Soil P in a forested seabird colony: inventories, parent material contributions, and N : P stoichiometry

Abstract

Guano from breeding seabirds provides a large external source of nutrients to the soils of breeding colonies. However, little is known of guano P retention relative to N or the relative importance of guano and soil parent material as P sources. Soil profile N and P inventories (0–0.60 m, n = 4; 0–0.36 m, n = 1) and guano N and P concentrations were measured at a Westland petrel colony, and the parent material contributions of P were calculated using Ca, Al, Fe, Ti, and Zr as reference elements. Median inventories (0–0.60 m) were 1.49 kg N/m2 and 332 × 10–3 kg P/m2, the N result being similar to that from a seabird colony on peat soil where N retention was very low. Calculated parent material contributions were smallest (32–66% of soil P) when based on Ca and largest (47–102% of soil P) when based on Zr. Contributions were similar for Al, Fe, and Ti; Al (41–87% soil P) was selected for subsequent calculations. Regardless of the reference element, parent material therefore contributed a large part of soil P. Phosphorus in excess of parent material supply (Pexcess) was significantly correlated with soil C, implying that guano P is held primarily in organic form. The median soil N : P molar ratios were 9.6 : 1 based on total P and 11.2 : 1 based on Pexcess, compared with ratios for Westland petrel guano of 4.1 : 1 (when birds were consuming fisheries waste) and 16.4 : 1 (when fisheries waste was replaced by fish). The similarity between soil and guano N : P ratios implies that both N and P are lost from soil at similar rates, although volatilisation of N would enrich soil drainage water in P. Calculations using guano deposition rates from the literature yielded P residence times of 4–15 years (Pexcess) and 11–41 years (total P), consistent with a highly dynamic soil system.

Stable isotopic (δ15N, δ13C) analysis of wood in trees growing in past and present colonies of burrow‐nesting seabirds in New Zealand. I. δ15N in two species of conifer (Podocarpaceae) from a mainland colony of Westland petrels (Procellaria westlandica), Punakaiki, South Island

“…The residence time of soil N and P in the study area is only c. 4-15 years (Hawke 2005), which implies that soil nutrient status will respond over comparatively short timescales to changes in the patterns of nutrient deposition by petrels. These changes could be caused by a combination of factors.…”

“…For this analysis, pseudoreplication may be an issue, because each series of cores was sampling different parts of the same tree. Initially, we assumed that all our wood δ 15 N results were independent, given that each sample from each tree was well separated in time, and that nutrient residence times in the colony soils are short (Hawke 2005). A further factor supporting the assumption of independence is that the lifetime of root hairs is brief, although significant solute uptake probably also occurs through woody roots.…”

“…; Hawke & Powell 1995), with petrel-derived P being held in organic form (Hawke 2005). N concentrations are 0.4-0.9%, with soil δ 15 N values of c. 14‰ (Harrow et al 2006).…”

“…An aerial photograph of the study area is given in Hawke (2004). The colony has a maximum possible age of 740-960 calendar years (Hawke 2004), and was the location of our previous studies on soil chemistry (Hawke 2005;Harrow et al 2006) and isotopic enrichment of the forest community (Hawke & Holdaway 2005). The vegetation included tree ferns (Cyathea spp.…”

Surface soil chemistry at an alpine procellariid breeding colony in New Zealand, and comparison with a lowland site

“…The burrowing activities of petrels and shearwaters result in thorough mixing of colony soil (Bancroft et al 2005a) from the surface to burrow depth (c. 0.6 m; Warham 1996) with ensuing dispersion of marine nutrients through the soil profile (Hawke & Newman 2004;Hawke 2005). The consequent enhancement of soil nutrient status has long been recognised (e.g., Leamy & Blakemore 1960) although net retention of seabird nutrients in colony soil is low (Hawke & Newman 2004).…”