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1. chinaXiv:201605.00530 [pdf]

Initial soil C and land‐use history determine soil C sequestration under perennial bioenergy crops

Rebecca L. Rowe; Aidan M. Keith; Dafydd Elias; Marta Dondini; Pete Smith; Jonathan Oxley; Niall P. McNamara
Subjects: Biology >> Botany >> Plant ecology, plant geography

In the UK and other temperate regions, short rotation coppice (SRC) and Miscanthus x giganteus (Miscanthus) are two of the leading ‘second-generation’ bioenergy crops. Grown specifically as a low-carbon (C) fossil fuel replacement, calculations of the climate mitigation provided by these bioenergy crops rely on accurate data. There are concerns that uncertainty about impacts on soil C stocks of transitions from current agricultural land use to these bioenergy crops could lead to either an under- or overestimate of their climate mitigation potential. Here, for locations across mainland Great Britain (GB), a paired-site approach and a combination of 30-cm- and 1-m-deep soil sampling were used to quantify impacts of bioenergy land-use transitions on soil C stocks in 41 commercial land-use transitions; 12 arable to SRC, 9 grasslands to SRC, 11 arable to Miscanthus and 9 grasslands to Miscanthus. Mean soil C stocks were lower under both bioenergy crops than under the grassland controls but only significant at 0–30 cm. Mean soil C stocks at 0–30 cm were 33.55 ± 7.52 Mg C ha−1 and 26.83 ± 8.08 Mg C ha−1 lower under SRC (P = 0.004) and Miscanthus plantations (P = 0.001), respectively. Differences between bioenergy crops and arable controls were not significant in either the 30-cm or 1-m soil cores and smaller than for transitions from grassland. No correlation was detected between change in soil C stock and bioenergy crop age (time since establishment) or soil texture. Change in soil C stock was, however, negatively correlated with the soil C stock in the original land use. We suggest, therefore, that selection of sites for bioenergy crop establishment with lower soil C stocks, most often under arable land use, is the most likely to result in increased soil C stocks.

submitted time 2016-05-04 Hits890Downloads499 Comment 0

2. chinaXiv:201605.00510 [pdf]

An interyear comparison of CO2 flux and carbon budget at a commercial‐scale land‐use transition from semi‐improved grassland to Miscanthus x giganteus

Jon P. McCalmont; Niall P. McNamara; Iain S. Donnison; Kerrie Farrar; John C. Clifton-Brown
Subjects: Biology >> Botany >> Plant ecology, plant geography

A 6-ha field at Aberystwyth, UK, was converted in 2012 from semi-improved grassland to Miscanthus x giganteus for biomass production; results from transition to the end of the first 3 years are presented here. An eddy covariance sensor mast was established from year one with a second mast added from year two, improving coverage and providing replicated measurements of CO2 exchange between the ecosystem and atmosphere. Using a simple mass balance approach, above-ground and below-ground biomass production are combined with partitioned CO2 fluxes to estimate short-term carbon deltas across individual years. Years one and two both ended with the site as a net source of carbon following cultivation disturbances, cumulative NEE by the end of year two was 138.57 ± 16.91 g C m−2. The site became a cumulative net sink for carbon by the end of June in the third growing season and remained so for the rest of that year; NEE by the end of year three was −616.52 ± 39.39 g C m−2. Carbon gains were primarily found in biomass pools, and SOC losses were limited to years one (−1.43 Mg C ha−1 yr−1) and two (−3.75 Mg C ha−1 yr−1). Year three saw recoupment of soil carbon at 0.74 Mg C ha−1 yr−1 with a further estimate of 0.78 Mg C ha−1 incorporated through litter inputs over the 3 years, suggesting a net loss of SOC at 3.7 Mg ha−1 from a 0- to 30-cm baseline of 78.61 ± 3.28 Mg ha−1, down 4.7%. Assuming this sequestration rate as a minimum would suggest replacement of cultivation losses of SOC by year 8 of a potential 15- to 20-year crop. Potential coal replacement per hectare of harvest over the three-year study would offset 6–8 Mg of carbon emission, more than double the SOC losses.

submitted time 2016-05-04 Hits737Downloads412 Comment 0

3. chinaXiv:201605.00500 [pdf]

Land‐use change to bioenergy: grassland to short rotation coppice willow has an improved carbon balance

Zoe M. Harris; Giorgio Alberti; Maud Viger; Joe R. Jenkins; Rebecca Rowe; Niall P. McNamara; Gail Taylor
Subjects: Biology >> Botany >> Plant ecology, plant geography

The effect of a transition from grassland to second-generation (2G) bioenergy on soil carbon and greenhouse gas (GHG) balance is uncertain, with limited empirical data on which to validate landscape-scale models, sustainability criteria and energy policies. Here, we quantified soil carbon, soil GHG emissions and whole ecosystem carbon balance for short rotation coppice (SRC) bioenergy willow and a paired grassland site, both planted at commercial scale. We quantified the carbon balance for a 2-year period and captured the effects of a commercial harvest in the SRC willow at the end of the first cycle. Soil fluxes of nitrous oxide (N2O) and methane (CH4) did not contribute significantly to the GHG balance of these land uses. Soil respiration was lower in SRC willow (912 ± 42 g C m−2 yr−1) than in grassland (1522 ± 39 g C m−2 yr−1). Net ecosystem exchange (NEE) reflected this with the grassland a net source of carbon with mean NEE of 119 ± 10 g C m−2 yr−1 and SRC willow a net sink, −620 ± 18 g C m−2 yr−1. When carbon removed from the ecosystem in harvested products was considered (Net Biome Productivity), SRC willow remained a net sink (221 ± 66 g C m−2 yr−1). Despite the SRC willow site being a net sink for carbon, soil carbon stocks (0–30 cm) were higher under the grassland. There was a larger NEE and increase in ecosystem respiration in the SRC willow after harvest; however, the site still remained a carbon sink. Our results indicate that once established, significant carbon savings are likely in SRC willow compared with the minimally managed grassland at this site. Although these observed impacts may be site and management dependent, they provide evidence that land-use transition to 2G bioenergy has potential to provide a significant improvement on the ecosystem service of climate regulation relative to grassland systems.

submitted time 2016-05-04 Hits764Downloads405 Comment 0

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