Legacy effects from agricultural soils determine post-restoration wetland biogeochemistry

Restoring degraded peat soils to wetlands can be an efficient measure to sequester carbon, improve water quality, create habitat for wildlife, control flooding, and provide opportunities for recreation. Agricultural lands that are restored to wetlands can build soil and reverse land subsidence. On the other hand, restored wetlands can result in increased methane emissions, which can reduce the net benefits of the restoration in terms of their greenhouse gas balance. Using eddy covariance towers in five restored wetlands in the Sacramento-San Joaquin Delta of California, we see high carbon sequestration as well as some of the highest methane emissions measured in restored freshwater wetlands. Several studies have shown that the legacy effect of the previous land use can significantly alter the restoration trajectory. For example, high Fe conditions can temporarily reduce methane fluxes while high concentration of nutrients such as N and P can increase emissions via several pathways. Soil samples from restored wetlands in the Delta allow us to understand the legacy effects of soil nutrients and how they influence wetland development. For example, a wetland that was previously under arable use had significantly higher P content than those that were used as a pasture. This may also be one of the reasons why the wetland vegetation developed quickly after the restoration in a system with high P concentration. Both N and P are common nutrients that limit plant growth, but when the nutrient concentration in the soil is high we can expect high-productivity but low-diversity systems such as Typha marshes. Nutrient-rich conditions can result in a high C sequestration potential due to rapid plant growth. On the other hand, nutrient loading can cause the wetlands to become eutrophic, especially if there is continuous diffuse pollution from nearby agricultural lands. Eutrophic systems can lead to elevated methane fluxes due to the reduced oxygen content from increased algae growth, especially with rising temperature. In this study, we plan to synthesize and analyze the soil nutrient legacy effects on biogeochemical cycling at five restored wetlands in the Delta.