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

Qiu, S., McComb, A.J. and Bell, R.W. (2008) Ratios of C, N and P in soil water direct microbial immobilisation–mineralisation and N availability in nutrient amended sandy soils in southwestern Australia. Agriculture, Ecosystems & Environment, 127 (1-2). pp. 93-99.

Abstract

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