Competition between roots and soil micro-organisms for nutrients from nitrogen-rich patches of varying complexity

1 We used Lolium perenne plants grown in microcosms to investigate the responses of root demography, plant N capture, soil fauna populations and microbial community profiles to five organic patches containing the same amount of N but differing in their chemical and physical complexity and C : N ratio. All patches were dual labelled with N-15/C-13. Control patches contained the background sand : soil mix only. 2 There was rapid decomposition in, and plant N capture from, the patches of lowest C : N ratio. Early in the experiment C-13 was detected in the soil atmosphere and N-15 in the shoots. No C-13 enrichment was detected in the plant material. 3 The rate of root production was slowest in the most complex patch (L. perenne shoot material) but accelerated when patches were simpler and had lower C : N ratios. There was no difference in root mortality between treatments. 4 Nitrogen concentrations of shoots and roots and shoot biomass were greater in the N-containing patches than controls, except for the most complex patch, while root biomass did not differ with treatments. 5 Total plant N capture was 45-54% of that initially added in patches that had a C : N ratio < 4. However, in the most complex patch (C : N ratio c. 21 : 1) plants captured only 11% of the N added. 6 Biomass of microbial-feeding protozoa was related to soil NO3--N concentration in the patch but not to numbers of microbial-feeding nematodes. Patches of greater complexity increased the metabolic diversity of the microbial community (i.e. the number of substrates used in a Biolog GN plate) and altered the pattern of substrate utilization. 7 At harvest, the amount of patch-derived N estimated to be in the microbial biomass was much smaller (i.e. 7-13%) than in the plant tissues. Thus, plants were highly effective competitors with micro-organisms when capturing N supplied in patches with a low C : N ratio.