Soil nitrogen leaching losses in response to freeze–thaw cycles and pulsed warming in a temperate old field

Abstract

Climate warming and increased climate variability are both predicted to increase the frequency of soil freeze–thaw cycles in temperate regions. We exposed intact soil-plant mesocosms to freeze–thaw cycles and examined the effects on nitrogen leaching losses. Freezing treatments were performed by incubating the mesocosms in the soil with their tops exposed to air to impose freezing from the top down, such that realistic freezing rates and cycle amplitudes were experienced across the soil profile. Leaching events were then initiated by water addition the following day for both the freezing treatment and control mesocosms. While water addition alone explained the major part of soluble organic nitrogen leaching, nitrate leaching approximately doubled in response to freeze–thaw cycles, and nitrogen leaching remained high after 11 freeze–thaw cycles. In a second experiment, pulses of warming were applied in situ to mesocosms over fall, winter or spring, in order to melt snow, and thereby increase freeze–thaw cycling by exposing soils to diurnal fluctuations in air temperature. Warming pulses had little effect on sub-surface soil temperatures and no effect on soil nitrogen leaching. However, warming pulses over spring severely reduced the abundance of the legume Coronilla varia in the following growing season. Overall, the results of these experiments indicate that while increased soil freeze–thaw cycles combined with leaching events are capable of increasing soil nitrogen losses, warming pulses will only promote increased freeze–thaw cycles if they are followed by cold, snow-free weather. The strong effect of warming on the N-fixer C. varia highlights that changes in plant species composition in response to warming may have stronger implications for soil nitrogen dynamics than the direct effects of freeze–thaw cycles on soil nitrogen leaching losses.