We developed a novel method for non-destructive, micro-scale sampling of bacteria and showed that the population density of bacteria in the rhizosphere was inversely proportional to distance from the apex of Brassica napus roots and that the composition of communities was highly variable at the micro-scale. Imaging of ¹⁴C-labelled root exudates revealed that population density was not reflective of exudate availability. Microelectrodes revealed that pH at the root surface was highly variable at the micro-scale which, combined with similar observations for bacterial density/diversity, highlighted the importance of spatial scale for linking bacterial communities with their environment.

We have demonstrated that phytohormones are used by some plant parasitic nematodes to locate host roots and possibly their feeding site within roots.

We have used isotopomer studies to discriminate N2O from nitrification and denitrification and traced the changes in gene expression of key enzymes in the denitrification process through time using the DENIS laboratory system. Conceptual and functional models of P transport contributed to derivation of a fuzzy dynamic based approach for P loss from headwater catchment sites. Cost-curve' analyses of greenhouse gas and diffuse water pollutant (N and P) mitigations have been produced.

We have demonstrated that rewetting of pasture soil released large quantities of organic P to soil solution from microbial P; determined concentration-discharge dynamics, scaling up from plots to river channels and showing the diurnal cycling of riverine nitrate, nitrite, ammonium, P, pH and river flow; shown the impacts of storm events on P transport patterns at a range of scales; demonstrated the spatial variability of P in fields and catchments to highlight the difficulty of specifying soil status in models.

We have shown that, as soil dries from field capacity, its strength (resistance to root penetration) rather than low matric potential (lack of water) adversely affects crop growth. This is because soil strength can be sufficient to reduce root elongation even in relatively wet soils, which in turn affects shoot growth through root-shoot signalling: even at matric potentials as high as -50 kPa, the yield of field-grown wheat was less than in a well-watered soil.

Trace elements in soil and crops

Deficiencies of micronutrients such as Fe, Zn and Se affect more than three billion people worldwide. Cereals provide an important source of mineral micronutrients to humans. Using the Rothamsted Sample Archive, we were able to rigorously determine the micronutrient status of wheat grain and soil since 1845 on the same plots with contrasting treatments in the Broadbalk experiment. We found evidence of decreasing concentrations of the minerals Zn, Cu, Mn, Fe and Mg in grain particularly since modern short-straw varieties were grown in the mid 1960s (example of Zn, figure below). The decreasing trend was not due to a depletion of the minerals in soil, but was related to increasing grain yield and harvest index.

For more information, see our publication: Fan MS, Zhao FJ, Fairweather-Tait SJ, Poulton PR, Dunham SJ and McGrath SP. 2008 Evidence of decreasing mineral density in wheat grain over the last 160 years. Journal of Trace Elements in Medicine and Biology (doi:10.1016/j.jtemb.2008.07.002).

Concentrations of zinc in wheat grain from three treatments of the Broadbalk experiment over the last 160 years.

Arsenic mobilisation in paddy soil and uptake mechanism in rice

Arsenic poisoning affects millions of people worldwide, particularly in southeast Asia where arsenic-contaminated groundwater has been used for drinking and for irrigation. Rice is very efficient in arsenic accumulation, thus posing a potential health risk to people who eat a lot of rice. We have recently discovered the two important reasons why rice takes up so much arsenic, and opened the way for mitigation methods. We found that flooding of paddy soil led to a rapid mobilization of arsenite in soil solution, which is then available for rice uptake. Growing rice aerobically decreased arsenic accumulation in grain markedly (Figure 2). In collaboration with Professor Jian Feng Ma of Okayama University, Japan, we have identified two transporters involved in the uptake of silicon that also allow the passage of arsenite into rice plants (Figure 3). Supplying silicon to rice plants significantly decreased arsenic accumulation in rice shoots. The highly efficient uptake pathway for silicon in rice also allows the inadvertent uptake of arsenite from paddy soils, thus explaining why rice so efficiently accumulates arsenic.

For more information, see our publications: Ma JF, Yamaji N, Mitani N, Xu XY, Su YH, McGrath SP and Zhao FJ. 2008. Transporters of arsenite in rice and their role in arsenic accumulation in rice grain. Proceedings of the National Academy of Sciences of the United States of America 105: 9931-9935. Xu XY, McGrath SP, Meharg AA and Zhao FJ. 2008. Growing rice aerobically markedly decreases arsenic accumulation. Environmental Science and Technology 42: 5574-5579.

Growing rice aerobically markedly decreased arsenic accumulation in grain.

Mutation in the silicon transporter Lsi2 results in a massive decrease in arsenite uptake by rice.

Rice crop