N or out? Examining the effects of drought on nitrate retention.

Abstract

Models predict that future precipitation in the Midwestern United States will display increased year-to-yearvariablity in total rainfall, as well as more droughts interspersed with heavy rainfall events. These factors, along with anthropogenic changes in land use, are known to increase nitrate losses in many soil systems. This nitrate usually goes on to pollute surface waters and promote harmful algal blooms. Much of the past work on this topic has been done only at a watershed scale, in pursuit of understanding surface water pollution. Thus, there remains a gap in the understanding of these dynamics at the pedon scale. To investigate this gap, we collected 135 intact large soil mesocosms from across Kansas. We collected cores from similar soil series but from differing management strategies and from different levels of historical precipitation. We brought them together in a common garden style experiment design, and applied a two-level rainfall treatment, an extended drought period, and several heavy rainfall events. The concentration of exported leachate was very consistent across land uses, historical precipitation regimes, and across rainfall levels, but did show a large increase directly after a long drought. The soil itself did contain more nitrate in certain western soils, but was unresponsive to the rainfall treatment. We expect that droughts will predict nutrient loss patterns of the future, particularly when followed by a large rainfall event.