Soil acidification and imbalanced nutrient availability in Scots pine forest soils in the Netherlands : causes, extent and control

IntroductionSince the 1950s, in the Netherlands and several other western European countries an intensified livestock production system has developed. As a consequence, the quantities of NH x (NH 3 and NH4+) emitted into the atmosphere have increased. However, atmospheric NH x is eventually returned to the soil surface, which occurs through dry and wet deposition. In forests, dry deposition on leaf surfaces is enhanced by several processes, resulting in average deposited amounts of 30 kg NH 4 -N ha -1year -1. In addition, annually 10 kg NO 3 -N ha -1is deposited. Possible negative effects of increased NH 4 depositions are, first, soil acidification and, second, an imbalanced plant nutrition in the form of (relative) mineral nutrient deficiencies.The former effect may be caused by nitrification of atmospheric NH 4 . or by an increased excess of cation-over-anion uptake by tree roots. When more (or less) cations than anions are absorbed, plants maintain their internal charge balance by net excretion of H +(or OH -) ions. Therefore, an increased uptake of NH4+may increase the roots' H +production.Relative or absolute nutrient deficiencies may occur when N availability is excessively high relative to that of the mineral nutrients, or through increased mobilization and leaching of base cations, respectively.This thesis presents two lines of research, pertaining to these possible negative effects of increased NH x depositions in the Netherlands. The first one deals with the question whether or not nutrient uptake of trees can be considered an important source of soil acidification, relative to other H +sinks and sources. The second line of research was aimed at obtaining more understanding of forest fertilization and liming as possible tools to ameliorate acid-rain-induced nutritional disorders and soil acidification.Scots pine (Pinus sylvestris L.) was chosen as experimental tree species, because of its predominant contribution to the forested area in the Netherlands and other western European countries. A pot experiment was conducted with 3-y-old trees, and a forest fertilization experiment was carried out in a mature Scots pine stand in Harderwijk, the Netherlands.N uptake pattern of Scots pine, N mineralization and soil acidificationThe H +production due to nutrient acquisition of plant roots can indirectly be assessed by measuring the uptake of cations and anions. The H +production is calculated as the difference between the absorbed charge equivalents of cations and anions (C a -A a ). Most cations and anions are absorbed by plant roots in one distinct form. Therefore, their uptake charge balance can straightforwardly be derived from plant analysis. However, N can be taken up as NH4+or as NO 3-. These N sources can not be distinguished when assimilated to organically bound N. However, a distinction can be made when one of the N sources is labelled with the stable nitrogen isotope 15N, while the other one is applied in its natural 14N form. To investigate the feasibility of this approach, in a pilot-study a method to measure the uptake of 15NH 4 or 15NO 3 was tested with larch (Larix kaempferi) trees. In a modified form, this method was used to investigate the uptake pattern of 3-y-old Scots pine trees. In a pot experiment N was supplied in three NH 4 -N/NO 3 -N ratios, 3:1, 1:1 and 1:3, either as 15NH 4 + 14NO 3 or as 14NH 4 + 15NO 3 . After these applications, the NH 4 /NO 3 ratios in the soil solution were monitored. At each application ratio, Scots pine appeared to show an NH 4 /NO 3 uptake ratio 7 times wider than that in the soil solution. It could be estimated that Scots pine exhibits an acidifying uptake pattern as long as the contribution of nitrate to the N nutrition is lower than 70%. For the three application ratios this contribution was always < 70%.These data were used to estimate acidification occurring under field conditions. For the three application ratios H +production due to wood growth was estimated to range from 2.5 to 4.2 kmol ha -1y -1, and that due to wood and needle growth from 3.6 to 6.2 kmol ha -1y -1. However, for these estimates the acidification found in the pot experiment was adopted without modifying soil solution NH 4 /NO 3 ratios or plant N concentrations according to those which may be expected in the field. For more realistic estimates, N concentrations in the soil solution as found in the root zone of the field trial in Harderwijk (see below) were combined with N uptake data of mature Scots pine trees. Thus, the acidification due to nutrient uptake for above-ground growth appeared to be lower, i.e. 2.20 and 0.25 kmol ha -1y -1. for untreated and limed plots, respectively. The lower value for limed plots was caused by higher NO 3 concentrations in the soil solution of the limed soil, thus reducing the excess cation-over-anion uptake and the associated H +excretion of tree roots.Besides nutrient uptake, nitrification is an acidifying process as well, whereas ammonification is acid-consuming. Ammonification consumes 1 mol H +per mol NH 4 produced. Nitrification, if preceded by ammonification, produces 1 mol H +per mol NO 3 produced. The combined result of both processes, the formation of inorganic N from organic N, is referred to as N mineralization. Depending on the rates of these microbial N transformations, acidification due to nutrient uptake may either be (partly) compensated for or be corroborated. To investigate these rates, an in-situ incubation experiment was carried out in the control and limed plots of the field experiment. PVC tubes were driven into the forest floor and the top 22-cm mineral soil layer, and left in the field. This was done in seven subsequent incubation periods of each 7 or 8 weeks (384 days total). For each period, the changes in the amounts of NH 4 and NO 3 in the tubes were measured. To prevent the input of atmospheric N and N leaching, the undisturbed soil column in each tube was confined between two layers of ion exchange resin (IER). Annually, in the control and limed plots 38 and 73 kg N ha -1were mineralized, respectively. In both treatments approx. 65% of NH 4 from the ammonification process was nitrified. Therefore, mineralization was a slightly acidifying process. The annual acidification in the control and lime treatments amounted to 0.9 and 1.4 kmol H +ha -1, respectively; these amounts were not significantly different. Thus, acidification owing to nutrient uptake and mineralization adds up to 3.14 and 1.61 kmol H +ha -1y -1. for the untreated and limed Scots pine plots, respectively. The acidification due to direct nitrification of deposited NH 4 was not determined, but may be considerable because through this process 2 mol H +is produced per mol NH 4 nitrified.In situations of low atmospheric N inputs N is effectively retained in most ecosystems. This implies that N is eventually taken up in the form in which it is released by N mineralization. Hence, acidification due to N transformations would probably be negligible.Field experimentIn order to alleviate potential nutrient deficiencies as a result of high N depositions, in 1985-1988 a Scots pine forest (planted in 1960) was limed and fertilized with P, K and Mg in a 2 4factorial design and in a separate experiment with 5 liming levels and basic dressings of P, K and Mg. Liming induced many effects on the mineral soil and especially on the forest floor. It proved to be responsible for the loss of extractable K and Mg from the forest floor, probably by exchange against Ca. The residence time of added P and possibly Mg in the forest floor was increased by liming, probably due to a reduced solubility of the added fertilizers (triple superphosphate and kieserite). Single applications of 3000 kg lime ha -1in 1985 increased the forest floor pH to 6.2. In the subsequent 5 years, the pH gradually declined to 4.2. In the same period, the pH of the untreated forest floor declined from 4.0 to 3.1. Five years after application, lime had caused a small, but significant pH increase to a 50-cm depth. Extractable Al in the forest floor and mineral soil was lowered by liming. Until 1989, inorganic N in the forest floor was lowered and that in the mineral soil was raised by liming. This could be attributed to the formation of NO 3 in the limed plots, which is more mobile than NH 4 . Added K (potassium sulphate) was poorly retained in the forest floor and probably substantial quantities quickly leached to soil layers deeper than 50 cm.The soil solution composition at a 30-cm depth was monitored from autumn 1989 until spring 1992. In spring 1991, the soil solution at a depth of 1-1.5 m was analyzed. The applied K and Mg compounds were easily soluble, and hence the soil solution concentrations of K and Mg were increased at both depths in the PKMg-amended plots. Liming increased Ca and left Al unaffected at both depths. The Ca/Al ratios were low (i.e. around 0.2) in the root zones of the unlimed plots, which may impair root functioning. The NO 3 concentrations at 30 cm were high in winter and lower in summer. NO 3 was increased by liming at both depths. The largest increase at 30 cm occurred in the autumn/winter of 1990/91, i.e. a few months after windfelling and thinning. Increased NO 3 concentrations in the soil solution are commonly observed after ecosystem disturbance. The present research shows that liming may enhance such an effect. The additions of P, K and Mg also increased NO 3 , but this effect only started 2½ year after the last applications. This finding suggests that the nitrifying microflora gradually developed an ability to profit from an improved nutrient availability.Stem volume increments, measured at all individual trees, amounted to 16 m 3ha -1y -1. which is high for Scots pine. It is likely that high N depositions increased tree growth. These high growth rates suggest that negative effects of an increased N availability may develop in the near future. However, increased growth after application of mineral nutrients may indicate that a situation of balanced nutrition has been restored, so that-growth may be used as a parameter to quantify positive effects of fertilization and liming.In the period 1988-1991, fertilization with P and/or K increased stem volume growth by 0.9 and 2.2 m 3ha -1y -1. respectively, while Mg and/or lime had no effect. Correspondingly, P and K were the first nutrients to show increased foliar concentration after fertilization. However, foliar analysis at the start of the experiment indicated that in fact the P supply was insufficient while that of K was sufficient. This discrepancy may have two causes. First, the foliar-analysis-based conclusions concerning a forest's nutritional status may not yet be fully accurate, or second, growth responses may not correctly reflect changes in the nutritional status.Future perspectivesProbably long after N depositions will have been reduced according to presently stated prescriptions, ecosystems that have been exposed to high N depositions will contain more N than they originally did. Therefore, they may continue to be potential sources of NO 3 leaching and be characterized by soil acidification and an unbalanced nutrient supply. In N-enriched regions, production forests may effectively prevent NO 3 leaching if they are enabled to accumulate N at a rate similar to the N mineralization rate. This condition requires the maintenance of high growth rates. When oligtrophic biotopes are to be restored, excessive N must be removed. This may most effectively be carried out by temporarily maintaining productive vegetations from which biomass is frequently removed. Because high growth rates require a sufficient and balanced nutrition, for both situations fertilization with nutrients other than N may be necessary.Liming may be applied to improve soil pH and reduce Al concentrations. However, liming may have several adverse effects, e.g. increased soil heavy-metal concentrations, profound floristic changes and an increased N mineralization and NO 3 leaching. Moreover, lime (CaCO 3 ) is not very effective in improving deeper soil horizons. Special attention should therefore be paid to the development of substances that increase mineral soil pH and pAl without drastically increasing forest floor pH.