Effects of artificial aeration and iron inputs on the transformation of carbon and phosphorus in a typical wetland soil

Abstract

Artificial aeration changes the redox conditions at the soil surface. The introduction of iron (Fe) into wetlands can influence carbon (C) and phosphorus (P) cycling under the fluctuating redox conditions. However, artificial Fe introduced into wetlands is uncommon, and there are no Fe dose guidelines. We compared aerobic and anaerobic conditions to test the hypothesis that Fe addition can, although redox-dependent, affect P forms and the coupling of organic C. Twenty-four intact soil cores were collected randomly from a lacustrine wetland of Lake Xiaoxingkai. And representative and homogeneous seedlings of Glyceria spiculosa were collected. The incubation was designed with two treatment factors: Fe/P ratio (5 or 10) and high and low dissolved oxygen (DO) concentrations (> 6 and < 2 mg L−1, respectively). Four groups with three replicates were separated randomly and labeled as aerobic + plant treatment, anaerobic + plant treatment, and aerobic or anaerobic treatment (control). The DO concentrations were stratified, decreasing with soil depth and increasing with time, especially under aerobic conditions. The Eh values generally increased with fluctuations under aerobic conditions. The artificial aeration substantially changed the redox environment at the water–soil interface. Of the total P, 45% was in the reactive Fe-bound P, indicating that Lake Xiaoxingkai had high internal P loading. No significant differences were observed in total Fe, amorphous Fe, and organic C at the soil surface between the two Fe/P ratios; however, a significant difference in free Fe was observed. And soil amorphous Fe was found to be a significant correlation with soil organic C, indicating that iron oxides were related with the soil chemical properties. After short-term incubation, Fe addition can affect the cycling of major elements in wetlands, although this effect is redox dependent. Excessive Fe doses may result in regional environmental risks, such as eutrophication and C sinks of wetland ecosystems. Large-scale controlled experiments are needed to fully understand the behaviors of soil elements in wetlands.