Opposing effects of nitrogen and phosphorus on soil microbial metabolism and the implications for soil carbon storage

Nitrogen (N) and phosphorus (P) availability plays a crucial role for carbon cycling in terrestrial ecosystems. However, the effect of nutrient supply on soil organic matter decomposition and microbial metabolism is generally not well understood. In this study, we incubated soils with three contrasting nutrient regimes from each of three Swedish long-term agricultural experiments (>45 years, 7-30% clay), namely an unfertilised control (0NPK), high P but no N fertilisation (PK0N) and high N but no P fertilisation (N-0pk). In the laboratory, we amended all soils with no fertiliser or with N, P and N P, with and without glucose, and monitored CO2 and heat production over five hours. Significant effects of the treatments were observed when nutrients were added in combination with glucose. Averaged over all field treatments, Glucose + N addition reduced CO2 and heat production by 14% and 14%, respectively, compared with glucose addition alone, while glucose + P addition increased CO2 and heat production by 17% and 9%, respectively. Similar results were found comparing the contrasting long-term field-treatments: PK0N showed higher glucose-induced CO2 and heat production per unit SOC than 0NPK, while both variables were suppressed in N-0pk-fertilised soils. Basal respiration per unit soil organic carbon (SOC) proved to be linked to long-term losses in SOC stocks, which were highest in the PK0N-fertilised plots at all three sites. Combined analyses of field and laboratory treatments revealed that the suppressing effect of laboratory N-addition on respiration only occurred in N-deficient soils, which clearly indicates that long-term N-addition alleviated N-mining. In conclusion, N and P showed opposing effects on the microbial metabolic processes, including respiration. The observed effects were similar in the short- and long-term and across different sites, which suggests that direct physiological controls of nutrients on microbial metabolism strongly regulate SOC turnover. Short-term nutrient effects were only observed in combination with a labile C source and the N-effect was restricted to N-deficient soils. Therefore, we conclude that those findings might be more important for nutrient-poor but carbon-rich ecosystems exposed to sudden nutrient inputs in comparison with nutrient rich and relatively carbon poor agricultural systems. (C) 2016 Elsevier Ltd. All rights reserved.