Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

This study examines the effect of soil P status and N addition on the decomposition of C-14-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg(-1) and annual pasture production was 4,868 and 14,120 kg DM ha(-1) respectively. Soils were amended with C-14-labelled glucose (2,076 mg C kg(-1) soil), with anti without the addition of N (207 mg kg(-1) soil), and incubated for 168 days. During incubation, the amounts of (CO2)-C-14 respired, microbial biomass C and C-14, microbial biomass P, extractable inorganic P (P-i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived (CO2)-C-14 production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The C-14 that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in Pi in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of Pi were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass C-14 and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates.