Soybean nodulation and nitrogen fixation in response to soil compaction and surface straw mulching

Soil compaction affects nodulation and nitrogen fixation of soybean (Glycine max (L.) Merr.). The effects of compaction can be mediated by straw mulch applied at soil surface. To test these hypotheses a field experiment was carried out in 2008 on a Haplic Luvisol (south-eastern Poland). We grew soybean in treatments with different soil compaction levels: non-compacted (NC), moderately compacted (MC) (3 tractor passes) and strongly compacted soil (SC) (5 tractor passes). Each compaction level had treatments without and with surface chopped straw mulch at the rate of 0.5kgm⁻². Nodulation was evaluated by measurements of nodule-size distribution, number, weight, and diameter of nodules and symbiotic nitrogen fixation – by measurement of nitrogenase activity, nitrogen content in leaves, protein content in seeds and seed yield. Soil measurements included bulk density and nitrogen content in the form of N-NO₃ and N-NH₄. In general, nitrogenase activity decreased with increasing soil compaction level. At every compaction level, the nitrogenase activity was higher in the mulched when compared to the unmulched soil. Soil compaction and mulching enhanced the contribution of large nodules, those greater than 0.41cm and dry weight of individual nodules. Total nodule number and weight decreased in sequence: moderately, strongly and non-compacted soil. Mulch influenced nitrogenase activity and nodulation to a higher extent than compaction. An optimum soybean response in terms of seed and protein yields was observed under mulched MC and the worst – under unmulched SC. Both N content in leaves and nitrate content in soil were lower in mulched than in unmulched soil, irrespective of compaction. Bulk density and N-NO₃ increased with soil compaction level and N-NH₄ remained nearly unchanged. Therefore, our results supported the hypotheses that soil compaction affects nodulation and nitrogen fixation, and that compaction can be mediated by straw mulch. Further studies are needed to assess variability related with different soil and weather conditions.