Non-equilibrium dynamics of ecosystem processes in a changing world

Abstract

The relatively mild and stable climate of the last 10,000 years betrays a history of environmental variability and rapid changes. Humans have recently accelerated global environmental change, ushering in the Anthropocene. Meeting accelerating demands for food, energy, and goods and services has accelerated species extinctions, flows of reactive nitrogen and phosphorus, and warming of the atmosphere. I address the overarching question of how ecosystems will respond to changing and variable environments through several focused studies. To address how the changing environment affects the sizes and turnover rates of slowly and quickly cycling soil carbon pools, I analyzed the responses of grassland soils to simulated species diversity loss, increased deposition of nitrogen and increased atmospheric $\textrm{CO}_2$. Species diversity, nitrogen deposition and atmospheric $\textrm{CO}_2$ had no effect on the total soil carbon after 8 years of treatments. Although total soil carbon did not change, the rates of cycling in the fast and slow pools changed in response to elevated $\textrm{CO}_2$ and diversity loss treatments. Nitrogen treatments increased the size of the slowly cycling carbon pool. Precipitation variability has increased around most of the world since the industrial revolution. I used plant mesocosms in a greenhouse experiment to manipulate rainfall variability and mycorrhizal associations. In response to individual rewetting events, drier soils released more nitrate and total nitrogen than wetter soils. Ectomycorrhizal treatments slightly reduced the amount of total nitrogen and nitrate losses from rewetting events. Only ammonium increased in response to patterns of events and mycorrhizal treatments had no effect. The more than 9 billion people expected on the planet by 2050 require food, energy, shelter and other goods and services. Historically, producing those benefits has resulted in environmental damage, especially nitrogen pollution through agricultural fertilizers, atmospheric nitrogen deposition and human waste. I developed a model to test the effectiveness of various levers to reduce the environmental harms associated with meeting the needs of human well-being. The tested levers were mildly effective at reducing nitrogen yield from the baseline business as usual (BAU) scenario, but still resulted in at least 15\% greater nitrogen yield than the present.