Base cation biogeochemistry and weathering under oak and pine: a controlled long-term experiment

Large earthen-walled lysimeters at the San Dimas Experimental Forest in southern California present a unique opportunity to assess vegetation effects on biogeochemical processes and cation release by weathering in controlled soil-vegetation systems where archived samples of soil parent material are available for comparison. The lysimeters were filled in 1937 with homogenized fine sandy loam derived on site from the weathering of diorite, and planted in 1946 with scrub oak (Quercus dumosa) and Coulter pine (Pinus coulteri). Changes in base cation contents were measured in above-ground biomass, and total and exchangeable soil pools to a depth of 1 meter. All cations in the non-exchangeable soil pool decreased relative to the initial fill material, indicating release by weathering. Sodium and K were depleted from both exchangeable and non-exchangeable pools of the soils. Plant uptake of Na was minimal, whereas K storage in vegetation exceeded the loss from the exchangeable soil pool. In both soil-vegetation systems, but especially for oak, there was an increase in exchangeable Ca and Mg. For all base cations, storage in above-ground biomass was greater for oak, whereas losses by weathering from the non-exchangeable soil pool were greater under pine. Strong evidence supports biocycling as a controlling mechanism resulting in greater Ca and Mg release by weathering under pine. In addition, decreases in non-exchangeable Ca and Mg were strongly correlated to decrease in Si under oak, whereas no correlation was observed under pine. We conclude that weathering reactions or stoichiometry differed between vegetation types.Corresponding author     

Lessons from the Sandbox: Is Unexplained Nitrogen Real?

In their review of 24 studies of forest nitrogen (N) budgets, Binkley and others (2000) found that only one of them supported the conclusion that an N accumulation of more than 25 kg N ha⁻¹ y⁻¹ is possible without known symbiotic N₂–fixing plants. They contended that, given how well the N cycle is known, new N accumulation pathways are unlikely. They also concluded that the Hubbard Brook sandbox study (Bormann and others 1993) was insufficiently replicated and had low precision in vegetation and soil estimates. Here we reevaluate and extend the sandbox analysis and place the findings in a broader context. Using multiple methods of estimating vegetation N accumulation in pine sandboxes, we arrived at results that differed from the reported rates but still strongly supported large biomass N accumulation. The original study's conclusions about soil N changes were strengthened when new evidence showed that N accumulated in lower horizons and that the sandboxes were successfully homogenized at the beginning of the experiment. Unexplained ecosystem N accumulation ranged from about 40 to 150 kg ha⁻¹ y⁻¹, with 95% confidence intervals that did not include zero. No evidence was found that could balance the sandbox ecosystem N budgets without adding unexplained N. Unreplicated experiments, such as the sandboxes, can explore the possibility that N can accumulate in ways not explainable by mass balance analysis, but they cannot quantify the frequency and extent of the phenomenon. New studies should combine substantive microbiological, mass balance, and process research using multiple direct measures of N₂ fixation. Received 4 September 2001; accepted 3 April 2002.     

Olivine Weathering in Soil, and Its Effects on Growth and Nutrient Uptake in Ryegrass (Lolium perenne L.): A Pot Experiment

Mineral carbonation of basic silicate minerals regulates atmospheric CO(2) on geological time scales by locking up carbon. Mining and spreading onto the earth's surface of fast-weathering silicates, such as olivine, has been proposed to speed up this natural CO(2) sequestration ('enhanced weathering'). While agriculture may offer an existing infrastructure, weathering rate and impacts on soil and plant are largely unknown. Our objectives were to assess weathering of olivine in soil, and its effects on plant growth and nutrient uptake. In a pot experiment with perennial ryegrass (Lolium perenne L.), weathering during 32 weeks was inferred from bioavailability of magnesium (Mg) in soil and plant. Olivine doses were equivalent to 1630 (OLIV1), 8150, 40700 and 204000 (OLIV4) kg ha(-1). Alternatively, the soluble Mg salt kieserite was applied for reference. Olivine increased plant growth (+15.6%) and plant K concentration (+16.5%) in OLIV4. At all doses, olivine increased bioavailability of Mg and Ni in soil, as well as uptake of Mg, Si and Ni in plants. Olivine suppressed Ca uptake. Weathering estimated from a Mg balance was equivalent to 240 kg ha(-1) (14.8% of dose, OLIV1) to 2240 kg ha(-1) (1.1%, OLIV4). This corresponds to gross CO(2) sequestration of 290 to 2690 kg ha(-1) (29 10(3) to 269 10(3) kg km(-2).) Alternatively, weathering estimated from similarity with kieserite treatments ranged from 13% to 58% for OLIV1. The Olsen model for olivine carbonation predicted 4.0% to 9.0% weathering for our case, independent of olivine dose. Our % values observed at high doses were smaller than this, suggesting negative feedbacks in soil. Yet, weathering appears fast enough to support the 'enhanced weathering' concept. In agriculture, olivine doses must remain within limits to avoid imbalances in plant nutrition, notably at low Ca availability; and to avoid Ni accumulation in soil and crop.     

An approach at estimating present day base cation weathering rates: a case study for the Hermine watershed,Canada

A new methodological approach is described for estimating Ca, Mg and K fluxes from soil mineral weathering. This method combines Na flux in surface waters in the Hermine watershed with base cation (BC) concentrations to Na molar ratios from the soil weatherable pool obtained using sequential extraction method. Comparison of BC:Na molar ratios of the weatherable pool with those from other compartments of the watershed suggests possible accumulation of base cations in some areas of the watershed, while losses or minimal changes are observed in others. On average, present day Na weathering rates estimated using the watershed input–output budget method was 0.26 (range 0.16–0.36) kmol_c ha^?1 yearr^?1, over the period of 1995–2006. For Ca, Mg and K, present day weathering rates estimated with the new methodological approach averaged 0.44 (range 0.27–0.60), 0.11 (range 0.07–0.15) and 0.02 (range 0.01–0.02) kmol_c ha^?1 year^?1, respectively. These values are within the range of present day rates previously calculated for the same site and for forested soils from similar granitic environments using other methods. Candidate models for predicting BC weathering rates on individual annual observations were developed using Akaike’s information criterion. The best model includes the number of frost days (inverse relationship) and explained 51% of the variation in total BC weathering rates. The newly developed method may be applicable to other watersheds, providing yearly estimates of nutrient BC at the watershed scale.     

The effects of ferulic acid on the mineral nutrition of grain sorghum

The combined effects of whole-tree harvesting (WTH) and soil leaching by both acid deposition and naturally-produced carbonic acid were evaluated in a mixed oak and a loblolly pine forest growing on similar soils in the Ridge and Valley province of eastern Tennessee. It was hypothesized that nutrient export via WTH would be greater in a mixed oak stand than in the loblolly pine stand because of greater nutrient concentrations in oak and hickory species than in pine. This hypothesis was true for N,P, and particularly Ca at the time of harvest, but not for K or Mg. When expressed on an annual basis, exports of N,P,K, and Mg were greater in the loblolly pine site and only Ca export was greater in the mixed oak site. It was also hypothesized that the large accumulation of Ca in the oak and hickory vegetation would cause lower exchangeable Ca²⁺ in soils, and, consequently, lower Ca²⁺ leaching in the mixed oak site than in the loblolly pine site. This hypothesis was supported by the data, which indicated 340–370% more exchangeable Ca and 100% more Ca²⁺ leaching in the loblolly pine site than in the mixed oak site. Research sponsored by the U.S. Environmental Protection Agency under Interagency Agreement No. 79-D-X0533 and Biofuels and Municipal Waste Technology Division, U.S. Department of Energy, under Contract No. De-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. Publication No. 2933, Environmental Sciences Division, ORNL.     

Controls of Soil Spatial Variability in a Dry Tropical Forest

We examined the roles of lithology, topography, vegetation and fire in generating local-scale (<1 km²) soil spatial variability in a seasonally dry tropical forest (SDTF) in southern India. For this, we mapped soil (available nutrients, Al, total C, pH, moisture and texture in the top 10cm), rock outcrops, topography, all native woody plants ≥1 cm diameter at breast height (DBH), and spatial variation in fire frequency (times burnt during the 17 years preceding soil sampling) in a permanent 50-ha plot. Unlike classic catenas, lower elevation soils had lesser moisture, plant-available Ca, Cu, Mn, Mg, Zn, B, clay and total C. The distribution of plant-available Ca, Cu, Mn and Mg appeared to largely be determined by the whole-rock chemical composition differences between amphibolites and hornblende-biotite gneisses. Amphibolites were associated with summit positions, while gneisses dominated lower elevations, an observation that concurs with other studies in the region which suggest that hillslope-scale topography has been shaped by differential weathering of lithologies. Neither NO₃⁻-N nor NH₄⁺-N was explained by the basal area of trees belonging to Fabaceae, a family associated with N-fixing species, and no long-term effects of fire on soil parameters were detected. Local-scale lithological variation is an important first-order control over soil variability at the hillslope scale in this SDTF, by both direct influence on nutrient stocks and indirect influence via control of local relief.     

Cation distribution,cycling, and removal from mineral soil in Douglas-fir and red alder forests

Overstory species influence the distribution and dynamics of nutrients in forest ecosystems. Ecosystem-level estimates of Ca, Mg, and K pools and cycles in 50-year old Douglas-fir and red alder stands were used to determine the effect of overstory composition on net cation removal from the mineral soil, i.e. cation export from the soil in excess of additions. Net cation removal from Douglas-fir soil was 8 kg Ca ha^–1 yr^–1, 1 kg Mg ha^–1 yr^–1, and 0.3 kg K ha^–1 yr^–1. Annual cation export from soil by uptake and accumulation in live woody tissue and O horizon was of similar magnitude to leaching in soil solution. Atmospheric deposition partially off-set export by adding cations equivalent to 28–88% of cation export. Net cation removal from red alder soil was 58 kg Ca ha^–1 yr^–1, 9 kg Mg ha^–1 yr^–1, and 11 kg K ha^–1 yr^–1. Annual cation accumulation in live woody tissue and O horizon was three times greater than in Douglas-fir, while cation leaching in soil solution was five to eight times greater. The lack of excessive depletion of exchangeable cations in the red alder soil suggests that mineral weathering, rather than exchangeable cations, was the source of most of the removed cations. Nitric acid generated during nitrification in red alder soil led to high rates of weathering and NO₃-driven cation leaching.     

The effect of bacteria and fungi on chemical weathering and chemical denudation fluxes in pine growth experiments

Vascular plants and associated microbial communities affect the nutrient resources of terrestrial ecosystems by impacting chemical weathering that transfers elements from primary minerals to other ecosystem pools, and chemical denudation that transports weathered elements out of the system in solution. We performed a year-long replicated flow-through column growth experiment to isolate the effects of vascular plants, ectomycorrhiza-forming fungi and associated bacteria on chemical weathering and chemical denudation. The study focused on Ca²⁺, K⁺ and Mg²⁺, for which the sole sources were biotite and anorthite mixed into silica sand. Concentrations of the cations were measured in input and output solutions, and three times during the year in plant biomass and on exchangeable cation sites of the growth medium. Weathering and denudation fluxes were estimated by mass balance, and mineral surface changes, biofilm and microbial attachments to surfaces were investigated with scanning electron microscopy. Both bacteria and fungi increased weathering fluxes compared to abiotic controls. Without a host plant denudation rates were as large as weathering rates i.e. the weathering to denudation ratio was about one. Based on whole year fluxes, ectomycorrhizal seedlings produced the greatest weathering to denudation ratios (1.5). Non-ectomycorrhizal seedlings also showed a high ratio of 1.3. Both ectomycorrhizal hyphal networks and root hairs of non-ectomycorrhizal trees, embedded in biofilm (microorganisms surrounded by extracellular polymers), transferred nutrients to the host while drainage losses were minimized. These results suggest that biofilms localize both weathering and plant nutrient uptake, isolating the root-hypha-mineral interface from bulk soil solution.     

Short-term effects of thinning and liming on forest soils of pitch pine and Japanese larch plantations in central Korea

The influences of thinning (50% of standing density) and liming (Ca+Mg, 2 Mg ha⁻¹) on soil chemical properties were investigated for 2 years (2001, 2002) in 40-year-old pitch pine (Pinus rigida Mill.) and 44-year-old Japanese larch (Larix leptolepis Gord.) plantations established on similar soils. In general, soil properties varied significantly among plantations and treatments. For both plantations, thinning significantly increased soil organic C (SOC) concentrations whereas there were no significant changes in soil pH and Ca and Mg concentrations. In addition, thinning increased total soil N and Na concentrations for the pitch pine plantation and available P concentration for the Japanese larch plantation in the second year after the treatment. Liming did not affect soil chemical characteristics for the pitch pine plantation except for Na concentration. However, for the Japanese larch plantation, liming significantly increased soil pH and K, Ca and Mg concentrations and decreased SOC and total soil N concentrations. For both plantations, soil Al concentration did not change after thinning and liming and decreased exponentially with increased pH values. The increases in SOC and total soil N concentrations after thinning were possibly due to increases in decomposition of organic matter and root death. Although differences were not statistically significant, soil available P concentration tended to increase at early stages of liming for both plantations. These results suggested that thinning and liming seemed to regulate soil chemical properties for pitch pine and Japanese larch plantations established on similar soils.     

Nutrient cycling and foliar status in an urban pine forest in Athens, Greece

The nutrient cycling and foliar status for the elements Ca, Mg, K, N, P, S, Fe, Mn, Zn and Cu were investigated in an urban forest of Aleppo pine (Pinus halepensis) in 2004 in Athens, Greece in order to draw conclusions on the productivity status and health of the ecosystem. The fluxes of bulk and throughfall deposition were characterized by the high amounts of Ca, organic N and sulfate S. The magnitude of the sulfate S fluxes indicated a polluted atmosphere. The nutrient enrichment in throughfall was appreciable for ammonium N, P and Mn. The mineral soil formed the largest pool for all the elements followed by the forest floor, trunk wood and trunk bark. The understory vegetation consisting of annual plants proved important for storing N, P and K. Compared to current year needles of Aleppo pine in remote forests of Spain, the needles of the Aleppo pine trees in Athens had significantly higher concentrations of Ca, N, P and Cu and significantly lower concentrations of Mg and Zn. The soil had a high concentration of calcium carbonate and accordingly high pH values. When all inputs to the forest floor were taken into account, the mean residence time of nutrients in the forest floor followed the order Fe > Mn > Cu > Ca > Mg > P > Zn > N > K > S.     

Laccaria bicolor S238N improves Scots pine mineral nutrition by increasing root nutrient uptake from soil minerals but does not increase mineral weathering

The role of ectomycorrhizal fungi on mineral nutrient mobilization and uptake is crucial for tree nutrition and growth in temperate forest ecosystems. By using a “mineral weathering budget” approach, this study aims to quantify the effect of the symbiosis with the ectomycorrhizal model strain Laccaria bicolor S238N on mineral weathering and tree nutrition, carrying out a column experiment with a quartz/biotite substrate. Each column was planted with one Scots pine (Pinus sylvestris L.) non-mycorrhizal or mycorrhizal with L. bicolor, with exception of the abiotic control treatment. The columns were continuously supplied with a nutrient-poor solution. A mineral weathering budget was calculated for K and Mg. The pine shoot growth was significantly increased (73%) when plants were mycorrhizal with L. bicolor. Whatever their mycorrhizal status, pines increased mineral weathering by factors 1.5 to 2.1. No difference between non-mycorrhizal and mycorrhizal pine treatments was revealed, however, mycorrhizal pines assimilated significantly more K and Mg. This suggests that in our experimental conditions, L. bicolor S238N improved shoot growth and K and Mg assimilation in Scots pine mainly by increasing the uptake of dissolved nutrients, linked to a better exploration and exploitation of the soil by the mycorrhizal roots.     

Responses of two populations of an Iranian nickel-hyperaccumulating serpentine plant,Alyssum inflatum Nyar., to substrate Ca/Mg quotient and nickel

We selected two geographically close serpentine and non-serpentine populations of a Ni-hyperaccumulating plant (Alyssum inflatum) to investigate the influence of two common factors of serpentine soils: high Ni concentrations and low Ca/Mg quotients. Soils and plants were sampled from serpentine and non-serpentine substrates, and concentrations of Ca, Mg and Ni were measured. A hydroponic culture was used to compare growth and elemental composition responses of serpentine and non-serpentine plants to different Ca/Mg quotients and Ni concentrations in the nutrient solution. The Ca/Mg quotient for non-serpentine soils was 15 times higher than for serpentine soils, but there was no difference in the Ca/Mg quotient of plants from the two populations. In hydroponic culture, plants from both populations were able to survive at high Ca/Mg quotients. This result suggests that serpentine plants of A. inflatum do not necessarily need a substrate with a low Ca/Mg quotient for survival. Decreases in the Ca/Mg quotient in hydroponics decreased growth. The magnitude of this decrease was significantly greater in non-serpentine plants, suggesting a greater resistance of serpentine plants to low Ca/Mg quotients. Total Ni concentration in serpentine soils was 13 times higher than in non-serpentine soils, but ammonium nitrate-extractable concentrations of Ni in both soil types were similar. Ni concentrations in non-serpentine plants from their natural habitat were significantly lower than in serpentine plants, but there was no significant difference in Ni accumulation by plants of the two populations in hydroponic culture. However, increased concentrations of Ni in the hydroponic medium caused similar decreases in growth of both populations, indicating that Ni tolerance of the two populations was similar.     

Nutrient pools and fluxes of the ground vegetation in coniferous forests due to fertilizing,liming and amelioration

The effects of fertilization and amelioration treatments on some nutrient pools and fluxes of ground vegetation in mature pine and spruce stands on acid soils in South Germany are described. In N-limited pine forests with moderate canopy density and with Deschampsia flexuosa an additional N-accumulation in biomass of 20–40 kg ha^-1 occurred 3 years after pure N-fertilization. The N, P, K-cycling through ground vegetation was stimulated more than 10 years by a combined N + CaCO₃ + P treatment leading toa shift in dominance from cryptogams and Ericaceae towards Deschampsia flexuosa and ruderal species like Epilobium angustifolium. The effect of a lupine treatment (combined with initial soil preparation, liming and P supply) was far stronger than the effect of the other experimental procedures. But the fertilizer and amelioration effects on the herb layer of pine forests tended to decline after two decades for different reasons.The shade-tolerant ground vegetation in a nitrogen-saturated spruce forest was not able to prevent heavy additional nitrate losses from upper mineral soil after dolomitic liming. But the Ca, Mg and K fluxes through ground vegetation were strongly elevated in the third year after treatment.     

Effects of water and nutrient applications in a Scots pine stand to tree growth and nutrient cycling

In a Scots pine forest stand, demineralized water and a complete set of nutrients with water were applied to the soil by means of frequent irrigation for four years in order to eliminate water and nutrient shortage of the trees. Apart from this optimization, dissolved (NH₄)₂SO₄ was irrigated at a rate of 120 kg N ha^-1 y^-1 to create a situation of N excess. Effect of treatments on tree growth and chemical composition of soil water and vegetation were monitored. From the first treatment year onwards basal area growth increased by ca. 35% as a result of the increased water supply. Nutrient applications increased K and P concentrations in pine needles immediately, but growth was enhanced only in the fourth treatment year and coincided with an improved K supply. Most of the applied P and K was retained in the soil, and only 6% was recovered in the vegetation. Tree nutrient status did not respond on Ca and Mg applications, whereas Ca and Mg seepage losses were increased with ca. 5 kg ha^-1 y^-1. The applied NH₄ was mostly retained in the 0–20 cm surface soil and caused a drastic increase of Al in soil solution. Tree growth was stimulated initially by extra NH₄, but was hampered after three years obviously because of a decreased P nutrition. The applied base cations were absorped to the soil and the accompanying anions were leached, thus temporarily increasing the acidification of the soil solution.     

Calcium, potassium, and magnesium cycling in aMexican tropical dry forest ecosystem

We estimated the fluxes, inputs and outputs of Ca, K,and Mg in a Mexican tropical dry forest. The studywas conducted in five contiguous small watersheds(12–28 ha) gauged for long-term ecosystem research. A total of five 80 × 30 m plots were used for thestudy. We quantified inputs from the atmosphere,dissolved and particulate-bound losses, throughfalland litterfall fluxes, and standing crop litter pools. Mean cation inputs for a six-year period were 3.03 kg/ha for Ca, 1.31 kg/ha for K, and 0.80 kg/ha for Mg. Mean outputs in runoff were 5.24, 2.83, and 1.79 kg/ha, respectively. Calcium, K, and Mgconcentrations increased as rainfall moved through thecanopy. Annual Ca return in the litterfall (11.4 g/m²) was much higher than K (2.3 g/m²)and Mg (1.6 g/m²). Litterfall represented 99%of the Ca, 84% of the Mg, and 53% of the K, totalaboveground return to the soil. Calcium concentrationin standing litter (3.87%) was much higher than K(0.38%) and Mg (0.37%). These concentrations werehigher (Ca), lower (K), or similar (Mg) to those inlitterfall. Residence times on the forest floor were0.86, 1.17, and 1.77 yr for K, Mg, and Carespectively. Compared to the residence time fororganic matter at the site (1.31 yr), these resultssuggest slow mineralization for Ca in this ecosystem. Budget estimates were calculated for a wet and a dryyear. Results indicated that nutrients accumulated inthe dry but that nutrients were lost during the wetyear. Comparison of Ca, K, and Mg losses in streamwater with the input rates from the atmosphere for thesix-year period show that inputs are lower thanoutputs in the Chamela tropical dry forestecosystem.     

Major element chemistry,weathering and element yields for the Caura River drainage,Venezuela

The Caura River, a major tributary of the Orinoco River, was sampled at bi-weekly intervals for two years. Because the watershed is covered with undisturbed forest overlying a Precambrian shield, the water of the Caura River has low conductivity (mean, 15 uS/cm), contains small amounts of particulate material (mean, 11 mg/1), and is slightly acidic (median pH, 6.8). Concentrations of total dissolved solids vary less than two-fold in response to the seasonal ten-fold variation in discharge; concentrations of particulate material vary more (ten-fold) and are lowest at the time of peak discharge. Seasonal changes in concentrations of Si, major metal cations, and hydrogen ion are complementary to each other and indicate regular seasonal changes in weathering rates. Measurements of bulk atmospheric deposition and the observed runoff yield of Cl and S were used in estimating the basin-wide atmospheric deposition of major elements, which were as follows (kg/ha/yr): Ca, 1.3; Mg, 0.29; Na, 8.2; K, 1.0; Cl, 12; S, 2.8; P, 0.14. Element ratios show that terrestrial sources contribute strongly to the atmospheric deposition of Ca, K, S, and P. From the atmospheric contributions and runoff yields, watershed retention was computed for major elements with reference to Si. The watershed accumulates Al, Fe, and P, whereas losses of Ca, Mg, Na, K, and S originating from non-atmospheric sources exceed the relative loss rates of Si. The rock weathering rate based on Si is 1.8 cm/ 1,000 years. Although significant amounts of Ca, Mg, Na, and K are found in atmospheric deposition, the dominant influence on the mass balances of these elements is weathering rather than deposition. Weathering has a trivial influence on the cycles of Cl and S. Both atmospheric deposition and weathering are important in the mass balance of P. The ecosystem does not effectively conserve most biologically active elements (Ca, Mg, Na, K). The ecosystem conserves significant amounts of phosphorus (31% of total input), but probably by abiotic mechanisms.     

Production and nutrient dynamics of plant communities on a sub-Antarctic island

Summary Studies of plant standing crop and nutrient concentrations have enabled an assessment of the seasonal changes in nutrient standing stocks (the mass of nutrients per m²) in two mire-grasslands at Marion Island (46°54S, 37°45E). Mire-grasslands are an important component of the island's vegetation, occurring on very wet peats and dominated by graminoids and bryophytes. Peak aboveground standing stocks of N, P and K in the vascular plant species of the mire-grasslands mostly occurred earlier in the season than did peak aboveground biomass, implying that aboveground accumulation rates of these nutrients were greater than the rate of biomass accumulation. Maximum Ca standing stocks coincided in the season with peak shoot biomass. Depending on the plant species, peak Mg and Na standing stocks occurred either before, or later than, peak shoot biomass. Total (above-plus belowground) standing stocks of nutrients (N+P+K+Ca+Mg+Na) at the time of peak aboveground biomass were 51 g m^-2 at study mire 1 and 44 g m^-2 at study mire 2. The most abundant element in the vegetation was N, followed by K. The net quantities of most nutrients translocated into the aboveground growth were mostly greater than the seasonal mean standing stocks in the aerial biomass. Except for Ca, nutrient standing stocks in the vegetation of the mire-grasslands are in the upper part of the range reported for sub-Arctic and Arctic graminoid communities. They are more similar to standing stocks at oceanic moorlands, montane grasslands and heath communities. Low Ca concentrations occur in the plants so that Ca standing stocks are lower than in most comparable northern hemisphere communities. Pool sizes (i.e. total quantities contained in the plant/soil system to a depth of 25 cm) of N, P, K and Ca are in the lower part of the range reported for wet, graminoid-dominated tundra and tundra-like communities of the northern hemisphere.     

黄土丘陵区两种主要退耕还林树种生态系统碳储量和固碳潜力

黄土丘陵区是中华文明的起源地,而原有植被却遭受严重破坏。因此,自20世纪70年代末开始的三北防护林工程、退耕还林工程和天然林保护工程等大型生态恢复工程,在本区均有大面积分布。这些工程已经对生态恢复起到重要作用,并将对全球碳素循环起到积极作用。以黄土丘陵区的主要造林树种--油松(Pinus tabulaeformis Carr.)和刺槐(Robinia pseudoacacia L.)为研究对象,共设置样方28个,测定森林乔木、灌木、草本生物量及凋落物碳储量;钻取并分析土样516份,获得土壤有机碳储量。结合文献数据和农田碳储量数据,建立0-86年生油松林和0-56年生刺槐纯林生态系统碳储量-林龄序列;在此基础上分析造林对生态系统碳储量和固碳潜力的影响。结果表明,造林后的油松林和刺槐林生态系统的植被、凋落物及土壤碳储量逐渐增加;在没有人为干扰的情况下,19、27、36、86年生油松林生态系统碳储量分别为70.76、143.43、167.30、271.23-332.26 Mg/hm²;8、17、39年生刺槐林生态系统碳储量分别为80.37、94.08、140.77 Mg/hm²。受间伐干扰、45\,52年生油松林生态系统碳储量分别为136.42\,168.56 Mg/hm²,相对于没有人为干扰的油松林,其植被碳储量明显下降,而土壤碳储量保持稳定甚至升高。受乱砍滥伐干扰的71年生油松林和56年生刺槐林的生态系统碳储量分别为118.87\,76.99 Mg/hm²,相对于没有人为干扰的森林,其植被碳储量和土壤碳储量均呈明显下降趋势。种植油松林之后的86a时间内,其生态系统固碳潜力为211.61-272.64 Mg/hm²;而种植刺槐林、在39a时间内的生态系统固碳潜力为81.15 Mg/hm²。     

Biological versus geochemical control and environmental change drivers of the base metal budgets of a tropical montane forest in Ecuador during 15 years

To assess the susceptibility of the base metal budget of a remote tropical montane forest in Ecuador to environmental change, we determined the extent of biological control of base metal fluxes and explored the impact of atmospheric inputs and precipitation, considered as potential drivers of ecosystem change, on the base metal fluxes. We quantified all major base metal fluxes in a ca. 9.1 ha forested catchment from 1998 to 2013. Mean (±s.d.) annual flux to the soil via throughfall + stemflow + litterfall was 13800 ± 1500 mg m^?2 Ca, 19000 ± 1510 mg m^?2 K, 4690 ± 619 mg m^?2 Mg and 846 ± 592 mg m^?2 Na of which 22 ± 6, 45 ± 16, 39 ± 10 and 84 ± 33%, respectively, were leached to below the organic layer. The mineral soil retained 79–94% of this Ca, K and Mg, while Na was released. Weathering rates estimated with three different approaches ranged from not detected (ND) to 504 mg m^?2 year^?1 Ca, ND-1770 mg m^?2 year^?1 K, 287–597 mg m^?2 year^?1 Mg and 403–540 mg m^?2 year^?1 Na. The size of mainly biologically controlled aboveground fluxes of Ca, K and Mg was 1–2 orders of magnitude larger than that of mainly geochemically controlled fluxes (sorption to soil and weathering). The elemental catchment budgets (total deposition ? streamflow) were positive for Ca (574 ± 893 mg m^?2) and K (1330 ± 773 mg m^?2), negative for Na (?370 ± 1300 mg m^?2) and neutral for Mg (1.89 ± 304 mg m^?2). Our results demonstrate that biological processes controlled element retention for Ca, K and Mg in the biological part of the ecosystem. This was different for Na, which was mainly released by weathering from the study catchment, while the biological part of the ecosystem was Na-poor. The deposition of base metals was the strongest driver of their budgets suggesting that the base metal cycling of the study ecosystem is susceptible to changing deposition.     

Major element fluxes from a coniferous catchment in central Ontario, 1983–1999

Acid deposition over time scales of decades may deplete essential base cation (BC) reserves in soils to the extent that forest health may be affected. In order to assess the nutrient status of soils in central Ontario, input–output budgets for calcium (Ca), magnesium (Mg), potassium (K) and nitrogen (N) were calculated over a 17-year period (1983–1999) for a coniferous catchment in the Muskoka-Haliburton region. Inputs through deposition and weathering (BCs only), were compared with outputs through stream export and net accumulation in forest biomass. Despite a lack of forest growth at PC1, annual NO₃–N concentrations in the stream were low (<0.1mg/l) over the 17-year period, and over 80% of the atmospheric N input was retained in the catchment, indicating this catchment has not reached N-saturation. Stream export of Mg, and in particular Ca, exceeded input of these elements through deposition and weathering, indicating a net loss from the catchment over the 17-year period. Mass balance calculations indicated there was no net loss of K from the catchment. Soil re-sampling measurements confirmed large losses of Ca, but not Mg, and there were significant decreases in both NaCl-exchangeable Ca concentrations and soil pH between 1983 and 1999. The measured decline in soil Ca concentration amounted to a loss of approximately 85kg/ha Ca from the exchangeable pool over the 17-year period. Similarly, input–output budget calculations indicated a net loss of Ca from the catchment in the range of 76 to 88kg/ha between 1983 and 1999. Although the magnitude of Ca export decreased over the 17-year period, current stream export continues to exceed Ca input through deposition and weathering.