Shifting rainfall patterns are becoming more common in the humid tropics as a result of climate change. These changes were shown to significantly affect the redox state of humid tropical forest soils, disturbing the immobilization of carbon (C) and phosphorus (P, a key limiting nutrient in tropical forests) by soil microorganisms. However, microbial C:P ratios appear to be remarkably constrained, even across broad latitudinal differences in P content and availability. Because microbes are the base of the forest's food web, the effects of changing redox states on their C:P stoichiometry are crucial for predicting the response of tropical forests to climate change. Here we incubated tropical soils with substrates varying in their C:P stoichiometry and chemical lability to examine the effects of changing redox conditions on the response of soil microbes to mismatches in the stoichiometry of their resources and their demands.
We found that under oxic conditions microbial C:P ratios were highly constrained and similar to the global average (60:1), even when the C:P ratio of their substrates didn’t match microbial C:P stoichiometry. However, under anoxic conditions, microbial C:P ratios more closely reflected the C:P ratio of their substrates, as their ability to immobilize P from their surroundings decreased. Low microbial P immobilization triggered a consequent decrease in microbial carbon use efficiency (CUE). However, when substrate stoichiometry met microbial P demands, microbial CUE increased sharply towards values found under aerobic conditions, demonstrating the control of P on the C cycle in redox-sensitive humid tropical forests soils. NanoSIMS imaging of cells extracted from these soils revealed a narrow C:P ratio distribution when the C:P ratios were close or lower than the global average value. However, cells from anoxic soils had highly variable C:P ratios, particularly when the C:P ratio of their substrate was higher than their demands, suggesting that not all cells were equally sensitive to changes in redox conditions. The dramatic response of microbial stoichiometry to changing redox conditions that we observe suggest that tropical forests may be highly sensitive to changes in climate that affect soil oxygen availability and redox status, which will disturb the transfer of P and energy across the forest food web and affect its productivity.