Over N fertilization is a common problem for the winter wheat (Triticum aestivum L.)–summer maize (Zea mays L.) rotation system in the North China Plain. A field experiment which included control (no N), conventional N (Con. N) fertilization, and optimized N (Nmin) fertilization treatments, was conducted from 1999 to 2003 near Beijing, China. Soil nitrate (NO3) dynamics were measured and N balance was calculated for the period of the eight successive cropping seasons. Soil NO3–N in the 0‐ to 90‐cm profile for the Con. N treatment ranged from 157 to 700 kg ha−1 during the eight successive cropping seasons, much greater than those in the no N and optimized N treatments. Large amounts of soil NO3–N were detected in the 90‐ to 200‐cm layer under the conventional N fertilization treatment, especially in the summer maize season. For the Nmin treatment, the total amount of N applied was 511 kg N ha−1 in the eight successive crops as compared with 2400 kg N ha−1 of the Con. N treatment. Grain yields were not different between the fertilized treatments except for maize in 2003. Soil NO3–N in the root zone under conditions of optimized N fertilization was maintained at a relatively low level as compared with the Con. N treatment, therefore dramatically decreasing NO3–N movement to deeper soil profile. This study indicates that soil NO3 movement out of the effective crop root zone is an important pathway of N losses in this winter wheat–summer maize rotation system in the North China Plain and the optimized N fertilization by an improved Nmin method shows high potential of reducing N‐leaching losses.
Chemical immobilization, an in situ remediation method where inexpensive chemicals are used to reduce contaminant solubility in contaminated soil, has gained attention. We investigated the effectiveness of lime-stabilized biosolid (LSB), N-Viro Soil (NV), rock phosphate (RP), and anaerobic biosolid (AB) to reduce extractability and plant and gastrointestinal (GI) bioavailability in three Cd-, Pb-, and Zn-contaminated soils from smelter sites. Treated (100 g kg(-1) soil) and control soils were incubated at 27 degrees C and -0.033 MPa (0.33 bar) water content for 90 d. The effect of soil treatment on metal extractability was evaluated by sequential extraction, on phytoavailability by a lettuce bioassay (Lactuca sativa L.), on human GI availability of Pb from soil ingestion by the Physiologically Based Extraction Test. The largest reductions in metal extractability and phytoavailability were from alkaline organic treatments (LSB and NV). Phytotoxic Zn [1188 mg Zn kg(-1) extracted with 0.5 M Ca(NO3)2] in Blackwell soil (disturbed soil) was reduced by LSB, NV, and RP to 166, 25, and 784 mg Zn kg(-1), respectively. Rock phosphate was the only treatment that reduced GI-available Pb in both gastric and intestinal solutions, 23 and 92%, respectively. Alkaline organic treatments (LSB, NV) decreases Cd transmission through the food chain pathway, whereas rock phosphate decreases risk from exposure to Pb via the soil ingestion pathway. Alkaline organic treatments can reduce human exposure to Cd and Pb by reducing Zn phytotoxicity and revegetation of contaminated sites.
A gronomy J our n al • Volume 10 0 , I s sue 3 • 2 0 0 8 517 ABSTRACT Th e improved soil N min -based N management is a promising approach to precision N management, which determines the optimum side-dress N rates based on N target values and measured soil nitrate N content in the root soil layer at diff erent growth stages. A total of 148 on-farm N-response experiments, in seven key summer maize (Zea mays L.) production regions of North China Plain (NCP) from 2003 to 2005, were conducted to evaluate the N min -based N management compared to traditional farmer's N practices. Th e recommended N rates based on the improved soil N min method were not signifi cantly diff erent ( ≤31 kg N ha -1 ) from those determined by yield response curves (n = 13). Th e average N rate determined with the soil N min method (157 kg N ha -1 ) was signifi cantly lower than farmer's practice (263 kg N ha -1 ), while maize grain yield was 0.4 Mg ha -1 higher than farmer's N practice (8.5 Mg ha -1 ) across all sites (n = 148). As a result, the improved soil N min -based N management signifi cantly increased net economic gains by $202 ha -1 , reduced residual nitrate N content and N losses by 44 kg N ha -1 and 65 kg N ha -1 , respectively, and improved recovery N effi ciency, agronomic N effi ciency and N partial factor productivity by 16%, 6 kg kg -1 and 36 kg kg -1 , respectively, compared with farmer's N practice. We conclude that the improved soil N min -based N management can be applied for summer maize production in NCP for improved N use effi ciency and reduced environmental contamination.
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