Nitrogen studies on black soils form the Darling Downs, Queensland. I. Seasonal variations in moisture and mineral nitrogen fractions

Abstract

A study of the equilibrium levels of moisture, nitrate nitrogen, and ammonium nitrogen is reported on two representative black soils (developed from basaltic colluvium and alluvium respectively) from the Darling Downs for the years 1952-1954 inclusive. Both soils were under native grassland in 1952; a portion of the sampling area at each site was cultivated early in 1953 and left fallow for the rest of the sampling programme. The moisture contents of surface (0-6 in.) grassland and fallow plots varied directly with rainfall. Subsoil under grass showed low moisture contents which increased only after heavy rain but decreased thereafter, probably owing to transpiration. Under fallow, subsoil moisture was higher and showed significant fluctuations, suggesting that losses occurred by slow liquid flow or vapour movement a t moisture contents between field capacity and the wilting point. Ammonia-nitrogen contents at the surface were lower than for comparable soils in temperate regions and showed a significant decrease with depth; values for the corresponding fallow plots were similar in amount and varied similarly with depth. Surface soils showed minor fluctuations with season, being higher in summer. Under incubation conditions at 25°C, ammonia nitrogen rarely fell below 1.0 mg/kg oven-dry soil, which suggests that this is the minimum threshold concentration of ammonia nitrogen in these soils for the nitrification process. Nitrate nitrogen showed a significant rise in summer on the colluvial basaltic soil under grass. The soil developed on basaltic alluvium showed no such seasonal trend and contained uniformly low amounts of this fraction. Cultivation resulted in a sharp rise in nitrate nitrogen in both soils, the wetter alluvial soil showing a longer time lag. There was some evidence of nitrification in the 6-12 in. depth a t one site, but below this rises in nitrate nitrogen, which were correlated with rainfall and increases in moisture content, could be accounted for by leaching. Incubation tests gave rise to highly variable nitrate nitrogen contents and such tests were found unreliable in these soils, probably owing to imperfect aeration of the laboratory sample. Analyses for total nitrogen of several alluvial black soils from the same area showed a progressive decline in this constituent with increasing periods of cultivation; in the first 26 years of cultivation about 0.8 per cent. of the total nitrogen is lost per annum. This value agrees with the nitrate nitrogen accession in fallow subsoil, suggesting that leaching of nitrate formed at the surface is a major factor determining losses in total nitrogen. The implications of these findings are discussed.