Litter Quality and Microbes Explain Aggregation Differences in a Tropical Sandy Soil

Soil aggregates store most soil organic carbon (SOC), but how does litter quality influence their formation? We hypothesized varying litter quality to facilitate differences in aggregate formation by altering the seasonal development of microbial biomass (MB) C and N, with MB driving  aggregate development in a tropical sandy soil in Thailand. Aggregate development was studied in a long-term fallow experiment, receiving 10 Mg ha−1 annual applications of rice (Oryza sativa) straw (low N and polyphenols (PP)), groundnut (Arachis hypogaea) stover (high N, low PP), tamarind (Tamarindus indica) litter (medium N and PP), or dipterocarp (Dipterocarpus tuberculatus) leaf litter (low N, high PP) compared to a control. N-rich litter from groundnut and tamarind led to significantly higher MB, bulk soil C and aggregate C than dipterocarp, rice straw, and the control. Bulk soil C and small macroaggregates C of N-rich litter treatments increased about 7% in 30 weeks. Increasing MB N explained increasing small macroaggregate C and both, MB C or N were important covariates explaining temporal variations of C stored in themicroaggregates, in silt and clay. MB also explained temporal variations of aggregate fraction weights. With time, SMA C only increased in the N-rich groundnut and tamarind treatments, but decreased in other treatments. Connections of MB to aggregate C and weight substantiated the importance of microbial activity for aggregate formation and carbon sequestration. By promoting MB for longest time spans, medium-quality tamarind could best facilitateaggregate formation, and increase silt and clay C.