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Plant–soil feedbacks (PSFs) describe the effect of a plant species on soil properties, which affect the performance of future generations. Here we test the hypothesis that drought alters PSFs by reducing plant–microbe associations and nutrient uptake. We chose two grassland forb species, previously shown to respond differently to soil conditioning and drought, to test our hypothesis. We conditioned unsterilised grassland soil with one generation of each species, and left a third soil unconditioned. We grew a second generation consisting of each combination of plant species, soil, and drought in a full factorial design, and measured soil microbial community and nutrient availability. Scabiosa columbaria displayed negative PSF (smaller plants) under non-droughted conditions, but neutral under drought, suggesting that drought disrupts plant–soil interactions and can advantage the plant. Photosynthetic efficiency of S. columbaria was reduced under drought, but recovered on rewetting regardless of soil conditioning, indicating that PSFs do not impede resilience of this species. Sanguisorba minor showed positive PSFs (larger plants), probably due to an increase in soil N in conspecific soil, but neutral PSF under drought. PSF neutralisation appeared to occur through drought-induced change in the soil microbial community for this species. When S. minor was planted in conspecific soil, photosynthetic efficiency declined to almost zero, with no recovery following rewetting. We attributed this to increased demand for water through higher demand for nutrients with positive PSF. Here we show that drought neutralises PSFs of two grassland forbs, which could have implications for plant communities under climate change.

Plant–soil feedbacks (PSFs) describe the effect of a plant species on soil properties, which affect the performance of future generations. Here we test the hypothesis that drought alters PSFs by reducing plant–microbe associations and nutrient uptake. We chose two grassland forb species, previously shown to respond differently to soil conditioning and drought, to test our hypothesis. We conditioned unsterilised grassland soil with one generation of each species, and left a third soil unconditioned. We grew a second generation consisting of each combination of plant species, soil, and drought in a full factorial design, and measured soil microbial community and nutrient availability. Scabiosa columbaria displayed negative PSF (smaller plants) under non-droughted conditions, but neutral under drought, suggesting that drought disrupts plant–soil interactions and can advantage the plant. Photosynthetic efficiency of S. columbaria was reduced under drought, but recovered on rewetting regardless of soil conditioning, indicating that PSFs do not impede resilience of this species. Sanguisorba minor showed positive PSFs (larger plants), probably due to an increase in soil N in conspecific soil, but neutral PSF under drought. PSF neutralisation appeared to occur through drought-induced change in the soil microbial community for this species. When S. minor was planted in conspecific soil, photosynthetic efficiency declined to almost zero, with no recovery following rewetting. We attributed this to increased demand for water through higher demand for nutrients with positive PSF. Here we show that drought neutralises PSFs of two grassland forbs, which could have implications for plant communities under climate change.

Resource capture and utilisation were studied in two agroforestry systems at the International Centre for Research in Agroforestry (ICRAF) Research Station at Machakos, Kenya. The agroforestry systems examined contained two contrasting tree species, leucaena (Leucaena leucocephala (Lam.) de wit) and grevillea (Grevillea robusta), and the C3 and C4 crops, cowpea (Vigna unguiculata) and maize (Zea mays, Katumani composite). The leucaena-based trial was established in November 1989 and the trees were grown with ten maize crop rows on either side of a pruned hedgerow (HM) or unpruned tree row (LM). A sole maize control (SM) was also grown. Paired sets of treatments were irrigated to eliminate below-ground competition for water (HMI, LMI and SMI respectively). Interception of photosynthetically active radiation (PAR) by leucaena and maize was measured on a row-wise basis in all treatments at 7-10 day intervals using a sunfleck ceptometer. Sap flux was measured for the maize and both pruned and unpruned leucaena using heat balance gauges. Results are presented for the 1992 April-July rainy season. Total PAR interception was 30 % greater in LM and LMI than in the SM and SMI sole maize treatments. However, little more than 30 % of the light intercepted by the LM and LMI systems was captured by the crop component, and competition for light alone reduced maize yields by over 30 %. Total water uptake by the LM leucaena and maize comprised 60 % of the seasonal rainfall (237 mm) as compared to 30 % for sole maize. However, as for light interception, only 30 % of the water transpired in LM was used by the intercropped maize, and competition from the trees for soil water reduced maize yields at distances of over 6 m from the leucaena. The leucaena was more effective at resource capture, yet less efficient in resource utilisation since it exhibited a lower dry matter:radiation quotient and a lower transpired water:dry matter ratio than maize. Thus the leucaena in the agroforestry s

The work reported here aimed to provide a comprehensive database of core information to support the development and validation of process-based models of resource capture and growth in semi-arid overstorey agroforestry systems. Intensive field studies were carried out in Kenya over a 30 month period and the results obtained were combined with data from a previous project to produce a dataset spanning a 4.5 year period. This dataset was then used to verify output from the HyPAR model. Allometric procedures developed from the pipe model theory (Lott et al., 1998) were used to estimate tree growth non-destructively throughout the observation period. Significant differences in tree size between the sole (Td) and dispersed agroforestry (CTd) treatments were established during the first 130 days after planting, probably because of competition with the associated crops. The above-ground biomass and trunk length and taper characteristics of the CTd trees remained inferior to those of Td trees throughout the observation period, seriously undermining the economic potential of this agroforestry system. The biomass and grain yield of CTd understorey crops were similar to the corresponding sole crops during the first three seasons, but were negligible in three of the final four seasons, with maize yields reaching 50 % of the equivalent sole crop values only when seasonal rainfall was well above average. This observation suggests that water availability was the primary limitation for CTd maize during the final seasons of the trial, a conclusion supported by the superior performance of maize grown under net enclosures which simulated tree shade in the absence of below-ground competition. Cowpea and maize were grown concurrently in two seasons to examine the impact of grevillea on C4 and C3 crops with contrasting responses to shade; biomass and grain yield were less affected in cowpea than in maize. The tree canopy in the dispersed agroforestry (CTd) treatment reduced the daily mea