Microbial carbon use efficiency of litter with distinct C/N ratios in soil at different temperatures, including microbial necromass as growth component

Abstract

An incubation study was carried out to investigate the effects of litter quality, i.e. 15 N-labelled maize (C/N of 25.5) and Rhodes grass (C/N of 57.8) leaf litter on microbial carbon use efficiency (CUE) and priming effects in a moderate alkaline soil at two different temperatures (15 and 25 °C). CUE values were calculated from the isotopic composition of the particulate organic matter (POM) recovered as an index for the amount of non-decomposed litter. This approach allows the inclusion of microbial necromass growth components in the calculation of CUE values. Additionally, the soil was incubated for 10, 20, and 30 days to determine the optimum incubation period. Soil microbial CUE values of maize and Rhodes grass leaf litter, including microbial necromass C in the calculation of CUE, varied around 0.61, regardless of litter type, temperature, and incubation period. However, the optimum incubation time is between 20 and 30 days, depending on temperature. The strong priming effect on autochthonous soil organic carbon (SOC) mineralization was apparently not caused by N mining, as it was similar for both litter qualities. It most likely resulted from SOC being used by microbial co-metabolism. The litter-induced true priming effect was accompanied by a significant increase in autochthonous POM. The current approach, including microbial necromass as growth component, has been shown to be a strong tool for investigating CUE values and priming effects after application of litter and harvest residues to soil, probably under all environmental conditions.

Microbial Mechanisms of the Priming Effect over 12 Years of Different Amounts of Nitrogen Management

“…Microbes show a negative correlation between SOC mineralization and microbial CUE [9,11]. This was proven by the significant negative relationship between CUE and the PE (Figures 1d and 3) [8,51].…”

“…However, the input of crop residues on arable lands was reported to either increase or decrease SOC mineralization, which is known as the priming effect (PE) [6][7][8][9]. Based on previous in situ and laboratory experiments, scientists have reported that the PE can be positive or negative depending on biotic or abiotic factors [10,11]. Among these influences, some studies have proven that nutrient availability is crucial for regulating the PE in arable soils [8,12].…”

“…Extracellular enzymes can simultaneously facilitate SOM mineralization [9,10]. Furthermore, "microbial nutrient mining" and "microbial stoichiometric decomposition" are mainly performed by K strategists and R strategists, respectively [8,11]. These results mainly focused on the short-term effects of the combined inputs of N fertilizer and crop residues on the PE.…”

“…Microbes absorb residual C and use it to acquire energy, composite microbial biomass, proliferate, and to maintain their metabolism. The ratio of residual C used for microbial growth is defined as the microbial C use efficiency (CUE) [11]. Higher CUE corresponds to the high-efficiency growth of microbes and relatively low emissions of C from the substrate into the atmosphere [12,13].…”

“…Soil N availability is a dominant factor driving microbial CUE. For example, Marie et al [11] suggested that soil amended with N benefited from both an increase in the residual C absorbed by the microbes and a reduction in the SOC loss triggered by the PE when compared with soil under conditions of limited N. Additionally, a significant negative relationship between CUE and the PE under low-N treatment conditions was recently reported [14,15]. N fertilizer can increase the CUE by decreasing the stoichiometric C-to-N imbalance [13][14][15][16].…”