From Biology of Alder ERTY OF: Proceedings of Northwest Scientific Association Annual Mee t4■::::),...;ADE HEAD EXPERIMENTAL FOREST April 14-15, 1967 Published 1968 AND SCENIC RESEARCH AREA OTIS, OREGON Nitrogen transformations in soils beneath red alder and conifers W. B. Bollen, Abstract Oregon State University Corvallis, Oregon Transformations of nitrogen in organic matter in the soil are essential to and plant nutrition because nitrogen in the form of proteins and other organic K. C. Lu compounds is not directly available. These compounds must undergo Forestry Sciences Laboratory microbial decomposition to liberate nitrogen as ammonium (NH -I-4) and Pacific Northwest Forest nitrate (NO3 ), which can then be absorbed by plant roots. and Range Experiment Station Nitrogen transformations, particularly nitrification, are rapid in soils under Corvallis, Oregon coastal Oregon stands of red alder (Alnus rubra Bong.); conifers — Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla Raf) Sarg.), and Sitka spruce (Picea sitchensis (Bong.) Carr.); and mixed stands of alder and conifers. Nitrification is especially rapid in the F layer beneath alder stands despite a very low pH. These findings are from a study of contributions to the nitrogen economy by red alder conducted at Cascade Head Experimental Forest near Otis, Oregon (Chen, 1965). 1 Procedures Ammonifying capacity of the different samples was compared by testing duplicate 50-g portions, ovendry basis, of each with peptone equivalent to 1,000 ppm nitrogen. These samples, and samples of untreated soil, were incubated for 35 days at 28 C. Moisture was adjusted to 50 percent of the water- holding capacity. At the close of incubation, each sample was analyzed for NH+4, NO-2, and NO-3 nitrogen and for pH. To determine nitrification rate, duplicate 50-g portions, ovendry basis, of each sample were variously treated with ammonium sulfate at 200 ppm nitrogen, with and without CaCO 3 to satisfy lime requirement, and without these additions. Each portion was moistened to 50 percent of water-holding capacity and incubated for 28 days at 28 C. They were then analyzed for NO2, NO-3 , and pH. Ammonium nitrogen was determined by distilling 10.00 g samples of soil, ovendry basis, with phosphate buffer at pH 7.4 (Nichols and Foote, 1931). One hundred ml of distillate were collected in 30 ml saturated boric acid solution and titrated with N/14 sulfuric acid, using methyl red-bromcresol green indicator. Nitrite was determined on 1:5 aqueous extracts of soil clari- This investigation was supported in part by the National Science Foundation, Grant No. GB-3214. 141 fied with cupric acetate and calcium hydroxide (Harper, 1924), by a modi- fied Ilosvay method (American Public Health Association, 1955), using 1-naph thy lamine, sulfanilic acid, and sodium acetate buffer. The phenoldisulfonic acid procedure was used to determine NO-3 in another portion of the extract (Harper, 1924). A glass electrode was used to measure pH of 1:5 aqueous suspensions of soil; readings were made while the suspensions were stirred. Dunns titration procedure using calcium hydroxide (Dunn, 1943) was employed to deter- mine lime requirement. Kjeldahl nitrogen was determined by a modified AOAC method (Associ- ation of Official Agricultural Chemists, 1960); Hibbards mixture and a selenized granule were used in the digestion. Total carbon analyses were made on 100-mesh air-dry samples, following the high temperature combus- tion procedure of Allison, Bollen, and Moodie (1965). Cation exchange capacity and exchangeable cations were determined by the ammonium acetate method (Schollenberger and Simon, 1945). All results are expressed on the basis of ovendry soil. Results Although samples were collected and tested at seasonal intervals, the readers attention is directed to results of March sampling in which analyses included cation exchange capacity and exchangeable cations of the samples (Table 1). Table 2 shows the variation in different forms of nitrogen with season. Nitrite nitrogen was always less than 2 ppm, so these values are not reported. The F layer and All horizon beneath alder contained significantly greater amounts of NO-3 and Kjeldahl nitrogen and had narrower C:N ratios, despite their low acidity, than similar samples from beneath conifer stands. Cation exchange capacities were similar in both sets of samples, but the alder All horizon is higher in exchangeable hydrogen and lower in exchangeable cal- cium. These findings agree closely with those of Franklin et al. (1968), and results with respect to calcium also agree with Yamayas comparison (1968) of A horizons beneath Alnus inokumai, Cryptomeria japonica, and Castanea crenata on an Ando soil in Japan. Yamaya attributed the lower exchangeable calcium under alder to more rapid absorption of calcium by alder than by conifers or other broadleaved trees. It seems, however, that the absorbed calcium would be recycled to a large extent by decay of fallen leaves and branches. Less exchangeable calcium in the more acid soil beneath alder could be due to leaching following replacement of calcium by hydrogen ions. Ammonification is the first step in mineralization of organic nitrogen compounds, and the process is generally rapid. Ammonification of native organic matter (Table 3) was greatest in soil from the alder All horizon — about twice the rate in the corresponding horizon beneath coni- fers. Although total nitrogen was higher in the alder F layer, the percentage of nitrogen ammonified was the same as for the conifer F layer. Differences in 142 TABLE 1. Analyses of F layers and All horizons beneath alder and conifer stands on Astoria silty clay loam soil Exchangeable cations Nitrogen Stand Water- Cation exchange Kjel- C/N and layer holding mg++ or horizon capacity pH capacity Na+ K+ Ca++ H+ NH+4 NO; dahl ratio Percent MI/100 g MI/100 g Ppm Ppm Percent Alder: F 420 3.6 82 1.1 1.64 10.0 9.5 59.3 15 146 2.05 17 F 230 4.1 70 - - - - - 55 98 1.32 15 All 200 3.9 68 2.5 8.1 8.6 3.8 45.1 5 67 .87 16 Conifer. F 290 5.1 71 .9 1.44 8.2 9.8 50.8 25 28 1.02 22 All 206 5.3 69 4.9 5.2 16.0 12.5 30.4 25 24 .77 21 Sample collected in September; all others in March. transformation of added peptone were minor, total ammonification being near 40 percent except in the conifer All horizon in which nearly one- half of the additive was accounted for as ammonium-plus-nitrate. Much of the ammonium liberated during 35 days incubation was nitrified, so this additional NO -3 must be included in calculating the total nitrogen ammonified. Results of analyses made after only 5 days incubation, not presented here, showed that, during the first few days of incubation, ammonification was more extensive in the less acid conifer soil. Nitrifying capacity of the soils sampled, independent of ammonification, is shown in Table 4. Nitrification of native nitrogen was generally slow. The highest nitrification, 1.4 percent, was in the alder All horizon samples, and the lowest, 0.3 percent, was in the F layer under alder. Nitrification was slightly greater in samples from the conifer F layer than for the conifer All horizon. Over 90 percent of added ammonium sulfate was nitrified in alder F layer samples, but only 27.5 percent was nitrified in conifer F layer samples. In All horizons, the order was reversed, nitrification being much lower in alder soil, probably because of very low pH. Added calcium carbonate markedly increased nitrification in both alder samples but had little influence on those from beneath conifer stands. Moreover, much of the native nitrogen in the alder F layer was nitrified, as indicated by the 293.5 percent obtained when added CaCO3 increased pH from 3.5 to 4.5. An increase in exchangeable cal- cium could also have been important. Discussion These results show that soil beneath alder is higher in NO- 3 and in nitrify- ing capacity than that under conifers. Differences in ammonifying capacity 143 TABLE 2. Ammonium, nitrate, and Kjeldahl nitrogen in soil under red alder, conifer, and mixed stands at different seasons April July September March Mean Kjel- Kjel- Kjel- Kjel- Kjel- Soil NH4 NO3 dahl NH4 NO3 dahl NH4 NO-3 dahl NH4 NO3 dahl NH4 NO3 dahl Ppm Ppm Percent Ppm Ppm Percent Ppm Ppm Percent Ppm Ppm Percent Ppm Ppm Percent F layer Alder 163 271 1.76 20 129 1.29 55 98 1.32 15 146 2.05 63 161 1.61 Mixed — 5 113 1.43 60 160 1.77 5 131 1.28 69 135 1.49 Conifer 85 79 1.06 10 20 .64 38 10 .89 25 28 1.02 40 34 .90 All horizon Alder 40 140 1.13 0 47 .74 25 33 .82 5 67 .86 18 72 .89 Mixed — 10 22 .70 40 61 .82 5 61 .64 14 36 .72 Conifer 20 71 .89 0 15 .53 8 18 .70 25 24 .77 12 32 .72 TABLE 3. Ammonification in soil beneath alder and conifer stands after 35 days incubation at 28°C Nitrogen Kjel- Total at Nitrogen Soil and treatment pH dahl NH4 NO3 Total 0 day ammonified Percent Ppm Ppm Percent Alder F layer 3.5 2.05 240 280 520 322 169 0.9 plus peptone 3.8 - 563 360 923 403 40.3 Conifer F layer 4.3 1.02 30 308 338 244 94 .9 plus peptone 4.3 - 330 400 730 392 39.2 Alder All horizon 3.6 .87 58 248 306 111 195 2.2 plus peptone 4.0 450 288 738 432 43.2 Conifer All horizon 4.6 .77 10 180 190 81 109 1.4 plus peptone 4.7 - 415 268 683 493 49.3 At 1,000 ppm N.