Root proliferation, soil fauna and plant nitrogen capture from nutrient-rich patches in soil

Abstract

We investigated interactions between plant roots, protozoa and nematodes after addition of patches containing inorganic or organic nitrogen in order to determine whether root proliferation could explain the capture of N by the plant from the patch. Decomposition of a ¹⁵N/¹³C, dual-labelled, organic patch in the absence of plant roots was also examined. In the decomposing patch the amounts of ¹³C and ¹⁵N remaining co-varied and both declined with time. Nematode numbers increased. However, 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 lateral to emerge from the main seminal root axis of Lolium perenne L. seedlings had no effect on root growth compared with controls 16 d after addition. Protozoan biomass increased. Furthermore, log protozoan biomass and NO₃⁻ concentrations of the growth medium were significantly (P<0·05) and positively related. Plant response (i.e. biomass production, N capture and root length) to an added organic patch was examined using five different grass species (Festuca arundinacea L., Phleum pratense L., Poa pratensis L., Dactylis glomerata L. and L. perenne). Total plant biomass was significantly (P<0·05) repressed by an organic patch. Plant N content was reduced when an organic patch was present but N concentrations were greater. Roots were generally slow to proliferate within the patch but there was a significant (P<0·05) species×patch interaction for root length within the patch at harvest and in the 2-cm band below it. However, ¹⁵N capture by the plants was not related to mean root length duration. All species captured similar amounts of ¹⁵N (c. 3–5%) at harvest as a percentage of the initial ¹⁵N 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 organic patch as a percentage of the plants' total N, however, did differ among species. Substantial amounts (>62%) of the ¹⁵N initially added remained in the patch at harvest. Much less (c. 13–21%) ¹³C remained in the patch. Protozoan biomass and nematode numbers increased significantly (P<0·05) in the organic patch, although the relationship between the two groups was not significant. As in the inorganic N study, the relationship between log protozoan biomass and NO₃⁻ concentrations in the soil was significantly positive. We conclude that, when grown in monoculture, plants' N capture from an organic patch is not a simple function of root proliferation. External factors, not plant attributes, are more important in controlling patch exploitation.