Soil Microbial Necromass Research Articles

Soil microbial carbon pump: Mechanism and appraisal

The soil microbial carbon pump (MCP) conceptualizes a sequestration mechanism based on the process of microbial production of a set of new organic compounds, which carry the carbon from plant, through microbial anabolism, and enter into soil where it can be stabilized by the entombing effect. Understanding soil MCP and its related entombing effect is essential to the stewardship of ecosystem services, provided by microbial necromass in the formation and stabilization of soil organic matter as well as its resilience and vulnerability to global change. The mechanism and appraisal of soil MCP, however, remain to be elucidated. This lack of knowledge hampers the improvement of climate models and the development of land use policies. Here, I overview available knowledge to provide insights on the nature of the soil MCP in the context of two main aspects, i.e., internal features and external constraints that mechanistically influence the soil MCP operation and ultimately influence microbial necromass dynamics. The approach of biomarker amino sugars for investigation of microbial necromass and the methodological limitations are discussed. Finally, I am eager to call new investigations to obtain empirical data in soil microbial necromass research area, which urgently awaits synthesized quantitative and modeling studies to relate to soil carbon cycling and climate change.

Earthworms act as biochemical reactors to convert labile plant compounds into stabilized soil microbial necromass

Earthworms co-determine whether soil, as the largest terrestrial carbon reservoir, acts as source or sink for photosynthetically fixed CO2. However, conclusive evidence for their role in stabilising or destabilising soil carbon has not been fully established. Here, we demonstrate that earthworms function like biochemical reactors by converting labile plant compounds into microbial necromass in stabilised carbon pools without altering bulk measures, such as the total carbon content. We show that much of this microbial carbon is not associated with mineral surfaces and emphasise the functional importance of particulate organic matter for long-term carbon sequestration. Our findings suggest that while earthworms do not necessarily affect soil organic carbon stocks, they do increase the resilience of soil carbon to natural and anthropogenic disturbances. Our results have implications for climate change mitigation and challenge the assumption that mineral-associated organic matter is the only relevant pool for soil carbon sequestration.

Plant and microbial biomarkers suggest mechanisms of soil organic carbon accumulation in a Mojave Desert ecosystem under elevated CO2

We investigated how properties of soil organic matter (SOM) were altered after 10 years exposure to elevated atmospheric CO2 concentration ([CO2]) in a Mojave Desert ecosystem, using plant and microbial biomarkers. We focused on roles of Larrea tridentata, the dominant evergreen shrub which form islands of fertility, and biological soil crusts which have extensive cover in plant interspace. Soils to 5 cm in depth were collected under L. tridentata and plant interspace, and biological soil crusts to 0.5 cm in depth under three cover types, Pleuraphis rigida, a C4 grass, shrubs and plant interspace. Soil organic carbon contents were not significantly different between elevated and ambient [CO2]. However, significantly higher abundance of n-alkanes, a major constituent of foliage wax material, occurred in the elevated compared to ambient [CO2] plots in the soils collected under L. tridentata, but no such difference was found in the soils at plant interspace or the biological soil crusts. There was no significant difference in abundance of microbial phospholipid fatty acids between the CO2 treatments in the soils of either cover types. However, neutral lipid fatty acid abundance was significantly higher under elevated than ambient [CO2] in the soils under L. tridentata, whereas no such significant difference was observed at plant interspace. These results emphasize important roles of the dominant shrubs in SOM formation under elevated [CO2] in arid ecosystems. Elevated [CO2] stimulated growth of L. tridentata in wet years, and aboveground litter deposition via senescence contributed to SOM formation in islands of fertility. In addition, elevated [CO2] stimulated soil microbial turnover rates in rhizosphere of L. tridentata, which left more soil microbial necromass, a major SOM source. We concluded that responses of dominant shrubs to elevated [CO2] can stimulate SOM formation in arid ecosystems, but biological soil crusts may have limited capacity.

Alterations of pathways in fertilizer N conservation and supply in soils treated with dicyandiamide, hydroquinone and glucose

Nitrogen (N) immobilization by microorganisms and NH4+ fixation by soil minerals are common reactions responsible for fertilizer N retention in soils. However, the relationship between microbial immobilization and NH4+ fixation remains unclear to date, and the availability of immobilized or fixed fertilizer N has yet to be compared. Accordingly, we conducted a 96-day incubation experiment to study the effects of the nitrification inhibitor dicyandiamide (DCD), the urease inhibitor hydroquinone (HQ), and glucose adding on 15N-labeled urea-N partitioning in different N pools and on the subsequent remineralization of immobilized N and release of fixed NH4+. Glucose significantly increased N retention in soil but decreased the availability of urea-derived N because a great proportion of urea-derived N was transformed into soil microbial necromass N (SMNN). In the non-glucose treatments, the effects of the fixed NH4+ pool on the conservation and supply of urea-derived N were 1.6-fold and 2.7-fold greater on average than those of the organic N pool (including soil microbial biomass N and SMNN), respectively, from the 12th day to the end of the incubation. In the glucose treatments, the corresponding effects of the organic N pool were 3.7-fold and 3.0-fold greater than those of the fixed NH4+ pool. Both inhibitors raised urea-derived fixed NH4+ but exhibited different influences on urea-derived organic N; in particular, DCD input increased urea-derived organic N, whereas HQ input decreased this parameter. The combination of glucose and DCD further decreased the availability of urea-derived N, but HQ alleviated the decline of urea-derived N availability induced by glucose input. Microbial immobilization and nitrification comparably contributed to the release of urea-derived fixed NH4+ in soil treated with urea alone. In the presence of glucose, microbial immobilization was the principal driving force of fixed NH4+ release, and this tendency was further enhanced by DCD but mitigated by HQ. The partitioning of released fixed NH4+ between the organic N pool (immobilization) and the mineral N pool (nitrification) can be clarified by comparing the path coefficients under different conditions. These results provide valuable information for combining the abiotic and biotic processes in N cycling after fertilizer N application and for quantifying N transformation in soils.

Effect of glucose addition on the fate of urea-15N in fixed ammonium and soil microbial biomass N pools

This study aims to determine the effect of glucose addition on the fate of urea-15N in fixed NH4+ and soil microbial biomass N (SMBN) pools in a 96-day incubation experiment. The contributions of organic N (including SMBN and soil microbial necromass N) and fixed NH4+ pools to mineral N were also assessed. Glucose addition significantly improved 15N recovery but decreased the availability of urea-derived N. A great portion of urea-derived N was immobilized and then transformed into microbial necromass. The effect of fixed NH4+ on the conservation and supply of urea-derived N were greater than those of organic N pool in the non-glucose treatment, in contrast to that in the glucose treatment. From the sixth day to the end of the incubation, the amount of urea-derived fixed NH4+ release was 1.8-fold higher than that of urea-derived organic N mineralization in the treatment without glucose. By contrast, the latter was 3.1-fold higher than the former under glucose addition. Moreover, path coefficient analysis showed that the release of fixed NH4+ was promoted by microbial immobilization and nitrification in the absence of glucose, whereas the release was primarily induced by microbial immobilization in the presence of glucose. The partitioning of the released fixed NH4+ in the mineral or organic N pools was identified using path coefficients. This study provides very helpful information for quantification of fertilizer N transformation in soil and combines the abiotic and biotic processes in the N cycle.