Influence of cropping systems on greenhouse gas emissions

Summary

Globally, agricultural soils are the largest source of total anthropogenic nitrous oxide (N2O); a potent greenhouse gas (GHG). In agricultural soils N2O is produced as an intermediate in microbially mediated nitrification and denitrification and also represents a pathway of soil nitrogen (N) loss. Specific management practices such as fertilization, inclusion of grass-clover leys and catch crops would be expected to have different effects on soil C input, biological, chemical and physical properties and, consequently, crop production and soil N2O emissions.
This study was mainly based on a monitoring programme conducted at a long-term experiment in Denmark. The effects of fertilization, catch crops and grass-clover leys on cereal production, soil properties and GHG emissions in arable cropping systems were evaluated. In addition, a diverse dataset of field measurements that included above- and below-ground dry matter accumulation and N concentrations in winter wheat, soil water content and mineral N concentrations, soil heterotrophic CO2 respiration and N2O emissions was used to test two process-based models (i.e. FASSET and MoBiLE-DNDC).
In the studied systems, increased C input from manure and catch crops enhanced N transformation processes in organic fertilizer-based systems. Microbial activity and gas diffusivity were higher in systems based on organic compared to inorganic fertilizer. Annual cumulative soil N2O emissions were similar across systems. However, high N2O emissions per N applied were observed in organic systems. High N2O emissions occurred during periods when high soil nitrate levels coincided with high soil temperature or high soil water content. With regards crop productivity, the incorporation of catch crops prior to sowing in combination with application of animal manure enhanced grain and N yields of low-input organic systems.
The two process-based models were generally capable of simulating most seasonal trends of measured soil N2O emissions. However, annual soil N2O emissions in all systems were overestimated by the two models. Both models had problems in simulating soil mineral N, which seemed to originate from deficiencies in simulating degradation of soil organic matter, incorporated residues of catch crops and organic fertilizers. To improve the performance of the two models in simulating N2O emissions, organic matter decomposition parameters need to be revised.
The significance of this study in terms of food production and GHG emissions is that by adjusting crop rotations, organic cropping systems can contribute to food security and soil quality without enhanced GHG emissions. However, to improve productivity and reduce N2O emissions from organic systems priority may need to be placed on increasing N use efficiency. The two studied process-based models were capable of simulating trends in soil N2O emissions; however, with a few revisions, especially modelling of organic matter decomposition, these two models could be useful for simulating and up-scaling of GHG emissions from site to regional/national scales.