Jalal D. Jabro

Soil nitrogen mineralization during laboratory incubation: dynamics and model fitting

Abstract Soil nitrogen mineralization kinetics were studied for eight treatments of two soils in an aerobic long-term (30 wk) incubation experiment. Soil mineral-N (NH 4 + and NO 3 − ) in the leachates was measured every week during the first 9 wk and every 2 or 3 wk thereafter. The NH 4 + percentage of the mineral-N ranged between 85 and 99% for all treatments in the first week of incubation and remained high (> 80%) in several treatments until the end of wk 4. Starting at wk 7, NH 4 + concentrations were negligible in all treatments. The net N mineralization rate was 15–24 mg N kg −1 wk −1 during the first 4–6 wk and 2–5 mg N kg −1 wk −1 from wk 8 until the end of the incubation. Four models, (i) a one-component, first-order exponential model (the single model), (ii) a two-component, first-order exponential model (the double model), (iii) a one-component, first-order exponential model including a constant term (the special model), and (iv) a hyperbolic model, were fit to the cumulative mineral-N vs time data using a non-linear regression procedure. The goodness of fit of the four models depended on the duration of incubation. With 30 wk data, the double and special models were significantly better than the other two models; with the first 15 wk data, the four models had essentially the same goodness of fit for seven out of eight treatments. The values of the regression parameters derived from each model also depended on the incubation duration. Results from this study show that the pool size and mineralization rate parameters in the different models are merely mathematically-defined quantities obtained from the kinetic analysis of the net N mineralization and do not represent any rigorously-defined pool sizes of potentially-mineralizable N and their mineralization rate constants in the soils.

Evaluation of nitrogen version of leachm for predicting nitrate leaching

The abilities of the Richards and convectiondispersion equations approach (LEACHNR) and the capacity model approach (LEACHNA) of the nitrogen version (LEACHN) of the LEACHM model to simulate nitrate leaching were evaluated using field data from a 5-year nitrate leaching experiment conducted in central Pennsylvania on Hagerstown silt loam soil (fine, mixed, mesic, Typic Hapludalf). Nitrate leaching losses from N-fertilized and manured corn below the 1.2-m depth were measured with zerotension pan lysimeters. Three N-fertilized and manured treatments for 1988–1989, 1989–1990, and 1990–1991 and two N-fertilized treatments for 1991–1992 and 1992–1993 were used from the leaching experiment to evaluate both approaches of LEACHN. The individual monthly simulations of nitrate leaching were compared with the mean of pan efficiency corrected-measured data for these 5 years. Both approaches of the model were calibrated to the site conditions using the data of 1989–1990 and were then evaluated using 1988–1989, 1990–1991, 1991–1992 and 1992–1993 nitrate leaching data. Simulated results for the calibration year for both models were reasonably accurate (31 of 36 months simulated within the experimental 95% confidence limits). The statistical analysis used in this study indicated that both LEACHNA and LEACHNR adequately (91 to 120 months within the 95% confidence limits) predicted nitrate leaching below the 1.2-m depth for treatments in the refinement years. Much of the simulation error in some treatments in the refinement years seemed to be related to the sub-routine controlling soil nitrogen transformation processes and their rate constants in the model. The large deviations in NO−3-N leached in some winter months may be related, in part, to problems with simulated water flow associated with the frozen soil conditions and snow accumulation. The addition of a dual-pore water flow option (LEACHNA) to the nitrogen version of LEACHM did not improve prediction of nitrate leaching beyond the rooting zone of corn under Pennsylvania conditions.

Variability of field-saturated hydraulic conductivity in a Hagerstown soil as affected by initial water content

Field-saturated hydraulic conductivity (Kfs) is perhaps the single most important property for flow and transport-related phenomena in soils. Kfs is prone to variability because of swelling and shrinking of clay that results from differences in initial soil water content. The effects of initial soil water content on the variability of Kfs in a structured clayey soil were investigated in this study. The soil was classified as a Hagerstown silt loam (fine, mixed, mesic, Typic Hapludalf). The Kfs and initial water content were measured monthly from May 1994 to November 1995. The Guelph permeameter was used to measure Kfs. Three Kfs measurements were made in-situ on three corners of an equilateral triangle with sides of 3 m at a depth range of 0.35-0.40 m. The measurements were taken between the 23rd and 31st day of each month, depending on weather and soil conditions. A soil sample was collected from each hole at a depth of 0.35-0.40 m before making Kfs measurements in order to determine water content using the gravimetric method. Variations and fluctuations in Kfs values were observed throughout the duration of the study. Kfs values in June and July 1994 were about 4 to 15 times greater than in the spring, fall, and winter. In 1995, Kfs varied in the same manner as it did in 1994. Kfs values for the interval from June through September 1995 were much larger than in early spring, late fall, and winter. This variation in Kfs was attributed primarily to changes in soil structure and macroporosity caused by the swelling and shrinking of the clay associated with variations in soil water content throughout the year. A curvilinear logistic relationship (R 2 = 0.95) was found between the data of Kfs and initial soil water content.

Estimation of preferential movement of bromide tracer under field conditions

Abstract Leaching of agricultural chemicals from the root and vadose zones into groundwater is an important environmental concern. To procure a better understanding of the movement and transport of agricultural chemicals through the soil profile, a field research study was conducted to estimate bromide leaching losses under saturated conditions where preferential flow is occurring. The field data were then used to evaluate the LEACHM model. Eighteen double-ring infiltrometers were used to apply a pulse (100 mm depth) of bromide tracer on two previously saturated soils located in a karst region of southeastern Pennsylvania. Internal drainage over the next seven days resulted in nearly 51% of the applied Br − being leached to a depth below 0.80 m. The LEACHM model was used to simulate the amount of bromide leached in each infiltrometer. The model predicted, accurately, an average of 46% of the applied Br − leached below the 0.80 m depth. Mean values of bromide concentration in the soil profile were predicted within two standard deviations of the measured mean for all depths except for the 0.20–0.40 m depth increment where the model overpredicted the bromide concentration. The model predictions of Br − leached were tested against field measurements using several statistical tests. The LEACHM model performed adequately under preferential flow conditions, perhaps because the infiltration rate at each site was used as a model input. This, actually, is some measure of the macropore flow process and suggests that simple models such as LEACHM can be used in the field, as long as a distribution of infiltration rates is used as an input.

Predicting bromide leaching under field conditions using slim and macro

The updated versions of the SLIM and MACRO computer simulation models were evaluated using 3 years of bromide leaching data from a field experiment conducted on Hagerstown silt loam soil (fine, mixed, mesic Typic Hapludalf) with a well developed structure in central Pennsylvania. An application of potassium bromide (KBr) tracer was broadcast to 18 plots at 100 kg ha−1 at planting on May 13, 1988. Eighteen zero-tension pan lysimeters (0.456 m2) were placed at a depth of 1.2-m below the soil surface to collect gravitational water samples. Average pan collection efficiency was 52%, with a CV of 44%. Model simulations of Br− leaching for 1988, 1989, and 1990 were compared with the mean of pan efficiency-corrected measured data. There were no statistical differences between the SLIM and MACRO model predictions and, the experimental data for 1988 except for 1 or 2 months for each model. The SLIM model accurately predicted Br− leaching in 1989 but significantly underestimated Br− leaching from October 1990 through February 1991. The MACRO model significantly underestimated Br− leaching from May to November 1989 and performed well in 8 of 12 months in 1990. The overall simulations for 3 years of each model were compared with the overall field measurements of Br− leaching. The statistical evaluation criteria indicated that the SLIM model performed somewhat better than the MACRO model through the 3-year period. The results given in this study indicate that both models have good potential for conservative tracer simulation

Nitrogen Fertilizer Rate and Crop Management Effects on Nitrate Leaching from an Agricultural Field in Central Pennsylvania

Eighteen pan lysimeters were installed at a depth of 1.2 m in a Hagerstown silt loam soil in a corn field in central Pennsylvania in 1988. In 1995, wick lysimeters were also installed at 1.2 m depth in the same access pits. Treatments have included N fertilizer rates, use of manure, crop rotation (continuous corn, corn-soybean, alfalfa-corn), and tillage (chisel plow-disk, no-till). The leachate data were used to evaluate a number of nitrate leaching models. Some of the highlights of the 11 years of results include the following: 1) growing corn without organic N inputs at the economic optimum N rate (EON) resulted in NO3-N concentrations of 15 to 20 mg l in leachate; 2) use of manure or previous alfalfa crop as partial source of N also resulted in 15 to 20 mg l of NO3-N in leachate below corn at EON; 3) NO3-N concentration in leachate below alfalfa was approximately 4 mg l; 4) NO3-N concentration in leachate below soybeans following corn was influenced by fertilizer N rate applied to corn; 5) the mass of NO3-N leached below corn at the EON rate averaged 90 kg N ha (approx. 40% of fertilizer N applied at EON); 6) wick lysimeters collected approximately 100% of leachate vs. 40–50% collected by pan lysimeters. Coefficients of variation of the collected leachate volumes for both lysimeter types were similar; 7) tillage did not markedly affect nitrate leaching losses; 8) tested leaching models could accurately predict leachate volumes and could be calibrated to match nitrate leaching losses in calibration years, but only one model (SOILN) accurately predicted nitrate leaching losses in the majority of validation treatment years. Apparent problems with tested models: there was difficulty estimating sizes of organic N pools and their transformation rates, and the models either did not include a macropore flow component or did not handle macropore flow well.

Water Balance And Flow Patterns In A Fragipan Using In Situ Soil Block

A soil block (1.5 m × 1.5 m × 1.5 m) was excavated at a footslope position on a hillslope underlain by a fragipan horizon. The block vertical faces were sealed on three sides using bentonite, sand, and lumber, with horizontal pressure exerted from basement jacks. The downslope face of the block