Long-term soil water content and exchangeable Ca interact to stabilize organic matter

Organo-mineral interactions stabilize soil organic matter (SOM) by protecting from microbial enzymatic attack. Soil water content affects aggregation, mineral weathering, and microbial respiration, thus influencing the relative importance of SOM stabilization mechanisms. While the response of microbial respiration to momentary changes in water content is well established, it is unclear how microbial activity will impact stabilization mechanisms under different long-term moisture contents.

To understand how long-term soil moisture affects SOM stabilization mechanisms we studied fallow soils from upstate New York situated on a naturally occurring water content gradient. Wetter (but not saturated) soils contained more exchangeable Ca and had more strongly stabilized SOM, resulting in SOM accumulation. But it was not clear whether Ca-driven surface interactions or occlusion in micro-aggregates was more important, and if interactions with Fe and Al played a role in the Ca-poor soils. Also, the role of biotic drivers in SOM stabilization at different water contents was unknown.

We tested which mechanisms governed SOM stabilization by determining C and N contents and natural isotope abundances in particulate and mineral-associated organic matter fractions. We also extracted the C bound to Ca and to reactive Fe+Al phases. Wetter, Ca-rich soils had higher oPOM content, and in the heavy mineral fraction, higher relative concentrations of Ca-bound C, lower C:N values, and more oxidized C forms. In addition, wetter soils had greater microbial biomass. Together, these results showed that high long-term soil moisture increased microbial SOM cycling, and that processed SOM was better stabilized, in agreement with the recent notion that stable SOM consists of processed labile C. Additionally, higher soil moisture augmented the role of Ca in SOM stabilization over that of Al+Fe phases. We then manipulated the exchangeable Ca content and incubated soils with ¹³C¹⁵N labeled plant litter. Ca-amended soils emitted less CO₂ while incubated with litter, confirming that Ca is instrumental in SOM stabilization. Tracing the labeled isotopes in the gaseous phase and soil fractions will allow us to gain a clearer understanding of how water content and soil Ca interact to stabilize SOM.