Article: Effect of grazing on soil-water content in semiarid rangelands of southeast Idaho
Authors: Weber, K.T. Gokhale, B.S.
Published: Journal of Arid Environments 75 (2011) 464-470
This article is a first for the Holistic Management community in a number of regards. It is one of the first peer-reviewed journal articles to show a link between animal impact and increased soil moisture. It is one of the first to specifically use (simulated) holistic planned grazing as an experimental treatment. It is also one of the first studies to apply a suite of technological innovations, like GIS, GPS, and soil moisture probes, towards monitoring the effects of holistic planned grazing over the medium term.
The paper articulates the research goal:
The goal of this study was to determine if soil-water content is affected by land management decision (e.g. grazing and rest), within the semiarid sagebrush-steppe rangelands of southeast Idaho. Specifically, this study sought to experimentally determine if a positive relationship exists between soil-water content and litter cover and if land use treatments can be used to beneficially manipulate litter cover.
The experimental design involved the division of a largely homogenous (ie topography, soils, climate) experimental grazing range into three fenced treatments. The first was simulated holistic planned grazing (SHPG), “where cattle grazed at high density (66 AU/11 ha) for a short period of time (6 days) during the first week of June each year.” Under the second treatment, rest rotation (RESTROT), “cattle grazed at low density (300 AU/ 1467 ha) for long periods of time (30 days) during the month of May each year.” Recovery days for these two treatments were 359 days and 335 days, respectively. The final treatment was total rest (TREST), “where no livestock grazing has occurred since June 2005.”
A number of ecological indicators were monitored through the course of the study. Soil moisture was recorded using thirty-six Decagon ECH20 capacitance sensors buried at a depth of 100 mm; 12 probes were installed for each of the three treatments. Vegetation variables measured were percent shrub, grass, litter and bare ground, and biomass was also measured via clipping and weighing.
After data collection, a number of statistical tests were run on the data. First, the comparative results of percent litter cover:
Comparisons of percent litter cover revealed significant differences among all three treatments beginning in 2007 but no difference prior to this time. Pair-wise comparison showed significantly higher litter cover in the SHPG pasture compared to the RESTROT pasture in both 2007 and 2008, as well as higher litter cover in the SHPG pasture relative to than found in the TREST pasture in 2007.
A similar pattern emerged from the data set for soil moisture:
Comparisons of daily %VWC among treatment pastures indicate a significant difference when all treatments were compared at once. Pair-wise comparisons indicated %VWC was significantly higher in the SHPG pasture compared to the RESTROT and TREST pastures in 2006, 2007, and 2008. No difference in %VWC was found between the RESTROT and TREST pastures in either 2006 and 2007 although %VWC was higher in the RESTROT pasture in 2008.
Do environmental factors alone account for these changes in soil moisture? The authors explain that although “%VWC differs annually, it is differentially variable by pasture, suggesting both an environmental and a treatment influence.”
This study provides strong experimental evidence to support the effects of planned grazing and animal impact long applied by holistic management practitioners. Specifically, this study validates the use of animal impact to control the arrangement of residual biomass in relation to the soil surface. The authors summarize some of the physical and biological mechanisms at play:
Litter improves soil nutrients and structure while reducing erosion
Litter reduces soil surface temperature, which lowers surface evaporation and “impedes the volatilization of soil carbon, thereby reducing greenhouse gas emissions”
Grazed pastures have higher levels of organic matter particles in the soil, probably caused by increases in biological decomposition when litter is put in contact with the soil surface
The authors provide further insight:
The changes observed in the SHPG treatment pasture appear to be the result of several interactive affects (high-intensity/short-duration grazing, animal impact, and increase litter cover) that produced a positive feedback cycle which may ultimately improve the condition and sustainability of these rangelands.
And:
Indeed, the different grazing treatments may have altered the porosity and organic matter of the soil within each treatment pasture through differential production and decomposition of litter, thereby offering a likely explanation of how these soils were able to retain more water throughout the growing season.
While these final thoughts are conjecture on the part of the authors, it certainly would fortify their argument if soil organic matter had been measured, analyzed, and reported with the same rigor of soil moisture. Future replications of this study should include such measurements. After many years, some research evidence is emerging that now supports many of the experiential lessons learned by farmer and ranchers. One question arises: Why is this evidence emerging now, after many years of debate and inconclusive research? A principle reason is the fact that Dr. Weber understands the principles embodied in Holistic Management and planned grazing, and has applied them in his experimental design. To date, few research designs have taken this approach. Another potential reason is our growing technological capacity to collect and analyze data. GIS/GPS technology allows us to randomize data collection points; digital soil moisture probes allow for the creation of very large, accurate data sets with relative ease; and statistical analysis software gives a single researcher the power to analyze massive amounts of data in very short time periods. With these new tools, which continue to grow and develop, monitoring and research should continue to experience major upgrades in the coming years, and should provide us with deeper insights into the functioning of grassland ecosystems and their ecological responses to management decisions.