Sustainable intensification of crop residue exploitation for bioenergy: Opportunities and challenges

Abstract Crop residue exploitation for bioenergy can play an important role in climate change mitigation without jeopardizing food security, but it may be constrained by impacts on soil organic carbon (SOC) stocks, and market, logistic and conversion challenges. We explore opportunities to increase bioenergy potentials from residues while reducing environmental impacts, in line with sustainable intensification. Using the case study of North Rhine‐Westphalia in Germany, we employ a spatiotemporally explicit approach combined with stakeholder interviews. First, the interviews identify agronomic and environmental impacts due to the potential reduction in SOC as the most critical challenge associated with enhanced crop residue exploitation. Market and technological challenges and competition with other residue uses are also identified as significant barriers. Second, with the use of agroecosystem modelling and estimations of bioenergy potentials and greenhouse gas emissions till mid‐century, we evaluate the ability of agricultural management to tackle the identified agronomic and environmental challenges. Integrated site‐specific management based on (a) humus balancing, (b) optimized fertilization and (c) winter soil cover performs better than our reference scenario with respect to all investigated variables. At the regional level, we estimate (a) a 5% increase in technical residue potentials and displaced emissions from substituting fossil fuels by bioethanol, (b) an 8% decrease in SOC losses and associated emissions, (c) an 18% decrease in nitrous oxide emissions, (d) a 37% decrease in mineral fertilizer requirements and emissions from their production and (e) a 16% decrease in nitrate leaching. Results are spatially variable and, despite improvements induced by management, limited amounts of crop residues are exploitable for bioenergy in areas prone to SOC decline. In order to sustainably intensify crop residue exploitation for bioenergy and reconcile climate change mitigation with other sustainability objectives, such as those on soil and water quality, residue management needs to be designed in an integrated and site‐specific manner.


Information on the case-study region
NRW is a highly productive and intensively cultivated region located in the central western part of Germany. It is the most populous and one of the most productive German states accounting for more than 20% of Germany's Gross Domestic Product (Staatskanzlei des Landes Nordrhein-Westfalen, 2016).

Figure S1. Location of NRW.
In NRW, annual rainfall varies between 600 mm in the lowland (north and west NRW) and 1,400 mm in the highlands (northeast and south NRW) and average temperature ranges between 5-10 °C (LWK NRW, 2014). The northwestern part of NRW (Munster and parts of Detmold and Dusseldorf) is mainly characterized by less fertile light soils. Soils are most fertile on the north eastern and southern plains. Figure S2. Texture (left) and organic carbon content (right) of soils in NRW. For the soil texture map we aggregated the 31 soil texture classes from the German soil-classification system (AG Boden, 2005) into light (sandy), medium (silty and loamy) and heavy (clayey) soils. The soil organic carbon (SOC) map shows baseyear (2000)(2001)(2002)(2003)(2004) SOC levels simulated by the MONICA model after model calibration. Figure S3. Combinations of pedoclimatic regions and rotations in NRW. The pedoclimatic regions are retrieved from Roßberg et al (2007)and the percentage of rotations per pedoclimatic region from Burkhardt and Gaiser (2010) based on statistical reports on cropland shares of major crops in 2007.

Fertilisation and cover crops
All fertilization strategies assume a crop-specific average N requirement to attain optimal growth and yield (target N in Table S2) derived from LWK NRW (2016). Part of this N demand is covered by the mineralization of soil organic matter and organic fertilizers. The remainder (i.e. target N -soil N supply) is applied as mineral N. The mineral N target is split into different fertilization events, with timing and relative dosage determined from recommendations (LWK NRW, 2016) (Table S2). The way soil N supply is estimated is scenario-dependent. For the rule-based option, soil N supply is provided by monitoring stations covering the study area (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007), which record the N availability in soil at the begin of vegetative growth for different combinations of crop sequences and soil type (Table  S3) (LWK NRW, 2018). The estimated soil supply is then adjusted by the effects of cover crops and organic N management following the official recommendations (LWK NRW, 2016). In specific, the presence of a cover crop prior to the main crop increases the supply by 20 kg N ha -1 and each livestock unit (corresponding to 100 kg N from organic fertilizer) increases it by 10 kg N ha -1 .
For the optimised option, we assume farmers are able to measure soil N supply at each fertilization event, and apply the precise amount of mineral N required to meet the target N in the rooted zone. The target N is partitioned among different events according to the same dosage rules used for the rule-based option (Table S2). Applied mineral N is adjusted to compensate for the difference between the event-specific target N (i.e. crop target N x dosage coefficient) and the mineral N content simulated in the rooted zone the day of fertilisation.
Regarding cover crop assumptions, we use mustard, as it is one of the most common ones in NRW. Sowing and harvest dates of the cover crop are set dynamically during the simulation (after the harvest of the previous main crop and no later than September 20 th ) in order to minimize the fallow period. Harvest of the cover crop occurs ten days before the date of sowing of the following main crop.
For the main crops, typical sowing and harvest dates were identified for each pedoclimatic region (Table S4). The former were used to set the fixed sowing date of each crop, whereas the latter allowed defining a latest date of harvest (i.e. 15 days after typical harvest date). Harvest is triggered either at maturity, if this is reached before the latest date, or at the latest date defined. Figure S4. Organic fertilisation levels in NRW. The map is based on the organic N balance of farms estimated at district level (LWK NRW, 2014,   Source: Bundessortenamt (2000). For each crop, missing dates (NA) were replaced by the average of crop operations in NRW pedoclimatic regions for which the data is available. For potato, we used the German average as retrieved from the German weather service (DWD) phenological database (DWD Climate Data Center, 2018).
Winter triticale dates are assumed as the average of winter wheat and winter rye. For the codes of the pedoclimatic regions see Figure S2.  (Stella et al., 2019).