Effects of elevated CO2, temperature and N fertilization on nitrogen fluxes in a temperate grassland ecosystem

The soil nitrogen cycle was investigated in a pre‐established Lolium perenne sward on a loamy soil and exposed to ambient and elevated atmospheric CO₂ concentrations (350 and 700 μL L^?1) and, at elevated [CO₂], to a 3 °C temperature increase. At two levels of mineral nitrogen supply, N– (150 kgN ha^?1 y^?1) and N+ (533 kgN ha^?1 y^?1), ¹⁵N‐labelled ammonium nitrate was supplied in split applications over a 2.5‐y period. The recovery of the labelled fertilizer N was measured in the harvests, in the stubble and roots, in the macro‐organic matter fractions above 200 μm in size (MOM) and in the aggregated organic matter below 200 μM (AOM). Elevated [CO₂] reduced the total amount of N harvested in the clipped parts of the sward. The harvested N derived from soil was reduced to a greater extent than that derived from fertilizer. At both N supplies, elevated [CO₂] modified the allocation of the fertilizer N in the sward, in favour of the stubble and roots and significantly increased the recovery of fertilizer N in the soil macro‐organic matter fractions. The increase of fertilizer N immobilization in the MOM was associated with a decline of fertilizer N uptake by the grass sward, which supported the hypothesis of a negative feedback of elevated [CO₂] on the sward N yield and uptake. Similar and even more pronounced effects were observed for the native N mineralized in the soil. At N–, a greater part of the fertilizer N organized in the root phytomass resulted in an underestimation of N immobilized in dead roots and, in turn, an underestimation of N immobilization in the MOM. The 3 °C temperature increase alleviated the [CO₂] effect throughout much of the N cycle, increasing soil N mineralization, N derived from soil in the harvests, and the partitioning of the assimilated fertilizer N to shoots. In conclusion, at ambient temperature, the N cycle was slowed down under elevated [CO₂], which restricted the increase in the aboveground production of the grass sward, and apparently contributed to the sequestration of carbon belowground. In contrast, a temperature increase under elevated [CO₂] stimulated the soil nitrogen cycle, improved the N nutrition of the sward and restricted the magnitude of the soil C sequestration.     

Effects of contrasting soil fertility on root mass, root growth, root decomposition and soil carbon under a New Zealand perennial ryegrass/white clover pasture

The contribution of below ground plant root tissue to soil carbon (C) pools is attracting considerable interest in the context of greenhouse gas mitigation options. A field experiment was conducted on a perennial ryegrass/white clover pasture in the Manawatu, New Zealand, to examine the effect of differing soil nitrogen (N) and phosphorus (P) fertility status on root dynamics. Root standing mass, shoot and root dry matter (DM) accumulation and root tissue decomposition were measured at 6–8 week intervals over one year at moderate (Olsen P?=?24, no added N) and high (Olsen P?=?49, 400 kgN ha^?1y^?1 added N) soil fertility levels. Shoot production was significantly greater in the high fertility treatment (2550 cf. 1890 gDM m^?2y^?1) but differences in root dynamics were confined to two periods in spring and winter. In late spring the pattern was for lower root mass (183 cf. 231 gDM m^?2 between 0–80 mm depth) and higher root production (0.71 cf. 0.52 gDM m^?2 d^?1 between 0–120 mm depth) under higher fertility. In winter the reverse was observed. There is some evidence that the soil type used in the root in-growth cores underestimated root production values for this site by a factor of approx. one third. Short-term differences between the two fertiity treatments in standing root mass and root production did not lead to treatment differences in topsoil C and N changes over four years. This may reflect insufficient separation in the two soil fertility treatments and a low overall root tissue input to soil organic matter.     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

Nitrogen deposition and plant species interact to influence soil carbon stabilization

Anthropogenic nitrogen (N) deposition effects on soil organic carbon (C) decomposition remain controversial, while the role of plant species composition in mediating effects of N deposition on soil organic C decomposition and long‐term soil C sequestration is virtually unknown. Here we provide evidence from a 5‐year grassland field experiment in Minnesota that under elevated atmospheric CO₂ concentration (560 ppm), plant species determine whether N deposition inhibits the decomposition of soil organic matter via inter‐specific variation in root lignin concentration. Plant species producing lignin‐rich litter increased stabilization of soil C older than 5 years, but only in combination with elevated N inputs (4 g m^?2 year^?1). Our results suggest that N deposition will increase soil C sequestration in those ecosystems where vegetation composition and/or elevated atmospheric CO₂ cause high litter lignin inputs to soils.     

A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize‐based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0–269 kg N ha^?1 yr^?1) that created a large range in crop residue inputs (3.60–9.94 Mg dry matter ha^?1 yr^?1), we provide the first agronomic assessment of long‐term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico‐chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra‐aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha^?1 yr^?1) and an excessive N rate (269 kg N ha^?1 yr^?1), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N₂O and NO₃ emissions exponentially increase when agronomic optimum N is surpassed.     

Large losses of soil C and N from soil profiles under pasture in New Zealand during the past 20 years

The conversion of two‐thirds of New Zealand's native forests and grasslands to agriculture has followed trends in other developed nations, except that pastoral grazing rather than cropping dominates agriculture. The initial conversion of land to pasture decreased soil acidity and elevated N and P stocks, but caused little change in soil organic C stocks. However, less is known about C and nutrient stock changes during the last two decades under long‐term pastoral management. We resampled 31 whole soil profiles in pastures spanning seven soil orders with a latitudinal range of 36–46°S, which had originally been sampled 17–30 years ago. We measured total C, total N, and bulk density for each horizon (generally to 1 m) and also reanalyzed archived soil samples of the same horizons for C and N. On average, profiles had lost significant amounts of C (− 2.1 kg C m⁻²) and N (− 0.18 kg N m⁻²) since initial sampling. Assuming a continuous linear decline in organic matter between sampling dates, significant losses averaged 106 g C m⁻² yr⁻¹ (P=0.01) and 9.1 g N m⁻² yr⁻¹ (P=0.002). Removal of C through leaching and erosion appears too small to explain these losses, suggesting losses from respiration exceed the inputs of photosynthate in the soil profile. These results emphasize that resampling soil profiles provide a robust method for detecting soil C changes, and add credence to the suggestion that soil C losses may be occurring in some temperate soil profiles. Further work is required to determine whether these losses are continuing and how losses might be extrapolated across landscapes to determine the implications for New Zealand's national CO₂ emissions and the sustainability of the implied rates of soil N loss.     

Fine litter input to terrestrial humus forms in Colombian Amazonia

A comparative litter fall study was made in five rain forest stands along a gradient of humus form development and soils in the Amazon lowlands of eastern Colombia. The total fine litter fall was highest in a plot on a well drained soil of the flood plain of the Caquetá River (1.07 kg · m^-2 · y^-1), lower in three plots on well drained upland soils (0.86, 0.69, and 0.68 kg · m^-2 · y^-1), and lowest in a plot on a poorly drained, upland podzolised soil (0.62 kg · m^-2 · y^-1). In the four upland plots, leaf litter fall patterns were highly associated, which points at climatic regulation. Litter resource quality, as represented by nutrient concentrations and area/weight ratio of the leaf litter fall, was comparatively high in the flood plain plot. In the upland plots, concentrations and fluxes of Ca, Mg, K, and P were as low as in oligotrophic central Amazonian upland forests. This questions generalisations that the western peripheral region of the Amazon basin should be less oligotrophic than central Amazonia. The upland plot on the podzolised soil showed the lowest concentrations and fluxes of N. Mean residence times of organic matter and nutrients in the L horizons hardly differed between the five plots, suggesting that edaphic properties and litter resource quality are of little importance in the first step of decomposition. Mean residence time of organic matter in all ectorganic horizons combined (estimated on the basis of litter input and necromass on the forest floor, and uncorrected for dead fine root input) varied from 1.0 y in the flood plain forest, 1.1–3.3 y in the well drained upland forests, and 10.2 y in the forest on the podzolised soil.     

Soil carbon fractions in grasslands respond differently to various levels of nitrogen enrichments

Background and Aims

Soil contains many different C fractions which have diverse physical and chemical compositions. Examining these differential soil C fractions in response to N enrichment is helpful for better understanding soil C changes under the predominantly increasing N deposition. In this study, we used a field N addition experiment in a grassland to explore the effects of various N enrichment levels on soil C fractions.

Methods

We conducted a field manipulative experiment which used a Latin square design with six N addition levels of 0, 2, 4, 8, 16 and 32 g N m^?2 year^?1 since 2003 in a semiarid grassland in northern China. Soil samples were collected in August (when plants have the greatest biomass), 2011. We measured C and N concentrations in soil light fraction, microbial biomass, extractable organic matter, heavy fraction, and total soil C and N.

Results

The results showed that total soil C and N, and heavy fraction C and N were not significantly affected by N addition after 9 years of treatments. In contrast, different N enrichment levels changed soil light fraction C and N, ranging from 4.3 to 27.7 % and 3.3–30.0 %, respectively. Moreover, both light fraction C and N had a nonlinear relationship with N addition rates, and the threshold for N-induced change in light fraction C and N was near 16 g N m^?2 year^?1 in this semiarid grassland. Increases of soil light fraction C and N primarily resulted from changes in biotic (N-stimulated aboveground biomass) and abiotic (soil temperature, moisture and pH) factors under N enrichment. Soil microbial biomass exponentially declined with increasing N, but extractable organic C showed a positive linear response to N enrichment rates. Changes in microbial biomass C and extractable organic C were primarily due to the reduced soil pH under N addition.

Conclusions

Our findings suggest that various soil C fractions differentially respond to elevated N, because different sets of biotic and abiotic factors regulate those fractions under N enrichment.     

Impact of Agricultural Land-use Change on Carbon Storage in Boreal Alaska

Climate warming is most pronounced at high latitudes, which could result in the intensification of the extensively cultivated areas in the boreal zone and could further enhance rates of forest clearing in the coming decades. Using paired forest‐field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Chronosequences showed large soil C losses during the first two decades following deforestation, with mean C stocks in agricultural soils being 44% or 8.3 kg m^?2 lower than C stocks in original forest soils. This suggests that soil C losses from land‐use change in the boreal region may be greater than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences indicated an increase in C storage during later decades after forest clearing, with 60‐year‐old grassland showing net ecosystem C gain of 2.1 kg m^?2 over the original forest. This increase in C stock resulted probably from a combination of large C inputs from belowground biomass and low C losses due to a small original forest soil C stock and low tillage frequency. Reductions in soil N stocks caused by land‐use change were smaller than reductions in C stocks (34% or 0.31 kg m^?2), resulting in lower C/N ratios in field compared with forest mineral soils, despite the occasional incorporation of high‐C forest‐floor material into field soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which could indicate that decomposition rates are more sensitive in forest soils than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes more widespread in the boreal region, the resulting C losses (51% or 11.2 kg m^?2 at the ecosystem level in this study) will induce a positive feedback to climatic warming and additional land‐use change. However, by selecting relatively C‐poor soils and by implementing management practices that preserve C, losses of C from soils can be reduced.     

Partitioning sources of soil-respired CO2 and their seasonal variation using a unique radiocarbon tracer

Soil respiration is derived from heterotrophic (decomposition of soil organic matter) and autotrophic (root/rhizosphere respiration) sources, but there is considerable uncertainty about what factors control variations in their relative contributions in space and time. We took advantage of a unique whole‐ecosystem radiocarbon label in a temperate forest to partition soil respiration into three sources: (1) recently photosynthesized carbon (C), which dominates root and rhizosphere respiration; (2) leaf litter decomposition and (3) decomposition of root litter and soil organic matter >1–2 years old. Heterotrophic sources and specifically leaf litter decomposition were large contributors to total soil respiration during the growing season. Relative contributions from leaf litter decomposition ranged from a low of ～1±3% of total soil respiration (6± 3 mg C m^?2 h^?1) when leaf litter was extremely dry, to a high of 42±16% (96± 38 mg C m^?2 h^?1). Total soil respiration fluxes varied with the strength of the leaf litter decomposition source, indicating that moisture‐dependent changes in litter decomposition drive variability in total soil respiration fluxes. In the surface mineral soil layer, decomposition of C fixed in the original labeling event (3–5 years earlier) dominated the isotopic signature of heterotrophic respiration. Root/rhizosphere respiration accounted for 16±10% to 64±22% of total soil respiration, with highest relative contributions coinciding with low overall soil respiration fluxes. In contrast to leaf litter decomposition, root respiration fluxes did not exhibit marked temporal variation ranging from 34±14 to 40±16 mg C m^?2 h^?1 at different times in the growing season with a single exception (88±35 mg C m^?2 h^?1). Radiocarbon signatures of root respired CO₂ changed markedly between early and late spring (March vs. May), suggesting a switch from stored nonstructural carbohydrate sources to more recent photosynthetic products.     

Interactive Effects of Fire, Soil Climate, and Moss on CO2 Fluxes in Black Spruce Ecosystems of Interior Alaska

Fire is an important control on the carbon (C) balance of the boreal forest region. Here, we present findings from two complementary studies that examine how fire modifies soil organic matter properties, and how these modifications influence rates of decomposition and C exchange in black spruce (Picea mariana) ecosystems of interior Alaska. First, we used laboratory incubations to explore soil temperature, moisture, and vegetation effects on CO₂ and DOC production rates in burned and unburned soils from three study regions in interior Alaska. Second, at one of the study regions used in the incubation experiments, we conducted intensive field measurements of net ecosystem exchange (NEE) and ecosystem respiration (ER) across an unreplicated factorial design of burning (2 year post-fire versus unburned sites) and drainage class (upland forest versus peatland sites). Our laboratory study showed that burning reduced the sensitivity of decomposition to increased temperature, most likely by inducing moisture or substrate quality limitations on decomposition rates. Burning also reduced the decomposability of Sphagnum-derived organic matter, increased the hydrophobicity of feather moss-derived organic matter, and increased the ratio of dissolved organic carbon (DOC) to total dissolved nitrogen (TDN) in both the upland and peatland sites. At the ecosystem scale, our field measurements indicate that the surface organic soil was generally wetter in burned than in unburned sites, whereas soil temperature was not different between the burned and unburned sites. Analysis of variance results showed that ER varied with soil drainage class but not by burn status, averaging 0.9 ± 0.1 and 1.4 ± 0.1 g C m⁻² d⁻¹ in the upland and peatland sites, respectively. However, a more complex general linear model showed that ER was controlled by an interaction between soil temperature, moisture, and burn status, and in general was less variable over time in the burned than in the unburned sites. Together, findings from these studies across different spatial scales suggest that although fire can create some soil climate conditions more conducive to rapid decomposition, rates of C release from soils may be constrained following fire by changes in moisture and/or substrate quality that impede rates of decomposition. Author contributions: JAO: performed research, analyzed data, contributed new methods, wrote the paper; MRT: designed laboratory study, performed research, analyzed data; JWH: designed field study, performed research; KLM: performed research; LEP: performed research, contributed new method; GS: performed research; JCN: performed research.     

Linking sequestration of 13C and 15N in aggregates in a pasture soil following 8 years of elevated atmospheric CO2

The influence of N availability on C sequestration under prolonged elevated CO₂ in terrestrial ecosystems remains unclear. We studied the relationships between C and N dynamics in a pasture seeded to Lolium perenne after 8 years of elevated atmospheric CO₂ concentration (FACE) conditions. Fertilizer‐¹⁵N was applied at a rate of 140 and 560 kg N ha^2?1 y^2?1 and depleted ¹³C‐CO₂ was used to increase the CO₂ concentration to 60 Pa pCO₂. The ¹³C–¹⁵N dual isotopic tracer enabled us to study the dynamics of newly sequestered C and N in the soil by aggregate size and fractions of particulate organic matter (POM), made up by intra‐aggregate POM (iPOM) and free light fraction (LF). Eight years of elevated CO₂ did not increase total C content in any of the aggregate classes or POM fractions at both rates of N application. The fraction of new C in the POM fractions also remained largely unaffected by N fertilization. Changes in the fractions of new C and new N (fertilizer‐N) under elevated CO₂ were more pronounced between POM classes than between aggregate size classes. Hence, changes in the dynamics of soil C and N cycling are easier to detect in the POM fractions than in the whole aggregates. Within N treatments, fractions of new C and N in POM classes were highly correlated with more new C and N in large POM fractions and less in the smaller POM fractions. Isotopic data show that the microaggregates were derived from the macro‐aggregates and that the C and N associated with the microaggregates turned over slower than the C and N associated with the macroaggregates. There was also isotopic evidence that N immobilized by soil microorganisms was an important source of N in the iPOM fractions. Under low N availability, 3.04 units of new C per unit of fertilizer N were sequestered in the POM fractions. Under high N availability, the ratio of new C sequestered per unit of fertilizer N was reduced to 1.47. Elevated and ambient CO₂ concentrations lead to similar ¹⁵N enrichments in the iPOM fractions under both low and high N additions, clearly showing that the SOM‐N dynamics were unaffected by prolonged elevated CO₂ concentrations.     

Effect of Increased Carbon Dioxide and Temperature on Runoff Chemistry at a Forested Catchment in Southern Norway (CLIMEX Project)

CLIMEX (Climate Change Experiment) is an integrated, whole-ecosystem research project that focuses on the response of forest ecosystems at the catchment scale to increased CO₂ and temperature. KIM catchment (860 m²) is completely enclosed by a transparent greenhouse, receives deacidifed “clean” rain, and has elevated CO₂ (560 ppmv) and elevated air temperature (3°–5°C above ambient). The uppermost 20% of the catchment is partitioned off, is not subject to changed CO₂ or temperature, and serves as an untreated control. Fluxes of nitrate and ammonium in runoff from KIM catchment increased from 2 mmol m^{− 2} y^{− 1} each in the 3 years before treatment to 6 and 3 mmol m^{− 2} y^{− 1}, respectively, in the 3 years after treatment (May 1994–April 1997), despite a 15 mmol m^{− 2} y^{− 1} decrease in N dry deposition due to the sealing of the walls to the enclosure. N flux in runoff from three reference catchments and the control section did not change. The net loss of inorganic N was thus about 20 mmol m^{− 2} treated soil y^{− 1}. There were no changes in organic N or total organic carbon in runoff. The ecosystem switched from a net sink to a net source of inorganic nitrogen (N). The increased loss of N may be due to accelerated decomposition of soil organic matter induced by higher temperature. Due to many decades of N deposition from long-range transported pollutants, the ecosystem prior to treatment was N saturated. If global change induces persistent losses of inorganic N on a regional scale, the result may be a significant increase in nitrate concentrations in fresh waters and N loading to coastal marine ecosystems. In regions with acid sensitive waters, such as southern Norway, the increased nitrate release caused by global change may offset improvements achieved by reduced sulfur and N deposition. Received 15 October 1997; accepted 18 November 1997.     

The effect of nitrogen addition on soil organic matter dynamics: a model analysis of the Harvard Forest Chronic Nitrogen Amendment Study and soil carbon response to anthropogenic N deposition

Recent observations indicate that long-term N additions can slow decomposition, leading to C accumulation in soils, but this process has received limited consideration by models. To address this, we developed a model of soil organic matter (SOM) dynamics to be used with the PnET model and applied it to simulate N addition effects on soil organic carbon (SOC) stocks. We developed the model’s SOC turnover times and responses to experimental N additions using measurements from the Harvard Forest, Massachusetts. We compared model outcomes to SOC stocks measured during the 20th year of the Harvard Forest Chronic Nitrogen Amendment Study, which includes control, low (5 g N m^?2 yr^?1) and high (15 g N m^?2 yr^?1) N addition to hardwood and red pine stands. For unfertilized stands, simulated SOC stocks were within 10 % of measurements. Simulations that used measured changes in decomposition rates in response to N accurately captured SOC stocks in the hardwood low N and pine high N treatment, but greatly underestimated SOC stocks in the hardwood high N and the pine low N treatments. Simulated total SOC response to experimental N addition resulted in accumulation of 5.3–7.9 kg C per kg N following N addition at 5 g N m^?2 yr^?1 and 4.1–5.3 kg C per kg N following N addition at 15 g N m^?2 yr^?1. Model simulations suggested that ambient atmospheric N deposition at the Harvard Forest (currently 0.8 g N m^?2 yr^?1) has led to an increase in cumulative O, A, and B horizons C stocks of 211 g C m^?2 (3.9 kg C per kg N) and 114 g C m^?2 (2.1 kg C per kg N) for hardwood and pine stands, respectively. Simulated SOC accumulation is primarily driven by the modeled decrease in SOM decomposition in the Oa horizon.     

Modelling temporal variability in the carbon balance of a spruce/moss boreal forest

A model of the daily carbon balance of a black spruce/feathermoss boreal forest ecosystem was developed and results compared to preliminary data from the 1994 BOREAS field campaign in northem Manitoba, Canada. The model, driven by daily weather conditions, simulated daily soil climate status (temperature and moisture profiles), spruce photosynthesis and respiration, moss photosynthesis and respiration, and litter decomposition. Model agreement with preliminary field data was good for net ecosystem exchange (NEE), capturing both the asymmetrical seasonality and short-term variability. During the growing season simulated daily NEE ranged from -4 g C m^-2 d^-1 (carbon uptake by ecosystem) to + 2 g C m^-2 d^-1 (carbon flux to atmosphere), with fluctuations from day to day. In the early winter simulated NEE values were + 0.5 g C m^-2 d^-1, dropping to + 0.2 g C m^-2 d^-1 in mid-winter. Simulated soil respiration during the growing season (+ 1 to + 5 g C m^-2 d^-1) was dominated by metabolic respiration of the live moss, with litter decomposition usually contributing less than 30% and live spruce root respiration less than 10% of the total. Both spruce and moss net primary productivity (NPP) rates were higher in early summer than late summer. Simulated annual NEE for 1994 was -51 g C m^-2 y^-1, with 83% going into tree growth and 17% into the soil carbon accumulation. Moss NPP (58 g C m^-2 y^-1) was considered to be litter (i.e. soil carbon input; no net increase in live moss biomass). Ecosystem respiration during the snow-covered season (84 g C m^-2) was 58% of the growing season net carbon uptake. A simulation of the same site for 1968–1989 showed = 10–20% year-to-year variability in heterotrophic respiration (mean of + 113 g C m^-2 y^-1). Moss NPP ranged from 19 to 114 g C m^-2 y^-1; spruce NPP from 81 to 150 g C m^-2 y^-1; spruce growth (NPP minus litterfall) from 34 to 103 g C m^-2 y^-1; NEE ranged from +37 to -142 g C m^-2 y^-1. Values for these carbon balance terms in 1994 were slightly smaller than the 1969–89 means. Higher ecosystem productivity years (more negative NEE) generally had early springs and relatively wet summers; lower productivity years had late springs and relatively dry summers.     

Connecting carbon and nitrogen storage in rural wetland soil to groundwater abstraction for urban water supply

We investigated whether groundwater abstraction for urban water supply diminishes the storage of carbon (C), nitrogen (N), and organic matter in the soil of rural wetlands. Wetland soil organic matter (SOM) benefits air and water quality by sequestering large masses of C and N. Yet, the accumulation of wetland SOM depends on soil inundation, so we hypothesized that groundwater abstraction would diminish stocks of SOM, C, and N in wetland soils. Predictions of this hypothesis were tested in two types of subtropical, depressional‐basin wetland: forested swamps and herbaceous‐vegetation marshes. In west‐central Florida, >650 ML groundwater day^?1 are abstracted for use primarily in the Tampa Bay metropolis. At higher abstraction volumes, water tables were lower and wetlands had shorter hydroperiods (less time inundated). In turn, wetlands with shorter hydroperiods had 50–60% less SOM, C, and N per kg soil. In swamps, SOM loss caused soil bulk density to double, so areal soil C and N storage per m² through 30.5 cm depth was diminished by 25–30% in short‐hydroperiod swamps. In herbaceous‐vegetation marshes, short hydroperiods caused a sharper decline in N than in C. Soil organic matter, C, and N pools were not correlated with soil texture or with wetland draining‐reflooding frequency. Many years of shortened hydroperiod were probably required to diminish soil organic matter, C, and N pools by the magnitudes we observed. This diminution might have occurred decades ago, but could be maintained contemporarily by the failure each year of chronically drained soils to retain new organic matter inputs. In sum, our study attributes the contraction of hydroperiod and loss of soil organic matter, C, and N from rural wetlands to groundwater abstraction performed largely for urban water supply, revealing teleconnections between rural ecosystem change and urban resource demand.     

Measurements and Modeling of Carbon and Nitrogen Cycling in Agroecosystems of Southern Wisconsin: Potential for SOC Sequestration during the Next 50 Years

Landmanagement practices such as no-tillage agriculture and tallgrass prairie restoration have been proposed as a possible means to sequester atmospheric carbon, helping to refurbish soil fertility and replenish organic matter lost as a result of previous agricultural management practices. However, the relationship between land-use changes and ecosystem structure and functioning is not yet understood. We studied soil and vegetation properties over a 4-year period (1995–98), and assembled measurements of microbial biomass, soil organic carbon (SOC) and nitrogen (N), N-mineralization, soil surface carbon dioxide (CO₂) flux, and leached C and N in managed (maize; Zea mays L.) and natural (prairie) ecosystems near the University of Wisconsin Agricultural Research Station at Arlington. Field data show that different management practices (tillage and fertilization) and ecosystem type (prairie vs maize) have a profound influence on biogeochemistry and water budgets between sites. These measurements were used in conjunction with a dynamic terrestrial ecosystem model, called IBIS (the Integrated Biosphere Simulator), to examine the long-term effects of land-use changes on biogeochemical cycling. Field data and modeling suggest that agricultural land management near Arlington between 1860 and 1950 caused SOC to be depleted by as much as 63% (native SOC approximately 25.1 kg C m⁻²). Reductions in N-mineralization and microbial biomass were also observed. Although IBIS simulations depict SOC recovery in no-tillage maize since the 1950s and also in the Arlington prairie since its restoration was initiated in 1976, field data suggest otherwise for the prairie. This restoration appears to have done little to increase SOC over the past 24 years. Measurements show that this prairie contained between 28% and 42% less SOC (in the top 1 m) than the no-tillage maize plots and 40%–47% less than simulated potential SOC for the site in 1999. Because IBIS simulates competition between C3 and C4 grass species, we hypothesized that current restored prairies, which include many forbs not characterized by the model, could be less capable of sequestering C than agricultural land planted entirely in monocultural grass in this region. Model output and field measurements show a potential 0.4 kg C m⁻² y⁻¹ difference in prairie net primary production (NPP). This study indicates that high-productivity C4 grasslands (NPP = 0.63 kg C m⁻² y⁻¹) and high-yield maize agroecosystems (10 Mg ha⁻¹) have the potential to sequester C at a rate of 74.5 g C m⁻² y⁻¹ and 86.3 g C m⁻² y⁻¹, respectively, during the next 50 years across southern Wisconsin. Received 28 December 1999; accepted 11 December 2000.     

Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

Response of Litter Decomposition to Simulated N Deposition in Disturbed, Rehabilitated and Mature Forests in Subtropical China

The response of decomposition of litter for the dominant tree species in disturbed (pine), rehabilitated (pine and broadleaf mixed) and mature (monsoon evergreen broadleaf) forests in subtropical China to simulated N deposition was studied to address the following hypothesis: (1) litter decomposition is faster in mature forest (high soil N availability) than in rehabilitated/disturbed forests (low soil N availability); (2) litter decomposition is stimulated by N addition in rehabilitated and disturbed forests due to their low soil N availability; (3) N addition has little effect on litter decomposition in mature forest due to its high soil N availability. The litterbag method (a total of 2880 litterbags) and N treatments: Control-no N addition, Low-N: −5 g N m⁻² y⁻¹, Medium-N: −10 g N m⁻² y⁻¹, and High-N: −15 g N m⁻² y⁻¹, were employed to evaluate decomposition_. Results indicated that mature forest, which has likely been N saturated due to both long-term high N deposition in the region and the age of the ecosystem, had the highest litter decomposition rate, and exhibited no significant positive and even some negative response to nitrogen additions. However, both disturbed and rehabilitated forests, which are still N limited due to previous land use history, exhibited slower litter decomposition rates with significant positive effects from nitrogen additions. These results suggest that litter decomposition and its responses to N addition in subtropical forests of China vary depending on the nitrogen status of the ecosystem.     

Soil carbon sequestration and land‐use change: processes and potential

When agricultural land is no longer used for cultivation and allowed to revert to natural vegetation or replanted to perennial vegetation, soil organic carbon can accumulate. This accumulation process essentially reverses some of the effects responsible for soil organic carbon losses from when the land was converted from perennial vegetation. We discuss the essential elements of what is known about soil organic matter dynamics that may result in enhanced soil carbon sequestration with changes in land‐use and soil management. We review literature that reports changes in soil organic carbon after changes in land‐use that favour carbon accumulation. This data summary provides a guide to approximate rates of SOC sequestration that are possible with management, and indicates the relative importance of some factors that influence the rates of organic carbon sequestration in soil. There is a large variation in the length of time for and the rate at which carbon may accumulate in soil, related to the productivity of the recovering vegetation, physical and biological conditions in the soil, and the past history of soil organic carbon inputs and physical disturbance. Maximum rates of C accumulation during the early aggrading stage of perennial vegetation growth, while substantial, are usually much less than 100 g C m^?2 y^?1. Average rates of accumulation are similar for forest or grassland establishment: 33.8 g C m^?2 y^?1 and 33.2 g C m^?2 y^?1, respectively. These observed rates of soil organic C accumulation, when combined with the small amount of land area involved, are insufficient to account for a significant fraction of the missing C in the global carbon cycle as accumulating in the soils of formerly agricultural land.