THE NITROGEN BUDGET OF A MOSS (POLYTRICHUM) ECOSYSTEM DEVELOPING ON GLACIAL OUTWASH SANDS IN NEW HAMPSHIRE

Nitrogen inputs, outputs, pools, and rates of net accumulation were examined in a 13-year old moss (Polytrichum) ecosystem in an effort to examine the phenomenon of unexplained nitrogen accumulation (UNA) frequently observed in many ecosystems. The ecosystem extended only 5 cm above and below the soil surface, and was developing upon glacial outwash in an abandoned sand pit. Total ecosystem biomass (living and dead) ranged from 14.9 {dollar}pm{dollar} 1.3 (S.E.) Mg/ha at a low biomass site to 30.1 {dollar}pm{dollar} 0.9 Mg/ha at a high biomass site, and total nitrogen content ranged from 229 {dollar}pm{dollar} 26 (S.E.) kg N/ha at the low biomass site to 699 {dollar}pm{dollar} 31 kg N/ha at the high biomass site. Most nitrogen was found in the soil (53-58%) and moss floor (26-27%), with the remainder in living moss biomass (10-14%) and dead moss biomass (4-8%). Net ecosystem nitrogen accumulation (including soil) from 1969 to 1982 was 10.1 {dollar}pm{dollar} 1.9 kg N/ha/yr at the low biomass site and 46.8 {dollar}pm{dollar} 2.3 kg N/ha/yr at the high biomass site.; Nitrogen inputs (kg N/ha/yr) to the ecosystem were: bulk precipitation, 6.1; nitrogen fixation, 0.4; windborne coarse particulate matter, 0.2. Outputs (kg N/ha/yr) were: hydrologic export, 0.2; gaseous effluxes (N{dollar}sb{lcub}2{rcub}{dollar}O + N{dollar}sb{lcub}2{rcub}{dollar}), 0.1.; A mass balance equation (N inputs {dollar}-{dollar} N outputs = N accumulation) indicated that the measured rates of inputs could not account for the observed accumulation rates, resulting in an UNA of 3.8 {dollar}pm{dollar} 2.2 kg N/ha/yr at the low biomass site and 40.5 {dollar}pm{dollar} 2.6 kg N/ha/yr at the high biomass site. The UNA at the low biomass site is thought to be explained by inputs from dry deposition (gaseous sorption and particle deposition) and cloudwater deposition, but the UNA at the high biomass site is thought to be overestimated, explained only in part by these inputs. Total ecosystem inputs, then, were estimated at 10.5 kg N/ha/yr.; The moss ecosystem was extremely efficient at retaining bulk precipitation nitrogen inputs; additions of {dollar}sp{lcub}15{rcub}{dollar}N-labelled artificial rainfall showed that soil and belowground living moss biomass retained most of the input. The amount of nitrogen retained by ecosystem components was generally related to the amount of organic matter in the component. Soil organic matter was shown to be important in controlling nitrogen retention by soil.; Development of the ecosystem is proposed to have begun with establishment of mosses upon the newly exposed sands. As moss biomass, and hence total ecosystem biomass increased, retention of nitrogen inputs also increased, further enhancing biomass accumulation. Increasing biomass is also thought to increase nitrogen inputs from dry deposition and cloudwater deposition, again enhancing biomass accumulation.