Publications

Soil inorganic C (SIC) has a huge stock and is supplemented by the bicarbonate-rich groundwater, irrigation water and seawater in the coastal Yangtze Delta area, where soil acidification may induce high CO2 efflux by neutralizing SIC. This study assessed whether and to what extent SIC generally contributed to surface or subsurface (50 cm) CO2 collected in typical croplands (a rice-wheat rotation paddy and an upland field) of this coastal region, and further investigated factors regulating SIC-derived CO2 efflux (E IC) by soil incubation. The E IC was separated from CO2 of organic sources (E OC) based on their distinct 13C isotopic signatures. Our results indicated that SIC contributed around 16 % to surface CO2 effluxes in the paddy field and 52 % to subsoil CO2 in the upland field. Despite the alkaline soil environments (with pH of 7–9), adding bicarbonate induced high CO2 effluxes comparable to those under glucose amendments, suggesting high reactivity of the dissolved SIC (HCO3 ) in E IC generation. Within a month, the 13C-labeled HCO3 (1 mg C g 1 soil) was almost completely converted to CO2 and induced extra E IC emission (84 % of the HCO3 input) from unlabeled sources. Bicarbonates also contributed > 40 % to total CO2 emission in soils amended with groundwater. However, the HCO3 -amended soil did not emit but strongly adsorbed CO2 when soil organic matter and microbial communities were destroyed by heating at 550 °C, implying the involvement of these organic components in HCO3 CO2 conversion. The E IC effluxes increased with temperature, N fertilization (due to acidity released in nitrification), microbial respiration (associated with straw addition or reinoculation rate of sterilized soil), and bicarbonate addition rate, but decreased with soil moisture. Multiple regression indicated that bicarbonate addition and N fertilization were most effective in increasing E IC. In conclusion, the SIC pools, particularly the bicarbonates, were active players in coastal soil C cycling, releasing CO2 fluxes susceptible to environmental, biological, and management influences. With the widespread acidification of calcareous soils under intensive N fertilization, special attention should be paid to not only the accelerated carbonate dissolution and leaching, but possible CO2 emissions resulting from the increased soil bicarbonates, in coastal agroecosystems.

Rhizodeposition represents a readily available C and energy source for soil microorganisms, that plays an important role in the regulation of C and nutrient cycling in ecosystems and exerts a strong influence on C sequestration. The dynamics of rice rhizo-C in soils and its allocation to microorganisms during rice growth, as well as the effects of nitrogen (N-NH4 +) fertilization are poorly understood, particularly with respect to the initial uptake of rhizo-C by microorganisms and its utilization during the entire growth period. To assess these two processes, rice plants were grown in pots with or without N fertilization (0 and 225 kg N-NH4 + ha−1), and 13C incorporation into microbial groups was traced by phospholipid fatty acids (PLFAs) analysis within 6 h after 13CO2 pulse labeling. Labeling was performed at five growth stages: tillering, elongation, heading, filling, and maturation. 13C incorporated into soil microbial biomass C changed rapidly at the beginning of the study period, before elongation, but remained stable thereafter. 13C incorporation into rhizosphere and bulk soil microbial biomass was higher with than without N addition. This stimulation was likely due to the excessive increase in phytomass formation and root exudates after N fertilization and the increased assimilate C input into the soil. Structural equation modelling suggested that N fertilization strongly affected carbon transfer between rhizosphere and non-rhizosphere. Hence, N-NH4 + application may not only increase rhizo-C flow into microorganisms but it may also increase the effect of rhizosphere on bulk-soil microorganisms and subsequent processes related to soil C-cycling.

As the primary microbial substrate after shoot cutting, the element stoichiometry of root-detritus (dying or dead roots) influences the enzyme activity in

Global warming and nitrogen (N) deposition promote the displacement of native plant species by neophytes which have similar ecological niches but stronger competitive abilities. It remains unclear how plants with different competitive abilities alter microbial growth and turnover in the rhizosphere under high and low N input. We hypothesized 1) slower microbial growth in the rhizosphere of plants with smaller roots and 2) restriction of microbial growth under low versus high N amendment. These hypotheses were tested on two strawberry species: Fragaria vesca (native species) and Duchesnea indica (an invasive plant in central Europe) grown under intra-specific and inter-specific competition at very low and high N levels. Species-specific traits of plant–microbial interactions mitigated N deficiency in the rhizosphere. At low N addition the native species F. vesca stimulated faster microbial growth and turnover than D. indica. F. vesca did this by increasing root mass and exudation at the expense of the shoots. In contrast, the invasive plant – D. indica – did not increase root mass under low N amendment, but did increase its N uptake rate. This resulted in N deficiency, retarding microbial growth and turnover in the rhizosphere, as revealed by the dominance of slow-growing microorganisms. A low N level in the soil promoted root growth and rhizodeposition and thus accelerated microbial turnover correspondingly to increasing root mass. Fast N uptake by roots, however, may lead to N deficiency and did retard microbial growth in the rhizosphere. In conclusion, the plant species with the stronger competitive ability at low N level controls the microbial community in the rhizosphere.

No abstract is provided for this article.

The conversion of land use is a crucial factor in the dynamics of soil carbon stocks. The widespread abandonment of cropland that began in Russia during th

Temperature sensitivity (Q10 ) of soil organic matter (SOM) decomposition is an important parameter in models of the global carbon (C) cycle. Previous studies have suggested that substrate quality controls the intrinsic Q10 , whereas environmental factors can impose large constraints. For example, physical protection of SOM and its association with minerals attenuate the apparent Q10 through reducing substrate availability and accessibility ([S]). The magnitude of this dampening effect, however, has never been quantified. We simulated theoretical Q10 changes across a wide range of [S] and found that the relationship between Q10 and the log10 -transformed [S] followed a logistic rather than a linear function. Based on the unique Holocene paleosol chronosequence (7 soils from ca. 500 to 6900 years old), we demonstrated that the Q10 decreased nonlinearly with soil age up to 1150 years, beyond which Q10 remained stable. Hierarchical partitioning analysis indicated that an integrated C availability index, derived from principal component analysis of DOC content and parameters reflecting physical protection and mineral association, was the main explanatory variable for the nonlinear decrease of Q10 with soil age. Microbial inoculation and 13 C-labelled glucose addition showed that low C availability induced by physical protection and minerals association attenuated Q10 along the chronosequence. A separate soil incubation experiment indicated that Q10 increased exponentially with activation energy (Ea ) in the modern soil, suggesting that SOM chemical complexity regulates Q10 only when C availability is high. In conclusion, organic matter availability strongly decreased with soil age, whereas Michelis-Menten kinetics defines the Q10 response depending on C availability, but Arrhenius equation describes the effects of increasing substrate complexity.