Carbon sequestration and turnover in soil under the energy crop Miscanthus: repeated ¹³C natural abundance approach and literature synthesis

Abstract

The stability and turnover of soil organic matter (SOM) are a very important but poorly understood part of carbon (C) cycling. Conversion of C 3 grassland to the C 4 energy crop Miscanthus provides an ideal opportunity to quantify medium-term SOM dynamics without disturbance (e.g., plowing), due to the natural shift in the d 13 C signature of soil C. For the first time, we used a repeated 13 C natural abundance approach to measure C turnover in a loamy Gleyic Cambisol after 9 and 21 years of Miscanthus cultivation. This is the longest C 3 -C 4 vegetation change study on C turnover in soil under energy crops. SOM stocks under Miscanthus and reference grassland were similar down to 1 m depth. However, both increased between 9 and 21 years from 105 to 140 mg C ha À1 (P < 0.05), indicating nonsteady state of SOM. This calls for caution when estimating SOM turnover based on a single sampling. The mean residence time (MRT) of old C (>9 years) increased with depth from 19 years (0-10 cm) to 30-152 years (10-50 cm), and remained stable below 50 cm. From 41 literature observations, the average SOM increase after conversion from cropland or grassland to Miscanthus was 6.4 and 0.4 mg C ha À1 , respectively. The MRT of total C in topsoil under Miscanthus remained stable at~60 years, independent of plantation age, corroborating the idea that C dynamics are dominated by recycling processes rather than by C stabilization. In conclusion, growing Miscanthus on C-poor arable soils caused immediate C sequestration because of higher C input and decreased SOM decomposition. However, after replacing grasslands with Miscanthus, SOM stocks remained stable and the MRT of old C 3 -C increased strongly with depth.

Measured and modelled effect of land‐use change from temperate grassland to Miscanthus on soil carbon stocks after 12 years

“…There was also no difference between carbon stocks at the two sampling points (T 6 and T 12 ) suggesting a reasonably stable carbon state. However, this is in contrast to Zang et al () where soil organic matter increased between sampling occasions (9 and 21 years after Miscanthus planting). This difference may be as a result of different soil pH and nutrient levels, or the slightly cooler (annual average air temperature 6.7°C vs. 10.4°C) and wetter (annual average precipitation 1,074 mm vs. 654 mm) climate in this study, which could all influence Miscanthus ‐derived carbon (Zimmermann et al, ).…”

“…In this new analysis, unlike Zatta et al (), we did find a reduction in soil carbon stock at T 6 compared to T 0 but the breakdown by hybrid confirmed that the difference was only significant for a single hybrid (at T 6 and T 12 , Figure ). The overall reduction in carbon from T 0 to T 12 , of 8 Mg/ha, is within the range +4 to −9 Mg/ha reported in other grassland to Miscanthus field‐based studies (Clifton‐Brown et al, ; Schneckenberger & Kuzyakov, ; Zang et al, ; Zimmermann et al, ). There was also no difference between carbon stocks at the two sampling points (T 6 and T 12 ) suggesting a reasonably stable carbon state.…”

“…Although new Miscanthus C 4 carbon replaced pre‐existing C 3 carbon, this was not at a high enough rate to completely offset losses by the end of year 12. The impact of LUC on SOC generally differs with soil depth (Poeplau & Don, ; Rowe et al, ; Zang et al, ). In this study, it was found that between T 6 and T 12 C mis increased in the top layer, although SOC also declined (Figure ).…”

“…Land‐use change from arable crop production to Miscanthus generally shows an increase or no change in SOC, whereas, in contrast, it has been found that Miscanthus plantations have lower or similar SOC when compared to grassland controls (Qin, Dunn, Kwon, Mueller, & Wander, ). However, to date, most studies have taken grassland sites adjacent to Miscanthus plantations as representative of pre‐cultivation conditions (Clifton‐Brown, Breuer, & Jones, ; Foereid, Neergaard, & Høgh‐Jensen, ; Rowe et al, ; Schneckenberger & Kuzyakov, ; Zang et al, ; Zimmermann, Dauber, & Jones, ), and while the use of such sites where soil and climate conditions are similar can provide a reasonable indication they may not accurately replicate baseline SOC stocks (McCalmont, Hastings, et al, ; Richter et al, ). Therefore, there is a need to reduce some of the uncertainty around the impact of this LUC from grassland to Miscanthus on SOC (Whitaker et al, ), especially over the longer term.…”

Nitrogen fertilization raises CO2 efflux from inorganic carbon: A global assessment

“…After this period, the new equilibrium between the plant C input, organic C stocks in soil and CO 2 losses is nearly reached and no further CO 2 losses from SOC are expected, at least until new major land‐use changes. Furthermore, in contrast to SOC pools with a comparatively short turnover rate of years to decades (Hsieh, ; Neff et al., ; Zang et al., ), the long turnover rate of SIC (ca. 85,000 year – without anthropogenic impact) (Schlesinger, ) makes CaCO 3 dissolution due to acidity induced by N fertilization, a unidirectional source of CO 2 efflux during human history.…”