In situ changes in gross N transformations in bare soil after addition of straw

In situ N-15 techniques were used to quantify gross mineralisation, immobil isation and nitrification of N in bare soil nonamended and after incorporat ion of wheat straw (8 t ha(-1)). Experimental plots were confined within op en-ended cylinders inserted into the tilled layer to 20 cm depth of a loamy soil (Typic Hapludalf). Measurements of gross N fluxes and C mineralisatio n were made during the year following incorporation of residues (September 1993-October 1994). At each date of measurement, inorganic N was added to s oil by multiple injections of (NH4NO3)-N-15-N-15 (atom % excess = 50.0) to a soil depth of 17.5 cm. The amount of NH4+ -N, NO3--N, organic N and their N-15 isotopic composition were determined 30 min after N-15 injections (t( 0)), and 2-3 d after in situ incubation (t(1)). N transformation rates were estimated using a numerical model, Fluaz. In the nonamended soil, gross mi neralisation in the 0-20 cm layer varied between 0.23 mg N kg(-1) soil d(-1 ) in December and 0.81 mg N kg(-1) soild(-1) in July. Immobilisation rate v aried similarly and was about one-third of gross mineralisation. Gross flux es corrected for the effects of soil temperature and water content, and exp ressed as a function of normalised time (day at 10 degrees C and optimal wa ter potential) varied much less. The mean rate of gross mineralisation was 0.42 mg N kg(-1) soil normalised d(-1), and for immobilisation it was 0.13 mg N kg(-1) soil normalised d(-1), resulting in net mineralisation of 0.29 mg N kg(-1) soil normalised d(-1). Gross mineralisation and immobilisation were both increased by straw addition, with rates of 0.99 and 1.98 mg N kg( -1) soild(-1) respectively, observed for the first measurement in September , 7 d after incorporation. Immobilisation and mineralisation decreased stea dily with time. One year after straw addition, gross immobilisation and min eralisation rates were similar to those measured in nonamended soil. This w as consistent with the changes in rate of straw-C mineralisation and with t he depletion of the coarse fraction in soil, determined in parallel. (C) 19 98 Elsevier Science Ltd. All rights reserved.