Factors influencing nitrogen mineralisation in Taita hill soil, a central yellow-brown earth, under grazed pasture

Abstract

Mineralisation of soil organic N in Taita hill soil sampled from four sites under grazed pasture that had been fertilised annually with molybdic superphosphate and lime was studied by laboratory techniques. Sieving of the soil, holding at 24°c for a few hours, and brief incubation after freezing and thawing all gave small increases in mineral-N (Min-N) content. Over four seasons, the production of Min-N by 0-for 28 days averaged 180 and 140 ppm at 60% water-holding capacity and field-moisture content respectively; the corresponding values for 4-8 cm depth samples were 60 and 45 ppm respectively. Considerable variation between and within sites occurred; seasonal variations were also appreciable with field-moist samples. The 0-4 cm depth samples mineralised proportionately more of their total N than did the 4-8 cm depth samples; thev also tended to produce proportionately more NO3-N. Generally, the conversion of NH4-N to NO3-N was rapid at 24°c, but it decreased with soil depth and with decreasing incubation temperature. Temperature coefficients (Q10) for N mineralisation over the 5°c to 24°c range averaged about 2.5 for all samples. Results are considered in relation to a fertiliser N trial conducted immediately adjacent to these sites; adequate mineralisation of soil N for plant growth was indicated. The limitations of the incubation technique for generally assessing the availability of N in hill soils are, however, recognised and briefly discussed. © 1977 Taylor & Francis Group, LLC.