dissolved organic carbon contribution from hillslope soils to a headwater stream

The downward flux of dissolved organic carbon (DOC) in the vadose zone was examined using “artificial rain” experiments on a soil lysimeter at the White Clay Creek watershed in southeastern Pennsylvania. Refractory DOC (RDOC) and labile biodegradable DOC (BDOC) were transported from the organic rich soil layer and eluted from the base of a soil lysimeter during the experimental water applications. A physically based, distributed model that considers water flow and the transport of heat, RDOC and labile BDOC was developed and used to explore the hypothesis that the delivery of DOC to the water table occurs primarily in the riparian zone. Macropores were considered by adopting a dual permeability modeling approach. The calibrated model successfully replicated the temporal dynamics of a bromide tracer, and of RDOC and labile BDOC eluted from the soil lysimeter during the experiments. An estimated annual DOC delivery to groundwater in the watershed was obtained by scaling up the vertical one‐dimensional model to the watershed. The results suggest that the subsurface contribution of DOC accounts for about 72% of the total annual export from the watershed, and the riparian zone contributes more than 90% of total DOC in groundwater.