The dynamics of soil, fertilizer and biologically-fixed nitrogen in soil-plant systems

© 1997 Dr. Phillip Michael ChalkNitrogen is a constituent of protein and hence is a key element in plant, animal and human nutrition. Low soil N concentrations and attendant N deficiencies are commonplace in world agriculture. Low protein in much of the Australian wheat crop is symptomatic of the overall low N status of our arable soils, a result of either intrinsic soil infertility or of poor soil management under intensive cereal culture. The most intensive exploitation of natural soil fertility over the past century has been under continuous cereal-bare fallow rotations on the cracking clays of the Victorian Wimmera, the Riverine plains of south-eastern Australia, and large areas of northern New South Wales, south-eastern and central Queensland. Historically, the n reserves of the Red brown Earths of New South Wales and South Australia were similarly exploited. The legume-based pasture ley has been a traditional and important component in the attempt to maintain soil fertility in the agriculture of temperate Australia, but the intensification of cropping in response to economic factors has reduced the benefit of the pasture phase. Consequently, the use of nitrogen fertilizers to alleviate acute N deficiency in our arable soils continues to increase. Soil nitrogen presents a complex chemistry because of the involvement of several discrete phases (organic, inorganic, gaseous) in a multitude of processes which can occur simultaneously, and which can be either chemically, biologically or physically-mediated. Nitrogen is a dynamic element, subject to losses via several pathways, given suitable agronomic, edaphic and environmental conditions. Thus it has often been reported that the recovery of fertilizer N by crops is low, and major research programmes have been directed towards improving the efficiency of N fertilizer use. On the other hand, N can be readily gained by the soil-plant system through the agency of biological N2 fixation, and a great deal of effort has similarly been devoted to identifying constraints and fording ways to maximize this input. The measurement of processes which transfer, subtract or add N to the soil-plant system has been a formidable scientific challenge. Identification and quantification of such processes are required before their role in the system N-balance can be properly assessed, and strategies devised to manage the nutrient more efficiently in agriculture. Much of the work collated in this dissertation was based on this philosophy. The processes which are generally considered to be the most difficult to measure in the field are denitrification, NH3 volatilization and biological N2 fixation. However, on closer examination it becomes apparent that field-estimation of rates of net N mineralization or immobilization and nitrification is equally difficult. (From Introduction