The influence of inorganic matrices on the decomposition of straw

Abstract

The decomposition of straw (St) in the absence or presence of inorganic matrices [sand (S), sand+kaolin (S+K), loamy sand (LS)] with and without added N (in the form of urea) was followed by chemical and spectroscopic methods over 24 weeks. Inorganic matrices had a significant effect on C mineralisation from day 14 onwards, and at the end of the incubation (day 168), the relative proportion of C remaining was least in 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 mineralisation, possibly by physical protection of the straw, but the addition of N to the S+K matrix negated this effect. Added N increased the 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 incubation; however, if only the -N and +N treatments were compared, there were significant differences between inorganic matrices throughout tile incubation. For the -N and +N treatments, the St+LS-N and St+N treatments had the lowest C : N ratios at day 168, respectively.

Treatments with straw only had biomass C concentrations an order of magnitude higher than those of the inorganic matrix treatments. However, when biomass C was expressed as a proportion of total organic C, the St+S+K+N treatment was significantly higher than all other treatments, suggesting that clay was protecting the microbial biomass against predation by other organisms.

Solid state ¹³C CP/MAS NMR spectra of whole samples from each treatment al the end of the incubation suggested that straw incubated in inorganic matrices was more decomposed (as determined by the O-alkyl-C : alkyl-C ratio) than the straw only treatments, despite the occurrence of the same amount of, or less, mineralisation. Either there are differences in the decomposition/mineralisation balance between treatments or alkyl-C has been synthesised by the microbial biomass and preserved 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 latter explanation is probably true.