TPEN 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 TPEN (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.