Assessing and minimising P leaching in Chinese agricultural soils.
Rothamsted workers: Phil Brookes.
Collaborators: Dr. Qimei Lin, Agricultural University of China; Professor Jinshui Wu, Institute of Sub-Tropical agriculture, Changsha, China; Professor Jialong Lu, Mr. Xueyun Yang, Nortth-West University, China.
Funding: Royal Society, British Council, BBSRC, Academica Sinica, Chinese Academy of Science.
Background to Project.
Until fairly recently, it was considered that the leaching of P from agricultural soils to surface and ground waters was negligible in most situations, exceptions including very sandy soils with high P loads. However, results from the Broadbalk Continuous Wheat Experiment at Rothamsted Research showed that this assumption was wrong. We found more than 3 mg total P l-1 in the drains of Broadbalk, at 65 cm below the soil surface, with drainage water P concentrations being linearly related to soil Olsen P from about 40 mg P kg-1 soil, until the maximum soil P concentrations were reached.
Although P is fixed very strongly onto the soil colloids, in all soil tested, there was a soil P concentration, well below P saturation, (around 40 mg Olsen P kg-1 soil for Broadbalk - see above) where there was a marked increase in the solubilisation of P in drainage waters. We found that similar results were obtained in laboratory extractions of soils using 0.01 M CaCl2. This suggests that CaCl2 may serve as an 'indicator' of soil P concentrations where leaching losses in drainage water may occur. These P concentrations in drainage water are not of economic significance to the farmer. However, since eutrophication may commence as low as 0.01 mg P l-1 they are certainly environmentally important and need to be both understood and quantified. In China thereis often a huge overuse of P
Approach
There are very few field experiments with suitable land drainage, such as Broadbalk. We will use relationships between CaCl2-P and Olsen P to investigate different Change-Points in Chinese agricultural soils. Other work will use lysimeters to check the validity of this approach. The aim is to get more data on the P status of Chinese agricultural soils and the risk of P loss from soil to water.