Soil biodiversity is instrumental for ecosystem functions such as nutrient cycling and carbon (C) storage. However, as anthropogenic disturbance places soil biodiversity at risk, we may witness disruptions in these essential ecosystem services. Despite its significance, understanding and quantifying shifts in soil biodiversity and their impacts on ecosystem functioning remains challenging. My dissertation addresses three knowledge gaps regarding the use of free-living nematodes as bioindicators of soil biodiversity and function in agroecosystems. First, I assess the relationship between nematode community assemblage and soil C pools and assess how these dynamics shift through time in contrasting agroecosystems. Regenerative agriculture can enhance soil food web structure through improved soil health; however, we have yet to understand if this is true over a long-term period. In chapter 1, I assess the effects of long-term regenerative agriculture practices on soil food web structure through quantifying free-living nematodes in 1991 and 2021 at the W.K. Kellogg Biological Station Long-term Ecological Research site. I found that after 20-years, nematode communities shifted from bacterivore to fungivore dominance in perennial systems. Soil C accumulation was also four times greater after 20-years but only in the early successional and a mown grassland systems. This decadal study demonstrates that the long-term maintenance of perenniality and diversity alters soil food web structure and drives soil C accumulation in agricultural systems. Second, I explore resistance and resilience of soil food webs to drought in a perennial vs. annual row crop. The impact that drought duration has on the soil food webs is seldom investigated, and even less is known regarding the role that agricultural management has on soil food web resistance and resilience to drought. In chapter 2, I aim to 1) understand how management intensity impacts the resistance of nematode communities to drought and 2) assess how the immediate alleviation of drought impacts soil food web resilience in contrasting agroecosystems. This study was conducted at the W.K. Kellogg Biological Station Long-term Ecological Research Site, where three rainfall manipulations (drought, variable, and control) were induced in two systems (early successional and no-till row-crop). Sampling for nematode communities was conducted before drought was imposed (pre-drought), six-weeks after drought was induced (peak-drought), and two days after rewetting (post-drought). I found that nematode communities in early successional systems were both resistant and resilient to drought. However, no-till systems were less resistant to drought stress, whereby fungivore r and K strategist nematode abundances declined under increased drought stress. Additionally, the alleviation of drought indicated that while early successional systems remained resilient to drought, no-till systems were slow to recover post-drought. Overall, this chapter demonstrates that reduced management intensity within agroecosystems is a valuable option for fostering soil food webs that are resistant to drought. Third, I assess how trophic level interactions within the soil food web influence N cycling. Bacterivore nematodes play a vital role in the nitrogen (N) cycle through their trophic interactions with bacterial communities, and their direct excretion of plant available ammonium. Here I 1) explore how the presence and absence of dominant bacterivore nematodes with different life-history strategies impact soil N pools and plant N use, and 2) assess how bacterial trophic channels interact with soil nitrogen use efficiency under the presence of varying bacterivore nematode species. This greenhouse microcosm experiment was conducted using soil collected from an organic farm that was defaunated. Microcosms were treated with four different nematode inoculums: Acrobeloides nanus (A.nanus), Rhabditid intermedia (R,intermedia), a co-inoculation of both species, and no nematodes. A.nanus and R.intermedia vary in their life-history strategies. The results from this study demonstrate that nematode diversity through co-inoculation can significantly increase organic nitrogen pools and soil nitrate. Additionally, co-inoculum treatments drove significant relationships between total nematode abundance and root N, aboveground biomass, and root biomass. I also found that co-inoculations of bacterivore nematodes enhance nitrogen use efficiency (NUE) and impact -diversity metrics of bacteria. Overall, results indicate that a diversity of bacterivore nematodes, which vary in life-history traits, is essential for overall N cycling and NUE. Taken together, these results indicate that free-living nematodes are highly connected to sustained ecosystem functioning and serve as valuable bioindicators of climatic disturbance and shifts in agricultural management practices. Moreover, this work supplies evidence that the conservation of soil biodiversity is essential for maintaining soil health and ecological function.
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