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.     

Cowpea N rhizodeposition and its below-ground transfer to a co-existing and to a subsequent millet crop on a sandy soil of the Sudano-Sahelian eco-zone

Nitrogen (N) rhizodeposition by cowpea (Vigna unguiculata (L.) Walp) is potentially a large N source in cropping systems of Sub-Saharan Africa. A field experiment was conducted to measure cowpea N rhizodeposition under the conditions of the Sudano-Sahelian zone using direct ¹⁵N labelling techniques to trace the amount of deposition and its transfer to associated and subsequent crops. Half of the total cowpea crop N was located below-ground at plant maturity, which exceeded 20 kg?N ha^?1 when intercropped with millet. Only 15% of the below-ground cowpea N was recovered in roots, while 85% was found in the rhizodeposited pools. The experiment demonstrated that direct below-ground N transfer occurred from cowpea to millet in intercrop at a rate of 2 kg?N ha^?1 over the growing season. Forty percent of the 25 kg below-ground N that the cowpea crop left at harvest were identifiable in the top 0.30 m soil in the beginning of the next planting season 7 months later; a pool still present at the end of that second season. Thus, the subsequent crop of millet (Pennisetum glaucum (L.) R. Br.) only recovered 2.5 kg?N ha^?1 from the below-ground cowpea pre-crop N during this growth season. The role and potential of cowpea as N provider has been underestimated in the past by ignoring the large proportion of N contained in its rhizodeposits. However, information is needed to determine how losses of the rhizodeposited N can be minimized to fully harness the potential of cowpea as N provider in agro-ecosystems of the region.     

Understanding the variability in soybean nitrogen fixation across agroecosystems

Legume-based cropping systems have the potential to internally regulate N cycling due to the suppressive effect of soil N availability on biological nitrogen fixation. We used a gradient of endogenous soil N levels resulting from different management legacies and soil textures to investigate the effects of soil organic matter dynamics and N availability on soybean (Glycine max) N₂ fixation. Soybean N₂ fixation was estimated on 13 grain farm fields in central New York State by the ¹⁵N natural abundance method using a non-nodulating soybean reference. A range of soil N fractions were measured to span the continuum from labile to more recalcitrant N pools. Soybean reliance on N₂ fixation ranged from 36% to 82% and total N₂ fixed in aboveground biomass ranged from 40 to 224 kg N ha^?1. Soil N pools were consistently inversely correlated with % N from fixation and the correlation was statistically significant for inorganic N and occluded particulate organic matter N. However, we also found that soil N uptake by N₂-fixing soybeans relative to the non-nodulating isoline increased as soil N decreased, suggesting that N₂ fixation increased soil N scavenging in low fertility fields. We found weak evidence for internal regulation of N₂ fixation, because the inhibitory effects of soil N availability were secondary to the environmental and site characteristics, such as soil texture and corresponding soil characteristics that vary with texture, which affected soybean biomass, total N₂ fixation, and net N balance.     

Quantifying below-ground nitrogen of legumes. 2. A comparison of 15N and non isotopic methods

Accurate information on below-ground nitrogen (N) of legumes is necessary for quantifying legume effects on soil N pools and on the N economies of crops following legumes in rotation systems. We report a series of glasshouse pot experiments to determine below-ground N (BGN) of the four legumes, fababean (Vicia faba), chickpea (Cicer arietinum), mungbean (Vigna radiata) and pigeonpea (Cajanus cajan) using both ¹⁵N shoot-labelling and ¹⁵N-labelled soil isotope-dilution methods, a mass N balance approach and the physical recovery of nodulated roots. Data from the ¹⁵N shoot-labelling experiment were manipulated in different ways in an attempt to counter errors associated with uneven ¹⁵N enrichment of roots and nodules. Values for BGN as percent of total plant N based on the physical recovery of nodulated roots ranged from 4 to 15%. With ¹⁵N shoot-labelling, a total of 8.11 mg ¹⁵N was supplied to each pot (six plants) as 0.5% ¹⁵N urea using either leaf-flap (fababean, mungbean and pigeonpea), petiole (chickpea) or leaf-tip (wheat) feeding. Calculations based on measurement of ¹⁵N enrichments of harvested plant parts and root-zone soil suggested that BGN represented 39% of total plant N for fababean, 53% for chickpea, 20% for mungbean and 47% for pigeonpea. The value for wheat was 60%. Adjustment for uneven nodulation patterns on the roots and nodule ¹⁵N depletion, resulting in different ¹⁵N enrichments between nodulated and unnodulated roots, reduced the fababean value to 37% and chickpea to 42%. Values using the other methods were generally in the same range, viz. 15–57% (simple ¹⁵N balance), 11–52% (soil ¹⁵N dilution) and 30–52% (mass N balance). We conclude that physical recovery of roots was the most inaccurate method for estimating BGN. Average values for BGN as percent of total plant N using all isotopic and mass N balance methods were 30% for fababean, 48% for chickpea, 28% for mungbean, and 43% for pigeonpea.¹⁵N shoot-labelling may be the best method for quantifying BGN of field-grown plants. The methodology is simple, apparently accurate provided care is taken in obtaining representative nodulated root samples and, unlike the soil ¹⁵N dilution method, does not require pre-treatment of the soil with ¹⁵N enriched material.     

Quantifying below-ground nitrogen of legumes

Quantifying below-ground nitrogen (N) of legumes is fundamental to understanding their effects on soil mineral N fertility and on the N economies of following or companion crops in legume-based rotations. Methodologies based on ¹⁵N shoot-labelling with subsequent measurement of ¹⁵N in recovered plant parts (shoots and roots) and in the root-zone soil have proved promising. We report four glasshouse experiments with objectives to develop appropriate protocols for in situ ¹⁵N labelling of the four legumes, fababean (Vicia faba), chickpea (Cicer arietinum), mungbean (Vigna radiata) and pigeonpea (Cajanus cajan). Treatments included ¹⁵N-urea concentration (0.1–2.0% w/w), feeding technique (leaf-flap and petiole), leaflet/petiole position (top and bottom of shoot) and frequency of feeding (one and two occasions). ¹⁵N-labelling via the leaf-flap was best for fababean, mungbean and pigeonpea, whilst petiole feeding was best for chickpea, in all cases at the lower-stem nodes 3 or 4 using 0.2 mL volumes of 0.5% urea (98 atom% ¹⁵N excess). Fed leaflets and petioles were removed within 2 weeks of labelling. Uneven ¹⁵N enrichment of the nodulated roots because of effects of the less-enriched nodules meant that root derived N in soil would be overestimated if recovered roots were more heavily nodulated than unrecovered roots. One possible solution would be to assume crown nodulation of the plants. Thus, recovered roots would be nodulated; root-derived N remaining in soil may be without nodules. The ratios of nodulated root to unnodulated root enrichments could then be used as an adjustment in the calculations, i.e. in the case of fababean and chickpea, by dividing calculated root-derived N in soil by 1.12 (fababean) and 1.56 (chickpea).     

Influence of fertilizers on nitrogen mineralization and utilization in the rhizosphere of wheat

Plant roots and microorganisms play an important role in the soil N cycle and plant N nutrition through the release of extracellular enzymes. In the present greenhouse pot experiment, wheat (Triticum aestivum) seedlings were grown in a fluvo-aquic soil (Udifluvent) to investigate N mineralization and utilization in the rhizosphere of wheat. The soil received chemical fertilizer (¹⁵N-labeled urea), chemical fertilizer plus manure (common urea + ¹⁵N-labeled swine manure) or no N. Plant roots were separated from the soil with a nylon cloth, and 1-mm increments of soil moving laterally away from roots were analyzed for N, microbial C, and the activities of invertase, urease and protease. Chemical fertilizer plus manure promoted wheat growth and N absorption significantly compared with chemical fertilizer. ¹⁵N from both chemical fertilizer and swine manure accumulated significantly in the rhizosphere soil within 5 mm of the roots. Fertilized N could thus move easily laterally towards roots and there was no indication that movement through the soil limited plant N supply. A large proportion of fertilizer N was lost from the soil during the wheat growing period, and N utilization efficiency was 24% for chemical fertilizer and 30% for swine manure. In addition, faster rates of N mineralization, larger amounts of microbial C, and increased activities of invertase, urease and protease occurred in the rhizosphere compared with other parts of the soil. There was a significant correlation between microbial C and N mineralization rate (r?=?0.968, P?<?0.01) in the whole soil. Microbial C also showed significant positive correlations with activities of invertase (r?=?0.892, P?<?0.01) and protease (r?=?0.933, P?<?0.01). Further study showed that adding manure into soil increased microbial C and the activities of invertase and protease; adding urea stimulated urease activity in the same soil. Changes in soil enzyme activities in response to N fertilizers could be considered indicators for different fertilizer managements.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

13CO2示踪不同化学形态氮素添加对高寒草甸植物光合碳分配的影响

外源氮素(N)输入陆地生态系统后会引起植物和土壤各碳库的变化,但是对不同化学形态氮素的长期输入如何影响光合碳在植物组织、土壤、土壤呼吸中的分配及转运知之甚少,尤其是对于氮输入引起光合碳分配变化进而作用于植物和土壤碳库的机制的认识还非常匮乏。基于在青藏高原矮嵩草草甸开展的不同化学形态氮素添加的长期实验,利用¹³C示踪方法揭示了光合碳在植物地上、地下组织的分配,及其随时间在土壤中的滞留和随土壤呼吸的释放。研究结果表明,外源氮素添加10年后,与对照未添加氮素处理相比,氨态氮处理下的地上生物量增加了49.5%,氨态氮处理下的地下生物量增加了111.3%。土壤中滞留的¹³C整体呈下降趋势,氨态氮处理下的土壤碳库显著高于硝态氮处理下的值。不同处理下的土壤呼吸中¹³C的滞留量随时间呈指数衰减的变化趋势,其中,硝态氮处理下的¹³C衰减最快。¹³C同位素标记后第1天测定植物茎和叶内的¹³C约占刚刚标定完茎和叶内¹³C的80%,不同处理之间没有显著性差异...     

Belowground carbon input and translocation potential of fodder radish cover-crop

We compared the soil C input potential of a common catch-crop (fodder radish) established in 6-year-old direct-drilled (DD) plots with adjacent conventionally tilled (CT) plots on a Danish sandy loam soil by use of ¹⁴C-isotope labelling techniques. Intact monoliths of soil with actively growing fodder radish seedlings were extracted in Autumn of 2008 from DD and CT field plots and labelled with ¹⁴CO₂ at different time intervals during fodder radish growth. Labelled monoliths were then sampled 6 and 100 days after termination of labelling by clipping above-ground biomass at soil level and separating below-ground components into macro-roots and macro-root-free soil at 0?C10, 10?C25 and 25?C45 cm soil depth. Using fodder radish ¹⁴C data and the preceding spring barley biomass yield data we estimated C input from the spring barley-fodder radish cycle in addition to evaluating the effect of the removal of spring barley harvestable straw on soil C input. Potential soil C input under straw removal scenarios with and without an established fodder radish crop was also evaluated. Relative to other depths, over 70% of labelled below-ground C was found in the 0?C10 cm soil depth in both DD and CT treatments for each of the two samplings. For both macro-root and macro-root-free soil and in both tillage treatments, labelled C decreased significantly with depth (P?<?0.05). A decline of labeled C in macro-root but an increase of labeled C in macro-root-free soil was observed from day 6 to day 100 for both tillage treatments. Over the autumn-winter growing period, total below-ground C input by fodder radish within the 0?C45 cm soil depth was approximately 1.0 and 1.2 Mg C ha^?1 for CT and DD, respectively. We used data from 100 days after labelling, which coincided with the incorporation of the field fodder radish biomass, to estimate that the total fodder radish contribution to below-ground C after biomass incorporation would range between 1.6 and 1.7 Mg C ha^?1 for DD and CT, respectively. The figures for spring barley straw removal with fodder radish establishment would be between 4.9 and 5.1 Mg C ha^?1, while with no fodder radish establishment, C input to the soil would range between 3.2 Mg C ha^?1 and 3.4 Mg C ha^?1, which is approximately 0.6 Mg C ha^?1 lower than the 4 Mg C ha^?1 biomass C input required to maintain long-term soil organic C. In comparison, under straw retention and fodder radish catch-crop establishment the total spring barley and fodder radish C input would be approximately 6.1 and 6.5 Mg C ha^?1 for DD and CT, respectively. We conclude that fodder radish catch-crops have a potential for mitigating against soil C depletion resulting from export of cereal straw to other uses.     

Differential effects of warming and nitrogen fertilization on soil respiration and microbial dynamics in switchgrass croplands

The mechanistic understanding of warming and nitrogen (N) fertilization, alone or in combination, on microbially mediated decomposition is limited. In this study, soil samples were collected from previously harvested switchgrass (Panicum virgatum L.) plots that had been treated with high N fertilizer (HN: 67 kg N ha^?1) and those that had received no N fertilizer (NN) over a 3‐year period. The samples were incubated for 180 days at 15 °C and 20 °C, during which heterotrophic respiration, δ¹³C of CO₂, microbial biomass (MB), specific soil respiration rate (R_s: respiration per unit of microbial biomass), and exoenzyme activities were quantified at 10 different collections time. Employing switchgrass tissues (referred to as litter) with naturally abundant ¹³C allowed us to partition CO₂ respiration derived from soil and amended litter. Cumulative soil respiration increased significantly by 16.4% and 4.2% under warming and N fertilization, respectively. Respiration derived from soil was elevated significantly with warming, while oxidase, the agent for recalcitrant soil substrate decomposition, was not significantly affected by warming. Warming, however, significantly enhanced MB and R_s indicating a decrease in microbial growth efficiency (MGE). On the contrary, respiration derived from amended litter was elevated with N fertilization, which was consistent with the significantly elevated hydrolase. N fertilization, however, had little effect on MB and R_s, suggesting little change in microbial physiology. Temperature and N fertilization showed minimal interactive effects likely due to little differences in soil N availability between NN and HN samples, which is partly attributable to switchgrass biomass N accumulation (equivalent to ~53% of fertilizer N). Overall, the differential individual effects of warming and N fertilization may be driven by physiological adaptation and stimulated exoenzyme kinetics, respectively. The study shed insights on distinct microbial acquisition of different substrates under global temperature increase and N enrichment.     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.     

Nitrogen addition reduces soil respiration in a mature tropical forest in southern China

Response of soil respiration (CO₂ emission) to simulated nitrogen (N) deposition in a mature tropical forest in southern China was studied from October 2005 to September 2006. The objective was to test the hypothesis that N addition would reduce soil respiration in N saturated tropical forests. Static chamber and gas chromatography techniques were used to quantify the soil respiration, following four‐levels of N treatments (Control, no N addition; Low‐N, 5 g N m^?2 yr^?1; Medium‐N, 10 g N m^?2 yr^?1; and High‐N, 15 g N m^?2 yr^?1 experimental inputs), which had been applied for 26 months before and continued throughout the respiration measurement period. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates found in the warm and wet growing season (April–September) and the lowest rates in the dry dormant season (December–February). Soil respiration rates showed a significant positive exponential relationship with soil temperature, whereas soil moisture only affect soil respiration at dry conditions in the dormant season. Annual accumulative soil respiration was 601±30 g CO₂‐C m^?2 yr^?1 in the Controls. Annual mean soil respiration rate in the Control, Low‐N and Medium‐N treatments (69±3, 72±3 and 63±1 mg CO₂‐C m^?2 h^?1, respectively) did not differ significantly, whereas it was 14% lower in the High‐N treatment (58±3 mg CO₂‐C m^?2 h^?1) compared with the Control treatment, also the temperature sensitivity of respiration, Q₁₀ was reduced from 2.6 in the Control with 2.2 in the High‐N treatment. The decrease in soil respiration occurred in the warm and wet growing season and were correlated with a decrease in soil microbial activities and in fine root biomass in the N‐treated plots. Our results suggest that response of soil respiration to atmospheric N deposition in tropical forests is a decline, but it may vary depending on the rate of N deposition.     

Carbon fluxes, nitrogen cycling, and soil microbial communities in adjacent urban, native and agricultural ecosystems

Urban ecosystems are expanding globally, and assessing the ecological consequences of urbanization is critical to understanding the biology of local and global change related to land use. We measured carbon (C) fluxes, nitrogen (N) cycling, and soil microbial community structure in a replicated (n=3) field experiment comparing urban lawns to corn, wheat–fallow, and unmanaged shortgrass steppe ecosystems in northern Colorado. The urban and corn sites were irrigated and fertilized. Wheat and shortgrass steppe sites were not fertilized or irrigated. Aboveground net primary productivity (ANPP) in urban ecosystems (383±11 C m^?2 yr^?1) was four to five times greater than wheat or shortgrass steppe but significantly less than corn (537±44 C m^?2 yr^?1). Soil respiration (2777±273 g C m^?2 yr^?1) and total belowground C allocation (2602±269 g C m^?2 yr^?1) in urban ecosystems were both 2.5 to five times greater than any other land‐use type. We estimate that for a large (1578 km²) portion of Larimer County, Colorado, urban lawns occupying 6.4% of the land area account for up to 30% of regional ANPP and 24% of regional soil respiration from land‐use types that we sampled. The rate of N cycling from urban lawn mower clippings to the soil surface was comparable with the rate of N export in harvested corn (both ～12–15 g N m^?2 yr^?1). A one‐time measurement of microbial community structure via phospholipid fatty acid analysis suggested that land‐use type had a large impact on microbial biomass and a small impact on the relative abundance of broad taxonomic groups of microorganisms. Our data are consistent with several other studies suggesting that urbanization of arid and semiarid ecosystems leads to enhanced C cycling rates that alter regional C budgets.     

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.     

Soil physical properties and carbon/nitrogen relationships in stable aggregates under legume and grass fallow

Short-season fallow with legumes and/or grasses can restore the soil organic C and nitrogen (N) and improve soil structure. In this study, we accessed the effects of 2-season legume and grass fallow on structural properties and C/N relationships in aggregates of a sandy loam soil. Two legumes (Calopogonium mucunoides and Centrosema pubescens), and two grasses (Guinea grass (Panicum maximum) and goose grass (Eleusine indica) were used. Results showed that Calopogonium and Centrosema increased soil total porosity and reduced soil bulk densities, while goose grass increased bulk density and reduced total porosity of the soils at 0–15 and 15–30?cm depths. Guinea grass significantly increased the saturated hydraulic conductivity (50.4?cm?h^?1) and water holding capacity of the soils. Aggregates, 4.75 to 0.5?mm were greater in Guinea grass and least in goose grass fallowed soils. Calopogonium increased macro-aggregates at 0–15?cm soils by 48%, and mean weight diameter (MWD) by 44%. Organic carbon in 0.5–0.25?mm and <0.25?mm aggregate sizes was higher in Guinea grass soils. Generally, grasses had 4-fold increases of C:N contents in dry aggregates. In conclusion, short-season fallow with Guinea grass, Calopogonium and Centrosema, increased soil C and N and protected them from losses in stable aggregates.     

Long term decomposition: the influence of litter type and soil horizon on retention of plant carbon and nitrogen in soils

How plant inputs from above- versus below-ground affect long term carbon (C) and nitrogen (N) retention and stabilization in soils is not well known. We present results of a decade-long field study that traced the decomposition of ¹³C- and ¹⁵N-labeled Pinus ponderosa needle and fine root litter placed in O or A soil horizons of a sandy Alfisol under a coniferous forest. We measured the retention of litter-derived C and N in particulate (>2 mm) and bulk soil (<2 mm) fractions, as well as in density-separated free light and three mineral-associated fractions. After 10 years, the influence of slower initial mineralization of root litter compared to needle litter was still evident: almost twice as much root litter (44% of C) was retained than needle litter (22–28% of C). After 10 years, the O horizon retained more litter in coarse particulate matter implying the crucial comminution step was slower than in the A horizon, while the A horizon retained more litter in the finer bulk soil, where it was recovered in organo-mineral associations. Retention in these A horizon mineral-associated fractions was similar for roots and needles. Nearly 5% of the applied litter C (and almost 15% of the applied N) was in organo-mineral associations, which had centennial residence times and potential for long-term stabilization. Vertical movement of litter-derived C was minimal after a decade, but N was significantly more mobile. Overall, the legacy of initial litter quality influences total SOM retention; however, the potential for and mechanisms of long-term SOM stabilization are influenced not by litter type but by soil horizon.     

Soil mineralizable nitrogen and soil nitrogen supply under two-year potato rotations

Two-year potato rotations were evaluated for their effects on soil mineralizable N and soil N supply. Pre-plant soil samples (0–15 cm) collected from the potato year after seven rotation cycles were used to estimate soil mineralizable N using a 24 week aerobic incubation. Potentially mineralizable N (N ₀ ) ranged from 102 to 149 kg N ha^?1, and was greater after pea/white clover and oats/Italian ryegrass than after oats by an average of 35 and 22%, respectively. Labile, intermediate and stable mineralizable N pools were increased after pea/white clover compared with oats, whereas only the stable mineralizable N pool was increased after oats/Italian ryegrass. Potato plant N uptake with no fertilizer applied was greater in potato-pea/white clover compared with the three other rotations (126 vs. average of 67 kg N ha^?1). Choice of rotation crop in potato production influences both the quantity and quality of soil mineralizable N.     

Soil microbial community structure in European forests in relation to forest type and atmospheric nitrogen deposition

The objective of the present study was to evaluate the combined effect of vegetation and N deposition on microbial community composition in forest soils. For this, microbial biomass and community structure were assessed by ester linked fatty acid methyl ester (EL-FAME) analyses for 12 European forest sites representing different forest types (coniferous/deciduous) and differing in annual N loads (2?C40 kg?N?ha^?1). Microbial community composition was affected by vegetation as indicated by a higher proportion of the marker for arbuscular mycorrhiza (AM) fungi??16:1 11???in deciduous forest soils (1.2%?C5.7% of total EL-FAMEs) compared to acidic coniferous forest soils (0.5%?C1.6%). The two pine forest sites investigated showed the highest proportion of fungi (up to 28% of total EL-FAMEs) and the lowest proportions of Gram-negative and Gram-positive bacteria of all study sites. Nitrogen deposition rates were highly correlated with the ratios of cyclopropyl fatty acids to their precursors (r?=?0.82; P?<?0.01) and of bacteria to fungi (r?=?0.71; P?<?0.05). The two sites with the highest N deposition (??32.3 kg?N?ha^?1a^?1) were depleted in the marker fatty acids for AM fungi and other fungi. Our findings suggest that vegetation has a pronounced effect on microbial community structure, but this effect is masked by high N inputs (>30 kg?N?ha^?1a^?1).     

Temporal patterns of net soil N mineralization and nitrification through secondary succession in the subtropical forests of eastern China

Linking temporal trends of soil nitrogen (N) transformation with shifting patterns of plants and consequently changes of litter quality during succession is important for understanding developmental patterns of ecosystem function. However, the successional direction of soil N mineralization and nitrification in relation to species shifts in the subtropical regions remains little studied. In this study, successional patterns of net soil N mineralization and nitrification rates, litter-fall, forest floor litter, fine root and soil properties were quantified through a successional sequence in the subtropical forests of eastern China. Net N mineralization rate was ‘U-shaped’ through succession: highest in climax evergreen broad-leaved forest (CE: 1.6?±?0.2 mg-N kg^?1 yr^?1) and secondary shrubs (SS: 1.4?±?0.2 mg-N kg^?1 yr^?1), lowest in conifer and evergreen broad-leaved mixed forest (MF: 1.1?±?0.1 mg-N kg^?1 yr^?1) and intermediate in conifer forest (CF: 1.2?±?0.1 mg-N kg^?1 yr^?1) and sub-climax forest (SE: 1.2?±?0.2 mg-N kg^?1 yr^?1). Soil nitrification increased with time (0.02?±?0.1, 0.2?±?0.1, 0.5?±?0.1, 0.2?±?0.1, and 0.6?±?0.1 mg-N kg^?1 yr^?1 in SS, CF, MF, SE and CE, respectively). Annual production of litter?fall increased through succession. Fine root stocks and total N concentration, soil total N, total carbon (C) and microbial biomass C also followed ‘U?shaped’ temporal trends in succession. Soil bulk density was highest in MF, lowest in CE, and intermediate in SS, CF and SE. Soil pH was significantly lowest in CE. Temporal patterns of soil N mineralization and nitrification were significant related to the growth of conifers (i.e. Pinus massoniana) and associated successional changes of litter-fall, forest floor, fine roots and soil properties. We concluded that, due to lower litter quality, the position of Pinus massoniana along the succession pathway played an important role in controlling temporal trends of soil N transformation.     

Modeling to discern nitrogen fertilization impacts on carbon sequestration in a Pacific Northwest Douglas‐fir forest in the first‐postfertilization year

This study investigated how nitrogen (N) fertilization with 200 kg N ha^?1 of urea affected ecosystem carbon (C) sequestration in the first‐postfertilization year in a Pacific Northwest Douglas‐fir (Pseudotsuga menziesii) stand on the basis of multiyear eddy‐covariance (EC) and soil‐chamber measurements before and after fertilization in combination with ecosystem modeling. The approach uses a data‐model fusion technique which encompasses both model parameter optimization and data assimilation and minimizes the effects of interannual climatic perturbations and focuses on the biotic and abiotic factors controlling seasonal C fluxes using a prefertilization 9‐year‐long time series of EC data (1998–2006). A process‐based ecosystem model was optimized using the half‐hourly data measured during 1998–2005, and the optimized model was validated using measurements made in 2006 and further applied to predict C fluxes for 2007 assuming the stand was not fertilized. The N fertilization effects on C sequestration were then obtained as differences between modeled (unfertilized stand) and EC or soil‐chamber measured (fertilized stand) C component fluxes. Results indicate that annual net ecosystem productivity in the first‐post‐N fertilization year increased by～83%, from 302 ± 19 to 552 ± 36 g m^?2 yr^?1, which resulted primarily from an increase in annual gross primary productivity of～8%, from 1938 ± 22 to 2095 ± 29 g m^?2 yr^?1 concurrent with a decrease in annual ecosystem respiration (R_e) of～5.7%, from 1636 ± 17 to 1543 ± 31 g m^?2 yr^?1. Moreover, with respect to respiration, model results showed that the fertilizer‐induced reduction in R_e (～93 g m^?2 yr^?1) principally resulted from the decrease in soil respiration R_s (～62 g m^?2 yr^?1).