Abstract
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.
Highlights
The amount of soil organic carbon (SOC) is a crucial indicator of soil fertility, especially in tropical sandy soils with low clay contents
We propose that microorganisms are actively driving aggregate formation through the following mechanisms: availability and quality of plant litter combined with favorable soil conditions, which in the studied experiment simultaneously occur in the beginning of the rainy season, facilitate different levels of microbial growth and excretion of binding agents
The treatment significantly affected microbial biomass (MB) C and MB N, but the absence of a significant interaction term between time and treatment suggested that MB C and MB N of different litter treatments developed at different levels but in parallel
Summary
The amount of soil organic carbon (SOC) is a crucial indicator of soil fertility, especially in tropical sandy soils with low clay contents. Due to lack of surface areas from clay and silt, have few mineral protection opportunities, aggregate formation may be crucial for maintaining SOC levels and to enable sustainable agriculture In such a soil, Puttaso et al (2013) could show that long-term application of medium-quality tamarind leaves led to almost double the amount of SOC stored compared to low-quality rice straw. The majority of this additional SOC was found in the aggregate fraction and was consumed by microbes upon aggregate destruction. They found that treatments, which had highest amounts of MB (Puttaso et al 2011), had the largest amounts of aggregates (Puttaso et al 2013)
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