Emission rates of N2O and CO2 from soils with different organic matter content from three long-term fertilization experiments—a laboratory study

Increasing organic matter stocks in soils reduce atmospheric carbon dioxide (CO 2 ), but they may also promote emissions of nitrous oxide (N 2 O) by providing substrates for nitrification and denitrification and by increasing microbial O 2 consumption. The objectives of this study were to determine the effects of fertilization history, which had resulted in different soil organic matter stocks on (1) the emission rates of N 2 O and CO 2 at a constant soil moisture content of 60% water-holding capacity, (2) the short-term fluxes of N 2 O and CO 2 following the application of different fertilizers (KNO 3 vs. farmyard manure from cattle) and (3) the response to a simulated heavy rainfall event, which increased soil moisture to field capacity. Soil samples from different treatments of three long-term fertilization experiments in Germany (Methau, Spröda and Bad Lauchstädt) were incubated in a laboratory experiment with continuous determination of N 2 O and CO 2 emissions and a monitoring of soil mineral N. The long-term fertilization treatments included application of mineral N (Methau and Spröda), farmyard manure + mineral N (Methau and Spröda), farmyard manure deposition in excess (Bad Lauchstädt) and nil fertilization (Bad Lauchstädt). Long-term addition of farmyard manure increased the soil organic C (SOC) content by 55% at Methau (silt loam), by 17% at Spröda (sandy loam) and by 88% at Bad Lauchstädt (silt loam; extreme treatment which does not represent common agricultural management). Increased soil organic matter stocks induced by long-term application of farmyard manure at Methau and Spröda resulted in slightly increased N 2 O emissions at a soil moisture content of 60% water-holding capacity. However, the effect of fertilization history and SOC content on N 2 O emissions was small compared to the short-term effects induced by the current fertilizer application. At Bad Lauchstädt, high N 2 O emissions from the treatment without fertilization for 25 years indicate the importance of a sustainable soil organic matter management to maintain soil structure and soil aeration. Emissions of N 2 O following the application of nitrate and farmyard manure differed because of their specific effects on soil nitrate availability and microbial oxygen consumption. At a soil moisture content of 60% water-holding capacity, fertilizer-induced emissions were higher for farmyard manure than for nitrate. At field capacity, nitrate application induced the highest emissions. Our results indicate that feedback mechanisms of soil C sequestration on N 2 O emissions have to be considered when discussing options to increase soil C stocks.