Hot Spots and Hot Moments of Soil Moisture Explain Fluctuations in Iron and Carbon Cycling in a Humid Tropical Forest Soil

Abstract

Soils from humid forests undergo spatial and temporal variations in moisture and oxygen (O2) in response to rainfall, and induce changes in iron (Fe) and carbon (C) biogeochemistry. We hypothesized that high rainfall periods stimulate Fe and C cycling, with the greatest effects in areas of high soil moisture. To test this, we measured Fe and C cycling across three catenas at valley, slope, and ridge positions every two days for a two-month period in a rainforest in Puerto Rico. Over 12 days without rain, soil moisture, FeII, rapidly reducible Fe oxides (FeIIIRR), and dissolved organic C (DOC) declined, but Eh and O2 increased; conversely, during a 10-day period of intense rain (290 mm), we observed the opposite trends. Mixed-effects models suggest precipitation predicted soil moisture, soil redox potential (Eh), and O2, which in turn influenced Fe reduction/oxidation, C dissolution, and mineralization processes. The approximate turnover time for HCl-extractable FeII was four days for both production and consumption, and may be driven by fluctuations in FeIIIRR, which ranged from 42% to 100% of citrate–ascorbate-extractable FeIII (short-range order (SRO)-FeIII) at a given site. Our results demonstrated that periods of high precipitation (hot moments) influenced Fe and C-cycling within day-to-week timescales, and were more pronounced in humid valleys (hot spots).