Expert discusses soil fertility and tropical forest productivity

Samantha Tracy, a YCNCC Science Communications Fellow, recently spoke with Wong about the impact of soil fertility on tropical forests and their responses to global change. The findings are published in the journal New Phytologist. Soil fertility refers to a soil's ability to sustain plant growth by supplying necessary resources (i.e., nutrients).

While the concept originates from agricultural science, the complexities of soil fertility's effects on forest growth and biomass are more intricate. In contrast to agricultural systems, forests host a greater number of plant species and have longer lifespans. Measuring soil fertility can be helpful in determining which fertilizers and quantities are required for crop production.

However, adding nutrients to forest ecosystems does not always produce the same results. Measuring soil fertility in forest systems is difficult, as various elements can limit growth, plant species have different nutrient demands, and the methodologies to measure nutrient availability to plants are still limited. Tropical forests are crucial to the global carbon cycle, storing approximately 50%–60% of the world's biomass carbon.

However, understanding what drives this biomass carbon and how it will react to global change is still unclear. Although soil fertility is expected to drive plant growth, some studies have indicated that soil fertility increases tree mortality rates. Nonetheless, there are no clear mechanisms to explain this relationship. To address these complexities, the researchers gathered a global team of scientists working in various tropical regions.

They recognized that defining soil fertility is a significant hindrance to large-scale syntheses across the tropics. One of the primary challenges was the lack of a clear relationship between soil fertility and other variables due to the undefined nature of soil fertility. To write the manuscript, the core workshop organizers drafted the initial versions, and the rest of the co-authors had multiple Zoom meetings and feedback sessions.

The initial goal was to understand how global change would affect tropical forest carbon cycling through soil fertility feedbacks. However, the researchers realized that before understanding these relationships, they needed to first comprehend the basic relationships between soil fertility and productivity and soil fertility and mortality.

Factors such as rising atmospheric carbon dioxide, climate stress, and land-use change are altering nutrient availability and forest responses, leading to complex outcomes. The researchers highlighted several important knowledge gaps in advancing understanding of tropical forest soil fertility and carbon cycling relationships. These include the role of soils in mortality mechanisms, the allocation of carbon above and below ground, how plant traits and species composition change across soil gradients, and the definition of soil fertility.

Since these patterns are often context-dependent, more network studies are needed to consistently measure these patterns across various sites. According to the researchers, collaborative and network science are crucial now more than ever.