Increased belowground carbon (C) transfer by plant roots at elevated CO 2 may change properties of the microbial community in the rhizosphere. Previous investigations that focused on total soil organic C or total microbial C showed contrasting results: small increase, small decrease or no changes. We evaluated the effect of 5 years of elevated CO 2 (550 ppm) on four extracellular enzymes: β-glucosidase, chitinase, phosphatase, and sulfatase. We expected microorganisms to be differently localized in aggregates of various sizes and, therefore analyzed microbial biomass (C mic by SIR) and enzyme activities in three aggregate-size classes: large macro- (> 2 mm), small macro- (0.25-2 mm), and microaggregates (< 0.25 mm). To estimate the potential enzyme production, we activated microorganisms by substrate (glucose and nutrients) amendment. Although C total and C mic as well as the activities of β-glucosidase, phosphatase, and sulfatase were unaffected in bulk soil and in aggregate-size classes by elevated CO 2 , significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO 2 were 1.2-1.9-fold higher than under ambient CO 2 . This indicates the increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO 2 . Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO 2 revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungal hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO 2 affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools.

Stimulation of microbial extracellular enzyme activities by elevated CO2 depends on soil aggregate size

Rice paddy agro-ecosystems play an important role in global carbon (C) sequestration. Because of flooding management, paddy soil experience periodical changes in oxygen availability, which may make soil organic carbon (SOC) mineralisation unique as compared to upland or other wetland ecosystems. However, at present, information about the relevant mechanisms involved in paddy SOC mineralisation is limited and unclear. We selected three paddy soils with variable iron (Fe) contents, which were either fumigated with chloroform (to reduce microbial biomass) or remained un-fumigated. Soils were incubated for 78 days in one of three treatments: alternating nonflooded–flooded (NF: moist for 0–30 days (oxygen-abundant) and flooded for 31–78 days (oxygen-limited)), continuously flooded (CF: oxygen-limited), and continuously anaerobically flooded (AF: oxygen-depleted). Fumigation reduced the microbial biomass C by more than 70%. Except for the nonflooded period in the NF treatment, the SOC mineralisation rate, at the late stage of each treatment, was significantly lower in the fumigated than in the un-fumigated soil. A multiple regressions showed that a reduction in dissolved organic C contents over time contributed to the cumulative SOC mineralisation only during the nonflooded period in the NF treatment. Furthermore, the labile C pool size was smaller in the AF treatment relative to the other treatments. These imply that dissolved substrates in oxygen-depleted paddy soil were of greater recalcitrance, most likely due to thermodynamic reasons. SOC mineralisation correlated with changes in the redox potential and the Fe2+ contents in the CF and AF treatments only. This indicates that under oxygen-limited and -depleted conditions, Fe played a significant role as an electron acceptor during SOC mineralisation. Correlation and linear regression analyses also suggest that Fe influenced dissolved organic C contents, and hydrolase and oxidative activities. Our findings show that SOC bioavailability is a rate-limiting factor for SOC mineralisation, but only under oxygen-abundant conditions. However, under oxygen-limited or -depleted conditions, microbial biomass, the recalcitrance of organic C compounds, and the availability of electron acceptors are key regulators in determining the intensity and rate of SOC mineralisation.

Oxygen availability determines key regulators in soil organic carbon mineralisation in paddy soils

Microorganisms maintain C:N stoichiometric balance by regulating the priming effect in long-term fertilized soils

Rice rhizodeposition plays an important role in carbon sequestration in paddy soils. However, the pathways through which rice rhizodeposits contribute to soil organic C (SOC) formation are poorly understood because of specific paddy soil conditions. Furthermore, microbial necromass has been largely ignored in studies examining the contribution of rhizodeposits to C sequestration during plant growth. To evaluate the contribution of microbial necromass to SOC formation via rhizodeposition, rice (Oryza sativa L.) plants were continuously labeled with 13CO2 for 38 days under ambient (aCO2, 400 μL L−1) or elevated CO2 (eCO2, 800 μL L−1) in a paddy field at two levels of N fertilization. The distributions of photosynthetic-13C in the shoots and roots, microbial communities, and SOC fractions were quantified. eCO2 increased plant growth and, consequently, the total 13C incorporated into the shoots, roots, and SOC compared to aCO2, while N fertilization (100 kg N ha−1) decreased root biomass and rhizodeposits in the soil and microbial pools, including living biomass (phospholipid fatty acids, PLFA) and microbial necromass (amino sugars). Rhizodeposits were initially immobilized mainly by bacteria and preferentially recovered in fungal necromass (glucosamine). While 13C incorporation into PLFAs was slightly increased during plant growth, 13C in microbial necromass increased greatly between the tillering and booting stages. Fungal necromass, which is less decomposable compared to bacterial residues, was the largest contributor to C sequestration with rhizodeposits via the mineral-associated SOC fraction, particularly under elevated CO2 without N fertilization. This study reveals the significance of the C pathways from rhizodeposits through fungal necromass and organo-mineral associations for the build up of SOC in paddy fields.

Rice rhizodeposition promotes the build-up of organic carbon in soil via fungal necromass

Paddy soils have a much higher microbial biomass content than upland soils: A review of the origin, mechanisms, and drivers

In contrast to what is observed in aerobic uplands, microbial biomass and the presence of electron acceptors, such as iron oxides, play a crucial role in regulating soil organic C (SOC) mineralisation in paddy soils. However, the related underlying mechanisms are still poorly explored. We conducted an anaerobic incubation study to investigate changes in CO2 emissions from SOC and acetate (13C-labeleld) in response to iron oxide (ferrihydrite and goethite) addition in chloroform-fumigated and unfumigated paddy soils. The iron oxides, as electron acceptors, increased CO2 emissions from SOC with stronger impact under ferrihydrite than goethite addition. However, the acetate addition, as a preferable C source for reducing microbes, decreased SOC mineralisation and caused a negative priming effect. CO2 emission from both acetate and SOC was affected by microbial biomass change. In the acetate-treated soil, goethite in the fumigated soil (i.e. high microbial biomass) increased CO2 emissions from acetate, providing electron acceptors, and decreased CO2 emissions from SOC. Ferrihydrite accepted electrons and adsorbed acetate, resulting in a slight decline in CO2 emission from acetate. However, in the fumigated soil (i.e. low microbial biomass), both iron oxide additions reduced CO2 emissions from acetate and SOC and likely the dominant role of both iron oxides shifted from being electron acceptors to being adsorbents, thus limiting acetate accessibility to microorganisms. The results suggest that microbial biomass is a key driver in shifting the effects of iron oxides on organic C mineralisation in anaerobic paddy soils.

Contrasting response of organic carbon mineralisation to iron oxide addition under conditions of low and high microbial biomass in anoxic paddy soil

Rhizodeposits in rice paddy soil are important in global C sequestration and cycling. This study explored the effects of elevated CO2 and N fertilization during the rice growing season on the subsequent mineralization and retention of rhizodeposit-C in soil aggregates after harvest. Rice (Oryza sativa L.) was labeled with 13CO2 under ambient (400 ppm) and elevated (800 ppm) CO2 concentrations with and without N fertilization. After harvest, soil with labeled rhizodeposits was collected, separated into three aggregate size fractions, and flood-incubated for 100 d. The initial rhizodeposit-13C content of N-fertilized microaggregates was less than 65% of that of non-fertilized microaggregates. During the incubation of microaggregates separated from N-fertilized soils, 3%–9% and 9%–16% more proportion of rhizodeposit-3C was mineralized to 13CO2, and incorporated into the microbial biomass, respectively, while less was allocated to soil organic carbon than in the non-fertilized soils. Elevated CO2 increased the rhizodeposit-13C content of all aggregate fractions by 10%–80%, while it reduced cumulative 13CO2 emission and the bioavailable C pool size of rhizodeposit-C, especially in N-fertilized soil, except for the silt-clay fraction. It also resulted in up to 23% less rhizodeposit-C incorporated into the microbial biomass of the three soil aggregates, and up to 23% more incorporated into soil organic carbon. These results were relatively weak in the silt-clay fraction. Elevated CO2 and N fertilizer applied in rice growing season had a legacy effect on subsequent mineralization and retention of rhizodeposits in paddy soils after harvest, the extent of which varied among the soil aggregates.

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Legacy effect of elevated CO2 and N fertilization on mineralization and retention of rice (Oryza sativa L.) rhizodeposit-C in paddy soil aggregates

Karst is a widespread ecosystem with properties that affect the microbial activity and storage and cycling of soil organic carbon. The mechanisms underlying microbial resource availability in karst, which limit the microbial growth and activity in soil aggregates, remain largely unknown. We assessed the microbial resource limitations using exoenzymatic stoichiometry and key extracellular enzyme activities in bulk soil and aggregates in karst and non-karst forest soils. Soil organic carbon, total nitrogen, and microbial biomass carbon and nitrogen were significantly higher in bulk soil and the aggregate fractions in karst forests. However, the microbial biomass accumulation was higher in finer aggregates than in macroaggregate fractions. This may be attributed to the surface area of finer aggregates that increase the microbial C accumulation. In karst forests, the activity of extracellular enzymes β-d-glucosidase, β-N-acetylglucosaminidase, α-glucosidase, and α-d-1,4-cellobiosidase was two to three times higher in microaggregates (0.053–0.25 mm) and mineral fractions (<0.053 mm) than in macroaggregates. This coincided with the distribution of microbial biomass carbon and phosphorus in finer aggregate fractions. The microorganisms in bulk soil and aggregates in karst forests were largely co-limited by carbon and phosphorus and rarely by nitrogen and only by phosphorus in non-karst soils. The microbial phosphorus limitation in non-karst soils was alleviated in finer soil aggregates, while these fractions reflected slightly higher. microbial C limitations than bulk and other aggregates in karst forests. The patterns of microbial resource limitations in the bulk and aggregate fractions in karst ecosystems reflected the regulation of enzyme activity and soil organic carbon accumulation in finer aggregate fractions but not in other aggregates.

Microbial Resource Limitation in Aggregates in Karst and Non-Karst Soils

The invention discloses a method and device for distinguishing rice respiration, rice rhizospheric respiration and non-rhizospheric respiration. The method comprises the steps of A, planting rice; B, placing the rice on an open space irradiated by light when the rice grows to the growth period, and mounting a collecting device; C, starting an electric fan in an air chamber, uniformly mixing gases, and sequentially extracting greenhouse gases generated by rice plant respiration, rice rhizospheric soil respiration and rice non-rhizospheric respiration; and D, calculating the release fluxes of all parts after testing the concentrations of gas samples. The device comprises a rice respiration gas collecting chamber, a non-rhizospheric soil respiration gas collecting chamber and a rhizospheric soil respiration gas collecting chamber, wherein the rhizospheric soil respiration gas collecting chamber is inserted to a PVC base and is fixed through an organic glass plate by using screw buckles and rubber washers; the rice respiration gas collecting chamber is placed on the non-rhizospheric soil respiration gas collecting chamber and is sealed through screw buckles and rubber washers; and a gas collecting hole is sealed by using a soft rubber plug. The method is easy to realize, simple and convenient to operate and capable of accurately measuring and distinguishing rice respiration, rice rhizospheric respiration and non-rhizospheric respiration.

Anlei Chen

Papers

Contrasting response of organic carbon mineralisation to iron oxide addition under conditions of low and high microbial biomass in anoxic paddy soil

Legacy effect of elevated CO2 and N fertilization on mineralization and retention of rice (Oryza sativa L.) rhizodeposit-C in paddy soil aggregates

Microbial Resource Limitation in Aggregates in Karst and Non-Karst Soils

Method and device for distinguishing rice respiration, rice rhizospheric respiration and non-rhizospheric respiration