Fate of carbon and nitrogen from animal manure and crop residues in wet and cold soils

Organic matter transformations take place in snow-covered soils during winter but in ways still poorly understood. Given the generally high soil water content and possible formation of ice layers during this period, anaerobic zones could develop and have determinant effects on soil C and N dynamics. The fate of C and N under wet and cold conditions was monitored in a sandy loam and a clay soil which were either unamended (control) or amended with cattle manure, pig slurry, maize or alfalfa residues. Soil samples were incubated for 84 d during winter at constant water potential (−6 kPa) in sealed containers laid on the floor of an unheated greenhouse. The containers were covered with fibre glass wool to simulate a snow cover. The O2, CO2 and N2O concentrations were periodically monitored in the headspace of the containers, whereas K2SO4-extractable C, KCl-extractable NO3− and NH4+, and volatile fatty acids were quantified in soil sub-samples. Despite the low soil temperature (average of 1 °C), O2 consumption and CO2 production rates were significantly (P<0.05) increased by the amendments for the first 29 d of incubation. After 29 d, the O2 concentration fell below 6% and CO2 production rate decreased in all amended soils. We assumed that after 29 d aerobic respiration was replaced with fermentation processes in amended soils as indicated by respiratory quotients generally larger than 2, and by the progressive accumulation of volatile fatty acids. As O2 was consumed in sealed containers, nitrification decreased whereas ammonification continued and N2O production increased. As a result, soil NH4+ content increased whereas NO3− content was progressively depleted. After complete exhaustion of NO3−, the accumulated N2O disappeared and was probably reduced to N2. All measured parameters indicated that pig slurry and alfalfa stimulated soil microbial activities more than cattle manure and maize residues and these differences were partly related to the lignin-to-N ratio of the various amendments. Our data indicated that (i) significant C and N transformations occurred during winter in wet and cold soils amended with organic residues, (ii) under these conditions C and N dynamics were strongly influenced by O2 availability, and (iii) the rates at which C and N transformations occurred were partly determined by both C degradability and N availability of the organic amendments.