GRAZING AS AN INTEGRATIVE SOIL HEALTH STRATEGY: IMPACTS ON SOIL MICROBIAL DYNAMICS AND LABILE CARBON AND NITROGEN POOLS

Kaone  Lorraine Abanikannda

The semi–arid landscape of Northeast (NE) Washington (WA) includes small grain dryland, temperate evergreen forest biomes, and ephemeral riparian zones as principal agroecosystems. These ecosystems coexist within a shared climatic envelope, but they differ in vegetation inputs, soil moisture regimes, and land–use history. Even with literature evidence of world soil degradation, there is lack of reference data to guide comparisons of soil health measures across NE WA agroecosystems, which is a concern when we consider the unique soil characteristics and ecosystem services provided by each agroecosystem. Therefore, given the increasing interest in improving soil health and ecosystem services under these management and ecosystems, the overarching goal of this project was to provide data–driven insights into the impacts of diversified agroecosystems on the overall soil health. For two years, we tested controlled short duration cattle grazing as the main integrative soil health practice, which is increasingly adopted in NE WA agroecosystems. To account for other literature gaps such as climate variability on soil health, we also included a study that measured the interaction of cattle grazing and global change factors, including salinity, drought, and warming. We characterized the potential for soil health improvement by monitoring the soil microbial community composition and functional potential, as well as the labile soil carbon and nitrogen pools (i.e., microbial extractable carbon and nitrogen, active carbon levels, and mineralizable nitrogen). The first study of this dissertation was a field study that integrated 16S and ITS1 gene sequencing for microbial community and functional profiling, which was correlated to labile pools in all agroecosystems. While microbial community and labile fractions were primarily driven by agroecosystem type (vegetation x management), our results showed positive impacts from short–duration, high–intensity grazing with increased microbial activity and labile pools. For the second study, we utilized cattle grazing on soils that were then transitioned to an eight–week mesocosm study. The same integrative approaches were used for this study, although we measured ecosystem stability with resistance and robustness ecological concepts. Overall, this study revealed grazing as a biological anchor for soil microbial communities, with higher labile carbon and nitrogen pools under grazed groups. We observed erratic responses within the bacterial community in the first year, indicating lack of resistance, followed by a greater level of stability by the second year, indicating an emergence of robustness. Fungal communities showed more consistent responses and resistance to global change factors under grazing overall. Future work should study diverse WA soils and under large–scale field conditions.

GRAZING AS AN INTEGRATIVE SOIL HEALTH STRATEGY: IMPACTS ON SOIL MICROBIAL DYNAMICS AND LABILE CARBON AND NITROGEN POOLS

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