Plant uptake of P from OA could
Plant uptake of P from OA could also be affected by arbuscular mycorrhizal fungi (AMF). Approximately 80% of terrestrial plant species form symbiotic associations with AMF (Smith and Read, 2010) which can enhance plant P uptake (Facelli and Facelli, 2002, Pearson and Jakobsen, 1993, Tibbett, 2000). It is well established that at high levels of inorganic P fertiliser addition the percentage of roots colonised by AMF decreases (Abbott et al., 1984, Bolan et al., 1984, Treseder, 2004). However, it is unclear whether or not addition of P in the form of OA has a similar effect. Cavagnaro (2015) found that compost generally has a neutral or positive effect on arbuscular mycorrhizal (AM) colonisation; however, as compost application increases AM colonisation may decrease (Cavagnaro, 2014). If OA provide a more sustained release of plant-available P rather than the immediate increase provided by soluble fertilisers they could supply P to crops throughout the growing season without adversely affecting the formation of AM. This could be beneficial to crops such as wheat which require P for the entire growing season (Römer and Schilling, 1986).
The results of a study investigating the use of a range of OA as P amendments are reported here. There were five main research questions addressed in this study:
To answer these questions an incubation experiment and a plant growth experiment were conducted. We hypothesised that:
Discussion OA with high C:P ratios stimulate the microbial biomass, which immobilise P (Malik et al., 2013, Takeda et al., 2009). However, while the C:P ratio of the PIG-STR was about three-fold larger than that of the CHK-STR and CHK-SD, the PIG-STR resulted in greater plant P uptake. This suggests that P immobilisation was not a dominant mechanism in this plant growth study, and is further supported by our incubation results demonstrating no effect of OA on the size of the microbial AR-C155858 (MBC). It has been suggested that the critical P concentration needed in an amendment to avoid the negative effects of P immobilisation is between 2 and 3 g P kg−1 (Nziguheba et al., 1998, Six et al., 2014). In our study, apart from the COMP (2.5 g P kg−1), all OA had total P concentrations much higher than this (Table 1). However, in addition to the P concentration, the forms of C in OA can influence microbial biomass and hence immobilisation of P. The OA varied in their proportions of different C species as detected by NMR spectroscopy. The CHK-STR and CHK-SD samples were quite similar to each other, despite having different bedding materials, indicating that straw and sawdust are chemically similar in terms of C speciation. This has been found previously, with both straw and sawdust containing similar amounts of cellulose (70–75% of weight) and lignin (15–20% of weight; Lv et al., 2010). The COMP sample had the highest alkyl C:O-alkyl C ratio, indicating that much of the C present in the sample had already been degraded by microbes in the composting process (Baldock et al., 1997), which is why the C in a compost is often more stable than in its feedstock (Bernal et al., 1998). Skene et al. (1996) reported the alkyl:O-alkyl ratio of a straw based feedstock increasing from 0.2 to 0.4 after 168 days of composting. In our case, the compost had an alkyl:o-alkyl ratio of 0.6. This could explain why, while COMP had the lowest P concentration, and hence amendment with COMP induced the greatest addition of C, it did not result in a higher MBC in the incubation experiment compared with the other OA. The PIG-STR sample had a lower alkyl C:O-alkyl C ratio compared to the CHK-STR and CHK-SD samples, indicating that PIG-STR had been less degraded by microbes (Baldock et al., 1997). This is likely as the PIG-STR was collected directly from the farm whereas the CHK-STR and CHK-SD were collected from a waste collection/redistribution centre and therefore are likely to be older and more decomposed. This would suggest that the C in the PIG-STR would be more readily available to soil microbial biomass compared with the C in CHK-STR or CHK-SD. Therefore, the lack of an increase in MBC in the PIG-STR treatment in the incubation experiment (compared to the chicken litters) cannot be explained by the forms of C in the PIG-STR and is likely because the quantity of C added in all of the litter treatments was relatively small.