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.

Dissimilarity nitrate reduction to ammonium (DNRA) is of significance in agriculture ecosystems as the process is beneficial to N retention in soils. However, how fertilization regimes influence DNRA rates and functional microbes in agriculture was rarely estimated. In the present study, a 2-year pot experiment was conducted in two contrasting paddy soils to evaluate the effects of straw and nitrogen addition on DNRA process and the related functional microbes, using stable isotope tracer and molecular ecology techniques. The results showed that the abundance and transcription activity of nitrite reductase encoding gene (nrfA) involved in DNRA process and DNRA rates were significantly higher in alkaline soils than in acidic soils. Straw incorporation significantly enhanced nrfA gene abundance and transcription activity, with a greater effect in alkaline soil than in acidic soil. The rates of DNRA, abundance and transcription activity of nrfA gene positively correlated to soil C/N and C/NO₃⁻ induced by straw application. Sequencing analysis based on nrfA gene transcript showed that Deltaproteobacteria was the most dominant group in both soil types (30.9%-67.4%), while Gammaproteobacteria, Chloroflexi, Actinobacteria were selectively enriched by straw incorporation. These results demonstrated that DNRA activity can be improved by straw return practice in paddy soils while the effect will vary among soil types due to differentiated functional microbial communities and edaphic properties.

There is little experimental field evidence on how multiple essential land use intensification drivers (LUIDs), such as nitrogen (N) fertilization and mowing, interact to control ecosystem multifunctionality. Here, we conducted a 4-year field experiment in a meadow steppe in northeast China and evaluated the direct and indirect effects of mowing and N fertilization on a range of ecosystem functions associated with nutrient cycle, carbon stocks, and organic matter decomposition during the past 2 years of the experiment (2017 and 2018). Mowing had negative effects on the ecosystem multifunctionality index (EMF), carbon (C) cycle multifunctionality index (CCMF), and N cycle multifunctionality index (NCMF) in 2 years of sampling. However, in general, the responses of multifunctionality to N fertilization were rate-specific and year-dependent. N fertilization had positive effects on EMF, CCMF, NCMF, and phosphorus (P) cycle multifunctionality index (PCMF) in 2017, with the higher precipitation rate during the growing season, which was likely associated with the strong monsoon season. However, in 2018, EMF, CCMF, and NCMF increased at the lower N fertilization levels (≤10 g N m⁻² yr⁻¹), but decreased at higher N rates. N fertilization had consistent positive effects on PCMF in the 2 years of sampling. The effects of land use drivers on multifunctionality were indirectly influenced by bacterial biomass, plant richness, and soil moisture changes. Our results also indicated that the impacts of land use drivers on multifunctionality played an important role in maintaining a range of functions at low levels of functioning (<50% functional threshold). Low N fertilization levels (≤10 g N m⁻² yr⁻¹) were able to reduce the negative effects of mowing on ecosystem multifunctionality while promoting plant biomass (food for livestock) and C storage. These findings are useful for designing practical strategies toward promoting multifunctionality by managing multiple LUIDs in a meadow steppe.

Rapid litter turnover in tropical forests and during summer seasons might be due to increases in ligninolytic enzyme activities during warmer periods. We compared ligninolytic enzyme activity [lignin peroxidase (LiP), manganese peroxidase (MnP), and laccase (Lac)] in the organic layers of forest soils across a global climate gradient. As expected, MnP activities in fresh litter layers increased with increasing air temperature. Litter Mn/lignin ratios correlate positively with MnP activity and more rapid litter turnover in warmer climates. In contrast, LiP and Lac activities are regulated by site-specific conditions. Lac activity is commonly observed in less acidic fresh litter layers, while LiP activity localizes in acidified and lignin-rich deeper organic layers. The widespread occurrence of MnP and an increase in MnP activities in warmer climates support efficient lignin degradation in the tropics and during summer seasons. High Mn/lignin ratios in fresh litter could be an indicator of lignin degradability by MnP-producing fungi across global climate gradients.

Global warming is an increasingly serious ecological problem, we examined how the active autotrophic microbes in paddy soils respond to the elevated CO₂ and temperature. Here we employed stable isotope probing (SIP) to label the active bacteria using the soil samples from a fully factorial Simulated Climate Change (SCC) field experiment where soils were exposed to ambient CO₂ and temperature, elevated temperature, elevated CO₂, and both elevated CO₂ and temperature. Around 28.9% of active OTUs belonged to ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). Nitrosospira taxa was dominant in all soils and 80.4% of carbon-fixing bacteria under elevated temperature were classified as Nitrosomonas nitrosa. While no labeled NOBs were detected when temperature or CO₂ were elevated independently, diverse NOBs were detected in the ambient conditions. We found that elevated CO₂ and temperature had contrasting effects on microbial community composition, while relatively small changes were observed when CO₂ and temperature were elevated simultaneously. Summarily these results suggest that carbon-fixing bacteria can respond positively to elevated CO₂ concentrations, but when it’s accompanied with increase in the temperature this positive response could be weakened. Multiple abiotic factors thus need to be considered when predicting how microbial communities will respond to multiple climatic factors.

The ever-increasing atmospheric CO₂ concentration is a key driver of modern global warming. However, the low heat capacity of atmosphere and strong convection processes in the troposphere both limit heat retention. Given the higher heat capacity and CO₂ concentration in soil compared to the atmosphere, the direct contributions of soil to the greenhouse effect may be significant. By experimentally manipulating CO₂ concentrations both in the soil and the atmosphere, we demonstrated that the soil-retained heat and the slower soil heat transmission decrease the amount of heat energy leaking from the earth. Furthermore, the soil air temperature was affected by soil CO₂ concentration, with the highest value recorded at 7500 ppm CO₂. This study indicates that soil and soil CO₂, together with atmospheric CO₂, play a crucial role in the greenhouse effect. The spatial and temporal heterogeneity of soils and soil CO₂ should be further investigated, given their potentially significant influence on global climate change.

This study investigated the effect of C/N ratio, placement of plant residues, and leaching amounts on soil respiration, microbial biomass, and nutrient availability within four weeks after amendment. Young faba bean shoots (FB, C/N 7) and mature wheat straw (WH, C/N 80) were used as low and high C/N residue, respectively. Soil was unamended, mulched with FB or WH only, or mulched with one residue and mixed with the other residue. Leaching with 5 or 25 mL water was carried out on days 4, 12, and 20. Cumulative respiration and microbial biomass N were higher with 25 than 5 mL only in treatments with two residues. WH under FB mulch reduced N availability compared to FB mulch alone, whereas FB under WH increased N availability compared to WH mulch alone. When soils were leached with 25 mL water, available N in FB mulch over WH was lower on day 12, but higher later, compared to WH mulch over FB. In contrast, in WH mulch over FB microbial biomass N increased over time whereas available N decreased. In conclusion, the effect of C/N ratio of the mulch on soil available and microbial biomass N was greater with the higher leaching amount.