Ground Cover Supplement : GC Supplement - Soil Biology 2005
7 GROUND COVER SOIL BIOLOGY BRAD COLLIS oxygen and the dependence on moisture to reduce oxy- gen levels in soil, highest rates of non-symbiotic nitro- gen fixation in dryland agriculture soils mainly occur at moisture contents close to or higher than field capacity. In clay soils, micro-sites of low oxygen availability are present at moisture levels less than field capacity and still support non-symbiotic nitrogen fixation. Data collected showed that summer-dominant rain- fall zones (northern NSW) potentially supported high levels of nitrogen-fixing bacteria (25 to 35 kilograms of nitrogen per hectare from January to June). Soil mois- ture was not a limiting factor in this region at this time. In contrast, moisture availability seemed to be the key regulating factor for non-symbiotic nitrogen fixa- tion during this period in southern and western agricul- tural zones (less than 10 kg/ha from January to June). These areas have a Mediterranean-type climate with hot dry summers and cool wet winters. Hence in sum- mer after harvest, soil moisture not only limited general microbial activity but also the use of residual carbon in crop residues for nitrogen-fixing activity. With the onset of winter rains, potential nitrogen-fixing bacteria activity increased in the Mediterranean-type regions but temper- atures were not ideal and so activity was still limited. Cereal crop yields in the low-rainfall Mallee region of SA and the WA wheat-belt, range from one to two- tonnes a hectare only, so the levels of available carbon from crop residues could also limit non-symbiotic nitro- gen fixation in this zone. Nitrogen budget calculations based on 17 years of data for the long-term farming system trial at Avon, SA, indicated that up to 20kg of nitrogen/ha/year was attrib- uted to nitrogen-fixing bacteria. Integration of data using spatial analysis tools can help to estimate the potential for non-symbiotic nitrogen fixation, and key regulating factors in cropping regions. Such information is useful for agronomists and exten- sion officers to help explain changes in nitrogen status within paddocks or within specific farming systems and help in providing more accurate advice on nitrogen fertiliser requirements, particularly in low-input farming systems. It should also be useful for researchers to select the most potentially responsive areas for non-symbiotic nitrogen fixation studies. For more information: Dr Gupta Vadakattu, firstname.lastname@example.org, Dr Margaret Roper, CSIRO Plant Industry, email@example.com factor for increasing populations of nitrogen-fixing bacteria is the availability of crop residues with wide C:N ratios. Nitrogenase (nitrogen-fixing) activity consumes large amounts of microbial energy and therefore nitro- gen-fixing bacteria will most likely use available nitro- gen sources before resorting to fixation. Also, due to the low levels of biologically-available organic carbon in Australian soils, significant amounts of non-symbiotic nitrogen fixation can only occur near decomposing crop residues and in the rhizosphere (root zone). Therefore stubble retention is essential to gain maxi- mum benefits from non-symbiotic nitrogen fixation. Cellulose and hemicellulose (major components of cere- al stubble) and their decomposition products can serve as the energy source for all nitrogen-fixing bacteria. The level of carbon decomposition and therefore rates of non-symbiotic nitrogen fixation can also be determined by the degree of tillage. Other significant factors that can affect this are soil temperature, avail- ability of soil water, soil texture and pH. Due to the sensitivity of the nitrogenase enzyme to 'Land use appeared to have the greatest impact on the size of these bacteria populations'
GC Supplement - Value Chain
GC Supplement - Grain Storage 2005