Soil nitrous oxide flux following land‐use reversion from Miscanthus and SRC willow to perennial ryegrass

Abstract

Decarbonization of the world's energy supply is essential to meet the targets of the 2016 Paris climate change agreement. One promising opportunity is the utilization of second generation, low input bioenergy crops such as Miscanthus and Short Rotation Coppice (SRC) willow. Research has previously been carried out on the greenhouse gas (GHG) balance of growing these feedstocks and land-use changes involved in converting conventional cropland to their production; however, there is almost no body of work understanding the costs associated with their end of life transitions back to conventional crops. It is likely that it is during crop interventions and land-use transitions that significant GHG fluxes might occur. Therefore, in this study, we investigated soil GHG fluxes over 82 weeks during transition from Miscanthus and SRC willow into perennial ryegrass in west Wales, UK. This study captured soil GHG fluxes at a weekly time step, alongside monthly changes in soil nitrogen and labile carbon and reports the results of regression modelling of suspected drivers. Methane fluxes were typically trivial; however, nitrous oxide (N 2 O) fluxes were notably affected, reverted plots produced significantly more N 2 O than retained controls and Miscanthus produced significantly higher fluxes overall than willow plots. N 2 O costs of reversion appeared to be contained within the first year of reversion when the Mis- . Despite these significant increases, the carbon cost of energy contained in these perennial crops remained far lower than the equivalent carbon cost of energy in coal. K E Y W O R D Scrop reversion, energy crops, greenhouse gas balance, land-use change, Miscanthus, N 2 O, nitrous oxide,, Nutrients, perennial ryegrass, Willow

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

“…In this long‐term LUC study where initial SOC stocks are similar to that expected for temperate grasslands in this climate (; Kiely et al, ; McCalmont, McNamara, et al, ), we have seen decreases in SOC (compared to baseline levels, and between modelled predictions of grassland and Miscanthus ), which more than doubled a production cost LCA result (Table ). Similarly soil N 2 O emissions during crop establishment and reversion to the next crop have recently been shown to represent a significant portion of the greenhouse gas balance (Holder et al, ; McCalmont et al, ).…”

“…To consider the inclusion of other GHG costs relating to the LUC, the carbon cost of increased soil N 2 O emissions over the establishment to Miscanthus (4.13 Mg CO 2 ‐eq/ha [8.83 kg N 2 O‐N/ha], Holder et al, ), and reversion process back to grassland (3.41 Mg CO 2 ‐eq/ha [7.29 kg N 2 O‐N/ha], McCalmont et al, ), were converted to g CO 2 ‐eq/MJ using the cumulative 15 year yield. In both N 2 O studies, no fertilizer was used during the Miscanthus management or LUC, and emissions were estimated from weekly (over a 20 month period, McCalmont et al, ) or biweekly (over an 18 month period, Holder et al, ) static chamber sampling.…”

Soil N2O emissions with different reduced tillage methods during the establishment of Miscanthus in temperate grassland

“…This was converted to Mg CO 2 ‐eq/ha using yield estimate of 12 Mg DM ha −1 year −1 , or 120 Mg DM/ha for the full 10 year period (Larsen, Jørgensen, Kjeldsen, & Lærke, ). Whilst yields can vary and are typically reduced at the start and end of a crops’ lifetime, the figure used is taken as a representative mean for the 10 year time span, being at the lower end of a range of reported mean yields (Clifton‐Brown, Stampfl, & Jones, ; Larsen et al, ; McCalmont et al, ). The energy content used was 17.95 GJ/Mg DM (Felten, Fröba, Fries, & Emmerling, ).…”

How can miscanthus fields be reintegrated into a crop rotation?

“…This question of the reintegration or removal of former miscanthus fields has only been dealt with in a few studies. McCalmont et al (2018), for example, investigated the nitrous oxide emissions after a miscanthus removal. Dufossé, Drewer, Gabrielle, and Drouet (2014) analysed the effect of a miscanthus removal on soil nutrient stock, greenhousegas emissions and the yield of the following crop (wheat).…”

“…This, in turn, can lead to nitrate leaching (Seidel et al, 2009). A removal of permanent crops, such as grassland or miscanthus, may also lead to high nitrous oxide emissions (Dufossé et al, 2014;McCalmont et al, 2018;Pinto et al, 2004;Vellinga, Pol-van, & Kuikman, 2004).…”