Soil enzyme profile analysis for indicating decomposer micro-food web.

Highly diverse exoenzymes mediate the energy flow from substrates to the multitrophic microbiota within the soil decomposer micro-food web. Here, we used a "soil enzyme profile analysis" approach to establish a series of enzyme profile indices; those indices were hypothesized to reflect micro-food web features. We systematically evaluated the shifts in enzyme profile indices in relation to the micro-food web features in the restoration of an abandoned cropland to a natural area. We found that enzymatic C:N stoichiometry and decomposability index were significantly associated with substrate availability. Furthermore, the higher Shannon diversity index in the exoenzyme profile, especially for the C-degrading hydrolase, corresponded to a greater microbiota community diversity. The increased complexity and stability of the exoenzyme network reflected similar changes with the micro-food web networks. In addition, the gross activity of the enzyme profile as a parameter for soil multifunctionality, effectively predicted the substrate content, microbiota community size, diversity, and network complexity. Ultimately, the proposed enzymic channel index was closely associated with the traditional decomposition channel indices derived from microorganisms and nematodes. Our results showed that soil enzyme profile analysis reflected very well the decomposer food web features. Our study has important implications for projecting future climate change or anthropogenic disturbance impacts on soil decomposer micro-food web features by using soil enzyme profile analysis.

Examining the Shift in the Decomposition Channel Structure of the Soil Decomposer Food Web: A Methods Comparison.

Selecting the appropriate indicators and measuring time point numbers is important for accurately examining the shift in soil gross decomposition channel structure. Through a selected case study on a natural forest vs. rainfed arable system over a two-month-long experiment, the utility of three commonly employed indicators (fungi to bacteria ratio (F:B), fungivore to bacterivore ratio (FF:BF), and glucosamine to muramic acid ratio (GlcN:MurN)) were compared to reflect the shift in soil gross decomposition channel structure. The requirement of measuring the time point numbers for the three indicators was also assessed, and we suggest a potential methodology. Our results revealed that the GlcN:MurN ratio was more reliable for assessing the shifts in gross decomposition channel structure for long-term land use changes, while it was less sensitive to short-term drought compared with the other two indicators. The F:B ratio was more applicable than the FF:BF ratio for reflecting both long- and short-term changes. Furthermore, the reliability of the GlcN:MurN ratio was the least dependent on measuring time point numbers. We suggest the use of multiple indicators and the adoption of multiple measuring time points for the overall methodology.

Long-term effects of nitrogen fertilization on aggregation and localization of carbon, nitrogen and microbial activities in soil.

Long-term nitrogen (N) fertilization affects soil aggregation and localizations of soil organic carbon (SOC), N and microbial parameters within aggregates. The mechanisms of these N effects are poorly understood. We studied these processes in a loamy soil from a 23-year repeated N addition field experiment under a rice-barley rotation. Nitrogen fertilization increased plant productivity and the portion of large macroaggregates (>2mm). However, SOC contents in macro- and micro-aggregates remained constant despite an N-induced increase of 27% in root C input into soil. Therefore, N fertilization accelerated SOC turnover. Nitrogen addition increased total N (TN) content in bulk soil and two macroaggregates (>2, and 1-2mm), but not in microaggregates (<0.25mm). Also, N fertilization increased the phospholipid fatty acids (PLFAs) contents of fungi in the large macroaggregates, but not in the microaggregates. In contrast, the effect of N addition on contents of bacterial and total microbial PLFAs was not apparent. Nitrogen fertilization increased N-acetyl-ß-D-glucosaminidase (NAG) activities in the two larger macroaggregate size classes (>2, and 1-2mm), but not in the aggregates (<1mm). In both control and N fertilization, the large macroaggregates localized more TN, microbial PLFAs, and NAG activities than the microaggregates. In conclusion, long-term N fertilization not only directly promotes soil N resource but also indirectly improves soil structure by forming large macroaggregates, accelerates SOC turnover, and shiftes localization of microorganisms to the macroaggregates.

Community diversity, structure and carbon footprint of nematode food web following reforestation on degraded Karst soil.

We examined community diversity, structure and carbon footprint of nematode food web along a chronosequence of T. Sinensis reforestation on degraded Karst. In general, after the reforestation: a serious of diversity parameters and community indices (Shannon-Weinier index (H'), structure index (SI), etc.) were elevated; biomass ratio of fungivores to bacterivores (FFC/BFC), and fungi to bacteria (F/B) were increased, and nematode channel ratio (NCR) were decreased; carbon footprints of all nematode trophic groups, and biomass of bacteria and fungi were increased. Our results indicate that the Karst aboveground vegetation restoration was accompanied with belowground nematode food web development: increasing community complexity, function and fungal dominance in decomposition pathway, and the driving forces included the bottom-up effect (resource control), connectedness of functional groups, as well as soil environments.

[Effects of conservation tillage on the composition of soil exchangeable base].

Taking the soil in Zhangwu County of Liaoning Province as test object, a comparative study was made to understand the composition of soil exchangeable base under traditional tillage and 6-year conservation tillage (no-tillage plus straw mulch). Comparing with traditional tillage, conservation tillage increased the total amount of exchangeable base (SEB) and the contents of exchangeable K, Ca, and Mg in top (0-15 cm) soil, suggesting its positive effect in increasing soil nutrient holding capacity and buffering ability. This effect had a close relationship with the changes of soil organic matter and clay contents, according to correlation analysis. In addition, the K/SEB and Ca/Mg ratios were higher, while the (Ca+Mg)/SEB, Ca/K, and Mg/K ratios were lower under conservation tillage than under traditional tillage, illustrating that the effects of conservation tillage on soil exchangeable base were mainly presented in the relative enrichment of soil exchangeable Ca and K, especially K. Conservation tillage increased the stratification ratio (0-5 cm/5-15 cm and 0-5 cm/15-30 cm) of soil exchangeable K, Ca, and Mg, and SEB, suggesting the increase of the vertical variability of SEB in plough layer.

Four years of free-air CO2 enrichment enhance soil C concentrations in a Chinese wheat field.

Elevated atmospheric CO2 can influence soil C dynamics in agroecosystems. The effects of free-air CO2 enrichment (FACE) and N fertilization on soil organic C (Corg), dissolved organic C (DOC), microbial biomass C (Cmic) and soil basal respiration (SBR) were investigated in a Chinese wheat field after expose to elevated CO2 for four full years. The results indicated that elevated CO2 has stimulative effects on soil C concentrations regardless of N fertilization. Following the elevated CO2, the concentrations of Corg and SBR were increased at wheat jointing stage, and those of DOC and Cmic were enhanced obviously across the wheat jointing stage and the fallow period after wheat harvest. On the other hand, N fertilization did not significantly affect the content of soil C. Significant correlations were found among DOC, Cmic, and SBR in this study.