1996 Annual Report for AW96-014
The Impact of Riparian Vegetation Filters on Western Soil and Water Quality: Nonpoint-Source Pollutants from Range and Croplands
Summary
Objectives:
1. Determine the establishment rate, biomass production and nitrogen (N) and phosphorus (P) uptake by hybrid willow and grasses under greenhouse and field conditions of variable soil salinity.
2. Determine the impact of vegetation filters on N and P concentration and load in groundwater and streamflow.
3. Determine denitrification rates (spatially, seasonally, at various depths) in riparian soils before and after installation of vegetation filters, and to estimate the contribution of microbial denitrification to total nitrate removal from nonpoint sources before and after installation of vegetation filters.
4. Determine the physical effects of stubble height/grazing residue of grazed tall wheatgrass vegetation filters on sediment entrapment.
5. Determine differences in local and non-local landowner reaction to pro-active philosophy and strategy of the project and to determine the most effective methods of disseminating study results.
Abstract
Thirty cattle exclosures, each 15 by 30 meters (m), were constructed in the Horse Creek riparian area on the Rottman Ranch, Hawk Springs, Wyoming; 15 were on the rangeland side and 15 on the corn/alfalfa cropland side. Three replications (exclosures) per land type were planted with 1) hybrid willow cuttings in June 1997, or broadcast seeded in March 1997 to 2) tall wheatgrass, 3) creeping foxtail, 4) beardless wildrye, or 5) basin wildrye. All but three plantings failed to produce an acceptable stand because of excess salinity, drought, unfavorable seeding conditions, or seed with low germination rates under saline conditions. The stands were replanted in late winter and early spring 1998 after new seed and varieties are tested in a germinator under saline conditions. Jose tall wheatgrass establishment in a greenhouse study using Horse Creek saline and saline-sodic soils was greater than 92 percent under salinity levels of 1.6, 3.5, and 6.8 deciSiemens (dS), but hybrid willows establishment was 93, 52, and 0 percent, respectively. However, all surviving plants reached soil depths of 0.8 m within 150 days of growth and are therefore capable of extracting ground water during the first growing season.
Different N concentrations in both tall wheatgrass and hybrid willow subjected to different salinity levels, N and P soil water concentrations, and lengths of growing season indicate that grass and willow should be harvested and/or grazed every 90 to 120 days to maintain the optimum nitrate extraction plant vegetative stage and remove the maximum N from the riparian system. Grass and willow leaf, stem (willow only), and root weights, root distribution (%), and N concentrations per plant part and top and bottom soil column halves were determined for all treatments. Jose tall wheatgrass is the best adapted to high saline-sodic conditions; Garrison creeping foxtail was slow to establish, but has become very productive. Because hybrid willows were determined to be inappropriate for highly saline/sodic conditions, we planted hackberry, cotoneaster, New Mexico privet, coyote willow, and black locust upon recommendations from the local USDA NRCS office. All tree seedlings were significantly defoliated by a severe grasshopper population, but New Mexico privet and black locust were the most successful. Two, shallow (i.e., about a meter deep) groundwater wells were established in each exclosure. Groundwater and stream samples were collected in different seasons and analyzed for pH, salinity, nitrate, sulphate, and phosphate concentrations. All samples showed less than 10 mg nitrate per liter of water, or well below the maximum allowed by US EPA.
Specific Results
Of the four species of grasses planted, alkar tall wheatgrass produced more above ground biomass (mean = 195.8 g per 0.5 m2; SE 56.5) and more percent cover per plot (25% - 85%) than the other grasses combined. The maximum height attained by the various species ranged from approximately 1.5 m for Garrison creeping foxtail and beardless wildrye to 2.0 m for alkar tall wheatgrass to 2.2 m for magnar basin wildrye.
Salinity and pH determination of soil samples collected from each exclosure indicated an extremely high soil quality variability depending on the microsite conditions (e.g., soil age and texture, topography, depth to ground water). Therefore, interpretations of treatment (i.e., land type, plant species) effects on nutrient uptake and denitrification need to made using multivariant and co-variant analyses.
Ground water salinity levels from wells in treatment plots ranged from zero to six deciSiemens (dS). Salinity levels were highest in February 2000 and May 1999 but lowest August 1999. Salinity levels in Horse Creek ranged from one to two dS and in control wells from two to three dS. The pH levels in ground water from wells inside treatment plots ranged from 7.1 to 8.9. The pH in Horse Creek ranged from 7.9 to 8.9 and in ground water from control wells from 7.1 to 7.7.
Denitrification rates were extremely low under all conditions.
Effects of species, stubble height, and biomass were not determined because Horse Creek only flooded the lower ends of certain plots during one year of the project. The scouring action of ice flows and erosion, rather than deposition, of the inside bends of the stream make this objective unrealistic in this area.
Neighboring ranchers were impressed with the production from tall wheatgrass plantings, less so with the slow spread of creeping foxtail, and disappointed in the failure of hybrid willows to grow under saline conditions. Wildlife biologists visiting the study area stated that the persistent tall wheatgrass cover throughout the winter will be valuable for nesting cover during the spring nesting season.
Potential Benefits
The financial implications to ranchers of the project results to date are mixed. Indications are that riparian plantings should be fenced from grazing until established and then grazed in a rotation system to maintain the optimum vegetative stage for nutrient uptake from ground water. Fencing riparian pastures is extremely expensive because of their configuration. However, there are existing cost-share programs which encourage fencing riparian pastures for rotation grazing. In addition, greenhouse results indicate the capacity of both tall wheatgrass and hybrid willow to produce very high quality fodder that could be useful for flushing breeding animals or for feeding animals with high nutrient requirements.
Farmer Adoption and Direct Impact
On the basis of results to date, cooperating ranchers applied for and received a cost-share program involving fencing a portion of a riparian area for separate wetland and agroforestry experimentation.
Tall wheatgrass (and similar tall saline-tolerate grasses) in riparian areas should be test-harvested at different frequencies and/or seasons and analyzed for forage or hay quality to determine the optimum grazing/harvesting frequency to produce the combination of quantity and quality which best fits each rancher’s total forage needs.
This summary was prepared by the project coordinator for the 2000 reporting cycle.