Cong Tu

Arbuscular Mycorrhizal Fungi Increase Organic Carbon Decomposition Under Elevated CO2

A Fungal Culprit to Carbon Loss In some ecosystems, such as in the layer of soil containing plant roots, fungi, and bacteria, increased levels of CO2 should stimulate more efficient aboveground photosynthesis, which in turn should promote increased sequestration of organic carbon in soil through the protective action of arbuscular mycorrhizal fungi. However, in a series of field and microcosm experiments performed under elevated levels of CO2 thought to be consistent with future emissions scenarios, Cheng et al. (p. 1084; see the Perspective by Kowalchuk) observed that these fungi actually promote degradation of soil organic carbon, releasing more CO2 in the process. Counter to expectations, fungi associated with plant roots diminish the carbon pool in soil ecosystems under elevated levels of carbon dioxide. The extent to which terrestrial ecosystems can sequester carbon to mitigate climate change is a matter of debate. The stimulation of arbuscular mycorrhizal fungi (AMF) by elevated atmospheric carbon dioxide (CO2) has been assumed to be a major mechanism facilitating soil carbon sequestration by increasing carbon inputs to soil and by protecting organic carbon from decomposition via aggregation. We present evidence from four independent microcosm and field experiments demonstrating that CO2 enhancement of AMF results in considerable soil carbon losses. Our findings challenge the assumption that AMF protect against degradation of organic carbon in soil and raise questions about the current prediction of terrestrial ecosystem carbon balance under future climate-change scenarios.

Progressive N limitation of plant response to elevated CO2: a microbiological perspective

A major uncertainty in predicting long-term ecosystem C balance is whether stimulation of net primary production will be sustained in future atmospheric CO2 scenarios. Immobilization of nutrients (N in particular) in plant biomass and soil organic matter (SOM) provides negative feedbacks to plant growth and may lead to progressive N limitation (PNL) of plant response to CO2 enrichment. Soil microbes mediate N availability to plants by controlling litter decomposition and N transformations as well as dominating biological N fixation. CO2-induced changes in C inputs, plant nutrient demand and water use efficiency often have interactive and contrasting effects on microbes and microbially mediated N processes. One critical question is whether CO2-induced N accumulation in plant biomass and SOM will result in N limitation of microbes and subsequently cause them to obtain N from alternative sources or to alter the ecosystem N balance. We reviewed the experimental results that examined elevated CO2 effects on microbial parameters, focusing on those published since 2000. These results in general show that increased C inputs dominate the CO2 impact on microbes, microbial activities and their subsequent controls over ecosystem N dynamics, potentially enhancing microbial N acquisition and ecosystem N retention. We reason that microbial mediation of N availability for plants under future CO2 scenarios will strongly depend on the initial ecosystem N status, and the nature and magnitude of external N inputs. Consequently, microbial processes that exert critical controls over long-term N availability for plants would be ecosystem-specific. The challenge remains to quantify CO2-induced changes in these processes, and to extrapolate the results from short-term studies with step-up CO2 increases to native ecosystems that are already experiencing gradual changes in the CO2 concentration.

Root-Parasitic Nematodes Enhance Soil Microbial Activities and Nitrogen Mineralization

Obligate root-parasitic nematodes can affect soil microbes positively by enhancing C and nutrient leakage from roots but negatively by restricting total root growth. However, it is unclear how the resulting changes in C availability affect soil microbial activities and N cycling. In a microplot experiment, effects of root-parasitic reniform nematodes (Rotylenchulus reniformis) on soil microbial biomass and activities were examined in six different soils planted with cotton. Rotylenchulus reniformis was introduced at 900 nematodes kg−1 soil in May 2000 prior to seeding cotton. In 2001, soil samples were collected in May before cotton was seeded and in November at the final harvest. Extractable C and N were consistently higher in the R. reniformis treatments than in the non-nematode controls across the six different soils. Nematode inoculation significantly reduced microbial biomass C, but increased microbial biomass N, leading to marked decreases in microbial biomass C:N ratios. Soil microbial respiration and net N mineralization rates were also consistently higher in the nematode treatments than in the controls. However, soil types did not have a significant impact on the effects of nematodes on these microbial parameters. These findings indicate that nematode infection of plant roots may enhance microbial activities and the turnover of soil microbial biomass, facilitating soil N cycling. The present study provides the first evidence about the direct role of root-feeding nematodes in enhancing soil N mineralization.

Soil Microbial Responses to Elevated CO2 and O3 in a Nitrogen-Aggrading Agroecosystem

Climate change factors such as elevated atmospheric carbon dioxide (CO2) and ozone (O3) can exert significant impacts on soil microbes and the ecosystem level processes they mediate. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. The prevailing hypothesis, which states that CO2- or O3-induced changes in carbon (C) availability dominate microbial responses, is primarily based on results from nitrogen (N)-limiting forests and grasslands. It remains largely unexplored how soil microbes respond to elevated CO2 and O3 in N-rich or N-aggrading systems, which severely hinders our ability to predict the long-term soil C dynamics in agroecosystems. Using a long-term field study conducted in a no-till wheat-soybean rotation system with open-top chambers, we showed that elevated CO2 but not O3 had a potent influence on soil microbes. Elevated CO2 (1.5×ambient) significantly increased, while O3 (1.4×ambient) reduced, aboveground (and presumably belowground) plant residue C and N inputs to soil. However, only elevated CO2 significantly affected soil microbial biomass, activities (namely heterotrophic respiration) and community composition. The enhancement of microbial biomass and activities by elevated CO2 largely occurred in the third and fourth years of the experiment and coincided with increased soil N availability, likely due to CO2-stimulation of symbiotic N2 fixation in soybean. Fungal biomass and the fungi∶bacteria ratio decreased under both ambient and elevated CO2 by the third year and also coincided with increased soil N availability; but they were significantly higher under elevated than ambient CO2. These results suggest that more attention should be directed towards assessing the impact of N availability on microbial activities and decomposition in projections of soil organic C balance in N-rich systems under future CO2 scenarios.

A technique for assessing environmental impact risks of agricultural systems.

Conventional agriculture often aims to achieve high returns without allowing for sustainable natural resource management. To prevent environmental degradation agricultural systems must be assessed and environmental standards need to be developed. This study used a multi-factor approach to assess the potential environmental impact risk of six diverse systems: five production systems and a successional system or abandoned agronomic field. Assessment factors were soil quality status, amount of pesticide and fertilizer applied, and tillage intensity. The assessment identified the certified organic system and the BMP-conventional tillage system as high-risk systems mostly because fertilizer and tillage use were highest. Conversely, the BMP-no tillage and the crop-animal integrated system were characterized as low-risk mainly because of reduced tillage. The paper discusses assessment strengths and weaknesses, ways to improve indicators used, and the need for additional indicators. We concluded that with further development the technique will become a resourceful tool to promote agricultural sustainability and environmental stewardship, and assist policy making processes. Subject Matter

Soil Carbon, Nitrogen and Microbial Dynamics of Pasturelands: Impacts of Grazing Intensity and Planting Systems

Management intensity critically influences the productivity and sustainability of pasture systems through modifying soil microbes, and soil carbon (C) and nutrient dynamics; however, such effects are not well understood yet in the southeastern USA. We examined the effects of grazing intensity and grass planting system on soil C and nitrogen (N) dynamics, and microbial biomass and respiration in a long-term field experiment in Goldsboro, North Carolina, USA. A split-plot experiment was initiated in 2003 on a highly sandy soil under treatments of two grass planting systems (ryegrass rotation with sorghum-sudangrass hybrid and ryegrass seeding into a perennial bermudagrass stand) at low and high grazing densities. After 4 years of continuous treatments, soil total C and N contents across the 0–30 cm soil profile were 24.7% and 17.5% higher at the high than at the low grazing intensity, likely through promoting plant productivity and C allocation belowground as well as fecal and urinary inputs. Grass planting system effects were significant only at the low grazing intensity, with soil C, N, and microbial biomass and respiration in the top 10 cm being higher under the ryegrass/bermudagrass than under the ryegrass/sorghum-sudangrass hybrid planting systems. These results suggest that effective management could mitigate potential adverse effects of high grazing intensities on soil properties and facilitate sustainability of pastureland.

Irrigation and weed control alter soil microbiology and nutrient availability in North Carolina Sandhill peach orchards

Orchard management practices such as weed control and irrigation are primarily aimed at maximizing fruit yields and economic profits. However, the impact of these practices on soil fertility and soil microbiology is often overlooked. We conducted a two-factor experimental manipulation of weed control by herbicide and trickle irrigation in a nutrient-poor peach (Prunus persica L. cv. Contender) orchard near Jackson Springs, North Carolina. After three and eight years of treatments, an array of soil fertility parameters were examined, including soil pH, soil N, P and cation nutrients, microbial biomass and respiration, N mineralization, and presence of arbuscular mycorrhizal fungi (AMF). Three general trends emerged: 1) irrigation significantly increased soil microbial biomass and activity, 2) infection rate of mycorrhizal fungi within roots were significantly higher under irrigation than non-irrigation treatments, but no significant difference in the AMF community composition was detected among treatments, 3) weed control through herbicides reduced soil organic matter, microbial biomass and activity, and mineral nutrients, but had no significant impacts on root mycorrhizal infection and AMF communities. Weed-control treatments directly decreased availability of soil nutrients in year 8, especially soil extractable inorganic N. Weed control also appears to have altered the soil nutrients via changes in soil microbes and altered net N mineralization via changes in soil microbial biomass and activity. These results indicate that long-term weed control using herbicides reduces soil fertility through reducing organic C inputs, nutrient retention and soil microbes. Together, these findings highlight the need for alternative practices such as winter legume cover cropping that maintain and/or enhance organic inputs to sustain the soil fertility.

Long-term no-tillage and organic input management enhanced the diversity and stability of soil microbial community

Intensive tillage and high inputs of chemicals are frequently used in conventional agriculture management, which critically depresses soil properties and causes soil erosion and nonpoint source pollution. Conservation practices, such as no-tillage and organic farming, have potential to enhance soil health. However, the long-term impact of no-tillage and organic practices on soil microbial diversity and community structure has not been fully understood, particularly in humid, warm climate regions such as the southeast USA. We hypothesized that organic inputs will lead to greater microbial diversity and a more stable microbial community, and that the combination of no-tillage and organic inputs will maximize soil microbial diversity. We conducted a long-term experiment in the southern Appalachian mountains of North Carolina, USA to test these hypotheses. The results showed that soil microbial diversity and community structure diverged under different management regimes after long term continuous treatments. Organic input dominated the effect of management practices on soil microbial properties, although no-tillage practice also exerted significant impacts. Both no-tillage and organic inputs significantly promoted soil microbial diversity and community stability. The combination of no-tillage and organic management increased soil microbial diversity over the conventional tillage and led to a microbial community structure more similar to the one in an adjacent grassland. These results indicate that effective management through reducing tillage and increasing organic C inputs can enhance soil microbial diversity and community stability.

Atmospheric CO2 Enrichment and Reactive Nitrogen Inputs Interactively Stimulate Soil Cation Losses and Acidification

Reactive N inputs (Nr) may alleviate N-limitation of plant growth and are assumed to help sustain plant responses to the rising atmospheric CO2 (eCO2). However, Nr and eCO2 may elicit a cascade reaction that alters soil chemistry and nutrient availability, shifting the limiting factors of plant growth, particularly in acidic tropical and subtropical croplands with low organic matter and low nutrient cations. Yet, few have so far examined the interactive effects of Nr and eCO2 on the dynamics of soil cation nutrients and soil acidity. We investigated the cation dynamics in the plant-soil system with exposure to eCO2 and different N sources in a subtropical, acidic agricultural soil. eCO2 and Nr, alone and interactively, increased Ca2+ and Mg2+ in soil solutions or leachates in aerobic agroecosystems. eCO2 significantly reduced soil pH, and NH4+-N inputs amplified this effect, suggesting that eCO2-induced plant preference of NH4+-N and plant growth may facilitate soil acidification. This is, to our knowledge, the first direct demonstration of eCO2 enhancement of soil acidity, although other studies have previously shown that eCO2 can increase cation release into soil solutions. Together, these findings provide new insights into the dynamics of cation nutrients and soil acidity under future climatic scenarios, highlighting the urgency for more studies on plant-soil responses to climate change in acidic tropical and subtropical ecosystems.