Atmospheric bulk deposition of soluble phosphorus in Ashiu Experimental Forest, Central Japan: source apportionment and sample contamination problem

Atmospheric bulk depositions of soluble reactive phosphorus (SRP), soluble unreactive phosphorus (SUP), soluble total phosphorus (STP), Na, SO4, V and Mo were monitored for 3 years in Ashiu Experimental Forest in Central Japan. Relations among P, SO4, V and Mo data were analyzed to determine if any part of P deposition is derived from fossil fuel combustion. Contamination by local particles was evaluated and its influence on atmospheric P input estimates was discussed. The mean bulk depositions of SRP, SUP, STP, Na, SO4, V and Mo were 175, 76, 251, 156000, 40100, 10.3, and 1.52 μmol m−2 year−1, respectively. Bulk depositions of Na, nss-SO4, V and Mo were within the range of published data, while P deposition represented the lowest value found in literature. The low P deposition likely reflected the applied methodology to reduce sample contamination, i.e. (1) placement of samplers off the ground surface, (2) installation of multiple samplers, (3) rejection of samples with visible contamination, and (4) choosing the lowest values as the least contaminated. P and Na showed single peak during winter and spring, while nss-SO4, V and Mo showed two peaks during winter and during summer. Nss-SO4 and Mo data suggested that 59-89% of deposited STP was derived from fossil fuel combustion. Elemental composition of fossil fuels and airmass backward trajectories suggested that a large part of anthropogenic P originates from coal combustion in China. It was speculated that the rest (11-41%) of STP deposition might be predominantly attributed to contamination of local biogenic material. The result suggests that even after precautionary methodologies were employed, the data might still be affected by contamination. It should be remembered that one must take considerable care when interpreting P deposition data to evaluate atmospheric P input to terrestrial ecosystems.