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 biomas s C and N, extractable organic C and N, as well as C and N in particul ate organic matter fractions were measured frequently over 1 y in the upper 15 cm of the soil. Measurements were carried out weekly at the b eginning and with decreasing frequency as the experiment progressed. I n the early stage of decomposition, the soil surface CO2-evolution rat e and the N mineralization-immobilization rates were influenced primar ily by the C-to-N ratio and by the content of easily decomposable subs tances of the added plant materials. However, during the later stage t he lignin contents and the lignin-to-N ratios of the added plant mater ials were more significant for the intensity of the microbial turnover . N-balances were calculated for the N-turnover during periods with mi nimal 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 dif ferent qualities of the added plant materials may have been responsibl e for this substrate dependent effect. The amount of light particulate organic matter (> 100 mu m, rho < 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 por tion of added plant material that remains undecomposed in the soil. Th e total amount of decomposed added plant material C calculated from th e light particulate organic matter measurements was more than double t hat 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 imm obilized both in the soil microbial biomass as well as in a considerab le amount of microbial residual products. Therefore, it does not appea r that soil surface CO2-flux-measurements are useful for quantitative estimations of the absolute turnover of added plant residues in the so il within the duration of our study. The natural C-13-enrichment of th e added maize material was apparently sufficient for the calculation o f the maize-derived particulate organic matter in the light particulat e organic matter fraction of the soil. However, these measurements see med to be biased by an uneven distribution of the C-13-abundance in th e added maize material. (C) 1998 Elsevier Science Ltd. All rights rese rved.