Effects of nitrogen deposition and management practices on leaf litterfall and N and P return in a Moso bamboo forest

The effects of nitrogen (N) deposition and management practices on leaf litterfall and N and P return in Moso bamboo forest are not yet known. In this study, we investigated the effects of four levels of simulated N deposition, including low-N (N30, 30 kg ha^?1 year^?1), medium-N (N60, 60 kg ha^?1 year^?1), and high-N (N90, 90 kg ha^?1 year^?1), and a control with no N (N-free addition). The experiment was performed in a Moso bamboo forest under conventional management (CM) and intensive management (IM). The results showed that leaf litterfall and N and P return occurred mainly from March to June and accounted for 78.2–82.2, 78.5–82.1, and 85.6–94.6% of annual leaf litterfall, N return, and P return, respectively. Unlike CM, IM significantly increased leaf litterfall and N and P return. The positive effects were further amplified by low- and medium-N deposition, but not high-N deposition. The combination of low- and medium-N deposition and IM significantly increased N and P return, but not litterfall. Our results indicated that the interaction of anthropogenic management practices and N deposition need to be considered when estimating the effects of N deposition on the biogeochemical cycle of a forest ecosystem.     

Nitrogen deposition promotes ecosystem carbon accumulation by reducing soil carbon emission in a subtropical forest

Background and aims

Tropical and subtropical forests are experiencing high levels of atmospheric nitrogen (N) deposition, but the responses of such forests ecosystems to N deposition remain poorly understood.

Methods

We conducted an 8-year field experiment examining the effect of experimental N deposition on plant growth, soil carbon dioxide efflux, and net ecosystem production (NEP) in a subtropical Chinese fir forest. The quantities of N added were 0 (control), 60, 120, and 240 kg ha^?1 year^?1.

Results

NEP was lowest under ambient conditions and highest with 240 kg of N ha^?1 year^?1 treatment. The net increase in ecosystem carbon (C) storage ranged from 9.2 to 16.4 kg C per kg N added in comparison with control. In addition, N deposition treatments significantly decreased heterotrophic respiration (by 0.69–1.85 t C ha^?1 year^?1) and did not affect plant biomass. The nitrogen concentrations were higher in needles than that in fine roots.

Conclusions

Our findings suggest that the young Chinese fir forest is carbon source and N deposition would sequester additional atmospheric CO₂ at high levels N input, mainly due to reduced soil CO₂ emission rather than increased plant growth, and the amount of sequestered C depended on the rate of N deposition.     

Three-Year Soil Carbon and Nitrogen Responses to Sugarcane Straw Management

Green harvest sugarcane management has increased soil organic C and N stocks over time. However, emerging sugarcane straw removal to meet increasing bioenergy demands has raised concerns about soil C and N depletions. Thus, we conducted a field study in southeast Brazil over nearly three years (1100 days) for assessing soil C and N responses to increasing sugarcane straw removal rates. In order to detect the C input as a function of the different amounts of straw over three years, a field simulation was performed, where the original soil layer (0–0.30 m) was replaced by another from an adjacent area with low total C and δ¹³C. The treatments tested were as follows: (i) 0 Mg ha^?1 year^?1 (i.e., 100% removal), (ii) 3.5 Mg ha^?1 year^?1 (i.e., 75% removal), (iii) 7.0 Mg ha^?1 year^?1 (i.e., 50% removal), (iv) 14.0 Mg ha^?1 year^?1 (i.e., no removal), and (v) 21.0 Mg ha^?1 year^?1 (i.e., no removal + extra 50% of the straw left on the field). The results showed that sugarcane straw removal affected the soil C and total N pools. In the first 45 days of straw decomposition, a small but important straw-derived C portion enters into the soil as dissolved organic carbon (DOC). The lower the straw removal rate, the higher was straw-derived DOC content found into the soil, down to 0.50 m depth. After 3 years of management, keeping sugarcane straw on soil surface significantly increased C and N stocks within surface soil layer (0–0.025 m). Our findings suggest that under no straw removal management (i.e., 14 Mg ha^?1), approximately 364 kg ha^?1 of C and 23 kg ha^?1 of N are annually stored into this low-C soil. The contribution of the straw-derived C (C-C4) to the total soil C increases over time, which accounted for about 60% under no straw removal rate. The greatest contribution of the C storage preferentially occurs into the fraction of organic matter (<?0.53 μm) associated with soil clay minerals. We concluded that indiscriminate sugarcane straw removal to produce cellulosic ethanol or bioelectricity depletes soil C stocks and reduces N cycling in sugarcane fields, impairing environmental gains associated with bioenergy production. Therefore, this information, linked with other agronomic and environmental issues, should be taken into account towards a more sustainable straw removal management for bioenergy production in Brazil.     

Simulated nitrogen deposition significantly suppresses the decomposition of forest litter in a natural evergreen broad-leaved forest in the Rainy Area of Western China

Background and aims

Litter, an essential component of forest ecosystems, plays an important role in maintaining soil fertility, sequestering carbon (C) and improving soil biodiversity. However, litter decomposition is affected by increased nitrogen (N) deposition. Numerous reports have presented N deposition experiments in different forest ecosystems to investigate the effects of N deposition on litter decomposition, but the effects remain unclear, especially in ecosystems receiving increasingly higher levels of ambient N deposition. To address this gap, we performed a litterbag experiment to understand the effects of increasing N deposition on the litter decomposition process in natural evergreen broad-leaved forest in the Rainy Area of Western China.

Methods

A 2-year field litter decomposition experiment was conducted using the litterbag method. Four levels of N deposition were established: control (CK; 0 kg·N·ha^?1·year^?1), low N deposition (LN; 50 kg·N·ha^?1·year^?1), medium N deposition (MN; 150 kg·N·ha^?1·year^?1), and high N deposition (HN; 300 kg·N·ha^?1·year^?1). The simulated N depositions ranged from 50% to 320% of the ambient rate of wet N deposition.

Results

Simulated N deposition significantly increased the remaining mass, C, N, lignin and cellulose of the litter. The LN treatment decreased the remaining phosphorus (P); conversely, the HN treatment increased it. In the late stage of the study period, the mass remaining was positively closely correlated to the lignin and cellulose remaining during the decomposition process.

Conclusions

Simulated N deposition significantly suppressed the litter decomposition in the natural evergreen broad-leaved forest, despite the high rate of ambient N deposition, and the inhibitory effects increased with the N deposition levels. The suppressive effect of N deposition on litter decomposition may be primarily explained by the inhibition of lignin and cellulose degradation by the exogenous inorganic N. With ongoing N deposition in future, N deposition may have a potentially significant impact on C and N cycles in such forest ecosystems.

     

Fluxes of nitrogen and phosphorus in a gallery forest in the Cerrado of central Brazil

The Gallery forests of the Cerrado biome play a critical role in controlling stream chemistry but little information about biogeochemical processes in these ecosystems is available. This work describes the fluxes of N and P in solutions along a topographic gradient in a gallery forest. Three distinct floristic communities were identified along the gradient: a wet community nearest the stream, an upland dry community adjacent to the woodland savanna and an intermediate community between the two. Transects were marked in the three communities for sampling. Fluxes of N from bulk precipitation to these forests resulted in deposition of 12.6 kg ha^?1 y^?1 of total N of which 8.8 kg ha^?1 was as inorganic N. The throughfall flux of total N was generally <8.4 kg ha^?1 year^?1. Throughfall NO₃?CN fluxes were higher (7?C32%) while NH₄?CN and organic N fluxes were lower (54?C69% and 5?C46%) than those in bulk precipitation. The throughfall flux was slightly lower for the wet forest community compared to other communities. Litter leachate fluxes differed among floristic communities with higher NH₄?CN in the wet community. The total N flux was greater in the wet forest than in the dry forest (13.5 vs. 9.4 kg ha^?1 year^?1, respectively). The stream water had total N flux of 0.3 kg ha^?1 year^?1. The flux of total P through bulk precipitation was 0.7 kg ha^?1 year^?1 while the mean fluxes of total P in throughfall (0.6 kg ha^?1 year^?1) and litter leachate (0.5 kg ha^?1 year^?1) declined but did not differ between communities. The low concentrations presented in soil solution and low fluxes in stream water (0.3 and 0.1 kg ha^?1 year^?1 for N and P, respectively) relative to other flowpaths emphasize the conservative nutrient cycling of these forests and the importance of internal recycling processes for the maintenance and conservation of riparian and stream ecosystems in the Cerrado.     

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.     

Fate of ammonium 15N in a Norway spruce forest under long-term reduction in atmospheric N deposition

In the last decades, in particular forest ecosystems became increasingly N saturated due to elevated atmospheric N deposition, resulting from anthropogenic N emission. This led to serious consequences for the environment such as N leaching to the groundwater. Recent efforts to reduce N emissions raise the question if, and over what timescale, ecosystems recover to previous conditions. In order to study the effects on N distribution and N transformation processes under the lowered N deposition treatment, we investigated the fate of deposited NH₄ ⁺-¹⁵N in soil of a N-saturated Norway spruce forest (current N deposition: 34 kg ha^?1 year^?1; critical N load: 14 kg ha^?1 year^?1), where N deposition has been reduced to 11.5 kg ha^?1 year^?1 since 14.5 years. We traced the deposited ¹⁵N in needle litter, bulk soil, and amino acids, microbial biomass and inorganic N in soil. Under reduced N deposition, 123 ± 23% of the deposited N was retained in bulk soil, while this was only 72 ± 15% under ambient deposition. We presume that with reduced deposition the amount of deposited N was small enough to become completely immobilized in plant and soil and no leaching losses occurred. Trees receiving reduced N deposition showed a decline in N content as well as in ¹⁵N incorporation into needle litter, indicating reduced N plant uptake. In contrast, the distribution of ¹⁵N within the soil over active microbial biomass, microbial residues and inorganic N was not affected by the reduced N deposition. We conclude that the reduction in N deposition impacted only plant uptake and drainage losses, while microbial N transformation processes were not influenced. We assume changes in the biological N turnover to start with the onset of the decomposition of the new, N-depleted litter.     

Total and heterotrophic soil respiration in a swamp forest and oil palm plantations on peat in Central Kalimantan,Indonesia

Heterotrophic respiration is a major component of the soil C balance however we critically lack understanding of its variation upon conversion of peat swamp forests in tropical areas. Our research focused on a primary peat swamp forest and two oil palm plantations aged 1 (OP2012) and 6 years (OP2007). Total and heterotrophic soil respiration were monitored over 13 months in paired control and trenched plots. Spatial variability was taken into account by differentiating hummocks from hollows in the forest; close to palm from far from palm positions in the plantations. Annual total soil respiration was the highest in the oldest plantation (13.8 ± 0.3 Mg C ha^?1 year^?1) followed by the forest and youngest plantation (12.9 ± 0.3 and 11.7 ± 0.4 Mg C ha^?1 year^?1, respectively). In contrast, the contribution of heterotrophic to total respiration and annual heterotrophic respiration were lower in the forest (55.1 ± 2.8%; 7.1 ± 0.4 Mg C ha^?1 year^?1) than in the plantations (82.5 ± 5.8 and 61.0 ± 2.3%; 9.6 ± 0.8 and 8.4 ± 0.3 Mg C ha^?1 year^?1 in the OP2012 and OP2007, respectively). The use of total soil respiration rates measured far from palms as an indicator of heterotrophic respiration, as proposed in the literature, overestimates peat and litter mineralization by around 21%. Preliminary budget estimates suggest that over the monitoring period, the peat was a net C source in all land uses; C loss in the plantations was more than twice the loss observed in the forest.     

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.     

Nitrogen biogeochemistry of a mature Scots pine forest subjected to high nitrogen loads

Nitrogen (N) biogeochemistry of a mature Scots pine (Pinus sylvestris L.) stand subjected to an average total atmospheric N deposition of 48 kg ha^?1 year^?1 was studied during the period 1992–2007. The annual amount of dissolved inorganic nitrogen (DIN) in throughfall (TF) averaged 34 kg ha^?1 year^?1 over the 16-year monitoring period. The throughfall fluxes contained also considerable amounts of dissolved organic nitrogen (DON) (5–8.5 kg N ha^?1 year^?1), which should be incorporated in the estimate of N flux using throughfall collectors. Throughfall DIN fluxes declined at a rate of ?0.9 kg N ha^?1 year^?1, mainly due to the decreasing TF fluxes of ammonium (NH₄), which accounted for 70% to TF DIN. The decrease in TF DIN was accompanied by a decrease in DIN leaching in the seepage water (?1.6 kg N ha^?1 year^?1), which occurred exclusively as nitrate (NO₃ ^?). Nitrate losses in the leachate of the forest floor (LFH) equalled the TF NO₃ ^? delivered to the LFH-layer. On the contrary, about half of the TF NH₄ ⁺ was retained within the LFH-layer. Approximately 60% of the TF DIN fluxes were leached indicating that N inputs were far in excess of the N requirements of the forest. For DON, losses were only substantial from the LFH-layer, but no DON was leached in the seepage water. Despite the high N losses through nitrate leaching and NO _x emission, the forest was still accumulating N, especially in the aggrading LFH-layer. The forest stand, on the contrary, was found to be a poor N sink.     

Effect of nitrogen and phosphorus fertilization on the composition of rhizobacterial communities of two Chilean Andisol pastures

The effect of nitrogen (N) and phosphorus (P) fertilization on composition of rhizobacterial communities of volcanic soils (Andisols) from southern Chile at molecular level is poorly understood. This paper investigates the composition of rhizobacterial communities of two Andisols under pasture after 1- and 6-year applications of N (urea) and P (triple superphosphate). Soil samples were collected from two previously established sites and the composition of rhizobacterial communities was determined by denaturing gradient gel electrophoresis (PCR–DGGE). The difference in the composition and diversity between rhizobacterial communities was assessed by nonmetric multidimensional scaling (MDS) analysis and the Shannon–Wiener index. In Site 1 (fertilized for 1 year), PCR–DGGE targeting 16S rRNA genes and MDS analysis showed that moderate N application (270 kg N ha^?1 year^?1) without P significantly changed the composition of rhizobacterial communities. However, no significant community changes were observed with P (240 kg P ha^?1 year^?1) and N–P application (270 kg N ha^?1 year^?1 plus 240 kg P ha^?1 year^?1). In Site 2 (fertilized for 6 years with P; 400 kg P ha^?1 year^?1), PCR–DGGE targeting rpoB, nifH, amoA and alkaline phosphatase genes and MDS analysis showed changes in rhizobacterial communities only at the highest rate of N application (600 kg N ha^?1 year^?1). Quantitative PCR targeting 16S rRNA genes also showed higher abundance of bacteria at higher N application. In samples from both sites, the Shannon–Wiener index did not show significant difference in the diversity of rhizobacterial communities. The changes observed in rhizobacterial communities coincide in N fertilized pastures with lower soil pH and higher pasture yields. This study indicates that N–P application affects the soil bacterial populations at molecular level and needs to be considered when developing fertilizer practices for Chilean pastoral Andisols.     

Threshold responses of soil organic carbon concentration and composition to multi-level nitrogen addition in a temperate needle-broadleaved forest

Responses of soil organic carbon (SOC) cycling and C budget in forest ecosystems to elevated nitrogen (N) deposition are divergent. Little is known about the N critical loads for the shift between gain and loss of SOC storage in the old-growth temperate forest of Northeast China. The objective of this study was to investigate the nonlinear responses of SOC concentration and composition to multiple rates of N addition, as well as the microbial mechanisms responsible for SOC alteration under N enrichment. Nine rates of urea addition (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha^?1 year^?1) with 4 replicates for each treatment were conducted. Soil samples in the 0–10 cm mineral layer were taken after 3 years of N fertilization. Soil aggregate size distribution and SOC physical fractionation were performed to examine SOC dynamics. Phospholipid fatty acid (PLFA) technique was used to measure the abundance and structure of microbial community. Three years of N addition led to significant increases in the concentrations of soil particulate organic C and aggregate-associated organic C fractions only. The responses of total N and each labile SOC fraction to the rates of N addition followed Gaussian equations, with the N critical loads being estimated to be between 80 and 100 kg N ha^?1 year^?1. The change in SOC concentration (ΔSOC) was positively correlated with the changes in aggregate associated OC (r² > 0.80) and POC concentrations (r² > 0.50). Significant correlations among the concentrations of labile SOC fractions, the percentages of soil aggregates, and the abundances of microbial PLFAs were observed, which implies a close linkage between microbial community structure and SOC accumulation and stability. Our results suggest that increase in soil moisture and shift of microbial community structure could control the critical N load for the switch between C accumulation and loss. The current N deposition rate (~ 11 kg N ha^?1 year^?1) to the northeast China’s forests is favorable for soil C accumulation over the short term.     

Carbon cycling and net ecosystem production at an early stage of secondary succession in an abandoned coppice forest

Secondary mixed forests are one of the dominant forest cover types in human-dominated temperate regions. However, our understanding of how secondary succession affects carbon cycling and carbon sequestration in these ecosystems is limited. We studied carbon cycling and net ecosystem production (NEP) over 4 years (2004–2008) in a cool-temperate deciduous forest at an early stage of secondary succession (18 years after clear-cutting). Net primary production of the 18-year-old forest in this study was 5.2 tC ha^?1 year^?1, including below-ground coarse roots; this was partitioned into 2.5 tC ha^?1 year^?1 biomass increment, 1.6 tC ha^?1 year^?1 foliage litter, and 1.0 tC ha^?1 year^?1 other woody detritus. The total amount of annual soil surface CO₂ efflux was 6.8 tC ha^?1 year^?1, which included root respiration (1.9 tC ha^?1 year^?1) and heterotrophic respiration (RH) from soils (4.9 tC ha^?1 year^?1). The 18-year forest at this study site exhibited a great increase in biomass pool as a result of considerable total tree growth and low mortality of tree stems. In contrast, the soil organic matter (SOM) pool decreased markedly (?1.6 tC ha^?1 year^?1), although further study of below-ground detritus production and RH of SOM decomposition is needed. This young 18-year forest was a weak carbon sink (0.9 tC ha^?1 year^?1) at this stage of secondary succession. The NEP of this 18-year forest is likely to increase gradually because biomass increases with tree growth and with the improvement of the SOM pool through increasing litter and dead wood production with stand development.     

Enhanced decomposition of selenium hyperaccumulator litter in a seleniferous habitat��evidence for specialist decomposers?

Grassland canopy management (spring burn, mowing and residue removal in late-summer, or no management) and native tallgrass species composition (cool season mixture, warm season mixture, or combined cool and warm mixture) effects on C and N in aboveground biomass and soil were investigated at Brookings SD on a previously-plowed Barnes clay loam (fine-loamy, superactive, frigid Calcic Hapludoll). During the last 2 yr of the 9-yr experiment, shoot biomass was affected by canopy management with the burn (2,730 kg ha^?1) and mow (3,421 kg ha^?1) treatments containing less than no management (4,655 kg ha^?1). Burn treatment biomass contained 1,189 kg ha^?1 and 25 kg ha^?1 of C and N, mow contained 1,433 kg ha^?1 and 33 kg ha^?1 of C and N, while no management contained 2,014 kg ha^?1 and 39 kg ha^?1 of C and N, respectively. Soil C accumulation was independent of grass species composition. Soil C accumulation rates, which increased in strong linear fashion (r ² of 0.89 to 0.92) after initial grass establishment, were 387 kg C ha^?1 yr^?1, 503 kg C ha^?1 yr^?1, and 711 kg C ha^?1 yr^?1 for burn, mow, and no management treatments, respectively. Thus, grassland management methods used after conversion of cropland to grassland have important effects on grass biomass and soil C accumulation.     

Nitrogen budget and riverine nitrogen output in a rice paddy dominated agricultural watershed in eastern China

The nitrogen (N) budget calculation approach is a useful means of evaluating the impact of human activity on the N cycle. Field scale N budget calculations may ignore the interactions between landscapes, and regional scale calculations rely on statistical data and indirect parameters. Watershed scale budget calculations allow for a more direct quantification of N inputs and outputs. We conducted N budget calculations for a rice paddy-dominated agricultural watershed in eastern China for 2007?C2009, based on intensive monitoring of stream N dynamics, atmospheric deposition, ammonia (NH₃) volatilization and household interviews about N-related agricultural activities. The results showed that although total N input to the watershed was up to 280 kg N ha^?1 year^?1, riverine discharge was only 4.2 kg N ha^?1 year^?1, accounting for 1.5% of the total N input, and was further reduced to 2.0 kg N ha^?1 year^?1 after reservoir storage and/or denitrification removal. The low riverine N output was because of the characteristics of the rice paddy-dominated landscape, which intercepts run-off and enhances soil denitrification. The watershed actually purified the N in rainwater, as N concentrations in river discharge were much lower than those in rain water. Major N outputs included food/feed export, NH₃ volatilization from chemical fertilizer and manure, and emissions from crop residue burning. Net reactive gaseous emissions (emissions minus deposition) accounted for 5.5% of the total N input, much higher than riverine discharge. Therefore, the agricultural N cycle in such paddy-dominated watersheds impacts the environment mainly through gas exchange rather than water discharge.     

Ratios of carbon mass in nodules to other plant tissues in chickpea

A quantitative measurement of the mass and carbon (C) of nodules in legume crops will provide more accurate estimate of total C entering to the soil. This study quantified the ratios of C in roots and nodules in relation to above-ground plant tissue (AG) for chickpea (Cicer arietinum L.). The cultivars ‘CDC-Anna’ and ‘CDC-Frontier’ were grown in continuously-cropped no-till wheat stubble and conventionally-tilled summer fallow systems under three rates (0, 28 and 84 kg N ha^?1) of N fertilizers in Swift Current and Shaunavon, Saskatchewan, Canada, in 2004, 2005 and 2006. The AG biomass ranged between 4,680 and 7,250 kg ha^?1 and increased with the application of N fertilizer ≥28 kg N ha^?1. The nodule mass measured at the early flowering stage ranged between 143 and 355 kg ha^?1, accounting for 2 to 6% of the total AG biomass. Nodule mass decreased significantly from the early flowering to the late-flowering stages (3 wk between). The C value averaged from 1,970 to 2,640 kg ha^?1 in the AG parts, 866 to 1,161 kg ha^?1 in roots and 82 to 184 kg ha^?1 in nodules. The C value in the nodules was 32% greater for chickpea grown in the no-till system than in the tilled-fallow system. CDC-Frontier had 34% greater C value in AG and roots, and 76% greater in nodules than CDC-Anna. Below-ground C (roots plus nodules) accounted for 50% that of the AG tissue at N?=?0 kg ha^?1, and decreased to 45% as N increased to 84 kg ha^?1. At N?=?0 kg ha^?1, the C allocation among plant parts was in the ratio of 67: 29: 4, respectively, in the above-ground tissues: roots: nodules; at N?=?84 kg ha^?1, this ratio was shifted to 69: 30: 1. The quantitative C allocation coefficients can be of great value to modellers in estimating total C contribution to the soil by annual legumes.     

Management of Warm- and Cool-Season Grasses for Biomass on Marginal Lands: I. Yield and Soil Fertility Status

Switchgrass (Panicum virgatum L.), tall fescue [Lolium arundinaceum (Schreb.)], and reed canarygrass (Phalaris arundinacea L.) are known for high biomass productivity and for various traits that make these species more suitable for marginal environmental growing conditions. The goal of this study was to evaluate the impact of organic vs. inorganic fertilizer application on grass biomass production and soil nutrient status. Switchgrass, tall fescue, and reed canarygrass were established on a sandy soil and a clay soil at the Cornell University Willsboro Research Farm in Willsboro, NY. The experiment was a split-split plot randomized block design with six replicates. Sites were whole plots, grass species were subplots, and fertility treatments were sub-subplots. The six treatments were (1) 168 kg ha^?1 of N fertilizer for cool-season grasses, 84 kg ha^?1 for switchgrass; (2) 56 kg ha^?1 of 0-46-0 P fertilizer plus N (#1); (3) 112 kg ha^?1 of 0-0-60 K fertilizer plus N (#1); (4) 89.6 Mg dairy manure ha^?1; (5) 44.8 Mg dairy manure compost ha^?1; and (6) no fertilizer applied (control plots). Switchgrass with a single harvest per season yielded on average 13.0 Mg ha^?1, while tall fescue and reed canarygrass averaged 8.4 and 7.7 Mg ha^?1, respectively, under two-cut systems. Switchgrass with no fertilization produced 84% of maximum yield of fertilized treatments. Application of a similar amount of organic N with fresh and composted dairy manure resulted in greater yields for fresh dairy manure. Organic fertilizers strongly impacted the P and K status of soils. Switchgrass is capable of high yields in marginal environments and can provide a land base for environmentally acceptable application of animal manure, although from a yield standpoint it is not very responsive to fertilizer applications.     

Modeling denitrification in a tile-drained, corn and soybean agroecosystem of Illinois, USA

Denitrification is known as an important pathway for nitrate loss in agroecosystems. It is important to estimate denitrification fluxes to close field and watershed N mass balances, determine greenhouse gas emissions (N₂O), and help constrain estimates of other major N fluxes (e.g., nitrate leaching, mineralization, nitrification). We compared predicted denitrification estimates for a typical corn and soybean agroecosystem on a tile drained Mollisol from five models (DAYCENT, SWAT, EPIC, DRAINMOD-N II and two versions of DNDC, 82a and 82h), after first calibrating each model to crop yields, water flux, and nitrate leaching. Known annual crop yields and daily flux values (water, nitrate-N) for 1993–2006 were provided, along with daily environmental variables (air temperature, precipitation) and soil characteristics. Measured denitrification fluxes were not available. Model output for 1997–2006 was then compared for a range of annual, monthly and daily fluxes. Each model was able to estimate corn and soybean yields accurately, and most did well in estimating riverine water and nitrate-N fluxes (1997–2006 mean measured nitrate-N loss 28 kg N ha^?1 year^?1, model range 21–28 kg N ha^?1 year^?1). Monthly patterns in observed riverine nitrate-N flux were generally reflected in model output (r ² values ranged from 0.51 to 0.76). Nitrogen fluxes that did not have corresponding measurements were quite variable across the models, including 10-year average denitrification estimates, ranging from 3.8 to 21 kg N ha^?1 year^?1 and substantial variability in simulated soybean N₂ fixation, N harvest, and the change in soil organic N pools. DNDC82a and DAYCENT gave comparatively low estimates of total denitrification flux (3.8 and 5.6 kg N ha^?1 year^?1, respectively) with similar patterns controlled primarily by moisture. DNDC82h predicted similar fluxes until 2003, when estimates were abruptly much greater. SWAT and DRAINMOD predicted larger denitrification fluxes (about 17–18 kg N ha^?1 year^?1) with monthly values that were similar. EPIC denitrification was intermediate between all models (11 kg N ha^?1 year^?1). Predicted daily fluxes during a high precipitation year (2002) varied considerably among models regardless of whether the models had comparable annual fluxes for the years. Some models predicted large denitrification fluxes for a few days, whereas others predicted large fluxes persisting for several weeks to months. Modeled denitrification fluxes were controlled mainly by soil moisture status and nitrate available to be denitrified, and the way denitrification in each model responded to moisture status greatly determined the flux. Because denitrification is dependent on the amount of nitrate available at any given time, modeled differences in other components of the N cycle (e.g., N₂ fixation, N harvest, change in soil N storage) no doubt led to differences in predicted denitrification. Model comparisons suggest our ability to accurately predict denitrification fluxes (without known values) from the dominant agroecosystem in the midwestern Illinois is quite uncertain at this time.     

Simulation of N2O emissions and nitrate leaching from plastic mulch radish cultivation with LandscapeDNDC

Radish is one of the major dry field crops in Asia commonly grown with plastic mulch and high rates of N fertilization, and potentially harming the environment due to N₂O emissions and nitrate leaching. Despite the widespread use of plastic mulch, biogeochemical models so far do not yet consider impacts of mulch on soil environmental conditions and biogeochemistry. In this study, we adapted and successfully tested the LandscapeDNDC model against field data by simulating crop growth, C and N turnover and associated N₂O emissions as well as nitrate leaching for radish cultivation with plastic mulch and in conjunction with different rates of N fertilization (465–765 kg N ha^?1 year^?1). Due to the sandy soil texture and monsoon climate, nitrate leaching with rates up to 350 kg N ha^?1 year^?1 was the dominant reason for overall low nitrogen use efficiency (32–43 %). Direct or indirect N₂O emissions (calculated from simulated nitrate leaching rates and IPCC EF_ind = 0.0075) ranged between 2 and 3 kg N ha^?1 year^?1, thus contributing an equal amount to total field emissions of about 5 kg N ha^?1 year^?1. Based on our results, emission factors for direct N₂O emissions ranged between 0.004 and 0.005. These values are only half of the IPCC default value (0.01), demonstrating the need of biogeochemical models for developing site and/or region specific EFs. Simulation results also revealed that changes in agricultural management by applying the fertilizer only to the rows would be an efficient mitigation strategy, effectively decreasing field nitrate leaching and N₂O emissions by 50–60 %.     

Quality and decomposition in soil of rhizome, root and senescent leaf from Miscanthus x giganteus, as affected by harvest date and N fertilization

To predict the environmental benefits of energy crop production and use, the nature and fate of biomass residues in the soil need to be quantified. Our objective was to quantify Miscanthus x giganteus biomass recycling to soil and to assess how harvesting time and N fertilization affect their characteristics and subsequent biodegradability. The quantification of aerial and belowground biomass and their sampling were performed on 2- and 3-year-old Miscanthus stands, either fertilized with 120 kg N ha^?1 year^?1 or not fertilized, in autumn (maximal biomass production) and winter (maturity). Plant biomass was chemically characterized (total sugars, Klason lignin, C/N) and incubated in optimum decomposition conditions (15°C, ?80 kPa) for 263 days, for C and N mineralization. Accumulation of carbon in rhizomes and roots was 7.5 to 10 t C ha^?1 and represented about 50% of total plant biomass C. Senescent leaves represented about 1.5 t C ha^?1 year^?1. All residues, especially the roots, had high lignin contents, while the rhizomes also had a high soluble content due to their nutrient storage function. The C mineralization rates were closely related to the chemical characteristics of the residue, higher sugar and lower lignin contents leading to faster decomposition, as observed for rhizomes.