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

Abstract

Increasing organic matter stocks in soils reduce atmospheric carbon dioxide (CO2), but they may also promote emissions of nitrous oxide (N2O) by providing substrates for nitrification and denitrification and by increasing microbial O2 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 N2O and CO2 at a constant soil moisture content of 60% water-holding capacity, (2) the short-term fluxes of N2O and CO2 following the application of different fertilizers (KNO3 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, Sproda and Bad Lauchstadt) were incubated in a laboratory experiment with continuous determination of N2O and CO2 emissions and a monitoring of soil mineral N. The long-term fertilization treatments included application of mineral N (Methau and Sproda), farmyard manure + mineral N (Methau and Sproda), farmyard manure deposition in excess (Bad Lauchstadt) and nil fertilization (Bad Lauchstadt). Long-term addition of farmyard manure increased the soil organic C (SOC) content by 55% at Methau (silt loam), by 17% at Sproda (sandy loam) and by 88% at Bad Lauchstadt (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 Sproda resulted in slightly increased N2O emissions at a soil moisture content of 60% water-holding capacity. However, the effect of fertilization history and SOC content on N2O emissions was small compared to the short-term effects induced by the current fertilizer application. At Bad Lauchstadt, high N2O 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 N2O 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 N2O emissions have to be considered when discussing options to increase soil C stocks.