Turnover of carbon and nitrogen in a sandy loam soil following incorporation of chopped maize plants, barley straw and blue grass in the field

In a field experiment, chopped maize, barley straw and blue grass were incorporated into a sandy loam soil. Mineral N, soil microbial biomass C and N, extractable organic C and N, as well as C and N in particulate organic matter fractions were measured frequently over 1 y in the upper 15 cm of the soil. Measurements were carried out weekly at the beginning and with decreasing frequency as the experiment progressed. In the early stage of decomposition, the soil surface CO2-evolution rate and the N mineralization-immobilization rates were influenced primarily by the C-to-N ratio and by the content of easily decomposable substances of the added plant materials. However, during the later stage the lignin contents and the lignin-to-N ratios of the added plant materials were more significant for the intensity of the microbial turnover. N-balances were calculated for the N-turnover during periods with minimal N-leaching. Negative balances indicated that N disappeared into an unknown sink which is considered to be microbial residual products (e.g. empty hyphae, residues of dead microbial cells, cell exudates). The disappearance of N was most obvious in the barley straw treatment. The development of different microbial communities induced by the different qualities of the added plant materials may have been responsible for this substrate dependent effect. The amount of light particulate organic matter (>100 μm, ρ<1.4 g cm−3) and its C-to-N ratio decreased during the experiment. This confirms a previous study which identified light particulate organic matter as an indicator of the portion of added plant material that remains undecomposed in the soil. The total amount of decomposed added plant material C calculated from the light particulate organic matter measurements was more than double that of the plant material C evolved as CO2 from the soil surface. Our results suggest that a part of the decomposed plant material C was immobilized both in the soil microbial biomass as well as in a considerable amount of microbial residual products. Therefore, it does not appear that soil surface CO2-flux-measurements are useful for quantitative estimations of the absolute turnover of added plant residues in the soil within the duration of our study. The natural 13C-enrichment of the added maize material was apparently sufficient for the calculation of the maize-derived particulate organic matter in the light particulate organic matter fraction of the soil. However, these measurements seemed to be biased by an uneven distribution of the 13C-abundance in the added maize material.