managing soil organic matter - Grains Research & Development ...

33MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEavailability of the majority of inorganic fertilisersapplied to soil, which are transformed into nitrateby soil microbes before being taken up by plants.This includes urea which is either decomposedby enzymes or chemically hydrolyzed to produceammonia and carbon dioxide. The ammonia isthen converted by microbes into ammonium andsubsequently converted into nitrate by specialistmicroorganisms through a process known asnitrification.In Australia, inorganic mineral fertilisers oftenmake up as little as 20 per cent of crop uptakedue to relatively low fertiliser applications and poornitrogen use efficiency. Biological processes supplythe remainder and in some cases contribute up to80 per cent of crop nitrogen uptake (Angus 2001).Although direct uptake of ammonium fertilisersby plants can occur most nitrogen fertilisersapplied in an ammonium (NH 4 +) form areconverted to nitrate (NO 3 -) by the soil microbesand are then taken-up by plants in this form.Inorganic nitrogen moves readily in soil and isrequired in relatively large amounts at critical stages incrop growth such as terminal spikelet, which occursabout eight weeks after sowing, and during grain fill.In wheat, nitrogen deficiency early in the season limitstiller formation and spikelet and floret number, whichin turn reduces yield potential. Later in the seasonnitrogen deficiency can result in smaller or fewer grainand where sufficient moisture during grain filling inlower grain protein.Nitrogen cyclingSoil nitrogen is primarily determined via biologicalprocesses, which are influenced by rate limitingfactors such as soil pH, tillage, soil moisture andtemperature. Ammonium released from organicmatter mineralised by soil microbes determinesthe supply (rate and amount) of inorganic nitrogen.The rate at which nitrogen is immobilised withinsoil microbes and converted to nitrate is directlyproportional to microbial demands for nitrogen(Murphy et al. 2003) and determines the net amount(or surplus) of soil nitrogen that becomes available forplant uptake. While both plants and microorganismscan use ammonium a large proportion of it isconverted into nitrate. Once dissolved in solution,nitrate is more readily taken-up by plants, but is alsoeasily leached (see Figure 4.1).Plant-available nitrogen originates from fertiliserinput, nitrogen fixation and mineralisation of organicmatter. The fate of mineral nitrogen within the profileis the result of immobilisation, plant uptake, leachingand gaseous losses.