JPIFACCE-Agriculture, Food Security and Climate Change

Crop residues contribute to emissions of the GHG nitrous oxide (N2O). This gas is particularly worrying because of its long-lasting time (half-life 300 years). ResidueGas addressed the estimation of N2O emissions from residue amendments, and to what extent these emissions can be offset by increasing soil organic carbon (SOC) stocks with soil residue incorporation. This project was an international effort in collaboration with Denmark, France, Germany, Sweden and UK. The results of ResidueGas illustrate the importance of the large variation in the quality of crop residues and their management on N2O emissions. The project developed a large database of residue quality representing important aspects for addressing impacts of crop residues on greenhouse gas (GHG) emissions and soil fertility. A comprehensive collection and analysis (meta-analysis) of published field experiments showed that crop residue effects on N2O emissions were best predicted when the quality of the crop residues was also taken into account, rather than only the total N content in crop residues. However, due to functional linkage among quality attributes, a simplified index based on the stage of residue maturity provided the most robust approach to classify crop residues according to their potential to enhance N2O emissions. Thus, immature residues strongly stimulated N2O emissions, whereas mature residues had marginal effects on N2O. Immature residues are still vegetative active and usually contain green parts, while a typical mature residue would be cereal straw at harvest. These findings were corroborated by new experimental data from laboratory and field experiments. Results showed also that N2O emissions caused by roots were marginal compared to emissions caused by aboveground residues, which could not be explained by the biochemical composition only. This suggests that soil provides chemical and physical protection to roots due to their close contact with soil mineral particles. The experimental results also showed an interaction of crop residue with soil mineral N management, resulting in greater emissions from incorporation of immature residues when soil mineral N was high. Thus, use of chemical nitrogen fertilizer shortly after incorporation of immature residues should be avoided. The experiments showed that residue removal would be an effective mitigation option for immature crop residues, whereas for mature crop residues effects of management were smaller and inconsistent. Norwegian field studies showed that strong N2O emissions can occur from frozen soil under snow, during the snow melting period. Thus, avoiding ploughing in autumn can be a mitigation option in case of immature residues, as grassland and green manure. A field study showed that raising soil pH of acidic soils thorough liming had a great effect on N2O emissions derived from plant residues. Thus, a pH (pHaq ~6.0 ) closer to that generally recommended for crop production halved the N2O emissions associated with incorporation of immature residues compared with a soil pH suboptimal for crop production (pHaq ~5.4). Modelling of aboveground crop residue management for cropping systems across Europe under both current and projected future climate shows that long-term effects of residue management on GHG balances is determined by effects on N2O emissions. Changing rates of addition in residues leads to temporary changes in SOC over 10-15 years; however, effects of changes in residue management on N2O emissions persist and outweigh possibly effects on soil organic carbon (SOC) stocks. Furthermore, simulations show that residue removal has greater effects on N2O emissions than the method of its application, i.e. surface application or incorporation by ploughing. A questionnaire survey of farmers in Denmark and Norway showed that retention of aboveground crop residues was the most widely used option among management strategies to maintain or improve SOC. Consequently, removal of plant residues was considered negatively for ecosystem services as well as crop production. In conclusion: ResidueGas has proposed a new concept for quantifying N2O emissions, where the emission factor is determined by crop residue characteristics, distinguishing mature and immature residue types aboveground. The main crop types belonging to immature residue - cover crops, grasslands and vegetables - are important for the delivery of multiple ecosystem services. Thus, these aboveground residues should be managed judiciously to avoid their potentially high N2O emissions. In the case of acid soil, liming to maintain soil pH at a level optimal for most plant production is also an option for mitigating N2O emissions from plant residues. There is little empirical evidence in the literature on the specific effect of roots and other belowground residues on N2O emissions, thus our finding that N2O emission from roots is marginal needs validation in several soil and root types.

ResidueGas has successfully reached the key objectives. This included the following aspects that would have not been possible without the intense collaboration among project partners: 1) We achieved to develop the scientific basis for a new method for quantifying N2O emissions from crop residues and assessed its value for mitigation efforts based on literature and new empirical studies, 2) we were able to compare parameterization and simulation results of two process oriented models on site and EU scale, 3) we propose practical mitigation strategies. These findings are likely to impact favourably the management of crops residues towards reduced N2O emissions without impairing other ecosystems services, and also contribute to the improvement of N2O accounting from agriculture in a simple and transparent way.

Crop residues provide large inputs of carbon (C) and nitrogen (N) to soils and contribute to the net GHG balance of soils in different ways. They are included as a key component in national emissions inventories for nitrous oxide (N2O) from agriculture. Residue are also a major contributor to sustaining or enhancing soil organic carbon (SOC) and N contents and thus soil fertility. Depending on the amount of C and N in crop residues and their contributions to N2O emissions or to the SOC balance residues might increase or decrease the GHG footprint of agroecosystems. Studies have shown that N2O emissions from N in crop residues vary considerably depending on residue quality, residue management and soils. This is currently not reflected in emissions inventories or likely not sufficiently in simulation models. This makes current emission inventories uncertain and in many cases biased. Lack of knowledge and precise model estimation of N2O emissions and SOC storage from crop residues limits the design of improved crop management systems for net GHG emissions reductions. ResidueGas will document an improved methodology to quantify N2O emissions from agricultural crop residues management, including standards for estimating the amount of N in residues and improved emission factors for crop residue that include effects of residue quality, management, soils and climate on emissions. ResidueGas will further identify and communicate best practice for crop residue management strategies with respect to their net greenhouse gas effect in terms of N2O emissions and SOC storage. The transnational team in ResidueGas has the necessary breadth of high-level expertise to undertake the task, covering knowledge on relevant cropping systems, measurements of GHG emissions (N2O and SOC) at both field and lab scales, access to existing data on crop residues and emissions, expertise in biogeochemical modelling of GHG emissions and knowledge on GHG inventory reporting at farm and national scale.