Nitrogen pools and turnover in arable soils under different durations of organic farming: II: Source-and-sink function of the soil microbial biomass or competition with growing plants?

The following parameters were measured on seven field plots at 3 sites whic h had been under organic farming for different periods of time: mineral nit rogen (N-min) contents, in situ net nitrogen mineralization (N-net), soil m icrobial biomass carbon (C-mic), and nitrogen (N-mic) contents, and extract able organic N contents. The measurements were conducted every three weeks from spring 1995/ 1996 to autumn 1997. The objective was to test whether, u nder organic farming: 1) temporal fluctuations of N-mic contents over the c ourse of the year are indicative for a source-and-sink function for plant-a vailable N of the soil microbial biomass, and 2) temporal variations in N-m ic content can be related with in situ N.., or plant N uptake. N-min conten ts gradually increased after ploughing in autumn until late winter. During intensive plant growth in spring, values rapidly declined. In situ N-net fl uctuated only moderately and reached high values during intensive plant gro wth (May-July) as well as after soil cultivation in autumn. The C-mic and N -mic contents generally were low in winter, increased in spring and reached maxima in late spring or summer. In spring, the increase in C-mic contents preceded the increase in N-mic contents, resulting in elevated C-mic:N-mic ratios until shooting of winter wheat. This corresponds to an uptake of av ailable soil nitrogen by the plants at the expense of soil microorganisms. The subsequent increase in N-mic contents, coinciding with high plant N upt ake rates, indicates an enhanced, plant-induced N mobilization at that time . Possible mobilization mechanisms are discussed. Soil microbial biomass ex erted a source-and-sink function for extractable organic N on some of the f ield plots. Estimates of in situ N., measurements were neither correlated s ignificantly with soil microbial biomass N, N-mic flux, N,nic turnover, nor with plant N uptake. Lower N-mic turnover rates on 41 years versus 3 years organically managed fields indicate a stabilizing effect of organic farmin g on soil microflora.