Seasonal distribution of mineral nitrogen with particular reference to leaching

Abstract

Field estimates of seasonal changes in mineral nitrogen contents of nitrogen fertilized sandy soils were made at Toodyay, Meckering, Tammin, Merredin and Bodallin in the Western Australian wheatbelt during 1970. A model was used to compute water balances at each site and estimates were made of mineral nitrogen ( NH4+ and NO3-) in the soils and nitrogen in the crop. Leaching, as indicated by the distribution of ammonium and nitrate with depth in the soils, was considered in relation to the net infiltration of rainfall and drainage from the 90 cm depth of soil. Leaching of nitrate was responsible for loss of mineral nitrogen from sowing to the first sampling at Toodyay. It was unlikely that nitrate was leached below 90 cm at Meckering or Tammin, and drainage at Merredin and Bodallin was negligible. There was no drainage or net infiltration of rainfall at any site after the first sampling, between June 30 and August 11. The decreases in mineral nitrogen in the soils was about equal to the plant uptake of nitrogen in all situations where there was no leaching, except at Merredin. At that site, during the period between sowing and the second sampling (August 27), there were unexplained decreases in soil mineral nitrogen. The results show that water balance studies can give reasonable understanding of the leaching and redistribution of nitrate and ammonium in sandy soils, and hence may be used to predict for different seasonal conditions.