SOIL C AND N TURNOVER AFTER INCORPORATION OF CHOPPED MAIZE, BARLEY STRAW AND BLUE GRASS IN THE FIELD - EVALUATION OF THE DAISY SOIL-ORGANIC-MATTER SUBMODEL

The DAISY soil-organic-matter submodel was evaluated against independe nt data from a 1 year field study with incorporation (0-15 cm) of chop ped barley straw, blue grass and maize into a sandy loam soil. Investi gated parameters were soil respiration, soil mineral N, soil microbial biomass-C and N (SMB-C and N) dynamics and the predicted decompositio n of the added organic matter (AOM) measured as light particulate orga nic matter (LPOM > 100 mu m, rho < 1.4 g cm(-3)). Significant differen ces between values predicted from the model and measured values of soi l respiration, soil mineral N, and soil microbial biomass-C and N were observed in all treatments. However, the model predictions for the un amended soil and for the soil receiving barley straw were better than those for the two other treatments. Discrepancies, in the blue grass a nd in the maize treatment, led to suggestions for model improvements. A distinct short-term pulse of SMB growth observed immediately after i ncorporation of the plant materials was not predicted fully. However, the difference between the measured and the predicted SMB pools did no t induce a complementary difference for the mineral N pool. Soil micro bial residues (SMR), temporarily protected against recycling via the m icrobial turnover and mineralisation, are discussed as a possible sink for the N from SMB. The predicted dynamics of the less labile AOM1-po ol (initialised as water inextractable AOM) was correlated with the me asured amounts of LPOM from the added plant materials. A slight overes timation of measured LPOM by AOM1 in the initial period after incorpor ation of AOM was followed by a slight underestimation later on. This t rend might be attributed to the assumed constant C/N-ratio and turnove r rate during the simulated decay of AOM1, contrasting reality in whic h LPOM is changing, e.g. C/N-ratio and lignin content. The simple init ial partition of AOM into a water extractable part (AOM2) and a water inextractable part (AOM1), both parameterised with predetermined turno ver rates and utilisation efficiencies, calls for re-evaluation. Sugge stions are made to include the concept of SMR and the changing composi tion of LPOM into the DAISY model. (C) 1998 Elsevier Science B.V. All rights reserved.