Above‐ and belowground litter inputs in a temperate deciduous forest were altered for 20 yr to determine the importance of leaves and roots on soil C and soil organic matter (SOM) quantity and quality. Carbon and SOM quantity and quality were measured in the O horizon and mineral soil to 50 cm in five treatments (control, double litter [DL], no litter [NL], no roots [NR], no inputs [NI]). After two decades of doubled litter addition, soil C and SOM did not increase. However, leaf litter exclusions reduced soil C (O and mineral horizons combined) by 24% in NL and 33% in NI treatments. In the mineral soil, the largest declines occurred in the 0‐ to 10‐cm depth (0.93–2.01 kg C m−2), although losses were observed throughout the entire solum. The NR treatments showed no losses of C. Thermal characterization of SOM quality differed among treatments in the 0‐ to 10‐cm depth. Patterns of CO2 evolution during SOM combustion revealed differences in SOM quality between surface and deeper horizons. Our work shows that the sources of litter are important in controlling soil C. Leaf litter made important contributions to maintaining current stocks of soil C; increased leaf litter did not increase soil C, but decreases in litter inputs resulted in rapid soil C declines. Root litter may ultimately provide more stable sources of soil C. Management activities or environmental alterations that decrease litter inputs in mature forests can lower soil C content; however, increases in forest productivity and the resulting increased litter production seem unlikely to increase soil C sequestration.

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Effective soil management is critical to achieving climate change mitigation in plant-based renewable energy systems, yet limitations exist in our understanding of dynamic belowground responses to the cultivation of energy crops. To better understand the belowground dynamics following cultivation of a grassland in a high-yielding tropical perennial C4 grass in a zero-tillage production system, changes in soil carbon (C) pools were quantified, modeled, and projected and the chemical composition of the aggregate-protected pool was determined in support of the simulated dynamics. Multiple C pools with different ecosystem functions and turnover increased following cultivation: immediately available microbial substrate (measured as hot water-soluble C) and active C (determined through laboratory incubation) increased by 12 and 30% respectively over time and soil C accumulated significantly in multiple physical fractions. A more rapid and dynamic nature of multiple C pools and transfers between pools existed than is often assumed in belowground models used widely in the field to simulate soil C accumulation. Multiple indicators of fresh roots, including the more easily degraded lignin monomers and root-derived long chain substituted fatty acids, appeared in aggregate-protected pools of cultivated soils over time since planting. This rapid transfer of plant inputs through active and intermediate C pools into mineral-dominated pools is the ultimate outcome required for building soil C stocks. Initial model runs suggested that this is evident, even on a 2-year frame, in transfer rates of 0.485 and 0.890 from active to slow and slow to passive pools respectively. The rapid transfer of fresh root-derived input to stable pool suggests that soil C under zero-tillage management may be resilient to disturbances, such as replanting following a kill-harvest, that would otherwise result in losses from unprotected or readily available pools.

The complex structural and functional responses of agricultural soil microbial communities to the addition of carbonaceous compounds such as biochar remain poorly understood. This severely limits the predictive ability for both the potential enhancement of soil fertility and greenhouse gas mitigation. In this study, we utilized shotgun metagenomics in order to decipher changes in the microbial community in soil microcosms after 14 days of incubation at 23°C, which contained soils from biochar-amended and control plots cultivated with Napier grass. Our analyses revealed that biochar-amended soil microbiomes exhibited significant shifts in both community composition and predicted metabolism. Key metabolic pathways related to carbon turnover, such as the utilization of plant-derived carbohydrates as well as denitrification, were enriched under biochar amendment. These community shifts were in part associated with increased soil carbon, such as labile and aromatic carbon compounds, which was likely stimulated by the increased available nutrients associated with biochar amendment. These findings indicate that the soil microbiome response to the combination of biochar addition and to incubation conditions confers enhanced nutrient cycling and a small decrease in CO 2 emissions and potentially mitigates nitrous oxide emissions.Citation Yu J, Deem LM, Crow SE, Deenik J, Penton CR. 2019. Comparative metagenomics reveals enhanced nutrient cycling potential after 2 years of biochar amendment in a tropical oxisol. Appl Environ Microbiol 85:e02957-18. https://doi.

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