Fusuo Zhang

Enhanced nitrogen deposition over China

China is experiencing intense air pollution caused in large part by anthropogenic emissions of reactive nitrogen. These emissions result in the deposition of atmospheric nitrogen (N) in terrestrial and aquatic ecosystems, with implications for human and ecosystem health, greenhouse gas balances and biological diversity. However, information on the magnitude and environmental impact of N deposition in China is limited. Here we use nationwide data sets on bulk N deposition, plant foliar N and crop N uptake (from long-term unfertilized soils) to evaluate N deposition dynamics and their effect on ecosystems across China between 1980 and 2010. We find that the average annual bulk deposition of N increased by approximately 8u2009kilograms of nitrogen per hectare (Pu2009<u20090.001) between the 1980s (13.2u2009kilograms of nitrogen per hectare) and the 2000s (21.1u2009kilograms of nitrogen per hectare). Nitrogen deposition rates in the industrialized and agriculturally intensified regions of China are as high as the peak levels of deposition in northwestern Europe in the 1980s, before the introduction of mitigation measures. Nitrogen from ammonium (NH4+) is the dominant form of N in bulk deposition, but the rate of increase is largest for deposition of N from nitrate (NO3−), in agreement with decreased ratios of NH3 to NOx emissions since 1980. We also find that the impact of N deposition on Chinese ecosystems includes significantly increased plant foliar N concentrations in natural and semi-natural (that is, non-agricultural) ecosystems and increased crop N uptake from long-term-unfertilized croplands. China and other economies are facing a continuing challenge to reduce emissions of reactive nitrogen, N deposition and their negative effects on human health and the environment.

Effect of Si on the distribution of Cd in rice seedlings

Growth chamber studies were conducted to investigate the effects of silicon (Si) on the distribution of Cd in rice seedlings (Oryzasativa L., cv. Qiu Guang) grown hydroponically under toxic level of cadmium (Cd). Si added significantly alleviated the toxicity of Cd in aerobic rice seedlings. Si partly overcame the reduction in growth due to Cd. This amelioration was correlated with a reduction in Cd uptake. Si increased Cd accumulation in the roots and restricted the transport of Cd from roots to shoots, where the distribution of Cd in the shoots decreased by 33%. Si reduced the transport of Cd and the apoplastic fluorescence tracer PTS (tri-sodium-8-hydroxy-1, 3, 6-pyrenesulphonate) from roots to shoots by 23 and 36%, respectively. Energy-dispersive X-ray analysis (EDX) showed Cd was mainly deposited in the vicinity of the endodermis and epidermis, Si deposition was heavier in the vicinity of the endodermis than in the epidermis. Although the tracing result of fluorescein isothiocyanate-dextrans showed Si did not change epidermal wall porosity, the significant reduction of apoplastic PTS transport in +Si plants suggested that the heavy deposition of silica in the vicinity of endodermis might offer possible mechanisms by which silicon did at least partially physically block the apoplast bypass flow across the roots, and restrained the apoplastic transport of Cd. In addition, the effect of Si on the subcellular distribution and chemical form of Cd was investigated by fractionation. Si decreased the concentrations of Cd in shoots and roots, but did not remarkably change the distribution ratio of Cd in symplasm and apoplast. Mechanisms by which Si alleviates the toxicity of Cd in rice seedlings are discussed.

Crop Yields, Internal Nutrient Efficiency, and Changes in Soil Properties in Rice–Wheat Rotations Under Non-Flooded Mulching Cultivation

A field experiment was conducted for 5xa0years to examine the effects of non-flooded mulching cultivation on crop yield, internal nutrient efficiency and soil properties in rice–wheat (R–W) rotations of the Chengdu Plain, southwest China. Compared with traditional flooding (TF), non-flooded plastic film mulching (PM) resulted in 12 and 11% higher average rice (Oryza sativa L.) yield and system productivity (combined rice and wheat yields), and the trends in rice and wheat (Triticum aestivum L.) yields under PM were stable over time. However, non-flooded wheat straw mulching (SM) decreased average rice yield by 11% compared with TF, although no significant difference in system productivity was found between SM and TF. Uptakes of N and K by rice under PM were higher than those under TF and SM, but internal nutrient efficiency was significantly lower (N) or similar (K) under PM compared to SM and TF. This implies that more N and K accumulated in rice straw under PM. After 5-year rice–wheat rotation, apparent P balances (112–160xa0kgxa0ha−1) were positive under all three cultivation systems. However, the K balances were negative under PM (−419xa0kgxa0ha−1) and TF (−90xa0kgxa0ha−1) compared with SM (45xa0kgxa0ha−1). This suggests that higher K inputs from fertilizer, straw or manure may be necessary, especially under PM. After five rice seasons and four wheat seasons, non-flooded mulching cultivation led to similar (PM) or higher (SM) soil organic carbon (SOC), total N (TN) and alkali hydrolyzable N (AH-N) in the top 0–5 and 5–12xa0cm layers compared with TF. SOC, TN, AH-N and Olsen-P (OP) in the sub-surface layer (12–24xa0cm) were significantly higher under PM or SM than under TF, indicating that rice under non-flooded mulching conditions may fail to make use of nutrients from the subsoil. Thus, the risk of decline in soil fertility under non-flooded mulching cultivation could be very low if input levels match crop requirements. Our data indicate that PM and SM may be alternative options for farmers using R–W rotations for enhancement or maintenance of system productivity and soil fertility.

Wheat powdery mildew and foliar N concentrations as influenced by N fertilization and belowground interactions with intercropped faba bean

Wheat (Triticum aestivum L.) production by intercropping with faba bean (Vicia faba L.) has increased in popularity but is often associated with severe wheat powdery mildew (Blumeria graminis (DC.) Speer). Very little is known about the effects of below- and aboveground interspecific interactions on wheat nitrogen (N) nutrition and occurrence of wheat powdery mildew. A greenhouse pot experiment examined four N application rates and three belowground partition types (plastic film, nylon mesh partition or no partition) to study N nutrition and interactions between wheat and faba bean growing together. A field experiment investigated three N application rates and growth of wheat in monoculture and intercropped with faba bean with or without belowground plastic film partitions between wheat and faba bean. Disease incidence (DI) and disease severity index (DSI) were assessed at flowering stage and wheat leaves were sampled and analyzed for N. Foliar N was enhanced substantially by N addition in greenhouse and field conditions and also by belowground interactions (no partition compared with plastic film partition) in the pot experiment (all Pxa0<xa00.001). There was a significant synergistic effect between N rate and belowground interactions on the enhancement of wheat N uptake (Pxa0<xa00.01) in the pot experiment. DI and DSI of mildew increased markedly with increasing N rate in both experiments (all Pxa0<xa00.001). In the pot experiment DI and DSI showed no marked differences among belowground partitions (both Pxa0>xa00.05) but belowground interactions had different effects under different N rates, limiting disease occurrence under 0, 0.1 and 0.2xa0g N kg−1 soil but promoting disease with 0.05xa0g added N kg−1 soil. In the field experiment DI and DSI showed no significant differences between wheat monoculture and intercropping (both Pxa0>xa00.05). However, the contributions of below- and aboveground interactions to disease control were different under different N rates, with interspecific root interactions increasing DI and DSI under different N rates and aboveground interactions increasing DI and DSI under zero-N application but decreasing DI and DSI at 150 and 300xa0kg N ha−1. The data suggest that the microclimate in the field and biological control mechanisms due to belowground interactions in wheat–faba bean associations may influence the incidence and severity of wheat powdery mildew.

Total nitrogen deposition at key growing stages of maize and wheat as affected by pot surface area and crop variety

Atmospheric nitrogen (N) deposition is a serious problem on the North China Plain (NCP) because it imposes a considerable nutrient burden on the local environment. However, it also makes a substantial contribution to agricultural crop N requirements. The integrated total N input (ITNI) system is a method to quantify total atmospheric N deposition by using 15N-labeled monitor plants grown in pots. The effect of pot surface area and variety of indicator plant on the amount of airborne N input quantified by the ITNI system was investigated in this study. Total N deposition to the soil-maize/soil-wheat plant system at key growth stages was also quantified to improve N-fertilizer recommendations. When indicator plants having a high space requirement were used a correction factor was needed and this could be obtained only by simulating commercial field conditions, especially plant density, because the factor depends largely on pot area or the difference in plant density between pot conditions and field conditions. The total airborne N input measured by the ITNI system was not influenced by the variety of monitoring plant. N deposition was 20–25xa0kgxa0N ha−1 during growth from three expanded leaf to ten expanded leaf and also from ten expanded leaf to maturity of maize. N deposition was 29.1xa0kgxa0N ha−1 between planting and the jointing stage and 10.1xa0kgxa0N ha−1 from jointing to maturity of wheat. This high measured N deposition indicates that N deposition should be taken into account when calculating the N fertilizer requirements of maize and wheat in this region.