ROOT PROLIFERATION, SOIL FAUNA AND PLANT NITROGEN CAPTURE FROM NUTRIENT-RICH PATCHES IN SOIL

We investigated interactions between plant roots, protozoa and nematod es after addition of patches containing inorganic or organic nitrogen in order to determine whether root proliferation could explain the cap ture of N by the plant from the patch. Decomposition of a N-15/C-13, d ual-labelled, organic patch in the absence of plant roots was also exa mined. In the decomposing patch the amounts of C-13 and N-15 remaining co-varied and both declined with time. Nematode numbers increased. Ho wever, protozoan biomass and inorganic N (NO3- and NH4+) availability did not significantly alter as decomposition of the patch progressed. Addition of inorganic N patches, as NH4NO3 solutions, to the first lat eral to emerge from the main seminal root axis of Lolium perenne L. se edlings had no effect on root growth compared with controls 16 d after addition. Protozoan biomass increased. Furthermore, log protozoan bio mass and NO3- concentrations of the growth medium were significantly ( P < 0.05) and positively related. Plant response (i.e. biomass product ion, N capture and root length) to an added organic patch was examined using five different grass species (Festuca arundinacea L., Phleum pr atense L., Poa pratensis L., Dactylis glomevata L,. and L. perenne). T otal plant biomass was significantly (P < 0.05) repressed by an organi c patch. Plant N content was reduced when an organic patch was present but N concentrations were greater. Roots were generally slow to proli ferate within the patch but there was a significant (P < 0.05) species x patch interaction for root length within the patch at harvest and i n the 2-cm band below it. However, N-15 capture by the plants was not related to mean root length duration. All species captured similar amo unts of N-15 (C. 3-5 %) at harvest as a percentage of the initial N-15 added in the organic patch. Similarly, the percentage of the total N captured from the patch was not related to the proportion of the root weight within the patch. The fraction of the captured N from the organ ic patch as a percentage of the plants' total N, however, did differ a mong species. Substantial amounts (> 62 %) of the N-15 initially added remained in the patch at harvest. Much less (c. 13-21 %) C-13 remaine d in the patch. Protozoan biomass and nematode numbers increased signi ficantly (P < 0.05) in the organic patch, although the relationship be tween the two groups was not significant. As in the inorganic N study, the relationship between log protozoan biomass and NO3- concentration s in the soil was significantly positive. We conclude that, when grown in monoculture, plants' N capture from an organic patch is not a simp le function of root proliferation. External factors, not plant attribu tes, are more important in controlling patch exploitation.