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 rapid development of agricultural biotechnology and release of new transgenic plants for agriculture has provided many economic benefits, but has also raised concern over the potential impact of transgenic plants on the environment. Considerable research has now been conducted on the effects of transgenic plants on soil microorganisms. These effects include unintentional changes in the chemical compositions of root exudates, and the direct effects of transgenic proteins on nontarget species of soil microorganisms. Most studies to date suggest that transgenic plants that have been released cause minor changes in microbial community structures that are often transient in duration. However, due to our limited knowledge of the linkage between microbial community structure and function, more work needs to be done on a case-by-case basis to further evaluate the effects of transgenic plants on soil microorganisms and soil ecosystem functions. This review summarizes the results of a variety of experiments that have been conducted to specifically test the effects of transgenic plants on soil microorganisms, and particularly examines the types of methods that are being used to study microbial interactions with transgenic plants.

To investigate whether brassinosteroids (BRs) could be used to alleviate chill-induced inhibition of photosynthesis in cucumber (Cucumis sativus L) during chilling and subsequent recovery, the effects of exogenously applied 24-epibrassinolide (EBR) on gas exchange, chlorophyll fluorescence parameters, and antioxidant enzyme activity were studied. Cucumber plants were exposed to chilling under low light (12/8 o C and 100 μmol m -2 s -1 PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18 o C and 600 μmol m -2 s -1 PPFD) for 6 days. Chilling significantly decreased the net photosynthetic rate (P N ) and stomatal conductance (g s ), and increased rate of O 2˙formation and H 2 O 2 and malondialdehyde (MDA) content in cucumber leaves, but did not influence the optimal quantum yield of PSII (F v /F m ). Chilling also decreased the effective quantum yield of PSII photochemistry (Φ PSII ) and photochemical quenching (q P ), but induced an increase in nonphotochemical quenching (NPQ), and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). High irradiance (600 μmol m -2 s -1 ) further aggravated the decrease in P N , g s , Φ PSII and q P , and enhanced the increase in reactive oxygen species (ROS) generation and accumulation in the first day of recovery after chilling. However, high irradiance induced a sharp decrease in F v /F m and NPQ, as well as the activities of SOD and APX on the first day of recovery. EBR pretreatment significantly alleviated chill-induced inhibition of photosynthesis during chilling stress and subsequent recovery period, which was mainly due to significant increases in g s , Φ PSII , q P and NPQ. EBR pretreatment also reduced ROS generation and accumulation, and increased the activities of SOD and APX during chilling and subsequent recovery. Those results suggest that EBR pretreatment alleviates the chill reduction in photosynthesis and accelerated the recovery rate mainly by increasing of the stomatal conductance, the efficiency of utilization and dissipation of leaf absorbed light, and the activity of the ROS scavenging system during chilling and subsequent recovery period. -light-adapted maximum fluorescence; F o -minimal fluorescence of dark-adapted state; F m -maximal fluorescence of dark-adapted state; F v /F m -optimal quantum yield of PSII; FM -fresh mass; g s -stomatal conductance; LT -low temperature; LTBR -low temperature; EBR-pretreatment; MDA -malondialdehyde; NT -normal temperature; NTBR -normal temperature/EBR-pretreatment; NPQ -nonphotochemical quenching; Φ PSII , -effective quantum yield of PSII photochemistry; P N -net photosynthetic rate; PPFD -photosynthetic photon flux density; q P -photochemical quenching coefficient; Rubisco -ribulose-1,5-bisphosphate carboxylase/oxygenase; ROS -reactive oxygen species; SOD -superoxide dismutase.