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

Abstract

The DAISY soil–organic-matter submodel was evaluated against independent data from a 1 year field study with incorporation (0–15 cm) of chopped barley straw, blue grass and maize into a sandy loam soil. Investigated parameters were soil respiration, soil mineral N, soil microbial biomass-C and N (SMB-C and N) dynamics and the predicted decomposition of the added organic matter (AOM) measured as light particulate organic matter (LPOM>100 μm, ρ<1.4 g cm −3). Significant differences between values predicted from the model and measured values of soil respiration, soil mineral N, and soil microbial biomass-C and N were observed in all treatments. However, the model predictions for the unamended soil and for the soil receiving barley straw were better than those for the two other treatments. Discrepancies, in the blue grass and in the maize treatment, led to suggestions for model improvements. A distinct short-term pulse of SMB growth observed immediately after incorporation of the plant materials was not predicted fully. However, the difference between the measured and the predicted SMB pools did not induce a complementary difference for the mineral N pool. Soil microbial residues (SMR), temporarily protected against recycling via the microbial turnover and mineralisation, are discussed as a possible sink for the N from SMB. The predicted dynamics of the less labile AOM1-pool (initialised as water inextractable AOM) was correlated with the measured amounts of LPOM from the added plant materials. A slight overestimation of measured LPOM by AOM1 in the initial period after incorporation of AOM was followed by a slight underestimation later on. This trend might be attributed to the assumed constant C/N-ratio and turnover rate during the simulated decay of AOM1, contrasting reality in which LPOM is changing, e.g. C/N-ratio and lignin content. The simple initial partition of AOM into a water extractable part (AOM2) and a water inextractable part (AOM1), both parameterised with predetermined turnover rates and utilisation efficiencies, calls for re-evaluation. Suggestions are made to include the concept of SMR and the changing composition of LPOM into the DAISY model.