Assessment of Soil Health under Native Warm-Season Grasses and Different Grazing Management

This research uses analysis of soil quality indicators (SQIs) to compare vegetation species and grazing management over the 2021 grazing season. The soil health effect of the native warm-season grasses (NWSG) big bluestem (BB) (Andropogon gerardii) mixed with indian grass (IG) (Sorghastrum nutans) (BBIG), and switchgrass (SG) (Panicum virgatum), inter-seeded with a 12 species biodiversity mix was investigated in a 5 pressure grazing system (no graze (NG), no rest (NR), early, middle, and late rest (ER, MR, LR)). Additionally, there is a need for inexpensive tools for land owners to assess soil quality, and a validation study was performed on a new rapid soil test (soil microBIOMETER®) for microbial biomass carbon (MBC) using correlation with chloroform fumigation-extraction (CFE).

Soil quality indicators are a proxy for carbon sequestration and soil resilience. Indicators analyzed included MBC using CFE and soil microBIOMETER®, labile carbon using permanganate oxidizable carbon (POxC), gravimetric water content, wet aggregate stability (WAS), bulk density (BD), and total organic carbon (TOC) over 4 years.

The data showed that when α = 0.1, TOC was increasing but was not significantly different between the years 2018, 2020, and 2022 over the entire NWSG pasture or within vegetation species. There was more TOC under BBIG and no difference in TOC between NG and NR. The BBIG portion of the pasture had consistently better values for SQI than SG. The data suggest that continuously grazing NWSG forage during the warm-season in East Tennessee at a stocking density of 2.4 – 4.2 AU/ha does not result in poorer soil quality than a NG or rotational grazing approach.

Soil microBIOMETER® had weak correlation with CFE at MBC valuesμgC/g soil. Correlation between the methods improved as measured MBC flush increased. Soil microBIOMETER® distinguished between vegetation species and provided a similar range of values as CFE, especially when adjusted by a factor of 0.55 at valuesμgC/g soil. Individual data points did not track closely between CFE and microBIOMETER®. Soil microBIOMETER® is a promising tool for rapid soil health assessment in the field when range of MBC or comparison of treatments is of interest.