Pre-Anthropocene nitrogen cycle: balanced mass and isotope fluxes

One of the features of the present-day global nitrogen cycle is the return to the atmosphere (taken as δ15N = 0 ‰) and surface waters of isotopically lighter N (δ15N < 0) primarily from denitrification on land and in the ocean. With the N residence time in the atmosphere of about 18 million years, the N mass and isotope fluxes were likely to be balanced at geologically longer time scales if the isotopic composition of the atmosphere did not vary. For Pre-Anthropocene time, we construct a balanced N cycle model of mass and isotopic fluxes, based on 14 nitrogen reservoirs in the domains of the atmosphere, land, and ocean. Atmosphere, the largest N reservoir, supplies N to land and ocean domains mainly by nitrogen fixation, deposition, and dissolution, and these fluxes are balanced by denitrification and volatilization back to the atmosphere. The land and ocean biotic reservoirs (N-fixing plants, non-N-fixing plant, and marine biota) interact with atmospheric N2 and dissolved inorganic nitrogen (DIN, consisting of N2, NO3-, and NH4+) in soil and ocean waters through N fixation and DIN uptake. Remineralization of dissolved organic N and particulate organic N in water and sediments produces nitrate and ammonia of DIN. Land and ocean domains are linked by river transport that carries both dissolved and particulate nitrogen to the oceanic coastal zone. As the reported δ15N values of reservoirs and fractionation factors (ɛ = δ15Nproduct - δ15Nsource) in natural systems vary greatly, the isotope-mass balances and fractionation factors in the cycle were obtained by numerical iteration of the flux equations. Within the mass-balanced N cycle model, the isotope fractionation factors of the interreservoir fluxes are sensitive to the δ15N reservoir values. The resultant fractionation factors (ɛ) are mostly within the reported ranges. In general, culture studies of N isotope fractionation suggest that the magnitude of fractionation is smaller (less negative) under N-limited growth conditions where biota utilizes the heavier nitrogen, making the isotopic fractionation smaller. Our balanced, integrated N-cycle model shows faster turnover times and greater isotopic fractionation in biologically mediated processes, consistent with the data on individual reservoirs and processes.