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  • Many processes can lead to

    2019-11-11

    Many processes can lead to changes in soil P pools over time. Plant-available P (when measured as DGT P) decreased over time for the INORG-P treatment (Fig. 1a). Orthophosphate can be stabilised with cations such as calcium to produce insoluble phosphates, reducing plant-available P (Bünemann et al., 2006, Holford, 1997, Bolland and Gilkes, 1998). Furthermore, it is possible that the addition of phosphoric TASIN-1 receptor (the material used in the INORG-P treatment) may have led to solubilisation of residual P in the soil. Changes in soil pH lead to carbonate dissolution and P solubilisation (Jalali and Zinli, 2011, Alt et al., 2013). However, in this experiment only a small amount of phosphoric acid was added to soil and it did not result in changes to the overall soil pH. Despite this, it is possible that some localised, transient changes in pH occurred. However, there is no research that suggests application of such small quantities of acid to a large amount of soil (i.e. 5 ml of acid solution to 1 kg soil) will have any significant effect on soil P availability. Moreover, given that the soil used in this experiment had a low calcium carbonate content (<0.2%) we believe that any effect will be negligible. While there was a decrease in plant-available P over time for the INORG-P treatment, this was not observed in the litter treatments. This could indicate that mineralisation and solubilisation were occurring at a similar rate to stabilisation in these treatments. The soil microbial biomass plays an important role in solubilisation and mineralisation of P (Hinsinger, 2001, Richardson, 2001) resulting in more P available to plants. While there were no differences in MBC among treatments in the incubation experiment, differences were observed in the plant growth experiment, with greater MBC values in treatments with a larger root biomass. Plants alter the soil microbial community around them which can result in a higher proportion of microbes that release P to the soil solution (Li et al., 2014). Plants with greater root biomass can supply more C to soil microbes from root turnover and root exudates. Additionally, particular groups of soil microbes, such as AMF, may have a disproportionate influence on plant P uptake. Arbuscular mycorrhizal fungi colonisation generally decreases with increasing available P in soil (Bolan et al., 1984, Treseder, 2004, Cavagnaro, 2014). Similarly, in the current study, colonisation (measured as % colonisation and as infected root length) was lowest in the INORG-P treatment. At this level of plant-available P colonisation was clearly suppressed by the plant. Colonisation can also be low when there are very low levels of plant-available P in soil (Bolan et al., 1984) as was observed in the CONT treatment. However, the COMP treatment had a significantly greater percent colonisation compared with the CONT treatment. This is unexpected, as the COMP treatment contained similar plant-available P as the CONT treatment, as determined by soil analysis in the incubation experiment (Colwell P and DGT P). While it is possible that the compost contained AMF spores, it is unlikely that this is the sole cause of the high level of colonisation, given the small amount of compost added to each pot. Similarly, Duong et al. (2012) found that addition of compost increased AM colonisation compared with a control. Despite the high colonisation rate in the COMP treatment this did not seem to benefit the plant in terms of P uptake, with plant P uptake in the COMP treatment similar to the CONT treatment. When colonisation was measured as infected root length, all of the OA treatments had more infected root length compared with the CONT treatment, with the CHK-SD treatment having significantly more infected root length compared with all other treatments except the PIG-STR treatment. The CHK-SD had a lower proportion of orthophosphate P and bicarbonate-extractable P compared to CHK-STR and PIG-STR, which could account for the higher rates of root length colonised. The higher infected root length in CHK-SD compared with CHK-STR could explain how plants in this treatment took up similar amounts of P than those in the CHK-STR treatment despite the evidence that CHK-STR should have higher P availability.