How ecosystems hold onto — and lose — carbon, nitrogen, and phosphorus

Biogeochemical Regimes of the Anthropocene

Our species has become extraordinarily good at liberating, fixing, transporting, concentrating, and redistributing elements. Modern economies depend on moving nutrients, metals, and synthetic compounds through the environment at enormous rates. The diversity of synthetic chemicals now present in soils, rivers, and sediments has increased rapidly over the past century, and in many places these compounds are interacting with ecosystems in ways we still poorly understand.

Working in ecosystems affected by mining, urbanization, and other forms of intensive land use has made us particularly interested in how chemical change reshapes ecosystem processes. In streams draining mined or urban landscapes, for example, we frequently observe sharp declines in biodiversity along with major shifts in microbial communities. One of the questions that has motivated our work is whether those biological changes translate into altered ecosystem metabolism and nutrient cycling. If particular microbial metabolic strategies are more sensitive to contaminants than others, disturbances may shift the balance of linked pathways such as denitrification or methane production.

These questions led to collaborations with environmental chemists, engineers, and toxicologists through the Center for the Environmental Implications of Nanotechnology. In that work we examined how engineered nanomaterials move through aquatic ecosystems and how their properties — particle size, surface chemistry, and interactions with organic matter — affect ecological responses. Working in that interdisciplinary setting also exposed our group to a wide range of analytical tools we had not previously used in ecosystem studies.

More recently, climate-driven changes to coastal landscapes have become a major focus of the lab's research program. Much of the North American Coastal Plain is currently experiencing rapid saltwater intrusion associated with sea level rise, land subsidence, drought, and storm events. One of the most visible outcomes is the spread of "ghost forests," where formerly forested wetlands are converted to standing dead trees as salinity increases. Through a combination of field studies and remote sensing, our group and collaborators are mapping where these transitions are occurring and examining how they affect soil carbon stocks, greenhouse gas emissions, and dissolved organic carbon export from coastal watersheds. This work has also drawn us into collaborations with researchers interested in the social and economic consequences of coastal ecosystem change.

Ecosystem Control Points in Space and Time

Biogeochemical activity in ecosystems is rarely evenly distributed across space or time. Instead, many processes occur at especially high rates within small features or brief windows in time. These locations and moments — often referred to as ecosystem control points — can have disproportionate influence on ecosystem function.

Our research has often focused on identifying where these control points occur and how processes operating at small spatial scales propagate through larger ecosystems. Earlier work in forest soils showed that microbial activity and nutrient turnover in rhizosphere soils can differ dramatically from the surrounding bulk soil. In aquatic systems we have shown that processes occurring within stream channels — areas that occupy only a small fraction of watershed area — can strongly influence the timing and magnitude of nutrient export from entire catchments.

More recent work from our group highlights the importance of biological structures that function as control points within stream ecosystems. In headwater streams of New England we have found that bryophyte mats play a surprisingly large role in stream productivity and nutrient cycling. These moss mats trap fine particulate material carried by flowing water and support dense algal communities that are otherwise nearly impossible to measure on the surrounding rock surfaces. Because headwater streams make up the majority of river network length globally, understanding the role of bryophytes could change how we think about nutrient uptake and energy flow at the scale of entire river networks. With new NSF funding we are now examining how bryophyte abundance varies among streams and how these patches influence biodiversity and ecosystem processes downstream.

Energetic and Stoichiometric Constraints on Ecosystem Function

Another long-standing theme of our work is the tight coupling between carbon (~energy) and nutrient cycles. Many of the most interesting questions in ecosystem biogeochemistry arise when we consider multiple elements simultaneously rather than treating carbon, nitrogen, or phosphorus in isolation.

In forest soils, we examined how carbon exuded by plant roots stimulates microbial mineralization of nitrogen. In wetlands, we studied how hydrologic shifts alter the relative export of nitrogen, phosphorus, and carbon from restored landscapes. In streams, we have examined how dissolved organic nitrogen interacts with inorganic nutrient pools to influence ecosystem metabolism.

This interest in coupled elemental cycles naturally led to work on ecosystem energetics in rivers. Stream organisms constantly alter the concentrations of oxygen and carbon dioxide in the water through photosynthesis and respiration. By measuring these changes over daily and seasonal timescales, we can estimate the metabolic balance of stream ecosystems.

Advances in environmental sensors have now made it possible to collect these measurements continuously. Through collaborative efforts such as the StreamPULSE project, we assembled high-frequency metabolic data from hundreds of rivers. These data allowed us to describe the energetic regimes of flowing waters and explore how factors such as light availability, hydrology, and nutrient supply influence ecosystem metabolism.

Building on that work, a current NSF Macrosystems project examines the timing, magnitude, and sources of greenhouse gases emitted from rivers included in the National Ecological Observatory Network. A key question is how much of the carbon dioxide and methane released from rivers originates from in-stream biological activity versus inputs from surrounding landscapes. Addressing that question requires integrating ecosystem metabolism measurements with watershed characteristics across many sites.

Disturbance as a Longitudinal and Long-Term Phenomenon

The influence of any landscape patch on downstream ecosystems depends strongly on its position within the watershed and its hydrologic connectivity with other patches. Disturbances that alter the supply or form of biologically important elements, therefore, tend to propagate along hydrologic flow paths.

This perspective has shaped much of our work on environmental disturbance. In urban watersheds, we examined how altered hydrology and nutrient loading propagate downstream through river networks. In Appalachian mining regions, we studied how large-scale landscape disturbance reshapes watershed hydrology, weathering processes, and aquatic biodiversity for decades after mining operations cease.

Coastal landscapes provide another example of disturbance unfolding over long time scales. In coastal wetlands, we have examined how drought-driven saltwater intrusion alters wetland biogeochemistry and greenhouse gas emissions long after the initial salinization event.

Long-term ecological records have been central to this work. We are closely involved in research at the Hubbard Brook Experimental Forest, where weekly streamwater and precipitation samples have been collected since the early 1960s. These data helped establish the existence of acid rain and later documented the effects of the Clean Air Act on watershed chemistry. Together with colleagues, we are now using these records to examine how climate variability, shifting snowpack dynamics, and the legacy of acid deposition are influencing stream metabolism, carbon cycling, and nutrient export.

A more recent direction in the lab's research program has been the synthesis of long-term watershed datasets across sites. Through the NSF-funded MacroSheds project, we assembled hydrologic and biogeochemical data from dozens of long-term watershed studies across the United States. The goal was simply to make these datasets easier to access and analyze across sites. The resulting database now allows researchers to explore patterns in watershed chemistry and hydrology across climate gradients and over multiple decades. Just as importantly, the project has helped train students and early-career scientists in working with large environmental datasets through workshops and collaborative analysis efforts.