Flow of nutrients and climate relevant gases in experimental rotations on three soil types

Summary

Within organic farming there is a continued effort to close nutrient cycles in order to improve nutrient use efficiency for crop production while also minimizing environmental impacts. In Denmark a long-term crop rotation experiment on three soil types, i.e., Jyndevad (4.5% clay, 2.0% C), Foulum (8.8% clay, 3.8% C) and Flakkebjerg (15.5% clay, 1.7% C), provides a platform for studies of agronomic and environmental aspects of organic farming. Selected results from investigations at these sites are presented.
In 2007 and 2008 one study determined soil microbial biomass N and mineralizable N on all three locations during the growing season in four selected rotations, three organic and one conventional; the objective was to evaluate the role of soil organic N dynamics for crop N supply. Consistent effects of crop rotation were observed across the three sites, but there were dramatic differences between soils in the ability to store N in labile organic pools.
The greenhouse gas balance of organic farming systems has become an important indicator of sustainability, and strategies to mitigate emissions are investigated. Nitrous oxide emissions were monitored in the field between October 2007 and September 2008 at Foulum and Flakkebjerg in the winter wheat crop included in all four rotations, which differed in manure strategy (livestock slurry, grass-clover green manure, or mineral N) and use of catch crops. At both sites fluxes were measured concurrently at least twice per month. Also, a second monitoring program of N2O emissions was conducted between March 2008 and May 2009 to investigate grass-clover co-digestion as a strategy to improve N use efficiency and reduce N2O emissions per unit product. Here, all crops in the rotation were represented in the monitoring program. Both studies indicate that N input rather than cropping system or soil management control the extent of N2O emissions.
Our limited ability to control N2O emissions may be due to the fact that N2O production is caused by complex interactions between C/N transformations and soil properties, including moisture. This has been investigated using intact and repacked soil from Foulum and Flakkebjerg sites. Adjusting the soil to well-defined soil water potentials with or without amendments serve to illustrate these interactions and the microbial response to nutrient supply and soil aeration.
Ammonia volatilization after livestock slurry application should be avoided as NH3 is an indirect source of N2O, and NH3 losses will reduce crop yields in N limited cropping systems. Livestock slurry treatment (anaerobic digestion, separation) and application methods (trailhose, direct injection) can influence the potential for NH3 volatilization by reducing the slurry-air contact, but with a possible trade-off with direct N2O emissions. Some results from a two-year field study, and their implications for manure management, are discussed.