J.M. Volk, J.L. Richardson, and R. Utter, NDSU Soils Science Department
W.T. Barker, NDSU Animal and Range Sciences Department
Table of Contents
Methods and Procedures
Initial Findings and Discussion
Mollisols, or black prairie soils, dominate the Missouri Coteau. The black color is due to soil organic carbon developed from decayed plant roots in the soil profile. Very little organic matter from the above-ground herbage is incorporated into the soil, hence its formation requires the penetration of roots deep into the soil. The natural fertility of these soils comes from the interaction of the mineral matter, microbes, and decaying roots that create soil organic matter. Many of the soil physical properties are derived from the soil organic matter, especially those that are related to water infiltration and water holding capacity.
The importance of this study stems from the impact of grazing on soil structure and soil aggregation, especially on water holding capacity, depth of water infiltration from rainfall events, and plant available moisture potential. Thus, the objectives of this study were to determine the effects of three land uses: intensive season-long grazing, rotational grazing, and idle land, on the soil physical properties at one time in the grazing season on a silty Bowbells Soil range site.
Methods and Procedures
Transverse transects were located perpendicular with drainage in three areas with similar topography and soil type, one on the season-long grazed site, the second on a rotationally grazed site, and the third on a idle landscape. All three pastures were located on the Central Grasslands Research and Extension Center (CGREC), near Streeter, North Dakota. A backhoe was used to dig the trenches which measured 27 feet long, 4.5 feet deep, and approximately 3 feet wide.
Within each soil trench, multiple soil samples were collected and analyzed for: aggregate stability, bulk density, texture, macro porosity (0.2 bar tension), micro porosity (15 bar tension), total carbon, calcium carbonate, plant available phosphorus, saturated hydraulic conductivity, and rooting biomass at depth. Eight sample depths were collected on each of three replications in each trench, at: 0-2, 2-5, 5-10, 10-20, 20-30, 30-40, 40-50, and 50-60 inches. The backhoe on the idle land had problems digging because of the severe compaction and a depth of 30 inches was all that could be achieved.
Initial Findings and Discussion
Rotational grazed treatments averaged 3241 lb/ac of forage production from 1985 to 1993 (Barker and Whitted 1993), with peak production in 2004 being 3517 lb/ac. The season-long pasture produced 2680 lb/ac, in 2004 and a 16-year peak production average of 2588 lb/ac (Patton unpublished data). We have no comparable data for production on idle land. Bio-diversity of pastureland, which is especially important in sustaining August and September grazing, was high in the rotationally grazed (30 species) and season-long pastures (39 species), but extremely low in the idle land (12 species), with an 85% dominance of smooth bromegrass.
Infiltration rates were measured with a modified double ring infiltrometer (30 inches outside diameter). The rotationally grazed site had the highest infiltration rate at 8 in/hr. The season-long and idle sites were similar with 1 in/hr and 2 in/hr, respectively. The saturated hydraulic conductivity, which is the measure of a soil's ability to transmit water did not follow suit. The saturated hydraulic conductivity was 1.5 in/hr on the season-long site and 0.5 in/hr on both the rotationally grazed and idle in the A horizon. The saturated hydraulic conductivity of the Bk horizon (20 inches) resulted in the season-long grazed site (0.5 in/hr) being higher than the idle site (0.005 in/hr). The rotationally grazed site (0.25 in/hr) was not statistically different from the season-long or idle land.
The most important soil property in our opinion is the macro porosity (0.2 bar tension) from 12 inches to 40 inches. Macro porosity below 20 inches was much higher in the rotationally grazed site than in the idle, which we were unable to dig with a backhoe past 30 inches because of virtually no macro pores. At 30 inches all treatments had 50 to 51 percent total porosity, yet the rotation grazing was higher in macro porosity, suggesting water can move down through this zone faster and be removed by plants easier. Because there are fewer macro pores in the season-long grazing and idle land plant roots are shallower and the stored water harder for plants to remove. The plants, therefore, have access to less stored soil moisture.
Aggregate stability was tested on all three sites at each depth. Each had approximately 97% water stable aggregates. This is in contrast to conventional tillage soils which may contain only 70% water stable aggregates, suggesting that these are subject to higher erosion rates and large water losses. Results demonstrate that the rooting depth and turnover of roots in rotationally grazed systems, caused by the disturbance of grazing followed by rest, creates higher macro porosity. These results amplify the soil structure and root development observations previously obtained and reported by us, along with previous studies by others, regarding infiltration and soil physical properties.
The initial findings in this study are summarized by stating that the rotationally grazed sites allow for high water infiltration and macro porosity to store and transport water. The season-long site is characterized by possessing the ability to store water in the soil profile, but lacks the ability to allow for water infiltration. The idle land resulted in low infiltration rates and very low saturated hydraulic conductivity. This in turn relates to little water entering the site and very little water movement throughout the soil profile, creating virtually zero macro porosity below 30 inches. This study will continue and more results will be available in 2005.