Grazing lands represent a globally significant land use, account for a large portion of global soil carbon, and thus could play a critical role in climate change mitigation. Despite their global importance, the factors that regulate soil carbon in subtropical pasture systems are relatively understudied. Here, we report findings from an in situ 13C/15N pulse-chase experiment conducted in subtropical pasture in Florida, USA. We aim to study perennial legume incorporation into warm-season pasture, an agronomic practice gaining increasing attention for both production and ecosystem service benefits. Specifically, we tested whether deposition of higher-quality (lower C:N ratio) legume plant tissue increases soil C and N pools more effectively than lower-quality C4 grass tissue, as suggested by recent theoretical frameworks that emphasize the positive role of microbial growth efficiency in increasing the fraction of organic inputs stabilized over the long term (e.g. the “MEMS hypothesis”).
Results/Conclusions
Over a 16-month chase period, we found that, while legume tissue cycles more rapidly through soil and plant pools, its labelled 13C/15N content is retained ~ 25% less efficiently in our pasture soils than the C4 grass tissue, challenging the notion that greater microbial efficiency should lead to greater organic matter stabilization. While previous research indicates that mineral-associated organic matter (MAOM) is stabilized over longer timeframes than particulate organic matter (POM), our pulse-chase experiment reveals that both fractions are heterogeneous, and likely feature similar residence times for C and N. Additionally, our experiment provides novel evidence that cation-bridging may be a critical mechanism of stabilization in both fractions, with Mehlich-3 extractable calcium explaining ~ 50% of the variation in C concentrations in our soils. Overall, our findings suggest that legume addition to warm-season pasture may benefit soil organic matter indirectly via N enrichment, but do not support the notion that higher-quality inputs increase either MAOM or POM more efficiently than lower-quality inputs in our subtropical pasture system.