THE INFLUENCE OF INORGANIC MATRICES ON THE DECOMPOSITION OF STRAW

The decomposition of straw (St) in the absence or presence of inorgani c matrices [sand (S), sand+kaolin (S+K), loamy sand (LS)] with and wit hout added N (in the form of urea) was followed by chemical and spectr oscopic methods over 24 weeks. Inorganic matrices had a significant ef fect on C mineralisation from day 14 onwards, and at the end of the in cubation (day 168), the relative proportion of C remaining was least i n the St+S+K+N, St+S+N and St+S-N treatments, and most in the St+LS+N and St+LS-N treatments. The LS and S+K matrices partially inhibited mi neralisation, possibly by physical protection of the straw, but the ad dition of N to the S+K matrix negated this effect. Added N increased t he mineralisation of C during the first 5 weeks but had little effect after this time. The addition of N increased the rate of change of the C:N ratio for the St+LS+N treatment in the first 16 weeks but did not significantly influence tile rate of change for the other treatments. When all the treatments were compared, inorganic matrices only had a significant effect on C:N ratios in the first 6 weeks of the incubatio n; however, if only the -N and +N treatments were compared, there were significant differences between inorganic matrices throughout tile in cubation. For the -N and +N treatments, the St+LS-N and St+N treatment s had the lowest C:N ratios at day 168, respectively. Treatments with straw only had biomass C concentrations an order of magnitude higher t han those of the inorganic matrix treatments. However, when biomass C was expressed as a proportion of total organic C, the St+S+K+N treatme nt was significantly higher than all other treatments, suggesting that clay was protecting the microbial biomass against predation by other organisms. Solid state C-13 CP/MAS NMR spectra of whole samples from e ach treatment al the end of the incubation suggested that straw incuba ted in inorganic matrices was more decomposed (as determined by the O- alkyl-C:alkyl-C ratio) than the straw only treatments, despite the occ urrence of the same amount of, or less, mineralisation. Either there a re differences in the decomposition/mineralisation balance between tre atments or alkyl-C has been synthesised by the microbial biomass and p reserved by the inorganic matrices. Examination of the alkyl-C region of the various spectra suggests that the alkyl-C which persists in the inorganic matrices is both plant and microbially derived; hence the l atter explanation is probably true.