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 "&N\"$C, dual-labelled, organic patch in the absence of plant roots was also examined. In the decomposing patch the amounts of "$C and "&N remaining co-varied and both declined with time. Nematode numbers increased. However, protozoan biomass and inorganic N (NO $ − and NH % + ) availability did not significantly alter as decomposition of the patch progressed. Addition of inorganic N patches, as NH % NO $ 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 NO $ − concentrations of the growth medium were significantly (P 0n05) 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 0n05) 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 0n05) speciesipatch interaction for root length within the patch at harvest and in the 2-cm band below it. However, "&N capture by the plants was not related to mean root length duration. All species captured similar amounts of "&N (c. 3-5 %) at harvest as a percentage of the initial "&N 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 "&N initially added remained in the patch at harvest. Much less (c. 13-21 %) "$C remained in the patch. Protozoan biomass and nematode numbers increased significantly (P 0n05) 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 NO $ − 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.

An arbuscular mycorrhizal inoculum enhances root proliferation in, but not nitrogen capture from, nutrient‐rich patches in soil

“…4b). Repression of root growth in the presence of an organic patch has been demonstrated previously by Hodge et al (1998). Cui & Caldwell (1996) reported that proliferation of Agropyron desertorum roots in nutrient-rich patches decreased when colonized by AM fungi, although the difference was only weakly significant (P l 0.07).…”

“…Phosphorus contents of root and shoot tissues from the last two destructive harvests (30 and 41 d) were determined after a triple acid digestion and using the molybdenum blue method (Allen, 1974). Subsamples of the soil from the different sections of each tube were used for moisture content determinations (105mC) and for total C, N, "$C and "&N analysis (see Hodge et al, 1998).…”

“…We expected AM colonization to influence organic N capture directly, rather than simply to increase absorbing surface area, because we have already shown that root proliferation has little effect on NH % + and NO $ − capture if plants are grown in monoculture (Hodge et al, 1998) ; thus the same might be expected for fungal hyphal proliferation. Nitrogen capture, rather than phosphorus, was chosen because the ability of AM fungi to capture N directly from complex organic sources remains a matter for debate (see Smith & Read, 1997).…”

Phytotoxicity, not nitrogen immobilization, explains plant litter inhibitory effects: evidence from solid‐state 13C NMR spectroscopy

“…This has been observed in the laboratory (Hodge et al, 1998), in natural ecosystems (Meier et al, 2009), and in many farming systems (Putnam, 1994). In a recent review of agricultural applications of organic matter, Bonanomi et al (2007) reported that soil amendment with plant residues inhibited plant growth in 119 of 1728 experiments (6.9%).…”

Selection for protection in an ant–plant mutualism: host sanctions, host modularity, and the principal–agent game

“…Many genera in the Agavaceae, of which Yucca is a member, abscise flowers after damage by insect feeding or oviposition (Sutherland 1987;Becerra & Lloyd 1992;Pellmyr 1997), despite not being associated with pollinating seed predators (see also Marr & Pellmyr 2003). Similarly, non-leguminous plants, which do not associate with rhizobia, respond to nitrogen soil availability via differential root investment, which would be a preadaptation for legume host sanctions if the trait occurs in the non-rhizobia-associated ancestors of legumes (Hodge et al 1998(Hodge et al , 1999Farley & Fitter 1999;Wijesinghe et al 2001). Thus, it is modularity that provides stronger evidence of adaptation.…”