No abstract

The estimation of the size and changes of soil organic carbon (SOC) stocks is of great importance for decision makers to adopt proper measures to protect soils and to develop strategies for mitigation of greenhouse gases. In this paper, soil data from the Second State Soil Survey of China (SSSSC) conducted in the early 1980s and data published in the last 5 years were used to estimate the size of SOC stocks over the whole profile and their changes in China in last 20 years. Soils were identified as paddy, upland, forest, grassland or waste-land soils and an improved soil bulk density estimation method was used to estimate missing bulk density data. In the early 1980s, total SOC stocks were estimated at 89.61 Pg (1 Pg 5 10 3 Tg 5 10 15 g) in China's 870.94 Mha terrestrial areas covered by 2473 soil series. In the paddy, upland, forest and grassland soils the respective total SOC stocks were 2.91 Pg on 29.87 Mha, 10.07 Pg on 125.89 Mha, 34.23 Pg on 249.32 Mha and 37.71 Pg on 278.51 Mha, respectively. The SOC density of the surface layer ranged from 3.5 Mg ha À1 in Gray Desery grassland soils to 252.6 Mg ha À1 in Mountain Meadow forest soils. The average area-weighted total SOC density in paddy soils (97.6 Mg ha À1 ) was higher than that in upland soils (80 Mg ha À1 ). Soils under forest (137.3 Mg ha À1 ) had a similar average area-weighted total SOC density as those under grassland (135.4 Mg ha À1 ). The annual estimated SOC accumulation rates in farmland and forest soils in the last 20 years were 23.61 and 11.72 Tg, respectively, leading to increases of 0.472 and 0.234 Pg SOC in farmland and forest areas, respectively. In contrast, SOC under grassland declined by 3.56 Pg due to the grassland degradation over this period. The resulting estimated net SOC loss in China's soils over the last 20 years was 2.86 Pg. The documented SOC accumulation in farmland and forest soils could thus not compensate for the loss of SOC in grassland soils in the last 20 years. There were, however, large regional differences: Soils in China's South and Eastern parts acted mainly as C sinks, increasing their average topsoil SOC by 132 and 145 Tg, respectively. In contrast, in the Northwest, Northeast, Inner Mongolia and Tibet significant losses of 1.38, 0.21, 0.49 and 1.01 Pg of SOC, respectively, were estimated over the last 20 years. These results highlight the importance to take measures to protect grassland and to improve management practices to increase C sequestration in farmland and forest soils.

The two-step nitrification process is an integral part of the global nitrogen cycle, and it is accomplished by distinctly different nitrifiers. By combining DNA-based stable isotope probing (SIP) and high-throughput pyrosequencing, we present the molecular evidence for autotrophic growth of ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and nitrite-oxidizing bacteria (NOB) in agricultural soil upon ammonium fertilization. Time-course incubation of SIP microcosms indicated that the amoA genes of AOB was increasingly labeled by 13 CO 2 after incubation for 3, 7 and 28 days during active nitrification, whereas labeling of the AOA amoA gene was detected to a much lesser extent only after a 28-day incubation. Phylogenetic analysis of the 13 C-labeled amoA and 16S rRNA genes revealed that the Nitrosospira cluster 3-like sequences dominate the active AOB community and that active AOA is affiliated with the moderately thermophilic Nitrososphaera gargensis from a hot spring. The higher relative frequency of Nitrospira-like NOB in the 13 C-labeled DNA suggests that it may be more actively involved in nitrite oxidation than Nitrobacter-like NOB. Furthermore, the acetylene inhibition technique showed that 13 CO 2 assimilation by AOB, AOA and NOB occurs only when ammonia oxidation is not blocked, which provides strong hints for the chemolithoautotrophy of nitrifying community in complex soil environments. These results show that the microbial community of AOB and NOB dominates the nitrification process in the agricultural soil tested. The ISME Journal (2011) 5, 1226-1236; doi:10.1038/ismej.2011.5; published online 17 February 2011Subject Category: microbial ecology and functional diversity of natural habitats

Rising CO2 levels may induce nutritional deficits (protein, minerals, and vitamins) in the highest rice-consuming countries.

No abstract

With regard to measuring nitrous oxide (N 2 O) emissions from biological sources, there are three most widely adopted methods that use gas chromatograph with an electron capture detector (GC-ECD). They use: (a) nitrogen (N 2 ) as the carrier gas (DN); (b) ascarite as a carbon dioxide (CO 2 ) trap with DN (DN-Ascarite); and (c) a mixture gas of argon and methane as the carrier (AM). Additional methods that use either a mixture of argon and methane (or of CO 2 and N 2 ) as a make-up gas with the carrier nitrogen or soda lime (or ascarite) as a CO 2 trap with the carrier helium have also been adopted in a few studies. To test the hypothesis that the use of DN sometimes considerably biases measurements of N 2 O emissions from plants, soils or soil-plant systems, experiments were conducted involving DN, AM and DN-Ascarite. When using DN, a significant relationship appeared between CO 2 concentrations and the apparent N 2 O concentrations in air samples. The use of DN led to significantly overestimated N 2 O emissions from detached fresh plants in static chamber enclosures. Meanwhile, comparably lower emissions were found when using either the DN-Ascarite or AM methods. When an N 2 O flux (from a soil or a soil-plant system), measured by DN in combination with sampling from the enclosure of a static opaque chamber, was greater than 200 μg N m −2 h −1 , no significant difference was found between DN and DN-Ascarite. When the DN-measured fluxes were within the ranges of <−30, −30-0, 0-30, 30-100 and 100-200 μg N m −2 h −1 , significant differences that amounted to −72, −22, 5, 38 and 64 μg N m −2 h −1 , respectively, appeared in comparison to DN-Ascarite. As a result, the DN measurements in rice-wheat and Plant Soil (vegetable fields overestimated both annual total N 2 O emissions (by 7-62%, p<0.05) and direct emission factors for applied nitrogen (by 6-65%). These results suggest the necessity of reassessing the available data determined from DN measurements before they are applied to inventory estimation. Further studies are required to explore appropriate approaches for the necessary reassessment. Our results also imply that the DN method should not be adopted for measuring N 2 O emissions from weak sources (e.g., with intensities less than 200 μg N m −2 h −1 ). In addition, we especially do not recommend the use of DN to simultaneously measure N 2 O and CO 2 with the same ECD.

Atmospheric CO(2) enrichment generally stimulates plant photosynthesis and nutrient uptake, modifying the local and global cycling of bioactive elements. Although nutrient cations affect the long-term productivity and carbon balance of terrestrial ecosystems, little is known about the effect of CO(2) enrichment on cation availability in soil. In this study, we present evidence for a novel mechanism of CO(2)-enhancement of cation release from soil in rice agricultural systems. Elevated CO(2) increased organic C allocation belowground and net H(+) excretion from roots, and stimulated root and microbial respiration, reducing soil redox potential and increasing Fe(2+) and Mn(2+) in soil solutions. Increased H(+), Fe(2+), and Mn(2+) promoted Ca(2+) and Mg(2+) release from soil cation exchange sites. These results indicate that over the short term, elevated CO(2) may stimulate cation release from soil and enhance plant growth. Over the long-term, however, CO(2)-induced cation release may facilitate cation losses and soil acidification, negatively feeding back to the productivity of terrestrial ecosystems.

No abstract