Soil biology’s bright future
In Uganda, University of New Hampshire undergraduate student Michael Casazza crosses a river with local help to access a forest on the edge of Kibale National Park. Soils from the forest and nearby agricultural fields are being compared in a study of the socioeconomic factors driving agricultural sustainability in a region with one of the planet’s highest rates of population growth.
It’s an exciting time to be a soil biologist; as we develop new tools, connect increasingly to other disciplines, and expand our funding opportunities, we are showing that soil biological processes strongly affect the environmental quality and productivity of ecosystems both regionally and globally. With these discoveries we are making ourselves increasingly relevant in a world where natural resources are limited, and it’s not hard to see the signs of our field’s success. Consider, for example, the rising impact factors of journals such as Biogeochemistry and Soil Biology & Biochemistry and the diversity of academic institutions that now employ soil biologists. A quick look at the current Soil Science Society of America jobs page shows four academic jobs with soils in soil biology, but only one in soil physics, and none in soil chemistry.
Our field is finally hot but, as one of my colleagues who’s a soil physicist reminds me, these things ebb and flow. The top spot in soil science was once occupied by soil physicists helping restore soil ecosystems suffering from massive soil losses via erosion; by soil pedologists describing soil characteristics and their potential uses for an expanding human population; and by soil chemists pushing crop yields to new heights. While impactful, highly relevant research is ongoing in these areas of soil science, the momentum seems to be with soil biology. Here I explore two thoughts about how we can maintain this momentum. The first relates to expanding research efforts in agricultural systems, and the second to how important it is for soil biologists to embrace the different sub disciplines within soil science.
Cover crops like the ryegrass shown here are grown in the offseason between cash crops. They diversify simple rotations and provide a number of aboveground benefits such as reducing insect and disease pressure. However, plant diversification effects on belowground processes in both agricultural and unmanaged plant communities remain poorly understood.
Soil biologists have been making important contributions to understanding nutrient cycles at global scales and to understanding ecosystem responses to pollution, climate and other stressors at local and regional scales. In answer to the question ‘where next’, there are as many answers as there are soil biologists but my vote goes with the need to better understand soil biological processes in agricultural systems. There is no ecosystem type that our society depends upon more or, arguably, that harms its surrounding environment more, and yet they remain underappreciated and poorly understood. For example, agricultural systems are the principal source of global nitrous oxide emissions and pollute our waterways, but how can we sustain and increase crop productivity in soils that lose little N to the environment remains unanswered. Soil biology has an important role to play in answering these questions.
Because of their highly simplified plant communities, intense soil disturbance, and high rates of nutrient inputs, agricultural systems provide a testing ground for some of ecology’s most pressing questions, including relationships between plant communities and belowground ecosystem functions. In my lab, we have been comparing rotations that produce more than one crop over time to monocultures to test questions about soil responses to diversifying plant communities (e.g. McDaniel et al. 2013; Figure 1). Different rotation crop types can also be compared in order to separate the effects of diversity, per se, from the effects of including specific crop functional groups, such as legumes. Recent studies have also taken advantage of the inherent differences in decomposer communities between agriculture and grassland soils to provide new insights into decomposition dynamics (e.g. Wickings et al. 2012), while my own and other’s labs have contributed to our understanding of how nitrogen influences soil organic matter (Gillespie et al. 2013; Grandy et al. 2013). Our understanding of soil ecosystems is improved by these studies, which will also help guide management practices on the 40% of the planet’s land currently used for agricultural production.
My second point relates to the dependence of soil biology on the other branches of soil science, so when disciplines such as pedology are deprioritized we lose important context for our work. The loss of pedologists at land grant universities has many causes but is dismaying, especially when considered in light of its impact on soil biology and other soils research. Among other things, pedologists often bring a perspective that spans broad spatial and temporal scales that many soil biologists don’t routinely consider (Schimel and Chadwick, 2013). Even within the broad area of soil biology, important areas such as faunal ecology and taxonomy are being overwhelmed by current interests in microbes. Microbes may be the engines in the soil, but both microbes and the processes they mediate are strongly controlled by factors outside the realm of traditional soil microbiology. While soil biogeochemistry captures some of the interdisciplinary connection inherent in soil biology, it’s not a substitute for maintaining strengths in all areas of soil science.
Student training in soil biology is also negatively impacted by the slow drain of faculty from sub-disciplines that aren’t trendy, something I’ve come to appreciate recently. When I was a mid-career PhD student about 10 years ago I was attending the biannual meeting of the Soil Ecology Society where several accomplished elder statesmen were lamenting the poor training many soil microbiology graduate students were receiving. They argued that graduate students were crossing over into soil microbiology from other backgrounds (e.g. zoology) without ever developing a comprehensive background in soil science. At the time my opinion was that these students were bringing a fresh perspective unencumbered by outdated classes. While I still see this student crossover into soils as a positive, I also now see the value in some formal background in the foundations of soil science. Since my student days, I have been continuously amazed by how complex the soil matrix can be. I’ve observed that many of our most important discoveries in soil biology are related to the interactions between this complex matrix and the organisms inhabiting it, and thus come to appreciate the importance of taking at least a few classes in different areas of soil science. Recently, I’ve sent new PhD students without backgrounds in soil science to the Summer Soil Institute at Colorado State University (http://soilinstitute.nrel.colostate.edu/), where they explore hands-on the physical, biological, and chemical components of soil. This short course is not a substitute for full-length classes in soil science but it helps students from different backgrounds begin to transition into becoming soil scientists.
This and the next generation of soil biologists appear well positioned to play an important role in solving our planet’s environmental crisis (Figure 2), which is inextricably linked to population growth and our ever-increasing demand for agricultural products. To make the most of our opportunity requires engagement with the entire breadth of soil science sub disciplines to critical answer questions in agricultural and other key ecosystems.