Ratios of C, N and P in soil water direct microbial immobilisation–mineralisation and N availability in nutrient amended sandy soils in southwestern Australia

Microbial processes regulate soil nutrient availability to plants through mineralisation and immobilisation. It is less clear why in some circumstances microbial activity results in net-immobilisation of nutrients, in others net-mineralisation. In this study, we used a full combination of C, N and P (7 treatments and a control), which resulted in a wide range of C/N (1–167) and C/P (7–651) ratios in two sandy soils of southwestern Australia, to test microbial response to the amendments in relation to N-immobilisation and -mineralisation behaviour. After 5 days of amended incubation, soil water N was measured and net-immobilisation or -mineralisation estimated as the difference in soil water N before and after incubation. Microbial respiration, soil water C and P concentrations were also monitored. Microbial respiration was generally coupled with net-N-immobilisation. While all C-supply treatments such as C, CN, CP and CNP resulted in net-N-immobilisation, the “non-C” amendments including N, P and NP and the blank favoured net-N-mineralisation. Overall, C/N ratio appears more critical than C/P in directing microbial N-immobilisation/mineralisation hence the soil water N, with N remaining in soil water decreasing exponentially with increasing initial C/N. Such an inverse relationship was, however, tied to non-P-limiting conditions. It is suggested that the immediate availability of C, N, and P and their relative supply compared to that of the cell physiological demands may underlie microbial responses, and in the short-term, determine the direction of the two opposing microbial processes, N-mineralisation and -immobilisation.