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

摘要

The following parameters were measured on seven field plots at 3 sites which had been under organic farming for different periods of time: mineral nitrogen (N-min) contents, in situ net nitrogen mineralization (N-net), soil microbial biomass carbon (C-mic), and nitrogen (N-mic) contents, and extractable organic N contents. The measurements were conducted every three weeks from spring 1995/ 1996 to autumn 1997. The objective was to test whether, under organic farming: 1) temporal fluctuations of N-mic contents over the course of the year are indicative for a source-and-sink function for plant-available N of the soil microbial biomass, and 2) temporal variations in N-mic content can be related with in situ N.., or plant N uptake. N-min contents gradually increased after ploughing in autumn until late winter. During intensive plant growth in spring, values rapidly declined. In situ N-net fluctuated only moderately and reached high values during intensive plant growth (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 available soil nitrogen by the plants at the expense of soil microorganisms. The subsequent increase in N-mic contents, coinciding with high plant N uptake rates, indicates an enhanced, plant-induced N mobilization at that time. Possible mobilization mechanisms are discussed. Soil microbial biomass exerted a source-and-sink function for extractable organic N on some of the field plots. Estimates of in situ N., measurements were neither correlated significantly 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 farming on soil microflora.