Effects of CO2 and nitrogen fertilization on vegetation and soil nutrient content in juvenile ponderosa pine

This paper summarizes the data on nutrient uptake and soil responses in opentop chambers planted with ponderosa pine (Pinus ponderosa Laws.) treated with both N and CO₂. Based upon the literature, we hypothesized that 1) elevated CO₂ would cause increased growth and yield of biomass per unit uptake of N even if N is limiting, and 2) elevated CO₂ would cause increased biomass yield per unit uptake of other nutrients only by growth dilution and only if they are non-limiting. Hypothesis 1 was supported only in part: there were greater yields of biomass per unit N uptake in the first two years of growth but not in the third year. Hypothesis 2 was supported in many cases: elevated CO₂ caused growth dilution (decreased concentrations but not decreased uptake) of P, S, and Mg. Effects of elevated CO₂ on K, Ca, and B concentrations were smaller and mostly non-significant. There was no evidence that N responded in a unique manner to elevated CO₂, despite its unique role in rubisco. Simple growth dilution seemed to explain nutrient responses in almost all cases.There were significant declines in soil exchangeable K⁺, Ca²⁺, Mg²⁺ and extractable P over time which were attributed to disturbance effects associated with plowing. The only statistically significant treatment effects on soils were negative effects of elevated CO₂ on mineralizeable N and extractable P, and positive effects of both N fertilization and CO₂ on exchangeable Al³⁺. Soil exchangeable K⁺, Ca²⁺, and Mg²⁺ pools remained much higher than vegetation pools, but extractable P pools were lower than vegetation pools in the third year of growth. There were also large losses of both native soil N and fertilizer N over time. These soil N losses could account for the observed losses in exchangeable K⁺, Ca²⁺, Mg²⁺ if N was nitrified and leached as NO ₃ ^– .     

The chemical composition of throughfall beneath oak, birch and pine canopies in Northwest Germany

The chemical composition of rainwater is altered upon its passage through tree canopies. In order to investigate how rainwater chemistry is affected by canopy-dependent processes in characteristic forest types of Northwest German sandy lowland regions – oak–birch-forests, Betula pubescens Ehrh. swamp forests, and stands of Pinus sylvestris L. – comparative studies on the chemical composition of throughfall were carried out at seven forest sites, situated in close proximity within a nature reserve. Additionally, rainwater was sampled at three heathland sites for analysis of open-field precipitation and at three sites along an oak–birch-forest edge. Throughfall concentrations of most of the parameters analysed were significantly higher than open-field concentrations, especially with regard to electric conductivity, NH₄-N, K⁺, and KMnO₄-index. Ion concentrations in throughfall were the lowest in a 10-year-old stand of Betula pendula Roth. and Pinus sylvestris and in a Betula pubescens swamp forest and were highest beneath a stand of Pinus sylvestris. Except for Na⁺, Cl⁻, and NO₃⁻, ion concentrations in both throughfall and open-field precipitation increased during the growing season (May–October). In throughfall, Ca²⁺, Mg²⁺, K⁺, and Mn²⁺ were strongly correlated. Enrichment ratios between throughfall and open-field deposition varied among sites and elements and were the highest for K^‰+, Mg^2‰+, and Mn^2‰+. Estimates of canopy leaching indicated high leaching rates of K^‰+ and Mn^2‰+ and moderate leaching of Mg^2‰+. The contribution of foliar leaching to throughfall deposition was higher at the deciduous than at the coniferous stands.     

Effects of [CO2] and nitrogen fertilization on soils planted with ponderosa pine

The effects of six years treatment with elevated [CO₂] (350, 525, and 700 μl l^-1) and nitrogen (N) (0, 10, and 20 g N m^-2 yr^-1) on soils, soil solution, and CO₂ efflux in an open-top chamber study with ponderosa pine (Pinus ponderosa Laws.) are described. The clearest [CO₂] effect was in year 6, when a pattern of lower soil N concentration and higher C/N ratio with elevated [CO₂] emerged. Statistically significant effects of elevated [CO₂] on soil total C, extractable P, exchangeable Mg²⁺, exchangeable Ca²⁺, base saturation, and soil solution HCO₃ ^- and NO₃ ^- were also found in various treatment combinations and at various times; however, these effects were inconsistent among treatments and years, and in many cases (P, Mg²⁺, Ca²⁺, base saturation) reflected pre-treatment differences. The use of homogenized buried soil bags did not improve the power to detect changes in soil C and N or help resolve the inconsistencies in soil C patterns. Nitrogen fertilization had the expected negative effects on exchangeable Ca²⁺, K⁺, and Mg²⁺ in year 6, presumably because of increased NO₃ ^- leaching, but had no consistent effect on soil C, N, or extractable P. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of forest type on throughfall deposition and seepage flux: a review

Converting deciduous forests to coniferous plantations and vice versa causes environmental changes, but till now insight into the overall effect is lacking. This review, based on 38 case studies, aims to find out how coniferous and deciduous forests differ in terms of throughfall (+stemflow) deposition and seepage flux to groundwater. From the comparison of coniferous and deciduous stands at comparable sites, it can be inferred that deciduous forests receive less N and S via throughfall (+stemflow) deposition on the forest floor. In regions with relatively low open field deposition of atmospheric N (<10 kg N ha⁻¹ year⁻¹), lower NH₄⁺ mean throughfall (+stemflow) deposition was, however, reported under conifers compared to deciduous forest, while in regions with high atmospheric N pollution (>10 kg N ha⁻¹ year⁻¹), the opposite could be concluded. The higher the open field deposition of NH₄⁺, the bigger the difference between the coniferous and deciduous throughfall (+stemflow) deposition. Furthermore, it can be concluded that canopy exchange of K⁺, Ca²⁺ and Mg²⁺ is on average higher in deciduous stands. The significantly higher stand deposition flux of N and S in coniferous forests is reflected in a higher soil seepage flux of NO₃⁻, SO₄²⁻, K⁺, Ca²⁺, Mg²⁺ and Al(III). Considering a subset of papers for which all necessary data were available, a close relationship between throughfall (+stemflow) deposition and seepage was found for N, irrespective of the forest type, while this was not the case for S. This review shows that the higher input flux of N and S in coniferous forests clearly involves a higher seepage of NO₃⁻ and SO₄²⁻ and accompanying cations K⁺, Ca²⁺, Mg²⁺ and Al(III) into the groundwater, making this forest type more vulnerable to acidification and eutrophication compared to the deciduous forest type.     

Simulated effects of atmospheric sulfur deposition on nutrient cycling in a mixed deciduous forest

The effects of three S deposition scenarios — 50% reduction, no change, and 100% increase — on the cycles of N, P, S, K, Ca, and Mg in a mixed deciduous forest at Coweeta, North Carolina, were simulated using the Nutrient Cycling model (NuCM). The purpose of this exercise was to compare NuCM's output to observed soil and streamwater chemical changes and to explore NuCM's response to varying S deposition scenarios. Ecosystem S content and SO₄ ^2– leaching were controlled almost entirely by soil SO₄ ^2– adsorption in the simulations, which was in turn governed by the nature of the Langmuir isotherm set in the model. Both the simulations and the 20-year trends in streamwater SO₄ ^2– concentration suggest that the ecosystem is slowly becoming S saturated. The streamwater data suggest S saturation is occurring at a slower rate than indicated by the simulations, perhaps because of underestimation of organic S retention in the model. Both the simulations and field data indicated substantial declines in exchangeable bases in A and BA soil horizons, primarily due to vegetation uptake. The correspondence of model output with field data in this case was a result of after-the-fact calibration (i.e. setting weathering rates to very low values) rather than prediction, however. Model output suggests that soil exchangeable cation pools change rapidly, undergoing annual cycles and multi-decade fluctuations.Varying S deposition had very little effect upon simulated vegetation growth, nutrient uptake, or N cycling. Varying S deposition strongly affected simulated Ca²⁺. Mg²⁺, K⁺, and P leaching but caused little change in soil exchangeable pools of Ca²⁺ K⁺, or P because soil exchangeable pools were large relative to fluxes. Soil exchangeable Mg²⁺ pools were reduced by high rates of S deposition but remained well above levels sufficient for tree growth. Although the total soil pools of exchangeable Ca²⁺ and K⁺ were only slightly affected by S deposition, there was a redistribution of Ca²⁺ and K⁺ from upper to lower horizons with increasing S deposition, causing increased base saturation in the deepest (BC) horizon. The 100% increased S deposition scenario caused increasing peaks in simulated Al³⁺ concentrations in A horizons after 25 years as a result of large seasonal pulses of SO₄ ^2– and lowered base saturation. Simulated soil solution Al³⁺ concentrations remained well below toxicity thresholds for selected tree species at the site.     

杨树人工林生态系统降水再分配及主要离子特征

大气降水是森林生态系统养分输入的主要途径之一,对养分的生物地球化学循环有着重要的意义。对13年生杨树人工林林外雨、树干流、林内雨和地表径流等水文过程中的养分特征进行了调查分析,旨在了解该生态系统的养分输入与输出规律,为杨树人工林可持续经营提供依据。结果表明,从2013年11月至2014年10月,杨树人工林生态系统林外雨量为1154.1 mm,树干流量仅占大气降水量的2.3%,15.4%的大气降水被杨树人工林的冠层截留;林内雨、树干流与大气降水量(林外雨)的动态变化规律相似。各类降水年加权平均pH值表现为林内雨林外雨树干流;各类降水的离子浓度动态变化规律基本一致,即在降水量较小的11月至次年1月份,各阴阳离子的浓度普遍较高,在降水量较大的2—9月份,阴阳离子浓度普遍较低。SO_4~(2-)-S和Ca~(2+)分别是各类降水中的主要阴离子和阳离子;整体上,树干流的离子浓度林内雨大气降水;林内雨是养分输入的主要形式,通过林内雨输入林地较多的养分离子是Ca~(2+)和K~+,分别为70.83 kg hm~(-2)a~(-1)和63.31 kg hm~(-2)a~(-1);地表径流和土壤渗漏是养分输出的主要形式,输出林地较多的离子是Cl~-和Ca~(2+),分别为196.47 kg hm~(-2)a~(-1)和123.09 kg hm~(-2)a~(-1),其次为SO_4~(2-)-S、Mg~(2+)、Na~+、K~+;NH_4~+-N和NO_3~--N的输出量不足输出离子总量的1%。所以,从水文过程看,杨树人工林生态系统无机氮(NH_4~+-N和NO_3~--N)和K~+表现为净积累,净积累量分别为10.9 kg hm~(-2)a~(-1)和56.4 kg hm~(-2)a~(-1),其他离子表现为净损失,其中Cl~-的净损失量达179.8 kg hm~(-2)a~(-1)左右,其他离子损失量50 kg hm~(-2)a~(-1)。     

Composition and deposition of throughfall in a flooded forest archipelago

The sources of spatial and temporal variation and rates of nutrient deposition via throughfall were studied for 9 months in the Anavilhanas archipelago of the Negro River, Brazil. A total of 30 events was sampled individually for rain and throughfall chemistry in a 1-ha plot of flooded forest. Throughfall samples were collected in 40 collectors distributed in five parallel transects in the study plot, while rain was collected in 4 collectors in an adjacent channel. Volume-weighted mean (VWM) concentrations of solutes in rain were consistently lower than in throughfall, except for H⁺, NO ₃ ^– and NH ₄ ⁺ . Ratios of VWM concentrations of rain to throughfall indicated that K⁺, followed by Mg²⁺ and PO ₄ ^3– , were the most enhanced solutes as rain passed through the forest canopy. The deposition of solutes varied significantly among transects, except for Na⁺ and Ca²⁺, and was significantly correlated with maximum flooding depth, foliar nutrient content, soil fertility and canopy closure for most solutes. The concentrations of PO ₄ ^3– and most major ions were higher in throughfall compared to those in rain due to canopy exchange and dry deposition. In contrast, NO ₃ ^– , NH ₄ ⁺ and H⁺ were retained due to immobilization by leafy canopy and ion exchange processes. Solute inputs via throughfall (not including stemflow) to a floodplain lake (Lake Prato) of the archipelago accounted for 30 to 64% of the total for most solutes in the lake at high water, which indicates that throughfall is an important source of nutrients to the aquatic ecosystem of the Anavilhanas archipelago.     

Impacts of Water Input Manipulations on Fine Root Production and Mortality in a Mature Hardwood Forest

In order to examine the below ground response of a mature upland hardwood forest in the southeastern U.S., to increases and decreases in water inputs, the gross production, mortality, and net production of fine roots were examined over the first and third years of a long-term water manipulation experiment (Throughfall Displacement Experiment). Treatments involved a 33% decrease (DRY), 33% increase (WET), and ambient (AMB) levels of throughfall to the forest floor, begun in July, 1993. Video images of roots appearing on minirhizotron faces installed on both upper and lower slopes were recorded biweekly to a depth of 90 cm from April through October of 1994 and of 1996. Comparisons were made between treatments in amounts of new root elongation, root mortality, and calculated net root production. Minirhizotron observations during 1994 growing season, immediately following winter 1994 installation, revealed a strong effect of installation disturbance and were therefore not considered valid reflections of the response of the stand to the treatments. The 1996 data, on the other hand, exhibited absence of installation biases inherent in 1994 data because of a longer period since treatment initiation (2 2/3 yr vs. 8 mths), and favorable root growth conditions in all treatments during a greater portion of the year. The 1996 data were, therefore, considered realistic measures of below ground treatment responses. During 1996, net root production at 0-30 cm depth, at the upper slope positions, was significantly greater in DRY than in WET and AMB. Net root production was also greater at the lower slope position, but not significantly so. Treatment differences were the result of gross root production, as patterns of mortality did not differ across treatments. Nor were there significant treatment differences at depths below 30 cm. Whether trees in DRY produced more roots to replace root biomass lost during a previous drought year, or whether a new root:shoot ratio was beginning to develop in response to treatments, will require observations from the response of the stand in future years to be determined.     

Fifteen-year Change in Forest Floor Organic and Element Content and Cycling at the Turkey Lakes Watershed

To assess the long-term effects of atmospheric deposition on forest floor chemical composition, we took quantitative samplings of L-(Oi), F-(Oe), and H-(Oa) layers at an old-growth sugar maple–yellow birch stand on a till soil at the Turkey Lakes Watershed near Lake Superior, Ontario, Canada, in 1981 and 1996. We then assessed these samples for contents of organic matter (OM), total N, K, Ca, Mg, S, and Na, and exchangeable NH₄ ⁺, NO₃ ⁻, K⁺, Ca²⁺, Mg²⁺, SO₄ ²⁻, and Na⁺. Over the 15-year period, total OM and element contents remained unchanged, with the exception of N, which increased significantly from 61.3 kmol/ha in 1981 to 78.4 kmol/ha in 1996. On an area basis, there were significant increases in exchangeable Ca²⁺ (from 3.8 to 4.6 kmol/ha) and Na⁺ (from 0.05 to 0.08 kmol/ha) and decreases in exchangeable NH₄ ⁺-N (from 1.41 to 0.95 kmol/ha) and SO₄ ²⁻-S (from 1.29 to 0.96 kmol/ha). There were no significant differences in average annual litterfall OM, N, Ca, Mg, S or Na inputs between 1980 and 1985 and between 1992 and 1997. Average annual wet-only SO₄ ²⁻-S deposition during 1981–86 was 0.30; during 1992–97, it was 0.21 kmol/ha. Annual wet-only NO₃ ⁻-N averaged 0.33 kmol/ha during 1981–86 and was similar during 1992–97. Throughfall was less rich in SO₄ ²⁻ and Ca²⁺, Mg²⁺, and Na⁺ during 1992–97 than earlier. Throughfall NH₄ ⁺ and NO₃ ⁻ fluxes were unchanged. Efflux of cations from the forest floor reflected reduced throughput of SO₄ ²⁻. Overall, the results suggest that in spite of atmospheric inputs, active biological processes—including litter input, fine-root turnover, and tree uptake—serve to impart stability to the mineral composition of mature sugar maple forest floor. Received 5 October 1999; accepted 25 October 2000.     

Effects of litter incorporation and nitrogen fertilization on the contents of extractable aluminium in the rhizosphere soil of tea plant (Camallia sinensis (L.) O. Kuntze)

The effects of litter incorporation and nitrogen application on the properties of rhizosphere and bulk soils of tea plants (Camellia sinensis (L.) O. Kuntze) were examined in a pot experiment. Total of 8 treatments included four levels of tea litter additions at 0, 4.9, 9.8, and 24.5 g kg^–1 in combination with two N levels (154.6 mg kg^–1 and without). After 18 months of growth the rhizosphere soil was collected by removing the soil adhering to plant roots and other soil was referred to as bulk soil. The dry matter productions of tea plants were significantly increased by N fertilization and litter incorporation. The effect of litter was time-depending and significantly decreased the content of exchangeable Al (Al_ex, by 1 mol L^–1 KCl) and Al saturation at 9 months after litter incorporation whereas soil pH was not affected, although the litter contained high Al content. After 18 months, the contents of extractable Al by dilute CaCl₂, CuCl₂ + KCl, NH₄OAC, ammonium oxalate and sodium citrate (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, Al_Oxal, and Al_Cit respectively) and Al_ex, were not affected by litter application, except that of Al_CaCl2 in the rhizosphere soil which was decreased following litter additions. Nitrogen fertilization with NH₄ ⁺ (urea and (NH₄)₂SO₄) significantly reduced soil pH, the contents of exchangeable Ca, K, Mg and base saturation while raised extractable Al levels (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, and Al_ex). In the rhizosphere soils exchangeable K accumulated in all treatments while exchangeable Ca and Mg depleted in treatments without litter application. The depletions of Ca and Mg were no longer observed following litter incorporation. This change of distribution gradients in rhizosphere was possibly due to the increase of nutrient supplies from litter decomposition and/or preferable root growth in soil microsites rich in organic matter. Lower pH and higher extractable Al (Al_CaCl2, Al_ex, and Al_NH4OAC) in the rhizosphere soils, regardless of N and litter treatments, were distinct and consistent in all treatments. Such enrichments of extractable Al in the rhizosphere soil might be of importance for tea plants capable of taking up large amounts of Al.     

Spatial variability of throughfall chemistry and selected soil properties as influenced by stem distance in a mature Norway spruce (Picea abies,Karst.) stand

Our aims were to investigate the spatial variability of throughfall chemistry and soil parameters as influenced by stem distance and to evaluate the implication of the observed systematic and random patterns for the sampling strategy.One hundred throughfall samplers with a sampling area of 106 cm² each were established in a systematic grid around 5 trees in a mature Norway spruce; site of the Fichtelgebirge (Germany). One hundred soil cores were taken with an auger of 50 cm² next to the throughfall samplers. Soil samples were stratified according to genetic soil horizons and analysed for pH, exchangeable NH₄ ⁺, SO₄ ^2– and total-S. Throughfall samples were collected over a period of 6 months. For each sampler an aliquod sample was mixed over the observation period and analysed for major ions.The spatial variability of the element concentrations in throughfall, expressed by the coefficient of variance, was 21–164%, depending on the element considered. For precipitation volume, the coefficient of variance was only 3%. The distance to the stem influenced most element concentrations in throughfall with increasing concentrations approaching the stem. Steepest gradients were observed in case of SO₄ ^2– and H⁺.The spatial variability of the investigated soil parameters was also very high with the exception of pH. The SO₄ ^2– content of the forest floor reflected the gradients observed in throughfall, while for the other investigated soil parameters and soil horizons no significant relations to stem distance were found.To determine site representative throughfall concentrations and soil properties with the sample volumes and time intervals we used, the number of samples required to get a statistical error of less than 10% (with 95% probability) can be very high. In case of throughfall, more than 100, and in case of the soil parameters, more than 300 replicates would be required.     

Seasonal and within-event dynamics of rainfall and throughfall chemistry in an open tropical rainforest in Rondônia,Brazil

Prolonged dry periods, and increasingly the generation of smoke and dust in partially-deforested regions, can influence the chemistry of rainfall and throughfall in moist tropical forests. We investigated rainfall and throughfall chemistry in a palm-rich open tropical rainforest in the southwestern Brazilian Amazon state of Rondônia, where precipitation averages 2300 mm year^?1 with a marked seasonal pattern, and where the fragmentation of remaining forest is severe. Covering the transition from dry to wet season (TDWS) and the wet season (WS) of 2004–2005, we sampled 42 rainfall events on event basis as well as 35 events on a within-event basis, and measured concentrations of DOC, Na⁺, K⁺, Ca²⁺, Mg²⁺, NH ₄ ⁺ , Cl^?, SO ₄ ^2? , NO ₃ ^? and pH in rainfall and throughfall. We found strong evidence of both seasonal and within-event solute rainfall concentration dynamics. Seasonal volume-weighted mean (VWM_S) concentrations in rainfall of DOC, K⁺, Ca²⁺, Mg²⁺, NH ₄ ⁺ , SO ₄ ^2? and NO ₃ ^? were significantly higher in the TDWS than the WS, while VWM_S concentrations in throughfall were significantly higher for all solutes except DOC. Patterns were generally similar within rain events, with solute concentrations declining sharply during the first few millimeters of rainfall. Rainfall and throughfall chemistry dynamics appeared to be strongly influenced by forest and pasture burning and a regional atmosphere rich in aerosols at the end of the dry season. These seasonal and within-event patterns of rainfall and throughfall chemistry were stronger than those recorded in central Amazônia, where the dry season is less pronounced and where regional deforestation is less severe. Fragmentation and fire in Rondônia now appear to be altering the patterns in which solutes are delivered to remaining moist tropical forests.     

The importance of initial exchangeable ammonium in the nitrogen nutrition of lowland rice soils

Summary The importance of initial exchangeable soil NH ₄ ⁺ in nitrogen nutrition and grain yield of rice was studied in a number of representative lowland rice soils in the Philippines. The initial exchangeable soil NH ₄ ⁺ +fertilizer N plotted against nitrogen uptake by the crop resulted in a highly significant linear relationship (R²=0.91), suggesting that the presence of exchangeable NH ₄ ⁺ in the soil at transplanting behaved like fertilizer nitrogen. The correlation between N fertilizer rate and N uptake by the rice crop was relatively poor (R²=0.73). On the other hand, relative grain yield was more closely correlated with the initial exchangeable soil NH ₄ ⁺ +fertilizer N than with fertilizer nitrogen applied alone. These results indicate that the initial exchangeable NH ₄ ⁺ in the soil contributed substantially to the nitrogen uptake of the crop.Critical nitrogen levels in the soil defined as the initial exchangeable soil NH ₄ ⁺ +fertilizer N at which the optimum grain yield (95% of the maximum yield) is obtained, varied from 60 to 100 kg N/ha in the wet season and from 100 to 120 kg N/ha in the dry season for the different fertilizer treatments. The results further suggest that the initial exchangeable soil NH ₄ ⁺ should serve as a guide in selecting an optimum nitrogen fertilizer rate for high grain yields.     

Can differences in microbial abundances help explain enhanced N2O emissions in a permanent grassland under elevated atmospheric CO2?

Long‐term effects of elevated atmospheric CO₂ on the ammonia‐oxidizing and denitrifying bacteria in a grassland soil were investigated to test whether a shift in abundance of these N‐cycling microorganisms was responsible for enhanced N₂O emissions under elevated atmospheric CO₂. Soil samples (7.5 cm increments to 45 cm depth) were collected in 2008 from the University of Giessen Free Air Carbon dioxide Enrichment (GiFACE), a permanent grassland exposed to moderately elevated atmospheric CO₂ (+20%) since 1998. GiFACE plots lay on a soil moisture gradient because of gradually changing depth to the underlying water table and labeled as the DRY block (furthest from water table), MED block (intermediate to water table), and WET block (nearest to water table). Mean N₂O emissions measured since 1998 have been significantly higher under elevated CO₂. This study sought to identify microbial and biochemical parameters that might explain higher N₂O emissions under elevated CO₂. Soil biochemical parameters [extractable organic carbon (EOC), dissolved organic nitrogen (DON), NH₄⁺, NO₃^?], and abundances of genes encoding the key enzymes involved in ammonia oxidation (amoA) and denitrification (nirK, nirS, nosZ) depended more on soil depth and block (underlying soil moisture gradient) than on elevated CO₂. Ammonia oxidation and denitrification gene abundances, relative abundances (ratios) of nirS to nirK, of nosZ to both nirS and to nirK, and of the measured soil biochemical properties DON and NO₃^? tended to be lower in elevated CO₂ plots as compared with ambient plots in the MED and WET blocks while the DRY block exhibited an opposite trend. High N₂O emissions under elevated CO₂ in the MED and WET blocks correlated with lower nosZ to nirK ratios, suggesting that increased N₂O emissions under elevated CO₂ might be caused by a higher proportion of N₂O‐producing rather than N₂O consuming (N₂ producing) denitrifiers.     

Effect of Sesbania,Azolla and rice straw incorporation on the kinetics of NH4, K,Fe, Mn,Zn and P in some flooded rice soils

In a greenhouse study, with and without rice plants, of five flooded Philippine rice soils whose organic C (OC) content varied from 0.5 to 3.6%, incorporation ofSesbania rostrata, Azolla microphylla and rice straw affected the kinetics of soil solution NH ₄ ⁺ −N, K⁺, Fe²⁺, Mn²⁺, Zn²⁺, and P. Sesbania and Azolla increased NH ₄ ⁺ −N concentration above the control treatment, whereas rice straw depressed it. In all soils Azolla released less NH ₄ ⁺ −N than Sesbania. The apparent net N release depended on the soil and ranged from 44–81% for Sesbania and 27–52% for Azolla. These effects persisted throughout the growth of IR36. Soil solution and exchangeable NH ₄ ⁺ −N increased initially but levelled off between 30 to 80 days and between 20 to 40 days after flooding (DF), respectively. With rice, soil solution NH ₄ ⁺ −N concentration, reached a peak at 15–40 DF and declined to very low levels (<4mg L⁻¹). In the 3 soils of low OC content nitrogen derived from green manure ranged from 34–53% and the apparent revovery of added green manure N varied from 29–67%. Almost all N released from both Azolla and Sesbania were recovered in the rice plant in all soils except Concepcion with only 77%. The concentration of K⁺, Fe²⁺, Mn²⁺ and P in the soil solution were higher with rice straw than Sesbania and Azolla in all soils except Hanggan which showed no change in Fe²⁺ and Mn²⁺ but increased K⁺ and P. In general, rice straw, Sesbania and Azolla decreased Zn²⁺ concentration in all soils.     

Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of Southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41?°S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy–soil–atmosphere interactions and to calculate input–output budgets. From May 1999 till April 2000, the experimental watershed received 8121?mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527?mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6?kg?ha^?1?year^?1 and 8.2?kg?ha^?1?year^?1 respectively. Occult deposition (clouds?+?fog) contributes for 40% to the atmospheric nitrogen input (bulk?+?occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9?kg?ha^?1?year^?1) and organic (5.2?kg?ha^?1?year^?1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2?kg?DIN?ha^?1?year^?1 and 5.6?kg?DON?ha^?1?year^?1). The low concentrations of NO-₃ and NH+₄ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.     

Soil nutrient dynamics in response to irrigation of a Panamanian tropical moist forest

We measured concentrations of soil nutrients (0–15 and 30–35 cm depths) before and after the dry season in control and dry-season irrigated plots of mature tropical moist forest on Barro Colorado Island (BCI) in central Panama to determine how soil moisture affects availability of plant nutrients. Dry-season irrigation (January through April in 1986, 1987, and 1988) enhanced gravimetric soil water contents to wet-season levels (ca. 400 g kg^–1 but did not cause leaching beyond 0.8 m depth in the soil. Irrigation increased concentrations of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), but it had little effect on concentrations of inorganic N (NH₄ ^+C, NO₃ ^– and S (SO₄ ^2–). These BCI soils had particularly low concentrations of extractable P especially at the end of the dry season in April, and concentrations increased in response to irrigation and the onset of the rainy season. We also measured the response of soil processes (nitrification and S mineralization) to irrigation and found that they responded positively to increased soil moisture in laboratory incubations, but irrigation had little effect on rates in the field. Other processes (plant uptake, soil organic matter dynamics) must compensate in the field and keep soil nutrient concentrations at relatively low levels.     

Effects of deforestation and cultivation on soil CEC and contents of exchangeable bases: A case study in Simlipal National Park,India

Deforestation in the tropics seems to be a serious problem probably because of the reduction in soil CEC and the consequent losses of nutrients from the soils. Here, changes in these parameters as influenced by deforestation as well as vegetative cover were studied; statistical methods were applied to interpret the results. Cultivation causes a significant reduction in CEC, total content of the exchangeable bases and exchangeable Ca2+ and Mg2+ levels compared to the adjoining unmanaged forest land. Levels of exchangeable K⁺ and Na⁺, however, do not change significantly. Evergreen forest soils have the highest levels of CEC, total exchangeable bases, exchangeable Ca²⁺ and K⁺. Deciduous forest, grassland and cultivated soils have statistically similar contents of exchangeable Ca²⁺, Mg²⁺, K⁺ and Na⁺. Exchangeable Mg²⁺, however, is not affected by vegetative cover. Soil CEC shows fairly good correlation with the organic carbon content only in evergreen forest soils. In others, organic carbon apparently does not influence CEC significantly. All soils show excellent correlation between their CEC and total exchangeable bases. It is concluded that for regeneration of weathered tropical soils, an evergreen cover provides the most effective means; deciduous vegetation or grass cover do not seem promising.     

Tree growth and soil acidification in response to 30 years of experimental nitrogen loading on boreal forest

Relations among nitrogen load, soil acidification and forest growth have been evaluated based on short‐term (<15 years) experiments, or on surveys across gradients of N deposition that may also include variations in edaphic conditions and other pollutants, which confound the interpretation of effects of N per se. We report effects on trees and soils in a uniquely long‐term (30 years) experiment with annual N loading on an un‐polluted boreal forest. Ammonium nitrate was added to replicated (N=3) 0.09 ha plots at two doses, N1 and N2, 34 and 68 kg N ha^?1 yr^?1, respectively. A third treatment, N3, 108 kg N ha^?1 yr^?1, was terminated after 20 years, allowing assessment of recovery during 10 years. Tree growth initially responded positively to all N treatments, but the longer term response was highly rate dependent with no gain in N3, a gain of 50 m³ ha^?1 stemwood in N2 and a gain of 100 m³ ha^?1 stemwood in excess of the control (N0) in N1. High N treatments caused losses of up to 70% of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺) in the mineral soil, along with decreases in pH and increases in exchangeable Al³⁺. In contrast, the organic mor‐layer (forest floor) in the N‐treated plots had similar amounts per hectare of exchangeable base cations as in the N0 treatment. Magnesium was even higher in the mor of N‐treated plots, providing evidence of up‐lift by the trees from the mineral soil. Tree growth did not correlate with the soil Ca/Al ratio (a suggested predictor of effects of soil acidity on tree growth). A boron deficiency occurred on N‐treated plots, but was corrected at an early stage. Extractable NH₄⁺ and NO₃^?were high in mor and mineral soils of on‐going N treatments, while NH₄+ was elevated in the mor only in N3 plots. Ten years after termination of N addition in the N3 treatment, the pH had increased significantly in the mineral soil; there were also tendencies of higher soil base status and concentrations of base cations in the foliage. Our data suggest the recovery of soil chemical properties, notably pH, may be quicker after removal of the N‐load than predicted. Our long‐term experiment demonstrated the fundamental importance of the rate of N application relative to the total amount of N applied, in particular with regard to tree growth and C sequestration. Hence, experiments adding high doses of N over short periods do not mimic the long‐term effects of N deposition at lower rates.     

Mineral nutrient economy in competing species of Sphagnum mosses

Bog vegetation, which is dominated by Sphagnum mosses, depends exclusively on aerial deposition of mineral nutrients. We studied how the main mineral nutrients are distributed between intracellular and extracellular exchangeable fractions and along the vertical physiological gradient of shoot age in seven Sphagnum species occupying contrasting bog microhabitats. While the Sphagnum exchangeable cation content decreased generally in the order Ca²⁺ ≥ K⁺, Na⁺, Mg²⁺ > Al³⁺ > NH₄ ⁺, intracellular element content decreased in the order N > K > Na, Mg, P, Ca, Al. Calcium occurred mainly in the exchangeable form while Mg, Na and particularly K, Al and N occurred inside cells. Hummock species with a higher cation exchange capacity (CEC) accumulated more exchangeable Ca²⁺, while the hollow species with a lower CEC accumulated more exchangeable Na⁺, particularly in dead shoot segments. Intracellular N and P, but not metallic elements, were consistently lower in dead shoot segments, indicating the possibility of N and P reutilization from senescing segments. The greatest variation in tissue nutrient content and distribution was between species from contrasting microhabitats. The greatest variation within microhabitats was between the dissimilar species S. angustifolium and S. magellanicum. The latter species had the intracellular N content about 40% lower than other species, including even this species when grown alone. This indicates unequal competition for N, which can lead to outcompeting of S. magellanicum from mixed patches. We assume that efficient cation exchange enables Sphagnum vegetation to retain immediately the cationic nutrients from rainwater. This may represent an important mechanism of temporal extension of mineral nutrient availability to subsequent slow intracellular nutrient uptake.