This article presents a case study of Lower Lough Erne, a humic, alkaline lake in northwest Ireland, and uses the radiocarbon method to determine the source and age of carbon to establish whether terrestrial carbon is utilized by heterotrophic organisms or buried in sediment. Stepped combustion was used to estimate the degree of the burial of terrestrial carbon in surface sediment. Δ 14 C, δ 13 C, and δ 15 N values were measured for phytoplankton, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). Δ 14 C values were used to indicate the presence of different sources of carbon, including bedrock-derived inorganic carbon, “modern,” “recent,” “subsurface,” and “subfossil” terrestrial carbon in the lake. The use of 14 C in conjunction with novel methods (e.g. stepped combustion) allows the determination of the pathway of terrestrial carbon in the system, which has implications for regional and global carbon cycling.
Soil testing to determine phosphorus (P) availability to crops is a well established process. Today, however, there is increasing emphasis on relating existing or new soil tests to the potential for P loss from soils to surface waters. The objective of this study was to determine how well short-term soil P measurements (water soluble P (WSP), Mehlich-1 P, degree of P saturation (DPS), and 1-day desorbable P) predicted long-term P release and P sorption in relation to soil properties. Topsoils and subsoils with widely differing properties were collected from four sites in Northern Ireland, the Republic of Ireland, and the U.S. mid-Atlantic coastal plain, with topsoils and subsoils sampled at each site. All soils were analyzed for water soluble P, Mehlich-1 P, oxalate extractable Al, Fe, and P (Alox, Feox, Pox), degree of P saturation (DPS = (Pox/0.5[Alox+Feox]) × 100, free [Alox+Feox] = 0.5[Alox+Feox]−Pox), long-term desorbable P (using Fe-oxide-filled dialysis membranes), and long-term P sorption for "remaining P sorption capacity" (from a solution maintained at 5 mg P L−1). Long-term desorbable P followed a pattern of initial fast P release followed by a slower release of P that was still in progress after 39 days. Water soluble P, Mehlich-1 P, and the DPS were all correlated with the cumulative amount of P desorbed in 39 days (r = 0.82*, 0.79*, and 0.83*, respectively). However, for short-term (1 day) desorbable P, correlations followed the order WSP (r = 0.94***) > DPS (r = 0.83*) > Mehlich-1 P (r = 0.72*). When P was added to the soils, all of the soils exhibited an initial period of rapid P sorption, followed by a period of slower sorption still in progress after 38 days. The soil components found to be related most closely to remaining P sorption capacity were free [Alox+Feox] (r = 0.73*) and free Alox (r = 0.80*), indicating that amorphous Fe and Al are the major soil components responsible for long-term (38 days) P sorption. Overall, a single oxalate extraction for Al, Fe, and P proved to be most useful for predicting both long-term P release, through calculation of the DPS and for predicting the ability of the soils to sorb more P by calculating free [Alox+Feox].
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