Methane oxidation in soils of two long-term fertilization experiments in Germany

Oxidation of CH4 was measured in intact soil cores collected from two arable sites located in the low rainfall area of central Germany. The soils were a loamy sand (Halle) and a loess black-earth (Bad Lauchstädt), treated with different mineral and organic fertilizers for 116 and 92 y, respectively. Soil cores were kept in sealed jars at 25°C for 48 h. The decrease in the initial CH4 concentration (10 μl l−1) followed first-order-kinetics and by log-transformation, individual oxidation rates could be calculated for each treatment and tested for significant differences. At the sandy site mineral N application caused a significant inhibition of CH4 oxidation with rates of 2.44 and 0.70 mg C m−2 d−1 in the PK (phosphate + potassium) and NPK (nitrogen + PK) treatments, respectively, under continuous rye, and 1.09 and 0.11 mg C m−2 d−1 in the same treatments under continuous maize. CH4 oxidation in the plot with long-term farmyard manure (FYM) applications was intermediate. A plot where long-term FYM application ceased 40 y ago showed no residual effect on CH4 oxidation. The CH4 oxidation rate was generally lower under maize than under rye with a mean of 0.31 compared to 0.85 mg C m−2 d−1. For the loess black-earth the effect of fertilization on CH4 oxidation differed depending on pH. With pH 8 mineral and organic N applications did not affect CH4 oxidation. However, at pH 6 mineral N was significantly inhibitory to methanotrophs with a mean rate of 0.04 vs 0.38 mg C m−2 d−1 without N fertilization. Extremely large FYM applications (100 and 200 t ha−1 y−1) to this loess soil inhibited CH4 oxidation significantly. It is assumed that in the sandy soil as well as in the acid subplot of the loess soil the suitable sites for methanotrophic bacteria were restricted and application of mineral N resulted in competition between methane- and ammonia-oxidizers. Thus, earlier results on the inhibitory effect of mineral N on CH4 oxidation were confirmed. However, there were exceptions where N fertilizer had no adverse effect upon CH4 uptake and interacting factors (e.g. pH) played a major role.