Ulery et al.
(1995) reported on the changes in soils in four large lysimeters filled with similar parent material over a 41 year period. The lysimeters were planted with monocultures of scrub oak (Quercus dumosa), chamise (Adenostoma fasiculatum), ceanothus (Ceanothus crassifolia), and Coulter pine (Pinus coulteri). As expected, the greatest N accumulation (to a 1 m depth) was under the N-fixing ceanothus (29 kg ha−1 yr−1), followed closely by oak (27 kg ha−1 yr−1), then chamise (15 kg ha−1 yr−1),and pine (10 kg ha−1 yr−1). The rates of N accretion under the non-fixing vegetation were not excessive compared to nearby measurements of inputs (23 kg ha−1 yr−1; Riggan et al. 1985), but increments in vegetation were not included in the study and thus total ecosystem increments could have been much greater than those reported. Bormann et al. (1993) reported on N increments over selleckchem a period of 5 years in a sandbox study at Hubbard Brook, New Hampshire, USA. In this study, excavated small plots were backfilled with find more sand obtained from a sand and gravel company to a depth of 1.3 m. Five cm of mixed topsoil were then added on top of the sand and tilled into
a depth of 20 cm. Soils were first sampled over one year after the topsoil was tilled in, at the time of planting of two N-fixers (Alnus glutinosa and Robinia pseudoacacia) and two pine species (Pinus resinosa and P. rigida). For this review, we report on the non-N-fixing species only. The authors reported unexplained N changes in vegetation + forest floor of 83 and 70 kg ha−1 yr−1 for Pinus resinosa and P. rigida, respectively, and concurrent changes in the 0–20 cm soils of −17 and −19 83 and 70 kg ha−1 yr−1, respectively. (Note that our calculations in Table 2 from their reported numbers differ slightly from these values.) Binkley et al. (2000) took issue with several aspects of this study, and concluded that the low precision precluded high confidence in the reported values. This prompted a response from Bormann et al. (2002) wherein they recalculated
their values, resulting in estimates of large increments of N in soil at the 20–135 cm depths (98 and 73 kg ha-1 yr-1 in P. resinosa and P. rigida, DNA ligase respectively) and less significant changes in estimates of vegetation and 0–20 cm soil changes. Bormann et al. (2002) concluded that they had very high confidence in their estimates of “unexplained” N accumulations. A rejoinder from Binkley (2002) expressed skepticism about the new calculations of unexplained soil N accumulations in the 20–135 cm depths. We will not take a position on that exchange, but merely report the numbers as part of the larger data set, noting the caveats described above. The case studies cited above give a very mixed picture of soil and ecosystem N accumulation.