Relationships between grassland community characteristics and soil properties along a grazing gradient in mixed-grass prairie

Background/Question/Methods

The extent that soil properties are correlated with grassland community composition and productivity has implications for understanding how grassland ecosystems are structured and for improving management of rangelands.  Here I explored these linkages over a livestock grazing gradient in a 0.3-ha site in eastern Montana and centrally located in a >22 million ha region of grazed grasslands.  Prior to measurement, the area was completely fenced off from livestock grazing.  Grassland composition and productivity was measured from 96 points, soil stability (mean weight diameter across six size classes and water stable aggregates [WSA] across three size classes) from 99 points, soil nutrients (soil organic matter, soil carbon, total nitrogen, and roots) at three depths and from 99 points, and soil physical properties (infiltration sorptivity and field-saturated infiltration) from 24 points, respectively.  The eight soil property variables were then used as predictors of total plant and gramanoid biomass of points across the grazing gradient.

Results/Conclusions

Plant total biomass was not correlated with any of the soil properties.  Gramanoid total biomass was positively correlated with field-saturated infiltration.  However, gramanoid total biomass was negatively correlated with the percent WSA of the largest size class of macroaggregates and not correlated with the other two.  Contrary to predictions, additional correlations among soil properties revealed that two of three measured size classes of percent WSA were negatively correlated with field-saturated infiltration and the third was not correlated with field-saturated infiltration.  These results indicate that measures of soil stability (i.e. percent WSA) may be either not correlated or negatively correlated to measures of plant and grass productivity and measures of soil physical structure (i.e. water infiltration).  This is problematic because prominent rangeland monitoring guides include soil stability tests as an indicator of degraded ecosystem function that result from severe management practices (e.g. overgrazing).  This is likely because soil stability is generally thought to be positively correlated with water infiltration and primary productivity and negatively correlated with surface erosion.  However, this research indicates that variation in water infiltration and primary productivity may be driven by other factors and decoupled from variation in soil stability.  This study reveals a need for more applied research across more systems and environmental gradients to further validate the appropriateness of individual soil variables as meaningful indicators of negative and positive effects of management practices on grassland productivity and sustainability.