Soil biota is the living component of soil organic matter (SOM), and plays a key role in the decomposition of SOM. Both soil biota and SOM are indicators of soil fertility and soil quality. However, they both are sensitive to soil disturbance. Although researchers developed various technologies to detect soil biota and SOM, they are mostly destructive and cause disturbance to soil, which may not reflect the actual situation of soil biota and SOM. Therefore, here we mostly focused on the non-destructive physical methods for estimating soil biota and SOM and discussed their advantages and disadvantages. These methods include but not limited to acoustic detection, radio frequency identification, radioactive tagging, hyperspectral sensing and electron energy loss spectroscopy. In addition, we pointed out the current research problems and the potential research directions for applications of physical methods in estimation of soil biota and SOM.
Graphene-based nanomaterials (GBNs) are likely to be entering the soil environment in increasing amounts via consumer products. However, the disturbance of bacterial communities and their associated ecological functions by GBNs remains elusive. We performed a soil incubation experiment with the addition of graphene oxide (GO) and reduced graphene oxide (RGO). The Illumina sequencing technique was used to investigate changes in bacterial communities, and the functional groups of the communities were analyzed using the functional annotation of prokaryotic taxa database. After 90 days of exposure, RGO induced a lower bacterial richness than GO. However, GO induced larger changes in community composition and functions than RGO. After exposure to GBNs, some of the functional groups associated with organic matter degradation and biogeochemical cycling of nitrogen and sulfur decreased. However, the functional group associated with aromatic compound degradation increased, possibly because GBNs contain rich aromatic hydrocarbon structures, which are tolerated by this functional group.
Soil organic matter turnover rates are typically estimated from mass loss of the material over time or from on rates of carbon dioxide production. In the study, we investigated a new way to characterize the concentration-dependent kinetics of amino acids used by measuring microbial uptake and mineralization of 14C-alanine. We measured the depletion from soil solution after additions 14C-alanine. The microbial uptake of 14C-alanine from soil solution was concentration dependent and kinetic analysis indicated the operation of at least three distinct alanine transport systems of differing affinities. Most of the 14C-alanine depletion from the soil solution occurred rapidly within the first 10 – 30 min of the incubation after 10 μM to 1 mM substrate additions. At alanine concentrations less than 250 μM, the kinetic parameters for Km and Vmax of the higher-affinity transporter were 60.0 μM and 1.32 μmol g−1 DW soil h−1, respectively. The mineralization of alanine was determined and the half-time values for the rapid mineralization process were 45 min to 1.5 h after the addition at alanine concentrations below 1 mM. The time delay after its uptake into microbial biomass suggested that alanine uptake and subsequent respiration was uncoupled pattern. The microbial N uptake rate was calculated by microbial mineralization, and an estimated the Km value of 1731.7±274.6 μM and Vmax value of 486.0±38.5 μmol kg−1 DW soil h−1. This study provides an alternative approach for measuring the rate of turnover of compounds that turnover very rapidly in soil.
In studies on the effects of mixing residues with different properties on decomposition rate and nutrient release, the extent of contact between the different residues is not known. In this study, we used an experimental design where crop residues were spatially separated by a layer of soil. Microcosms were set up using young faba bean residue (low carbon (C)/nutrient ratio, L) and mature barley straw (high C/nutrient ratio, H). The microcosms comprised of two caps of PVC tubes, each filled with moist soil. Between the two caps, there were three layers each separated from the others by fine nylon mesh with the middle layer being the moist interface soil. Microcosms had similar (H/H or L/L) or different (L/H) residue types, or only residue type (H/S or L/S) while the other cap had no residue. the interface soil. In treatments with only one residue, measured parameters, except MBP, were higher in L/S than H/S. In treatments with two residues, all parameters were lowest in H/H. In L/H compared to L/L after 14 days, available P and MBN were lower, available N was similar and MBP was higher. After 28 days, available P and N were lower in L/H than L/L, but MBP and MBN did not differ. In L/H, measured resin P, MBP and MBN were higher than expected whereas available N was lower. The experimental design used in this study allows assessing the effect of residues on properties of the soil between them.
The impact of forest microhabitats on physiochemical properties of the soil and that of microbial communities on tropical soils remain poorly understood. To elucidate the effect of tropical forest stand on leaf litter and soil microbial communities, we studied enzyme activities, microbial biomass, and diversity in three distinct microhabitats in terms of plant richness, diameter at breast height (DBH), and physiochemical properties of soil and litter, each associated with a different Vanilla sp. In the soil, positive correlations were found between electrical conductivity (EC) and total organic carbon (TOC) with phosphatase activity, and between nitrogen (N) and water-soluble carbon (WSC) content with urease activity (UA). In the litter, the water content was positively correlated with bacterial and fungal biomass, and N and WSC contents were positively correlated with fungal biomass. Positive correlations were found between plant richness and UA in the soil, plant richness and fungal biomass in the soil and litter, and DBH and fungal biomass in the litter. Amplicon sequencing revealed differences between microhabitats in the relative abundance of some fungal and bacterial taxa and in the bacterial community composition of both litter and soil. Bacterial richness and diversity were different between microhabitats, and, in litter samples, they were negatively correlated with DBH and plant richness, respectively. By contrast, none of the soil and litter physiochemical properties were significantly correlated with microbial diversity. Our results show that significant shifts in enzyme activity, microbial biomass, and diversity in the microhabitats were driven by key abiotic and biotic factors depending on the soil or litter sample type.
Herb residue vermicompost is thought to have high agriculture value, while its effects on soil microbial activities have not been fully understood. Here, soil microbial biomass, respiration and enzyme activities in soil planted with maize were compared among treatments amended with herb residue vermicompost at rates of 25, 50, 75 and 100 g kg−1, chemical NPK fertilizer and no fertilizer (the control). Our results showed that soil microbial biomass carbon, respiration, and alkaline phosphatase, urease, and invertase activities were greater in soil amended with herb residue vermicompost than the unfertilized control (P<0.05). Compared with chemical fertilizer, herb residue vermicompost increased soil urease and alkaline phosphatase activities at each application rate, promoted soil respiration and microbial biomass carbon at the application rates of 50, 75 and 100 g kg−1, and increased soil inverse activity at the application rates of 75 and 100 g kg−1. In conclusion, herb residue vermicompost supported greater soil microbial biomass, respiration and enzyme activities than conventional NPK fertilizer, and the effect was larger when higher rates of herb residue vermicompost were added.
The functional performance of soil ecosystems following disturbance determines ecosystem stability, and although contributions of bacterivorous nematodes to soil ecosystems are recognized, their roles in functional stability have received little attention. The objective of this study was to evaluate the roles of bacterivorous nematodes in functional stability following stress. In a factorial laboratory experiment, soil microcosms were prepared with two levels of nematode abundance, either an enriched abundance of bacterivores (Nema soil) or background abundance of nematodes (CK soil), and three levels of stress, copper, heat, or an unstressed control. The resistance and resilience of nematode abundance, as well as soil microbial function by determining decomposition of plant residues and microbial substrate utilization pattern using a BIOLOG microplate, were followed post stress. The relative changes of two dominant bacterivores, Acrobeloides and Protorhabditis, responded differently to stresses. The resistance and resilience of Protorhabditis were greater than that of Acrobeloides to copper stress during the whole incubation period, while both bacterivores only showed higher resilience under heat stress at the end of incubation. The enrichment of bacterivores had no significant effects on the soil microbial resistance but significantly increased its resilience to copper stress. Under heat stress, the positive effect of bacterivores on soil resilience was only evident from 28 days to the end of incubation. The differences in the responses of soil function to stress with or without bacterivores suggested that soil nematodes could be conducive to ecosystem stability, highlighting the soil fauna should be taken into account in soil sustainable management.
Collembola are among the most abundant and diverse soil animals contributing significantly to major ecosystem processes. Global climate changes in temperature and precipitation are likely to affect their community structure and functioning and this is likely to differ along altitudinal gradients. In this study, changes in richness, abundance, and body size of onychiurin Collembola with altitude have been investigated in the Changbai Mountain range of northeast China. Sampling was carried out on a 30 km long transect along forested slopes of the Changbai Mountains. Standardized samples were taken from 800 to 1700 m at seven altitudinal levels. More than 5000 specimens of Onychiurinae representing 13 species were collected, making Onychiuridae (with the sole subfamily Onychiurinae in Changbai) the most abundant Collembolan family in the area. The number of species of Onychiurinae slightly increased along the altitudinal gradient. The average number of species per sample, but not the total abundance, changed significantly but not monotonically with altitude. Body size of Onychiurinae species decreased significantly with increasing altitude contradicting Bergmann’s rule. Furthermore, the abundance of the three body-size groups differentially responded to increasing altitude, with the abundance of the large body-size group decreasing and the abundance of the small body-size group increasing. Our results suggest that the distribution patterns of Collembola along the altitudinal gradient are complicated and may be linked to taxonomic groups and bioclimatic zones.