Changes in soil microbial community, enzyme activities and organic matter fractions under long-term straw return in north-central China

Straw return is an effective management practice for improving soil fertility and sustaining crop productivity in agro-ecosystems. A field experiment was conducted to investigate the effects of short-term (three-year) crop straw return on soil organic carbon fractions and microbial communities in a rice–wheat cropping system at three sites from central to eastern parts of China. Two treatments were established at each site: no straw return (N) and straw return (S). Compared to N treatment, S treatment significantly increased concentrations of total organic carbon (TOC, except for Qujialing site), dissolved organic C (DOC) and microbial biomass C (MBC), but did not significantly affect the easily oxidizable C content in the three sites. Soil light fraction, light fraction organic C and particulate organic C were higher for S than N treatment by 28–52% in both Guangde and Jiangyan, while these values were not significantly different between N and S treatment in Qujialing. Soil microbial community compositions of all three sites were changed with straw return. Crop straw return significantly increased total phospholipid fatty acid (PLFA), bacterial biomass and actinomycete biomass by 52, 75 and 56% in Jiangyan, but had no significant effects on PLFAs in both Qujialing and Guangde, compared to N treatment. MBC and TOC were the two main factors affecting microbial communities under short-term crop straw return. Our results demonstrated that DOC and MBC were the most sensitive indictors for assessing changes of SOC under short-term straw return in the rice–wheat cropping system, and soil properties in response to straw return were different at the three sites.

Changes in soil microbial community and organic carbon fractions under short-term straw return in a rice–wheat cropping system

Microbial decomposers are responsible for the breakdown of organic matter (OM) and thus regulate soil carbon (C) stocks. During the decomposition of OM, microorganisms can use the assimilated C for biomass production or respire it as CO2, and the fraction of growth to total assimilation defines the microbial carbon-use efficiency (CUE). As such, CUE has direct consequences for how microbial decomposers affect the balance of C between atmosphere and soil. We estimated fungal and bacterial growth in C units in microcosm systems with submerged plant litter. We established conversion factors between bacterial and fungal growth to biomass and applied this to a dataset representing 9 different sites in temperate forest soils, temperate agricultural soils, and subarctic forest soils, to estimate growth rates of fungi and bacteria in units of C, to estimate the dominance of the two decomposer groups, and to compare these values to respiration to estimate the microbial CUE. We observed that fungal-to-bacterial growth ratios (F:B) ranged from 0.02 to 0.44, and that the fungal dominance was higher in soils with lower C:N ratio and higher C-quality. We found a negative exponential relationship between the dominance of fungi and the microbial CUE. CUE ranged from 0.03 to 0.30, and values clustered most strongly according to site rather than level of soil N. CUE was higher in soil with high C:N ratio and high C-quality. However, within each land-use type, a high mineral N-content did result in lower F:B and higher resulting CUE. In conclusion, a higher soil C-quality coincided with lower F:B and higher CUE across the surveyed sites, while a higher N availability did not. A higher N availability resulted in higher CUE and lower F:B within each site suggesting that site-specific differences such as the effect of plant community via e.g. plant litter and rhizosphere input, overrode the influence of N-availability.

Microbial growth and carbon use efficiency in soil: Links to fungal-bacterial dominance, SOC-quality and stoichiometry

Plant litter decomposition in the soil is governed by microorganisms such as bacteria and fungi that colonize lignocellulose residues during the decomposition process, and thus, the interplay of bacterial and fungal communities can yield insight into the lignocellulose decomposition dynamics Previous studies have mainly investigated litter decomposing communities in microcosms or ex-situ conditions or at a single soil ecosystem Here we conducted a 12 week-long litter decomposition experiment to explore how the temporal dynamics of soil enzyme activities and microbial communities are linked to litter decomposition under three different land use sites (forestland, farmland, and abandoned farmland) in Nanjing, China We found that litter decomposition in the forestland was the highest among the three land use sites Then, using a multifactorial approach, we showed that this higher decomposition rate in forest soils is determined by microbial communities with higher ligninolytic enzyme activities, higher diversity, and a less complex but more specialized network Chryseobacterium in bacteria, and Fusarium, Aspergillus and Penicillium in fungi were the keystone taxa in networks across three land use types We conducted subsequent culturing that further confirmed the strong decomposition ability and enzyme activities of these taxa, indicating their importance for microbial litter decomposition As such, this is one of the first studies to validate the role of keystone taxa for litter decomposition, and it demonstrates that co-occurrence network scores can be used for statistical identification of putative keystone taxa for further screening and linking to microbiome functioning Overall, we show that land use alters the composition and network structure of soil microbiota that determine the litter decomposition Our study also reveals that specialized keystone taxa are involved in the decomposition dynamics, and highlights an opportunity of harnessing such taxa for manipulating lignocellulose decomposition in soil ecosystems

Network analysis and subsequent culturing reveal keystone taxa involved in microbial litter decomposition dynamics

Change in straw decomposition rate and soil microbial community composition after straw addition in different long-term fertilization soils

It is generally accepted that during the early stages of residue decomposition, easily available compounds are decomposed, leading to a relative increase in more recalcitrant compounds in the later stages of decomposition and that these changes in substrate availability are associated with changes in microbial community composition. However most studies on residue decomposition are conducted over several weeks or months; little is known about the changes in microbial community composition in the first weeks of decomposition. To address this knowledge gap, we incubated wheat residues inoculated with a microbial suspension in mesh bags buried in sand for 30 days, with sampling on days 0, 2, 4, 6, 8, 10, 15, 20, 25 and 30. Of the C added with the residues, 10, 18 and 25% had been respired on days 10, 20 and 30, respectively. The sum of PLFAs (phospholipid fatty acids), as an indicator of microbial biomass, increased strongly in the first 4 days and then decreased. The concentration of bacterial fatty acids was maximal on days 2 and 4, whereas the concentration of fungal fatty acids peaked on day 15. Microbial community composition (based on PLFA patterns) changed rapidly, with significant changes in the first 8 days and from day 8 to day 20. There were no significant changes in microbial community composition after day 20. The concentration of water-soluble C decreased strongly in the first 8 days, suggesting that the rapid changes in microbial community in this period are related to the changes in water-soluble C. Residue C chemistry, assessed by 13C NMR spectroscopy, changed little during the incubation period. This study showed that microbial community composition in decomposing residues changes rapidly in the first 1–2 weeks, which is, at least partly, the result of competition for the easily available compounds in the water-soluble fraction. After depletion of the water-soluble compounds, the microbial community composition changes more slowly.

The microbial community composition changes rapidly in the early stages of decomposition of wheat residue

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The microbial community composition changes rapidly in the early stages of decomposition of wheat residue