Belowground Carbon Dynamics in Tropical Perennial C4 Grass Agroecosystems

Abstract

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.

Dynamic, Intermediate Soil Carbon Pools May Drive Future Responsiveness to Environmental Change

“…The lower temperature was the mean annual temperature for multiple field sites under study at the same time, and the higher temperature is ∼2°C higher than the mean annual temperature of the site and corresponds with the projected global temperature increase range scenarios for Hawaii (Giambelluca et al, 2013; IPCC, 2014). The details of the incubation may be found in Crow et al (2018), which reported the raw cumulative CO 2 efflux curves as a proxy for the amount of active C, readily accessible to microbes. Briefly, 10 g of soil (the dry weight equivalent) was brought to field capacity and placed in an incubation jar fitted with a septa and equilibrated for 48 h at the designated temperature within two C‐controlled environment chambers (Model 6021‐1, Caron Products & Services).…”

“…Finally, mineral‐associated C was isolated within the dense fraction. The details may be found in Crow et al (2018), which reported the C pool size of density fractions as representations of unprotected organic debris (free light fraction), aggregate‐protected organic matter (occluded light fraction), and mineral‐associated organic C (dense fraction). Briefly, 20 g of soil (dry weight equivalent) was fractionated in a 1.8‐g mL −1 solution of sodium polytungstate, and the free light fraction was isolated by gently shaking and removing the floating material by suction.…”

“…Total inputs of C to the soil are represented by I . This conceptual model was developed and tested previously for the in situ C measurements and reported on in Crow et al (2018) to confirm the rapid, dynamic transfer of root‐derived organic matter through multiple particulate C pools and into the mineral‐bound pool at this site.…”

“…Simulations were run forward to 2016 to predict potential changes in C stocks and pools over time as an effect of land use and temperature change, and to assess whether the system would gain or lose C during this timeframe. The result of the baseline simulation for the in situ C measurements is reported in Crow et al (2018). Here, we used the relative change in cycling rates from the incubation experiment to modify the cycling rates obtained in the optimization of the in situ data of density fractions (Table 1).…”

“…A follow‐up study in perennial grasslands in Hawaii showed that, where no additional plant inputs were present, the aggregate‐protected soil C pool declined over time, whereas in planted soils, the aggregate‐protected pool accumulated C, specifically, root residues from the recently cultivated grass inputs (Crow et al, 2018). These data suggest the presence of a dynamic component to the aggregate‐protected C pool that is sensitive on an annual timescale to changes in the soil environment and may affect the soil C response to future change.…”