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Plants significantly affect rates of carbon (C) turnover in soils, both because they are sources of carbon through exudation in the rhizosphere and litter-fall, and because rhizosphere microbes stimulated by roots also metabolize native soil carbon. Different plant species affect these components of soil carbon turnover in different ways, but the quantitative information on this is lacking for different ecosystems and soil-plant combinations. To compare the effects of grassland and forest plant species on the components of rhizosphere respiration in different soils, we grew ryegrass ( Lolium perenne) and radiata pine ( Pinus radiata D. Don) in two silt loam soils in pots in a glasshouse, and in seven samplings over 45 weeks measured total ( Rtotal), root ( Rroot) and root-free soil respiration ( Rrfs), the latter from respiration in unplanted controls. We calculated rhizosphere respiration ( Rrhizo), defined here as the net of that fuelled by native soil C and root-derived C, from Rtotal less Rroot+ Rrfs. We also measured plant growth and total, water-soluble and microbial biomass C in the soils at each sampling. Results showed that Rrfs decreased over the experimental period in both soils. Under ryegrass, Rroot, Rrhizo and Rtotal increased up to 14 weeks after planting and then stabilized, whereas under radiata pine, they continued to increase throughout the experiment. By the end of the experiment, the Rroot, Rrhizo and Rrfs components accounted for 49–58, 31–50 and 1–11% of soil total respiration under ryegrass, respectively, and 43–66, 29–53 and 1–5% under radiata pine. The greater Rroot, Rrhizo and Rtotal values under radiata pine were related to greater root biomass and root-derived organic C, and enhanced microbial mineralization of native soil organic C. [ABSTRACT FROM AUTHOR]

Vegetative conversion from grass to forest may influence soil nutrient dynamics and availability. A short-term (40 weeks) glasshouse experiment was carried out to investigate the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil phosphorus (P) availability in 15 grassland soils collected across New Zealand using 33P isotopic exchange kinetics (IEK) and chemical extraction methods. Results from this study showed that radiata pine took up more P (4.5–33.5 mg P pot-1) than ryegrass (1.1–15.6 mg pot-1) from the soil except in the Temuka soil in which the level of available P (e.g., E 1min P i, bicarbonate extractable P i) was very high. Radiata pine tended to be better able to access different forms of soil P, compared with ryegrass. There were no significant differences in the level of water soluble P (Cp, intensity factor) between soils under ryegrass and radiata pine, but the levels of Cp were generally lower compared with original soils due to plant uptake. The growth of both ryegrass and radiata pine resulted in the redistribution of soil P from the slowly exchangeable P i pool (E > 10m P i, reduced by 31.8% on the average) to the rapidly exchangeable P i (E 1min-1d P i, E 1d-10m P i) pools in most soils. The values of R/r1 (the capacity factor) were also generally greater in most soils under radiata pine compared with ryegrass. Specific P mineralisation rates were significantly greater for soils under radiata pine (8.4–21.9%) compared with ryegrass (0.5–10.8%), indicating that the growth of radiata pine enhanced mineralisation of soil organic P. This may partly be ascribed to greater root phosphatase activity for radiata pine than for ryegrass. Plant species × soil type interactions for most soil variables measured indicate that the impacts of plant species on soil P dynamics was strongly influenced by soil properties. [ABSTRACT FROM AUTHOR]