Labile organic carbon stocks and mineral-organic matter associations in soil: Role of anticipated changes in rainfall pattern

Anticipated changes in climate are expected to have significant effect on biogeochemical cycling of essential elements in the critical zone. In California, over the next 100 years, a significant shift in rainfall amount and timing is expected. There is currently a disagreement whether the anticipated increase in rainfall will fall as either additional winter rain or result in extension of the rainy season into late spring and early summer months. Here we present results from a field rainfall addition experiment that was setup to test the effects of increased amount of rainfall (20% over ambient) and timing (no addition = control, increased rainfall in the rainy season = winter addition, or increased rainfall during late spring and early summer = spring addition treatment). Specifically, in this presentation, we will discuss results from work looking into changes in amount and biochemical composition of free light soil organic matter fraction (fLF, <1.7g/cm3 in 0-5 and 5-10cm soil depths) and association of soil organic matter storage with Fe and Al oxides in soil (top 50cm soil). We found that the two treatments, winter vs. spring addition, have different effects on carbon storage and association of organic matter with iron and aluminum oxides in soil. Extension of the rainy season into late spring and summer months results in significant increase (20-40%) in the fraction of carbon that is stored as fLF, which was not observed in the winter treatment. Increase of rainfall amount during the already wet rainy season (winter addition treatment) leads to important changes in relationship of organic carbon with soil mineral that are critical for SOM stabilization. More than 35% of the variability in soil carbon storage in the control and spring treatments is explained by dithionite extractable (pedogenic) Fe, compared to <0.01% in the winter treatment plot. Similarly, more than 26% and 41% of the variability in carbon storage is explained by poorly crystalline iron and aluminum oxides in the control and spring treatments, respectively, compared to <5% in the winter treatment plots. Our results suggest that rapid shifts in rainfall patterns could have significant implications for not just soil carbon storage but also its stabilization.