Bottom Soil Layer Research Articles

Regional estimation of base flow and groundwater recharge in the Upper Mississippi river basin

Groundwater recharge and discharge (base flow) estimates from two methods were compared in the Upper Mississippi River basin (USGS hydrologic cataloging unit 07). The Upper Mississippi basin drains 491,700km2 in Illinois, Iowa, Missouri, Minnesota, and Wisconsin and outlets in the Mississippi River north of Cairo, Illinois. The first method uses the water balance components from the soil and water assessment tool model (SWAT). The model was used to simulate the daily water balance of approximately 16 soil/land use hydrologic response units (HRU) within each of the 131 USGS 8-digit watersheds. The water balance of each HRU is simulated with four storages: snow, soil (up to ten layers), shallow aquifer, and deep aquifer. Groundwater recharge is defined as water that percolates past the bottom soil layer into the shallow aquifer. Recharge is lagged to become base flow and can also be lost to ET. The second method consists of two procedures to estimate base flow and recharge from daily stream flow: (1) a digital recursive filter to separate base flow from daily flow and (2) a modified hydrograph recession curve displacement technique to estimate groundwater recharge. These procedures were applied to 283 USGS stations ranging in area from 50 to 1200km2. A smoothed surface was obtained using a thin plate spline technique and estimates were averaged for each 8-digit basin. Simulated flow was calibrated against average annual flow for each 8-digit. Without further calibration, simulated monthly stream flow was compared against measured flow at Alton, Illinois (445,000km2) from 1961–1980. To validate the model, measured and simulated monthly stream flow at Alton from 1981–1985 were compared with an R2 of 0.65. No attempt was made to calibrate base flow and recharge independent of total stream flow. Base flow and recharge from both methods were shown to be in general agreement. The filter and recession methods have the potential to provide realistic estimates of base flow and recharge for input into regional groundwater models and as a check for surface hydrologic models.

Maize and Soybean Tap, Basal, and Lateral Root Responses to a Stratified Acid, Aluminum‐Toxic Soil

Although crop seedling root systems are made up of several types of root (e.g., tap, basal, and lateral roots) there has been little attempt to determine their respective sensitivity to abiotic stresses. Most acidic agricultural fields are treated so that they are only slightly or non-Al-toxic in the topsoil with Al-toxicity in the subsoil—effectively stratifying the soil profile relative to Al-toxicity. To determine if there is differential root type sensitivity to Al-toxic soils, several maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] cultivars reported to be Al-tolerant, or sensitive, were grown in containers with a stratified soil. This assembled soil consisted of a non-Al-toxic, 100-ramdeep top layer and a 200-mm-deep Al-toxic bottom layer of Porter soil, a coarse-loamy, mixed, mesic Umbric Dystrochrepts, pH 4.5 and 92% AI saturation. Containers, backed with X-ray film, were exposed to a neutron beam to determine tap, basal, and lateral root lengths in both layers. Root lengths, relative to a limed control for each cultivar, in the Al-toxic bottom layer suggest that the tap, basal, and lateral roots of maize cv. CMS-36 are each Al-tolerant. The other “tolerant” maize and soybean cultivars had only one or two root types tolerant with the remaining root type(s), and all root types the sensitive cultivars, showing no growth in the Al-toxic bottom layer. Only two cultivars, maize cv. SA-6 and soybean cv. Perry, increased lateral root production in the non-toxic top layer, a classic avoidance response. This variation in root type response to an Al-toxic bottom layer observed among the previously identified Al-tolerant cultivars emphasizes the importance of considering different root types as distinct entities in studies of root function.

Effect of simulated acid precipitation on nitrogen mineralization and nitrification in forest soils

After exposure of samples of three forest soils (pH 3.4 to 3.9) from the Adirondacks region of New York to 60, 230, or 400 cm of simulated rain of pH 3.5 or 5.6 in 4, 14, or 24 weeks, respectively, the soil samples were separated into the 0 to 2 and 2 to 5 cm organic layers and further incubated. The rates of N mineralization in Woods soil exposed to the simulated precipitation were less for rain at pH 3.5 than at pH 5.6, but the inhibition decreased with increasing exposure of the 0 to 2 cm layer. In Panther soil, the rates of mineralization were usually not affected by the acidity of the simulated rain. In the upper layer of Sagamore soil, mineralization was not influenced by pH of the simulated rain, but the transformation was faster in the bottom layer of soil after prolonged exposure to simulated rain at pH 3.5 than at pH 5.6. The rate of nitrate formation in Panther and Woods soil amended with ammonium was inhibited by the more acid rain. Studies with 15NH4 indicated that ammonium was oxidized to nitrate even though ammonium levels did not decline or declined only slightly after prolonged exposure of Panther or Woods soil to rain at pH 3.5. The growth of orchardgrass in Panther and Woods soil was inhibited by the more acid simulated rain.

EFFECTS OF SOIL MOISTURE AROUND NODULES ON NITROGEN FIXATION BY WELL WATERED SOYBEANS

Soybeans (Glycine max (L.) Merr.) were grown at various soil moisture levels around nodules so effects on nitrogen fixation could be studied. Plants were grown in a growth room in 35-cm diam pots. Pots contained two layers of loam–sand mixture separated by a layer of coarse silica, intended to restrict capillary movement of soil moisture from the bottom to the top soil layer. At the beginning of seed development, pots received 200 ml water on the surface, 200 ml in the bottom layer through plastic tubes or 100 ml each way. Plants with good root development in the bottom soil layer maintained leaf water potentials greater than − 6 bars, while soil moisture around nodules varied from 4 to 20%. In three individual experiments, in which only plants with leaf water potentials greater than − 6 bars were considered, there was no relationship between soil moisture around nodules and mg N2[C2H2] fixed/g nodule dry weight × h. When results of two experiments with similar fixation means were combined, there was a barely significant (R2 =.19*) quadratic relationship, with very dry soil or soil near field capacity around nodules decreasing fixation slightly. Percent moisture in soil around nodules did not affect nodule moisture content, indicating that nodule moisture status was maintained if plants received adequate moisture from below the nodule zone.