Processes Controlling Movement, Storage, and Export of Phosphorus in a Fen Peatland

Abstract

Field and laboratory studies were conducted to determine the mechanisms controlling P movement, storage, and export from a minerotrophic peatland (fen) in central Michigan that had demonstrated high P removal from nutrient additions. An annual P budget completed for the fen ecosystem revealed that plant uptake requirements were 7—9 kg · ha—1 · yr—1, but 35% of aboveground P uptake by plants was returned to the peatland surface via litterfall. Permanent storage of organic P in peat ranged between 2 and 5 kg · ha—1 · yr—1 under natural levels of P input. Both microbial uptake and soil exchange capacity controlled the amount of P made available for plant growth. Fertilizer additions of 5.5 kg · ha—1 · yr—1 of P and 17 kg · ha—1 · yr—1 of N in the fen resulted in no significant (P < .05) increase in growth or nutrient uptake by emergent macrophytes as the litter—microorganism compartment (LMC) retained up to 84% of the added P in year 1. A doubling of the P fertilization level resulted in an LMC retention of only 57%. In year 2 the retention of P by the LMC dropped to 67 and 31% for the two fertilizer levels, respectively. Concurrent with decreases in LMC phosphorus retention were increased peat sorption of P, but plant growth responses and P uptake were negligible. Higher level fertilizer additions of 22 and 55 kg · ha—1 · yr—1 of P and 68 and 170 kg · ha—1 · yr—1 of N applied with minimal water additions resulted in significant (P < .05) increases in net primary productivity and P storage by Carex spp. Narrow—leaved sedge (Carex lasiocarpa, C. oligosperma, and C. aquatilis) removed as much as 61% of the P additions in year 1, with the LMC sorbing an additional 22%. Roots and rhizomes accounted for 81% of plant P storage in the higher fertilizer treatment, when surface water flow rates were reduced and fertilizer additions were sequestered in the root zone. However, seasonal dieback and leaching of P from aboveground standing plant material on the high fertilizer plots resulted in a fivefold increase of P flux to the water compartment. Microcosm 32 P studies indicated that most of the P added to the fen ecosystem was removed from the water column within the 1st h by microorganisms and fine sediments, and that sedge uptake was extremely low even 45 d after addition. Thus plant uptake of P is not a major factor in the rapid removal of low levels of newly added PO 4 in the fen. Selective biocide treatments used to separate the P uptake by bacteria and actinomycetes from that of fungi and yeasts in the fen surface water revealed that the latter group of microorganisms was the dominant group responsible for initial P removal. Biological uptake and abiotic sorption of P by the fine sediments in the surface waters were also shown to be of the same order of magnitude, but immobilization of P in the peat soil zone was mainly controlled by chemical sorption. Freezing of peat resulted in P release to the water column upon thawing, but concentrations returned to control levels within 24 h, suggesting minimal ecosystem losses of P in spring runoff. A Freundlich P adsorption maximum of 15 and 38 kg/ha was calculated for a 2 cm and 5 cm depth of peat adsorption, respectively. These soil P adsorption maxima are only 23% (2 cm) and 60% (5 cm) of annual wastewater P additions of 64 ± 14 kg · ha—1 · yr—1 and may account for the 26 and 42 kg/ha of P exported from the 19.5—ha test area in the fen during the 4th and 5th yr, respectively, of nutrient additions. Collectively, our field research and microcosm studies on the Houghton Lake fen suggest that soil adsorption and peat accumulation (i.e., phosphorus stored in organic matter) control long—term phosphate sequestration. But microorganisms and small sediments control initial uptake rates, especially during periods of low nutrient concentration and standing surface water. Carex P uptake increases later in the growing season during field fertilization, but algal populations in the fen water respond quickly and absorb significant amounts of P in areas where sewage effluent has been added. Both biotic and abiotic control mechanisms are thus functional in the peatland, and the proportional effect of each on P transfers is dependent on water levels, the amount of available P, fluctuating microorganism populations, seasonal changes in P absorption by macrophytes, and P soil adsorption capacity.