Multi-drivers of soil development

Soil P fractionation

Differences in soil P pools between three vegetation stands could be a combined effect of landscape hydrological gradients and external nutrient input by bird guano. Differences in total P concentration of about 400 mg kg-1 _{in the surface soil and about 200 mg kg}-1 _{in the top two horizons between three}

stands (Table 6.2, Figure 6.6), were much larger than could be accounted for by differences in leaching rates, based on losses of approximate 0.01% of total P yr-1 _{in the West Coast, New Zealand (Eger et al.,}

125 2011). It is possible there are differences in the total soil P pool that are independent of vegetation types. The present vegetation stands became established on the most suitable sections of this alluvial fan overlying sand plain, with the flax stand locates on the ending edge of the fan, while grassland and palm stand locate further inland (away shoreline) toward the alluvial source (Figure 1.3 in Chapter 1). Flax and palm vegetation are likely to be more capable than grassland of intercepting sediment flow. Associated with the alluvial fan formation, landscape pattern and hydrological gradients (Schaetzl and Thompson, 2015), the flax stand may have accumulated most sediment, followed by the palm stand, and then the grassland. The flax stand, being located at the lower end of the gradient receiving most sediments, may have had higher total soil P (Smeck, 1985).

Another factor to consider is that, both flax and palm stands would be expected to receive substantial amounts of bird guano during the flowering period. Bird guano usually contains high concentrations of N and P (Marion et al., 1994); the guano nutrient contents would be expected to be similar between flax- and palm-feeding frugivorous birds (Emerson & Roark, 2007). Although both plants can act like funnels, flax stands are denser and would tend to accumulate and concentrate more nutrients. This may explain why the flax stand had higher total soil P. Additionally, inputs of N from bird guano could also have contributed to higher mineral N status in the flax and palm stands.

However, differences in the proportional importance of individual P fractions of total P could also be attributed to a vegetation effect. It has been suggested that the dynamics of soil organic P is mainly driven by biological processes and changes related to vegetation development (Brandtberg et al., 2010; De Schrijver et al., 2012; Zhou et al., 2013). Vegetation development promotes soil weathering via organic acid production from plant litter decomposition and root exudation (Binkley & Giardina, 1998; Lambers et al., 2009). For example, the proportional importance of the organic-P fraction of total-P was evident in the flax and palm stands in the present study (Figure 6.3, 6.7). Since, palm and flax litters contain a high proportion of fibre (low quality), which is recalcitrant for decomposition (see Chapter 3); their root systems would be expected to play a more important role in promoting soil P dynamics and related weathering processes. Part of the mobile P that is released via guano sources and soil weathering might be fixed by the organic compounds released from root exudates or related to root turnover. Positive correlations between soil organic P and soil organic carbon content has also been found in earlier studies, for example by Brandtberg et al. (2010) and Hou et al. (2014). This is particularly evident in the profile soils of the present study, reflecting the deeper root systems of palm and flax. Differences in the proportional importance of soil organic P between palm and flax profiles could be explained by their different root morphologies (primary tap root system of palm versus extensive fibrous root system of flax) (Hinsinger, 2001; Lambers et al., 2006). However, a contrast view was given in Talkner et al. (2009), who they indicated that tree species only significantly impact the P turnover in the organic layer, while dynamics of P in mineral soil were mainly controlled by original soil

properties, in particular clay content. Detailed study of the differe nt effects of palm and flax systems on soil properties is needed.

Less soil weathering effects would have been promoted under grassland, with a lower proportion of organic-P, but a higher proportion of primary apatite P and acid-soluble P in the top two soil horizons. Soil secondary mineral P and occluded P fractions also related to the degree of soil weathering, were proportionally less important in the flax stand soil (Figure 6.7). Two possible causes might explain this. Firstly, flax rhizospheres could be more efficient in transformation of released-P into organic forms, rather than allowing it to be bound with Fe/Al minerals or else being occluded; even though transported sediments also bring about Fe and Al mineral accumulation. Slightly higher soil pH in the flax profile might also facilitate this transformation (Figure 6.5); it has been suggested that soil mobile P tends to bind or precipitate with Fe/Al minerals when soil pH is lower than 5.5 (Brady & Weil, 2008). A second possible explanation for less secondary mineral P and occluded P beneath flax is that accumulated alluvium sediments might have also contained soil organic matter that contributed to the transformation of soil released-P into organic P. This fits with the proposed sediment accumulation pattern between different stands, as discussed above. This type of competition of mobile P into different P fractions is known to be intensive within the P cycle (Tiessen et al., 1984). In addition, an argument against inherent differences between stands prior to vegetation establishment is provided by evidence from deeper soil horizons. Concentrations of primary apatite P and its proportional importance in the lower C horizons were getting close between three profiles, indicating a similar age (or starting point) for the flax, palm and grassland soils (Figure 6.6, 6.7).

Soil Al and Fe minerals

In addition to containing a larger soil P pool, the flax stand soil had significantly higher extractable Al and Fe concentrations in the Ah Bw and upper BC horizons. Accumulation of sediments due to a landscape hydrological gradient and external nutrient inputs by bird guano could also contributed to the enrichments of Al and Fe. However, this contribution from bird guano would be relatively small compared to N and P, since guano elemental composition results indicate that around 0.2% of Al and 0.5% of Fe comparing with 3.5% of N and 1.4-12% of P in seabird guanos (data measured in the next Chapter). Unfortunately, elemental data for guano from frugivorous birds feeding on palm and flax were not obtained in the present study.

Depth trends of Alp and Fep were related with soil organic carbon contents in the profile (Figure 6.5,

6.8). Pyrophosphate extraction mostly extracts (> 80%) organic-matter Al and organic-matter Fe, but is poor (< 10%) in the extraction of allophane- or imogolite-related Al and ferrihydrite- or goethite- related Fe (Parfitt and Childs, 1988), and this could explain relatively higher Fep/Feo and Alp/Alo ratios

6.5

Conclusion

(1) Substantial differences were recorded in soil pH, organic matter, P fractionation, and extractable-Al/Fe mineral contents between stands of flax, Nikau palm and grassland on the same gravel ridge at the PCRP site.

(2) Pre-existing differences in soil, prior to vegetation establishment were considered possible due to landscape hydrological gradients of alluvial fans.

(3) The findings indicate contributing effects from vegetation on soil chemistry; flax and palm vegetation stands have significantly promoted the soil development via mediating soil P transformations and Al/Fe mineral weathering.

(4) External nutrient inputs from guano deposition of nectar-feeding birds substantially contribute to the dynamics of soil N, P and Al/Fe minerals in the present study site;

(5) Differences in above- and belowground plant morphology between flax, palm and grassland also accounted for the observed soil pedogenic differences.