Environmental Control of Root Exudation of Low-Molecular Weight Organic Acids in Tropical Rainforests

The availability of phosphorus (P) can limit net primary production (NPP) in tropical rainforests growing on highly weathered soils. Although it is well known that plant roots release organic acids to acquire P from P-deficient soils, the importance of organic acid exudation in P-limited tropical rainforests has rarely been verified. Study sites were located in two tropical montane rainforests (a P-deficient older soil and a P-rich younger soil) and a tropical lowland rainforest on Mt. Kinabalu, Borneo to analyze environmental control of organic acid exudation with respect to soil P availability, tree genus, and NPP. We quantified root exudation of oxalic, citric, and malic acids using in situ methods in which live fine roots were placed in syringes containing nutrient solution. Exudation rates of organic acids were greatest in the P-deficient soil in the tropical montane rainforest. The carbon (C) fluxes of organic acid exudation in the P-deficient soil (0.7?mol?C?m^?2?month^?1) represented 16.6% of the aboveground NPP, which was greater than those in the P-rich soil (3.1%) and in the lowland rainforest (4.7%), which exhibited higher NPP. The exudation rates of organic acids increased with increasing root surface area and tip number. A shift in vegetation composition toward dominance by tree species exhibiting a larger root surface area might contribute to the higher organic acid exudation observed in P-deficient soil. Our results quantitatively showed that tree roots can release greater quantities of organic acids in response to P deficiency in tropical rainforests.     

Carbon allocation in a Bornean tropical rainforest without dry seasons

To clarify characteristics of carbon (C) allocation in a Bornean tropical rainforest without dry seasons, gross primary production (GPP) and C allocation, i.e., above-ground net primary production (ANPP), aboveground plant respiration (APR), and total below-ground carbon flux (TBCF) for the forest were examined and compared with those from Amazonian tropical rainforests with dry seasons. GPP (30.61 MgC ha^?1 year^?1, eddy covariance measurements; 34.40 MgC ha^?1 year^?1, biometric measurements) was comparable to those for Amazonian rainforests. ANPP (6.76 MgC ha^?1 year^?1) was comparable to, and APR (8.01 MgC ha^?1 year^?1) was slightly lower than, their respective values for Amazonian rainforests, even though aboveground biomass was greater at our site. TBCF (19.63 MgC ha^?1 year^?1) was higher than those for Amazonian forests. The comparable ANPP and higher TBCF were unexpected, since higher water availability would suggest less fine root competition for water, giving higher ANPP and lower TBCF to GPP. Low nutrient availability may explain the comparable ANPP and higher TBCF. These data show that there are variations in C allocation patterns among mature tropical rainforests, and the variations cannot be explained solely by differences in soil water availability.     

Root morphology and architecture respond to N addition in Pinus tabuliformis, west China

Belowground dynamics of terrestrial ecosystems are responding to global increases in anthropogenic N deposition with important consequences for productivity and ecosystem health. We compared root characteristics across five root orders in Pinus tabuliformis plantations treated for 3 years to a gradient of N addition (0–15 g m^?2 year^?1). In reference plots, the roots of P. tabuliformis were finer and with higher specific root length than reported for other pine species, suggesting severe N limitation. Addition of N resulted in slightly reduced fine root biomass and significant changes in root morphology, responses that were associated primarily with first and second order roots. In particular, root number, cumulative root length, individual root length, and specific root length all declined with increasing N addition for first and second order roots, with most of the responses elicited at <9 g m^?2 year^?1 N addition. These responses (1) support the concept of ephemeral root modules consisting of first and second orders and (2) are consistent with a change in functional demand from uptake to transport with increasing soil resource availability. Traditionally, fine roots have been identified by a somewhat arbitrary diameter cut-off (e.g., 1 or 2 mm); as an index of fine root function, diameter would fail to reveal most of the functional response.     

Comprehensive assessments of root biomass and production in a Kobresia humilis meadow on the Qinghai-Tibetan Plateau

Alpine meadow covers ca. 700,000 km² with an extreme altitude range from 3200 m to 5200 m. It is the most widely distributed vegetation on the vast Qinghai-Tibetan Plateau. Previous studies suggest that meadow ecosystems play the most important role in both uptake and storage of carbon in the plateau. The ecosystem has been considered currently as an active “CO₂ sink”, in which roots may contribute a very important part, because of the large root biomass, for storage and translocation of carbon to soil. To bridge the gap between the potential importance and few experimental data, root systems, root biomass, turnover rate, and net primary production were investigated in a Kobresia humilis meadow on the plateau during the growing season from May to September in 2008 and 2009. We hypothesized that BNPP/NPP of the alpine meadow would be more than 50%, and that small diameter roots sampled in ingrowth cores have a shorter lifespan than the lager diameter roots, moreover we expected that roots in surface soils would turn over more quickly than those in deeper soil layers. The mean root mass in the 0–20 cm soil layer, investigated by the sequential coring method, was 1995?±?479 g?m^?2 and 1595?±?254 g?m^?2 in growing season of 2008 and 2009, respectively. And the mean fine root biomass in ingrowth cores of the same soil layer was 119?±?37 g?m^?2 and 196?±?45 g?m^?2 in the 2 years. Annual total NPP was 12387 kg?ha^?1?year^?1, in which 53% was allocated to roots. In addition, fine roots accounted for 33% of belowground NPP and 18% of the total NPP, respectively. Root turnover rate was 0.52 year^?1 for bulk roots and 0.74 year^?1 for fine roots. Furthermore, roots turnover was faster in surface than in deeper soil layers. The results confirmed the important role of roots in carbon storage and turnover in the alpine meadow ecosystem. It also suggested the necessity of separating fine roots from the whole root system for a better understanding of root turnover rate and its response to environmental factors.     

Fine root biomass and turnover of two fast-growing poplar genotypes in a short-rotation coppice culture

Background and aims

The quantification of root dynamics remains a major challenge in ecological research because root sampling is laborious and prone to error due to unavoidable disturbance of the delicate soil-root interface. The objective of the present study was to quantify the distribution of the biomass and turnover of roots of poplars (Populus) and associated understory vegetation during the second growing season of a high-density short rotation coppice culture.

Methods

Roots were manually picked from soil samples collected with a soil core from narrow (75 cm apart) and wide rows (150 cm apart) of the double-row planting system from two genetically contrasting poplar genotypes. Several methods of estimating root production and turnover were compared.

Results

Poplar fine root biomass was higher in the narrow rows than in the wide rows. In spite of genetic differences in above-ground biomass, annual fine root productivity was similar for both genotypes (ca. 44 g DM m^?2 year^?1). Weed root biomass was equally distributed over the ground surface, and root productivity was more than two times higher compared to poplar fine roots (ca. 109 g DM m^?2 year^?1).

Conclusions

Early in SRC plantation development, weeds result in significant root competition to the crop tree poplars, but may confer certain ecosystem services such as carbon input to soil and retention of available soil N until the trees fully occupy the site.     

Fine root dynamics along a 2,000-m elevation transect in South Ecuadorian mountain rainforests

Fine root turnover plays an important role in the cycling of carbon and nutrients in ecosystems. Not much is known about fine root dynamics in tropical montane rainforests, which are characterized by steep temperature gradients over short distances. We applied the minirhizotron technique in five forest stands along an elevational transect between 1,050 and 3,060 m above sea level in a South Ecuadorian montane rainforest in order to test the influence of climate and soil parameters on fine root turnover. Turnover of roots with diameter <?2.0 mm was significantly higher in the lowermost and the uppermost stand (0.9 cm cm^?1 year^?1) than in the three mid-elevation stands (0.6 cm cm^?1 year^?1). Root turnover of finest roots (d?<?0.5 mm) was higher compared to the root cohort with d?<?2.0 mm, and exceeded 1.0 cm cm^?1 year^?1 at the lower and upper elevations of the transect. We propose that the non linear altitudinal trend of fine root turnover originates from an overlapping of a temperature effect with other environmental gradients (e.g. adverse soil conditions) in the upper part of the transect and that the fast replacement of fine roots is used as an adaptive mechanism by trees to cope with limiting environmental conditions.     

Phosphorus cycling in a small watershed in the Brazilian Cerrado: impacts of frequent burning

Plant productivity in many tropical savannas is phosphorus limited. The biogeochemical cycling of P in these ecosystems, however, has not been well quantified. In the present study, we characterized P stocks and fluxes in a well-preserved small watershed in the Brazilian Cerrado. As the Cerrado is also a fire-dominated ecosystem, we measured the P stocks and fluxes in a cerrado stricto sensu plot with complete exclusion of fire for 26 years (unburned plot) and then tested some predictions about the impacts of fire impacts on P cycling in an experimental plot that was burned three times since 1992 (burned plot). The unburned area is an ecosystem with large soil stocks of total P (1,151 kg ha^?1 up to 50 cm depth), but the largest fraction is in an occluded form. Readily extractable P was found up to 3 m soil depth suggesting that deep soil is more important to the P cycle than has been recognized. The P stock in belowground biomass (0?C800 cm) was 9.9 kg ha^?1. Decomposition of fine litter released 0.97 kg P ha^?1 year^?1. Fluxes of P through bulk atmospheric deposition, throughfall and litter leachate were very low (0.008, 0.006 and 0.028 kg ha^?1 year^?1, respectively) as was stream export (0.001 kg ha^?1 year^?1). Immobilization of P by microbes during the rainy season seems to be an important mechanism of P conservation in this ecosystem. Fire significantly increased P flux in litter leachate to 0.11 kg ha^?1 year^?1, and added 1.2 kg ha^?1 of P in ash deposition after fire. We found an increase of P concentration in soil solution at 100 cm depth (from 0.03 ??g l^?1 in unburned plot to 0.3 ??g l^?1 in the burned plot). In surface soils (0?C10 cm) of the burned plot, fire decreased the concentrations of extractable organic-P fractions, but did not significantly increase inorganic-P fractions. The reduction of extractable soil organic P in the burned plot in topsoil and the increase of P in the soil solution at greater depths indicated a reduction of P availability and may increase P fixation in deep soils. Repeated fire events over the long term may result in significant net loss of available forms of phosphorus from this ecosystem.     

Carbon cycling and sequestration in a Japanese red pine (Pinus densiflora) forest on lava flow of Mt. Fuji

Biometric-based carbon flux measurements were conducted in a pine forest on lava flow of Mt. Fuji, Japan, in order to estimate carbon cycling and sequestration. The forest consists mainly of Japanese red pine (Pinus densiflora) in a canopy layer and Japanese holly (Ilex pedunculosa) in a subtree layer. The lava remains exposed on the ground surface, and the soil on the lava flow is still immature with no mineral soil layer. The results showed that the net primary production (NPP) of the forest was 7.3 ± 0.7 t C ha^?1 year^?1, of which 1.4 ± 0.4 t C ha^?1 year^?1 was partitioned to biomass increment, 3.2 ± 0.5 t C ha^?1 year^?1 to above-ground fine litter production, 1.9 t C ha^?1 year^?1 to fine root production, and 0.8 ± 0.2 t C ha^?1 year^?1 to coarse woody debris. The total amount of annual soil surface CO₂ efflux was estimated as 6.1 ± 2.9 t C ha^?1 year^?1, using a closed chamber method. The estimated decomposition rate of soil organic matter, which subtracted annual root respiration from soil respiration, was 4.2 ± 3.1 t C ha^?1 year^?1. Biometric-based net ecosystem production (NEP) in the pine forest was estimated at 2.9 ± 3.2 t C ha^?1 year^?1, with high uncertainty due mainly to the model estimation error of annual soil respiration and root respiration. The sequestered carbon being allocated in roughly equal amounts to living biomass (1.4 t C ha^?1 year^?1) and the non-living C pool (1.5 t C ha^?1 year^?1). Our estimate of biometric-based NEP was 25 % lower than the eddy covariance-based NEP in this pine forest, due partly to the underestimation of NPP and difficulty of estimation of soil and root respiration in the pine forest on lava flows that have large heterogeneity of soil depth. However, our results indicate that the mature pine forest acted as a significant carbon sink even when established on lava flow with low nutrient content in immature soils, and that sequestration strength, both in biomass and in soil organic matter, is large.     

Fine root dynamics in a Norway spruce forest (Picea abies (L.) Karst) in eastern Sweden

The annual dynamics of live and dead fine roots for trees and the field layer species and live/dead ratios were investigated at a coniferous fern forest (Picea abies L. Karts) in Sweden. Our methods of estimating the average amount of fine roots involved the periodic sampling of fine roots in sequential cores on four sampling occasions. The highest live/dead ratio was found in the upper part of the humus layer for both tree and field-layer species and decreased with depth. Most tree fine roots on the four sampling occasions were found in the mineral soil horizon, where 86, 81, 85 and 89% of <1 mm and 89, 88, 89 and 92% of <2 mm diameter of the total amounts of live fine roots in the soil profile were found. The mean amounts of live fine roots of tree species for the total soil profile on the four sampling occasions was 317, 150, 139 and 248 g m^?2 for <1 mm and 410, 225, 224 and 351 g m^?2 for <2 mm diameter fine roots. The related amount of dead fine roots was 226, 321, 176 and 299 g m^?2 and 294, 424, 282 and 381 g m^?2, respectively. Average amounts of live and dead fine-roots and live/dead ratios from other Picea abies forest ecosystems were within the range of our estimates. The production of fine roots, <1 and <2 mm in diameter, estimated from the annual increments in live fine roots, was 207 and 303 g m^?2. The related accumulation of dead fine roots was 257 and 345 g m^?2, The turnover rate of tree fine roots <1 mm in diameter in the total soil profile amounted to 0.7 yr^?1 for live and 0.8 yr^?1 for dead fine roots. The related turnover rates for tree fine roots <2 mm were 0.4 yr^?1 and 0.7 yr^?1. Our data, although based on minimum estimates of the annual fluxes of live and dead fine roots, suggests a carbon flow to the forest soil from dead fine-roots even more substantial than from the needle litter fall. Fine-root data from several Picea abies forest ecosystems, suggest high turnover rates of both live and dead tree fine-roots.     

Microbial C, N and P in soils of Mediterranean oak forests: influence of season, canopy cover and soil depth

In Mediterranean ecosystems the effect of aboveground and belowground environmental factors on soil microbial biomass and nutrient immobilization-release cycles may be conditioned by the distinctive seasonal pattern of the Mediterranean-type climates. We studied the effects of season, canopy cover and soil depth on microbial C, N and P in soils of two Mediterranean forests using the fumigation-extraction procedure. Average microbial values recorded were 820 μg C g^?1, 115 μg N g^?1 and 19 μg P g^?1, which accounted for 2.7, 4.7 and 8.8% of the total pools in the surface soil, respectively. Microbial N and P pools were about 10 times higher than the inorganic N and P fractions available for plants. Microbial C values differed between forest sites but in each site they were similar across seasons. Both microbial and inorganic N and P showed maximum values in spring and minimum values in summer, which were positively correlated with soil moisture. Significant differences in soil microbial properties among canopy cover types were observed in the surface soil but only under favourable environmental conditions (spring) and not during summer. Soil depth affected microbial contents which decreased twofold from surface to subsurface soil. Microbial nutrient ratios (C/N, C/P and N/P) varied with seasons and soil depth. Soil moisture regime, which was intimately related to seasonality, emerged as a potential key factor for microbial biomass growth in the studied forests. Our research shows that under a Mediterranean-type climate the interaction among season, vegetation type and structure and soil properties affect microbial nutrient immobilization and thus could influence the biogeochemical cycles of C, N and P in Mediterranean forest ecosystems.     

Carbon sequestration in freshwater wetlands in Costa Rica and Botswana

Tropical wetlands are typically productive ecosystems that can introduce large amounts of carbon into the soil. However, high temperatures and seasonal water availability can hinder the ability of wetland soils to sequester carbon efficiently. We determined the carbon sequestration rate of 12 wetland communities in four different tropical wetlands—an isolated depressional wetland in a rainforest, and a slow flowing rainforest swamp, a riverine flow-through wetland with a marked wet and dry season, a seasonal floodplain of an inland delta—with the intention of finding conditions that favor soil carbon accumulation in tropical wetlands. Triplicate soil cores were extracted in these communities and analyzed for total carbon content to determine the wetland soil carbon pool. We found that the humid tropic wetlands had greater carbon content (P ≤ 0.05) than the tropical dry ones (96.5 and 34.8 g C kg^?1, respectively). While the dry tropic wetlands had similar sequestration rates (63 ± 10 g Cm^?2 y^?1 on average), the humid tropic ones differed significantly (P < 0.001), with high rates in a slow-flowing slough (306 ± 77 g Cm^?2 y^?1) and low rates in a tropical rain forest depressional wetland (84 ± 23 g Cm^?2 y^?1). The carbon accumulating in all of these wetlands was mostly organic (92–100%). These results suggest the importance of differentiating between types of wetland communities and their hydrology when estimating overall rates at which tropical wetlands sequester carbon, and the need to include tropical wetland carbon sequestration in global carbon budgets.     

Fine-root dynamics in sugi (Cryptomeria japonica) under manipulated soil nitrogen conditions

Aims

Nitrogen deposition affect fine-root dynamics, a key factor in forest carbon and nutrient dynamics. This study aimed to elucidate the effects of increased soil inorganic nitrogen (N) levels on the fine-root dynamics of Cryptomeria japonica, which is tolerant to excess N load.

Methods

An ammonium nitrate solution (28 kg ha^?1 month^?1) was applied for 3 years to plots (1 m?×?2 m) in a C. japonica plantation. The elongation and disappearance of the fine roots were examined using the minirhizotron technique.

Results

The N fertilization increased soil inorganic N content and lowered the soil pH. Fine-root elongation rates increased with fertilization, whereas patterns of their seasonal changes were not affected. The ratio of cumulative disappearance to cumulative elongation of fine roots was lower in the N-fertilized plots than in the control plots. The mean diameter of the fine roots was not affected by N fertilization.

Conclusions

Our results suggest that C. japonica can respond to increased levels of soil inorganic N by increasing both the production and residence time of the fine roots. However, the effects of the changing soil N content are less evident for the phenology and morphology of the fine roots in C. japonica.     

Fine-root morphological and growth traits in a Turkey-oak stand in relation to seasonal changes in soil moisture in the Southern Apennines, Italy

We investigated the effects of seasonal changes in soil moisture on the morphological and growth traits of fine roots (<2?mm in diameter) in a mature Turkey-oak stand (Quercus cerris L.) in the Southern Apennines of Italy. Root samples (diameter:?<0.5, 0.5?C1.0, 1.0?C1.5, and 1.5?C2.0?mm) were collected with the Auger method. Mean annual fine-root mass and length on site was 443?g?m^?2 (oak fine roots 321?g?m^?2; other species 122?g?m^?2) and 3.18?km?m^?2 (oak fine roots 1.14?km?m^?2; other species 2.04?km?m^?2), respectively. Mean specific root length was 8.3?m?g^?1. All fine-root traits displayed a complex pattern that was significantly related to season. In the four diameter classes, both fine-root biomass and length peaked in summer when soil water content was the lowest and air temperature the highest of the season. Moreover, both fine-root biomass and length were inversely related with soil moisture (p?<?0.001). The finest roots (<0.5?mm in diameter) constituted an important fraction of total fine-root length (79?%), but only 21?% of biomass. Only in this root class, consequent to change in mean diameter, specific root length peaked when soil water content was lowest showing an inverse relationship (p?<?0.001). Furthermore, fine-root production and turnover decreased with increasing root diameter. These results suggest that changes in root length per unit mass, and pulses in root growth to exploit transient periods of low soil water content may enable trees to increase nutrient and water uptake under seasonal drought conditions.     

Different nitrogen saturation thresholds for above-, below-, and total net primary productivity in a temperate steppe

Identifying the thresholds for the positive responses of total net primary productivity (NPP) to nitrogen (N) enrichment is an essential prerequisite for predicting the benefits of N deposition on ecosystem carbon sequestration. However, the responses of below-ground NPP (BNPP) to N enrichment are unknown in many ecosystems, which limits our ability to understand the carbon cycling under the scenario of increasing N availability. We examined the changes in above-ground NPP (ANPP), BNPP, and NPP of a temperate meadow steppe across a wide-ranging N addition gradient (0, 2, 5, 10, 20, and 50 g N m⁻² year⁻¹) during 5 years. Both ANPP and NPP increased nonlinearly with N addition rates. The N saturation threshold for ANPP (T_A) and NPP (T_N) was at the rate of 13.11 and 6.70 g N m⁻² year⁻¹, respectively. BNPP decreased with increasing N addition when N addition rates ˃5 g N m⁻² year⁻¹, resulting in much lower T_N than T_A. Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, and consequently on the earlier occurrence of N saturation threshold for NPP. Our results highlight the negative effects of soil N enrichment on NPP in natural grasslands super-saturated with N. Furthermore, by considering ANPP and BNPP simultaneously, our results indicate that previous findings from above-ground might have over-estimated the positive effects of N deposition on primary productivity.     

Short-term simulated nitrogen deposition increases carbon sequestration in a Pleioblastus amarus plantation

In order to understand the influence of nitrogen (N) deposition on the key processes relevant to the carbon (C) balance in a bamboo plantation, a two-year field experiment involving the simulated deposition of N in a Pleioblastus amarus plantation was conducted in the rainy region of SW China. Four levels of N treatments: control (no N added), low-N (50 kg N ha^?1 year^?1), medium-N (150 kg N ha^?1 year^?1), and high-N (300 kg N ha^?1 year^?1) were set in the present study. The results showed that soil respiration followed a clear seasonal pattern, with the maximum rates in mid-summer and the minimum in late winter. The annual cumulative soil respiration was 585?±?43 g CO₂-C m^?2 year^?1 in the control plots. Simulated N deposition significantly increased the mean annual soil respiration rate, fine root biomass, soil microbial biomass C (MBC), and N concentration in fine roots and fresh leaf litter. Soil respirations exhibited a positive exponential relationship with soil temperature, and a linear relationship with MBC. The net primary production (NPP) ranged from 10.95 to 15.01 Mg C ha^?1 year^?1 and was higher than the annual soil respiration (5.85 to 7.62 Mg C ha^?1 year^?1) in all treatments. Simulated N deposition increased the net ecosystem production (NEP), and there was a significant difference between the control and high N treatment NEP, whereas, the difference of NEP among control, low-N, and medium-N was not significant. Results suggest that N controlled the primary production in this bamboo plantation ecosystem. Simulated N deposition increased the C sequestration of the P. amarus plantation ecosystem through increasing the plant C pool, though CO₂ emission through soil respiration was also enhanced.     

Temporal changes in spatial pattern of fine-root mass and nutrient concentrations in Indian bamboo savanna

Abstract. Temporal variations in the spatial distribution of fine-root mass and nutrient concentrations were studied in recently harvested and mature bamboo savanna sites in the dry tropical Vindhyan region in India. The soil block method and root-free-soil cages were used to investigate fine-root dynamics. The mean annual fine-root biomass was 596 and 690 g/m² in harvested and mature sites, respectively. The fine-root net production calculated by different methods ranged from 486 to 749 g m^-2 yr^-1 in the harvested site and 485 to 875 g m^-2 yr¹ in the mature site. All fine-root mass fractions decreased with increase in distance from the base of bamboo clumps, and the herb root mass showed the reverse trend. Bamboo fine roots were better developed in the 10 - 20 cm soil depth and those of herbs in the upper 10 cm. The ingrowth of fine roots in root-free-soil cages showed maximum biomass accumulation during the rainy season (64.2 - 69.9 g m^-2 mo^-1) and minimum in the summer (4.5 - 7.5 g m^-2 mo^-1). The fine-root nutrient concentrations were strongly related to their diameter. The fine-root nutrient concentrations varied considerably in different seasons. The highest nutrient concentrations in all categories were recorded in summer followed by winter and rainy seasons. Nutrient concentrations in live roots were always greater than those found in dead roots in different diameter classes. We suggest the occurrence of nutrient retranslocation from senescent roots to surviving roots in bamboo savanna. Fine roots in the bamboo savanna increased as a function of N-mineralization and nitrification rates. This tendency further increased after the harvest of bamboo, suggesting the crucial role of fine roots in the bamboo savanna after the harvesting of bamboo culms.     

Soil fertility and fine root dynamics in response to 4 years of nutrient (N,P, K) fertilization in a lowland tropical moist forest,Panama

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m^?2 year^?1), phosphorus (P as superphosphate, 5 g P m^?2 year^?1) and potassium (K as KCl, 5 g K m^?2 year^?1) within 38 ha of old‐growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg^?1), low concentrations of Bray‐extractable phosphate (PO₄ = 2.2 mg kg^?1), and modest concentrations of KCl‐extractable nitrate (NO₃ = 5.0 mg kg^?1) and KCl‐extractable ammonium (NH₄ = 15.5 mg kg^?1). Added N increased concentrations of KCl‐extractable NO₃ and acidified the soil by one pH unit. Added P increased concentrations of Bray‐extractable PO₄ and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m^?2. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species‐rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.     

Evaluating the net primary productivity in the grasslands of southern China from 2001 to 2010 using a new land portfolio assessment model

Grassland ecosystems play important roles in the global carbon cycle. The net primary productivity (NPP) of grassland ecosystems has become the hot spot of terrestrial ecosystems. To simulate the NPP in the grasslands of southern China, we built a land portfolio assessment (LPA) model. The LPA model was named according to the framework and principle of this model. From the framework of the model aspect, it was mainly driven by two parameters: leaf area index (LAI) and photosynthesis accumulation (PA). LAI is an extremely important structural characteristic of grassland and directly related to the exchange of energy, CO₂ and mass at a variety of scales. PA is represented by the amount of net photosynthetic production based on fixed-point observation. From the principle of the model aspect, it is represented by the inherent implication of NPP and a part of land portfolio assessment. The results showed that the NPP values in the study area had a decreasing trend from east to west and south to north and that the mean NPP was 320 g C m^?2 year^?1 from 2001 to 2010. Correlations analysis showed that the correlation coefficient (r) between NPP and highest monthly mean temperature of a year was the maximum (0.6422), and the r value between NPP and annual precipitation was the minimum (0.3821). Using trial and error, the LPA model accurately simulated the NPP dynamics of southern China’s grassland ecosystem, and the results were biologically realistic.     

Potential nitrogen and carbon processing in a landscape rich in milldam legacy sediments

Recent identification of the widespread distribution of legacy sediments deposited in historic mill ponds has increased concern regarding their role in controlling land–water nutrient transfers in the mid-Atlantic region of the US. At Big Spring Run in Lancaster, Pennsylvania, legacy sediments now overlay a buried relict hydric soil (a former wetland soil). We compared C and N processing in legacy sediment to upland soils to identify soil zones that may be sources or sinks for N transported toward streams. We hypothesized that legacy sediments would have high nitrification rates (due to recent agricultural N inputs), while relict hydric soils buried beneath the legacy sediments would be N sinks revealed via negative net nitrification and/or positive denitrification (because the buried former wetland soils are C rich but low in O₂). Potential net nitrification ranged from 9.2 to 77.9 g m^?2 year^?1 and potential C mineralization ranged from 223 to 1,737 g m^?2 year^?1, with the highest rates in surface soils for both legacy sediments and uplands. Potential denitrification ranged from 0.37 to 21.72 g m^?2 year^?1, with the buried relict hydric soils denitrifying an average of 6.2 g m^?2 year^?1. Contrary to our hypothesis, relict hydric layers did not have negative potential nitrification or high positive potential denitrification rates, in part because microbial activity was low relative to surface soils, as indicated by low nitrifier population activity, low substrate induced respiration, and low exoenzyme activity. Despite high soil C concentrations, buried relict hydric soils do not provide the ecological services expected from a wetland soil. Thus, legacy sediments may dampen N removal pathways in buried relict hydric soils, while also acting as substantial sources of NO₃ ^? to waterways.     

Forest Productivity and Efficiency of Resource Use Across a Chronosequence of Tropical Montane Soils

We tested the hypothesis that plants adjust to nutrient availability by altering carbon allocation patterns and nutrient-use efficiency (NUE = net primary production [NPP] per unit nutrient uptake), but are constrained by a trade-off between NUE and light-use efficiency () = NPP per unit intercepted light). NPP, NUE and ) were measured in montane Metrosideros polymorpha forest across a 4.1 x 10⁶ yr space for time substitution chronosequence in which available soil N and P pools change with site age. Although the range of N and P availability across sites was broad, there was little difference in NPP between sites, and in contrast to theories of carbon allocation relative to limiting resources, we found no consistent relationships in production allocation to leaves, fine roots or wood. However, canopy nutrient pools and fluxes were correlated with the mass of fine roots per unit soil volume and there was a weak but positive correlation of NPP with LAI. Patterns of ) and NUE across the soil developmental sequence were opposite to each other. ) increased as nutrient availability and nutrient turnover increased, while NUE decreased in response to the same influences but reached its highest values where either N or P availability and turnover of both N and P were low. A negative correlation between ) and NUE supports the hypothesis that a trade-off exists between ) and leaf characteristics affecting NUE.