• Energy Research

When rock phosphates (RP) are used to remediate Pb-contaminated soils, their effectiveness is likely affected by their grain size. In this study, the effect of grain size of rock phosphate on the effectiveness of heavy metal immobilization in two contaminated soils was measured in pot experiment. Rock phosphate was used with four different grain sizes: <35, 35-72, 72-133 and 133-266microm. The application rate of rock phosphate in two soils was determined based on P/metals (Pb, Zn, Cu and Cd) molar ratio of 5.0 in the soils. The results showed that rock phosphate of the smallest grain size (<35microm) was superior to all of other grain sizes more than 35microm for reducing uptake in plant (Brassica oleracea L.) shoots for Cd (19.6-50.0%), Pb (21.9-51.4%) and Zn (22.4-34.6%), respectively, as compared with the soil without application of rock phosphate. Sequential extraction analysis indicated that rock phosphate was most effective for soil Pb to induced transformation from non-residual fractions to a residual fraction than that for Zn and Cd. Such transformation was probably through dissolution of Pb associated with exchangeable (EX), organic fraction (OC), acidic fraction (AC) and amorphous Fe and Al oxides-bound (OX) fraction and precipitation of pyromorphite-like minerals. Results suggested that the rock phosphate with small grain size was superior to that with large grain size for in situ remediation technology.

Soil environmental quality standards in respect of heavy metals for farmlands should be established considering both their effects on crop yield and their accumulation in the edible part. A greenhouse experiment was conducted to investigate the effects of chromium (Cr) on biomass production and Cr accumulation in carrot plants grown in a wide range of soils. The results revealed that carrot yield significantly decreased in 18 of the total 20 soils with Cr addition being the soil environmental quality standard of China. The Cr content of carrot grown in the five soils with pH>8.0 exceeded the maximum allowable level (0.5mgkg(-1)) according to the Chinese General Standard for Contaminants in Foods. The relationship between carrot Cr concentration and soil pH could be well fitted (R(2)=0.70, P<0.0001) by a linear-linear segmented regression model. The addition of Cr to soil influenced carrot yield firstly rather than the food quality. The major soil factors controlling Cr phytotoxicity and the prediction models were further identified and developed using path analysis and stepwise multiple linear regression analysis. Soil Cr thresholds for phytotoxicity meanwhile ensuring food safety were then derived on the condition of 10 percent yield reduction.

In spite of the importance of N management in agricultural production, closing the full nitrogen balance remains a challenge, mainly due to the uncertainties in both fluxes of nitrogen input and output. We analyzed N deposition and its influence on crop productivity and field nitrogen balance based on data from three of 15 years (1990–2005) of experiments in North China. The results showed that the average annual nitrogen deposition was 76, 80, and 94 kg N/ha at Changping, Zhengzhou, and Yangling in a wheat-maize rotation system, respectively. The deposited N could support a corresponding total biomass production (wheat plus maize) of 9.6, 10.6, and 8.8 Mg/ha with a total grain yield of 3.8, 4.8, and 3.7 Mg/ha, however, that did not cause a further decline in soil organic matter. N fertilizer application could increase total biomass (grain) by 244% (259%) and 74% (119%) for wheat and maize, respectively. Under optimal N management, N deposition accounted for 17–21% of the total N inputs, which affected significantly the recovery efficiency of applied N. N deposition showed a significant spatial variation in terms of the fraction of dry and wet depositions. On an annual average, N deposition roughly balanced out N losses due to NH3 volatilization and N2O loss from nitrification and denitrification. NH3 volatilization and NO3−-N leaching each accounted for 16–20% of the total N outputs. A system modeling approach is recommended to investigate the spatial variation of N leaching as affected by climatic conditions, and to fully account for the nitrogen fluxes. The N deposition derived from this study can be used as the background N input into the wheat-maize double cropping system for N balance.

AbstractLoss of plant diversity with increased anthropogenic nitrogen (N) deposition in grasslands has occurred globally. In most cases, competitive exclusion driven by preemption of light or space is invoked as a key mechanism. Here, we provide evidence from a 9‐yr N‐addition experiment for an alternative mechanism: differential sensitivity of forbs and grasses to increased soil manganese (Mn) levels. In Inner Mongolia steppes, increasing the N supply shifted plant community composition from grass–forb codominance (primarily Stipa krylovii and Artemisia frigida, respectively) to exclusive dominance by grass, with associated declines in overall species richness. Reduced abundance of forbs was linked to soil acidification that increased mobilization of soil Mn, with a 10‐fold greater accumulation of Mn in forbs than in grasses. The enhanced accumulation of Mn in forbs was correlated with reduced photosynthetic rates and growth, and is consistent with the loss of forb species. Differential accumulation of Mn between forbs and grasses can be linked to fundamental differences between dicots and monocots in the biochemical pathways regulating metal transport. These findings provide a mechanistic explanation for N‐induced species loss in temperate grasslands by linking metal mobilization in soil to differential metal acquisition and impacts on key functional groups in these ecosystems.

Nickel pollution imposes deleterious effects on soil ecosystem. The responses of soil microorganisms to long-term nickel pollution under field conditions remain largely unknown. Here, we used high-throughput sequencing to elucidate the impacts of long-term nickel pollution on soil bacterial communities in two contrasting agricultural soils. Our results found that the soil microbial biomass carbon consistently decreased along the nickel gradients in both soils. Nickel pollution selectively favored or impeded the prevalence of several dominant bacterial guilds, in particular, Actinobacteria showed tolerance, while Acidobacteria and Planctomycetes displayed sensitivity. Despite the apparent shifts in the bacterial community composition, no clear tendency in the bacterial diversity and abundance was identified along the nickel gradients in either soil. Collectively, we provide evidence that long-term nickel pollution shifted the soil bacterial communities, resulting in the decrease of microbial biomass although the bacterial diversity was not significantly changed.

Ethylenediaminetetraacetic acid (EDTA) is known to be a chelating agent and has been widely used for estimating the total extractable metals in soil. The effect of aging on EDTA-extractable cadmium (Cd) was investigated in five different soils at three Cd concentrations incubated for 180 days. The EDTA-extractable Cd rapidly decreased after incubated during 30–60 days, followed by slow processes, and for 90 days the EDTA-extractable Cd tended to be stable. The decrease in EDTA-extractable Cd may be due to precipitation/nucleation processes, diffusion of Cd into the micropores/mesopores, and occlusion within organic matter in soils. A semi-mechanistic model to predict the extractability of Cd during incubation, based on processes of Cd precipitation/nucleation, diffusion, and occlusion within organic matter, was developed and calibrated. The results showed that the processes of micropore/mesopore diffusion were predominant processes affecting the extractability of Cd added to soils, and were slow. However, the proportions of the processes of precipitation/nucleation and occlusion within organic matter to the non-EDTA-extractable Cd added to soils were only 0.03–21.0% and 0.41–6.95%, respectively. The measured EDTA-extractable Cd from incubated soils were in good agreement with those predicted by the semi-mechanistic model (R2 = 0.829). The results also indicated that soil pH, organic matter, and incubation time were the most important factors affecting Cd aging.