Abstract
AbstractThe demand for bioenergy has increased the interest in short‐rotation woody crops (SRWCs) in temperate zones. With increased litter input and ceased annual soil cultivation, SRWC plantations may become soil carbon sinks for climate change mitigation. A chronosequence of 26 paired plots was used to study the potential for increasing soil organic carbon (SOC) under SRWC willow and poplar after conversion from cropland (CR) on well‐drained soils. We estimated SOC stocks in SRWC stands and adjacent CR and related the difference to time since conversion, energy crop species, SOC stock of the adjacent CR (proxy for initial SOC of SRWC) and the fine soil percentage (<63 μm) (FS). Soil cores to 40 cm depth were sampled and separated by layers of fixed depths (0–5, 5–10, 10–15, 15–25 and 25–40 cm). Additionally, soils were sampled from soil pits by genetic horizons to 100 cm depth. Comparisons of SOC stocks by equivalent soil masses showed that mean SOC stocks in SRWC were 1.7 times higher than those of CR in the top 5 cm of the soil (P < 0.001). The differences between SRWC and CR remained significant for the plough layer (0–25 cm) by a factor of 1.2 (P = 0.003), while no changes were detectable for the 0–40 cm (P = 0.32), or for the entire 0–100 cm soil layer (P = 0.29). The SOC stock ratio, that is the ratio of SOC stock in SRWC relative to CR, did not change significantly with time since conversion, although there was a tendency to an increase over time for the top 40 cm (P = 0.09). The SOC stock ratio was negatively correlated to SOC in CR and FS percentage, but there was no significant difference between willow and poplar at any depth. Our results suggest that SOC stocks in the plough layer increase after conversion to SRWC.
Highlights
Biomass for bioenergy is one of the most flexible sources of renewable energy as it is storable and can be used for production of electricity, heat, and transport (Sartori et al, 2006)
To conform to this assumption, the selection of the sample sites was based on the following criteria: (i) the short-rotation woody crop (SRWC) should be planted on previous cropland with the same management history and similar soil properties as the adjacent cropland; (ii) the SRWC stands should not have been deep ploughed at conversion, as mixing of the plough layer with deeper soil layers would prevent comparisons between land uses when sampling to 40 cm depth; (iii) the SRWC and CR plots should be on welldrained soils with no anoxic conditions influencing the decomposition rate of organic matter; and (iv) sampled SRWC stands should, to the extent possible, cover a large range of years since conversion, a large range of different soil textures, and a wide geographical range within the region
The higher C concentration in the top layers of SRWC and the development of a C concentration gradient with soil depth is presumably due to cessation of tillage and increased litter inputs in SRWC compared to CR
Summary
Biomass for bioenergy is one of the most flexible sources of renewable energy as it is storable and can be used for production of electricity, heat, and transport (Sartori et al, 2006). The two types of systems are referred to as short-rotation woody crop (SRWC) systems, as earlier described by Agostini et al (2015). The ability of these systems to achieve high yields in relatively short time spans is their. It is hypothesized that they can offset greenhouse gas emissions through increased soil carbon (C) sequestration following landuse change of agricultural land (Lemus & Lal, 2005). SRWCs may not follow the same trajectory or timing as found during the early years after afforestation They involve fastergrowing species and are often grown with a denser stocking, and more frequent harvesting. Other studies report no change in SOC stocks during the first five to 19 years after conversion (Coleman et al, 2004; Sartori et al, 2007; Pacaldo et al, 2013; Rytter, 2016)
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