Effects of chemical composition on N,Ca, and Mg release during incubation of leaves from selected agroforestry and fallow plant species

Nitrogen, Ca and Mg release from leaves of ten selected plant residues with varying chemical compositions was studied under laboratory conditions. Three patterns of N-release were observed over a seven week incubation period: (a)Gliricidia sepium, Leucaena leucocephala, Mucuna pruriens andCentrosema pubescens leaves showed rapid N release, (b)Acioa barteri andDialium guineense leaves immobilized N, and (c)Alchornea cordifolia, Anthonata macrophylla, Cassia siamea andPterocarpus santalinoides leaves initially showed N immobilization which gradually changes to net mineralization after about four weeks of incubation. Nitrogen mineralization rate constant (k) ranged from –0.0018 (A. barteri) to 0.0064 day^–1 (G. sepium). Statistical analysis of data showed that N mineralization rate constants are significantly correlated with initial N, polyphenol and lignin contents of leaves. Nitrogen release increased with increasing N content and decreased with increasing contents of polyphenols and lignin.Addition of leaves from all species significantly increased soil exchangeable Ca and Mg levels.L. leucocephala, G. sepium, C. pubescens andM. pruriens showed relatively high Ca and Mg release rates. Calcium release rate was related to N release rate rather than to initial Ca content.     

The nitrogen mineralization rate of legume residues in soil as influenced by their polyphenol,lignin, and nitrogen contents

A 12-week greenhouse experiment was conducted to determine the effect of the polyphenol, lignin and N contents of six legumes on their N mineralization rate in soil and to compare estimates of legume-N release by the difference and ¹⁵N-recovery methods. Mature tops of alfalfa (Medicago sativa L.), round leaf cassia (Cassia rotundifolia Pers., var. Wynn), leucaena (Leucaena leucocephala Lam., deWit), Fitzroy stylo (Stylosanthes scabra Vog., var Fitzroy), snail medic (Medicago scutellata L.), and vigna (Vigna trilobata L., var verde) were incorporated in soil at the rate of 100 mg legume N kg^-1 soil. The medic and vigna were labeled with ¹⁵N. Sorghum-sudan hybrid (Sorghum bicolor, L. Moench) was used as the test crop. A non-amended treatment was used as a control. Net N mineralization after 12 weeks ranged from 11% of added N with cassia to 47% of added N for alfalfa. With the two legumes that contained less than 20 g kg^-1 of N, stylo and cassia, there was net N immobilization for the first 6 weeks of the experiment. The legume (lignin + polyphenol):N ratio was significantly correlated with N mineralization at all sampling dates at the 0.05 level and at the 0.01 level at 6 weeks (r²=0.866). Legume N, lignin, or polyphenol concentrations or the lignin:N ratio were not significantly correlated with N mineralization at any time. The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r²=0.692). The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant polyphenol needs to be standardized. Estimates of legume-N mineralization by the difference and ¹⁵N recovery methods were significantly different at all sampling dates for both ¹⁵N-labeled legumes. After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the ¹⁵N recovery method. This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from ¹⁵N recovery experiments.     

Impact of residue quality on the C and N mineralization of leaf and root residues of three agroforestry species

A laboratory incubation experiment with ¹⁵N labeled root and leaf residues of 3 agroforestry species (Leucaena leucocephala, Dactyladenia barteri and Flemingia macrophylla) was conducted under controlled conditions (25 C) for 56 days to quantify residue C and N mineralization and its relationship with residue quality.No uniform relation was found between the chemical composition of the above and below residues. The leucaena and dactyladenia roots contained more lignin (8 and 26% respectively) and less N (2.0 and 1.0% respectively) than the respective leaves (2 and 13% lignin and 2.9 and 1.4% N, respectively), whereas the differences between the lignin and N contents of the flemingia leaves and roots were not significant (4.6 and 3.0% lignin and 2.63 and 2.68% N, respectively). The leucaena leaves contained more polyphenols than the roots (6.4 and 3.6%), while the polyphenol content of the leaves and roots of the other residues was similar (5.0 and 5.1% for dactyladenia and 4.0 and 3.5% for flemingia).Three patterns of N mineralization could be distinguished. A first pattern, followed by residues producing the highest amounts of CO₂, showed an initial immobilization of soil derived N, followed by a net release of both soil and residue derived N after 7 days of incubation. A second pattern, followed by the flemingia leaf residues which produced intermediate amounts of CO₂ and had an intermediate quality, showed no significant immobilization of soil derived N, and significant mineralization of residue N. A third pattern, followed by both low quality dactyladenia residues, showed a low release of residue derived N and a continued inmobilization of soil derived N.Residue C mineralization was significantly (p<0.05) correlated with the residue lignin content, C-to-N ratio, and polyphenol-to-N ratio. The proportion of residue N mineralized (immobilized) after 56 days of incubation was significantly correlated with the residue N content (p<0.01) and the C-to-N ratio (p<0.05). The relations were quadratic, rather than linear. The ratio of the proportion of residue N mineralized (immobilized) over the proportion of residue C mineralized after 56 days was highly significantly correlated with the lignin content (p<0.01) and C-to-N (p<0.001), lignin-to-N (p<0.01), polyphenol-to-N (p<0.01) and (lignin+polyphenol)-to-N ratios (p<0.01) in a linear way. This indicates that due to the low availability of the residue C, relatively less N is immobilized for the very low quality residues ((lignin+polyphenol)-to-N ratio: 29.7) than for the residues with a relatively higher quality ((lignin+polyphenol)-to-N ratios between 3.3 and 12.5).     

Ammonia volatilization from tropical legume mulches and green manures on unlimed and limed soils

In the humid tropics, legumes are harvested and surface applied as mulch or incorporated as green manure. Studies on N dynamics and budgets from these systems report unaccounted losses of N. Ammonia volatilization may account for a significant percentage of these unexplained N deficits. The main objectives of this study were to: 1) determine the rate and amount of ammonia volatilization from organic amendments, both incorporated (green manure) and unincorporated (mulch), 2) compare ammonia volatilization of organic amendments on both acid (unlimed) and limed soils, and 3) correlate quality, i.e. polyphenolic and lignin concentration and carbon-to-nitrogen ratio, of the organic amendments with ammonia volatilization and net N mineralization. In an incubation experiment, ammonia volatilization losses and net N mineralization were measured from fresh leaflets of 10 legumes over a three-week period. Ammonia volatilization losses for the 10 species ranged from 3.4 to 11.8% of the total N applied in the organic amendment. Lignin content was negatively correlated to ammonia volatilization. Ammonia volatilized from mulches but not green manures, on both unlimed and limed soils, suggesting that ammonia volatilization is a surface phenomenon and not affected by soil pH. Net N mineralization was affected by species and soil pH, but was unaffected by placement (green manure or mulch). For the farmer in low-input agriculture where N tends to be limiting, volatilization losses of N from legume mulch systems could be on the same order of magnitude as crop removal.     

Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil

We studied C and N mineralisation patterns from a large number of plant materials (76 samples, covering 37 species and several plant parts), and quantified how these patterns related to biological origin and selected indicators of chemical composition. We determined C and N contents of whole plant material, in water soluble material and in fractions (neutral detergent soluble material, cellulose, hemicellulose and lignin) obtained by stepwise chemical digestion (modified van Soest method). Plant materials were incubated in a sandy soil under standardised conditions (15 °C, optimal water content, no N limitation) for 217days, and CO₂ evolution and soil mineral N contents were monitored regularly. The chemical composition of the plant materials was very diverse, as indicated by total N ranging from 2 to 59 mg N g^–1, (i.e. C/N-ratios between 7 and 227). Few materials were lignified (median lignin=4% of total C). A large proportion of plant N was found in the neutral detergent soluble (NDS) fraction (average 84%) but less of the plant C (average 46%). Over the entire incubation period, holocellulose C content was the single factor that best explained the variability of C mineralisation (r=–0.73 to –0.82). Overall, lignin C explained only a small proportion of the variability in C mineralisation (r=–0.44 to –0.51), but the higher the lignin content, the narrower the range of cumulative C mineralisation. Initial net N mineralisation rate was most closely correlated (r=0.76) to water soluble N content of the plant materials, but from Day 22, net N mineralisation was most closely correlated to total plant N and NDS-N contents (r varied between 0.90 and 0.94). The NDS-N content could thus be used to roughly categorise the net N mineralisation patterns into (i) sustained net N immobilisation for several months; (ii) initial net N immobilisation, followed by some re-mineralisation; and (iii) initially rapid and substantial net N mineralisation. Contrary to other studies, we did not find plant residue C/N or lignin/N-ratio to be closely correlated to decomposition and N mineralisation.     

Manipulation of quality and mineralization of tropical legume tree prunings by varying nitrogen supply

The effect of N supply on the quality of Calliandra calothyrsus and Gliricidia sepium prunings was studied in a glasshouse over a 7-month growing period. Increasing the concentration of N supplied from 0.625 to 10.0 mM NO₃-N resulted in increased N concentration but decreased polyphenol concentration, protein-binding capacity and C:N ratio of prunings from both species. Lignin concentration was not consistently altered by the N treatment. Mineralization of N from the prunings was measured over a 14-week period under controlled leaching and non-leaching conditions. The results indicated a strong interaction between legume species and concentration of N supply in their influence on N mineralization of the prunings applied to the soil. Differences in the %N mineralized were dictated by the quality of the prunings. The (lignin + polyphenol):N ratio was the pruning quality factor which could be used most consistently and accurately to predict N mineralization of the legume prunings incubated under leaching conditions, and the relationship was best described by a linear regression. Under non-leaching conditions, however, the protein-binding capacity appeared to be the most important parameter in determining the patterns of N release from the prunings studied. The relationship between the N mineralization rate constant and the protein-binding capacity was best described by a negative exponential function, y=0.078 exp(–0.0083x). The present study also indicated that the release of N from legume prunings containing a relatively high amount of polyphenol could be enhanced by governing the N availability conditions under which the plant is grown, for example whether or not it is actively fixing nitrogen. Estimates of pruning N mineralization after 14 weeks with the difference method averaged 6% (leaching conditions) and 22% (nonleaching conditions) more than with the ¹⁵N method for all legume prunings studied. The recovery of pruning by maize (4–38%) was well correlated with the % pruning N mineralized suggesting that incubation data closely reflect the pruning N value for a given catch crop under non-leaching conditions.     

模拟干旱和磷添加对热带低地雨林氮矿化过程的影响

土壤氮矿化作为氮转化的主要过程决定土壤供氮能力。热带森林生态系统往往受磷限制, 氮矿化过程对干旱的响应是否受磷限制的调控值得探讨。该研究以海南三亚甘什岭自然保护区热带低地雨林为研究对象, 利用2019年建立的林内穿透雨减少(50%)及磷添加双因素交互实验平台, 通过野外树脂芯原位培养法研究模拟干旱及磷添加对土壤无机氮(包括铵态氮和硝态氮)含量和氮矿化过程的影响。结果表明: 1)减雨处理显著降低了5和15 cm深度土壤的水分含量, 而对土壤温度没有显著影响。2)减雨处理和减雨与磷添加共同处理无论在旱季还是湿季对0-10 cm土壤无机氮含量均没有产生显著影响, 但磷添加处理在旱季显著降低了土壤硝态氮含量, 表明磷添加处理对氮有效性的影响主要体现在旱季, 而非湿季。3)干旱处理在旱季和湿季均显著降低了土壤净氨化速率和净氮矿化速率, 而磷添加处理和减雨与磷添加共同处理无论在旱季还是湿季对净氨化速率、净硝化速率和净氮矿化速率均没有产生显著影响, 结果表明了干旱能够显著降低土壤净氮矿化速率。4)土壤水分含量与土壤净氨化速率和净氮矿化速率呈显著正相关关系, 同时减雨处理显著影响了土壤净氨化速率与铵态氮含量的关系, 并且在铵态氮含量相等的情况, 随着干旱的影响净氨化速率下降得更快。这表明土壤水分含量变化是影响该研究样地土壤氮矿化的主要因素。上述研究结果说明, 降水变化对热带低地雨林中土壤氮矿化有重要影响, 短期磷添加没有显著影响, 减雨与磷添加对土壤氮矿化过程并没有交互效应。     

Leaf strategies and soil N across a regional humidity gradient in Patagonia

We analyzed leaf traits related to carbon-fixation, nutrient conservation strategies, and decomposability and their relationships with potential N-mineralization and microbial N in soil in 19 species of 5 dominant life forms growing in 40 sites across a regional humidity gradient in northern Patagonia. We hypothesized that (1) the shifting of species and life forms across the humidity gradient is related to a shifting in traits of green and senesced leaves with some overlapping among life forms, and (2) leaf traits associated with litter decomposition are related to the potential dynamics of soil-N across the humidity gradient. LMA in green leaves and P-resorption efficiency decreased with humidity while concentrations of lignin and total phenolics in green and senesced leaves and P concentration in senesced leaves increased with humidity. Soil C and N concentrations were positively correlated to humidity. Increasing soil N concentration was related to increasing rates of absolute (per unit soil mass) potential net N-mineralization and microbial-N flush. Relative (per unit N mass) potential net N-mineralization and microbial-N flush decreased with soil N and were inversely correlated to lignin concentration and C/N ratio in senesced leaves. We found overlapping in N concentration and C/N ratio in green and senesced leaves, P concentration in green leaves, and N resorption among species and life forms across the humidity gradient. We concluded that (1) leaf traits related to carbon fixation and the decomposition pathway significantly varied with humidity and were not overlapped between plant life forms from dry and humid habitats, (2) the largest overlapping among species and plant life forms across the gradient occurred in those leaf traits related to N conservation in the plant, and (3) life forms from humid habitats produce more recalcitrant litter that induce lower rates of relative potential net N mineralization (per unit N) than those of dry habitats.     

Foliage litter quality and annual net N mineralization: comparison across North American forest sites

The feedback between plant litterfall and nutrient cycling processes plays a major role in the regulation of nutrient availability and net primary production in terrestrial ecosystems. While several studies have examined site-specific feedbacks between litter chemistry and nitrogen (N) availability, little is known about the interaction between climate, litter chemistry, and N availability across different ecosystems. We assembled data from several studies spanning a wide range of vegetation, soils, and climatic regimes to examine the relationship between aboveground litter chemistry and annual net N mineralization. Net N mineralization declined strongly and non-linearly as the litter lignin:N ratio increased in forest ecosystems (r ² = 0.74, P < 0.01). Net N mineralization decreased linearly as litter lignin concentration increased, but the relationship was significant (r ² = 0.63, P < 0.01) only for tree species. Litterfall quantity, N concentration, and N content correlated poorly with net N mineralization across this range of sites (r ² < 0.03, P = 0.17–0.26). The relationship between the litter lignin:N ratio and net N mineralization from forest floor and mineral soil was similar. The litter lignin:N ratio explained more of the variation in net N mineralization than climatic factors over a wide range of forest age classes, suggesting that litter quality (lignin:N ratio) may exert more than a proximal control over net N mineralization by influencing soil organic matter quality throughout the soil profile independent of climate. Received: 16 December 1996 / Accepted: 8 February 1997     

Growth,Net Production,Litter Decomposition,and Net Nitrogen Accumulation by Epiphytic Bryophytes in a Tropical Montane Forest1

To understand the ecological roles of epiphytic bryophytes in the carbon (C) and nitrogen (N) cycles of a tropical montane forest, we used samples in enclosures to estimate rates of growth, net production, and N accumulation by shoots in the canopy, and litterbags, to estimate rates of decomposition and N dynamics of epiphytic bryophyte litter in the canopy and on the forest floor in Monteverde, Costa Rica. Growth of epiphytic bryophytes was estimated at 30.0–49.9 percent/yr, net production at 122–203 g/m²/yr, and N accumulation at 1.8–3.0 g N/m²/yr. Cumulative mass loss from litterbags after one and two years in the canopy was 17 ± 2 and 19 ± 2 percent (mean ± 1 SE) of initial sample mass, respectively, and mass loss from litter and green shoots in litterbags after one year on the forest floor was 29 ± 2 and 45 ± 3 percent, respectively. Approximately 30 percent of the initial N mass was released rapidly from litter in both locations. Nitrogen loss from green shoots on the forest floor was greater; about 47 percent of the initial N mass was lost within the first three months. There was no evidence for net N immobilization by litter or green shoots, but the remaining N in litter was apparently recalcitrant. Annual net accumulation of C and N by epiphytic bryophytes was estimated at 37–64 g C/m²/yr and 0.8–1.3 g N/m²/yr, respectively. Previous research at this site indicated that epiphytic bryophytes retain inorganic N from atmospheric deposition to the canopy. Therefore, they play a major role in transforming N from mobile to highly recalcitrant forms in this ecosystem.     

Kinetic parameters of nitrogen mineralization rates of leguminous crops incorporated into soil

Summary Fresh leguminous plant residues were incorporated into soil columns and incubated at 23°C for up to 20 weeks. The N released from specific fractions (foliage, stems, and roots) of each residue were monitored at specific time intervals. Relationships between organic carbon, total nitrogen, CN ratio, lipids, and lignin content of the plant materials and the cumulative amount of N mineralized in soil were investigated. Statistical analyses indicated that the rates of N mineralized were not significantly correlated with the organic C nor lipid content of the residues. However, the cumulative amount of N released was significantly correlated with the total N content of the plant material (r=0.93^***). The percentage of organic N of the legumes mineralized in soil ranged from 15.9 to 76.0%. The relationship between the percentage of N released and the CN ratio of the plant material showed an inverse cuvilinear response (r= 0.88^***). It was also evident that the composition of lignin in the residue influenced N mine-ralization rates of the leguminous organs incorporated into soil.There was a curvilinear relationship between the cumulative amount of N released from the residues and time of incubation. Nitrogen mineralization rates were described by first-order kinetics to estimate the N mineralization potential (N₀), mineralization rate constant (k), and the time of incubation required to mineralize one-half of N₀ (t_1/2). The kinetic parameters were calculated by both the linear least squares (LLS) and nonlinear least squares (NLLS) transformations. The N₀ values among the crop residues varied from –35 to 510 g Ng^–1 soil. Statistical analyses revealed that the N₀ values obtained by both LLS and NLLS methods were significantly correlated (r=0.93^***). The mineralization rate constants (k) of the residues ranged from 0.045 to 0.325 week^–1. The time of incubation required to mineralize one-half the nitrogen mineralization potential (t_1/2) of the legumes incorporated into soil ranged from 2.1 to 15.4 weeks.     

15N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function

Organic carbon (C) and nitrogen (N) are essential for heterotrophic soil microorganisms, and their bioavailability strongly influences ecosystem C and N cycling. We show here that the natural ¹⁵N abundance of the soil microbial biomass is affected by both the availability of C and N and ecosystem N processing. Microbial ¹⁵N enrichment correlated negatively with the C : N ratio of the soil soluble fraction and positively with net N mineralization for ecosystems spanning semiarid, temperate and tropical climates, grassland and forests, and over four million years of ecosystem development. In addition, during soil incubation, large increases in microbial ¹⁵N enrichment corresponded to high net N mineralization rates. These results support the idea that the N isotope composition of an organism is determined by the balance between N assimilation and dissimilation. Thus, ¹⁵N enrichment of the soil microbial biomass integrates the effects of C and N availability on microbial metabolism and ecosystem processes.     

Is there a feedback between N availability in siliceous and calcareous soils and<Emphasis Type="Italic"> Cistus albidus</Emphasis> leaf chemical composition?

The effects of the Mediterranean shrub Cistus albidus on N cycling were studied in two siliceous (granitic-derived and schistic-derived) and one calcareous soil differentiated by their texture and acidity. We aimed to find out whether soils under C. albidus were affected by the release of C compounds from the canopy, and whether phenolic compound production in C. albidus changed depending on the soil N availability. Calcareous soils, with higher clay content and polyvalent cations, had a higher organic matter content but lower net N mineralization rates than siliceous soils, and C. albidus growing therein were characterized by lower foliar N and phenolic compound concentrations. Under C. albidus, all types of soils had higher phenolic compound concentrations and polyphenol oxidase activity. C. albidus presence and leachate addition decreased net N mineralization and increased soil respiration in siliceous soils, and these changes were related to a higher soil C/N ratio under the canopy. In calcareous soils, however, no significant effects of plant presence on N cycling were found. In the studied plant-soil system it is not likely that higher phenolic compound concentrations were selected during evolution to enhance nutrient conservation in soil because (1) higher phenolic compound concentrations were not associated with lower soil fertilities, (2) C compounds released from C. albidus accelerated N cycling by increasing N immobilization and no evidence was found for decreased gross N mineralization, and (3) soil organic N content was more related to soil chemical and physical properties than to the effects of the C. albidus canopy.     

Decomposition of leaves of huisache (Acacia tortuoso) and mesquite (Prosopis spp) in soil of the central highlands of Mexico

In the central highlands of Mexico, mesquite (Prosopis spp) and huisache (Acacia tortuoso), N₂ fixing trees or shrubs, dominate the vegetation and are used in an alley cropping system to prevent erosion and restore soil fertility. We investigated how much the leaves of both trees contribute to dynamics of carbon (C) and nitrogen (N) in soil by adding leaves of both species to soil sampled under the canopy of mesquite and huisache, outside their canopy and from fields cultivated with maize at three different sites and monitoring microbial biomass C, production of carbon dioxide (CO₂), and dynamics of inorganic N (ammonium and nitrate) in an aerobic incubation. The soluble fraction and N content of the mesquite leaves were larger than in the huisache leaves, but lignin and polyphenol content were lower. Evolution of CO₂ increased 2.7-times when mesquite and 2.4-times when huisache leaves were added to soil. During all stages of decomposition and in all treatments, C mineralization of leaves from mesquite was greater than from huisache leaves. Mesquite leaves induced an increase in mineral N of 25.6 mg N kg^–1 soil after 56 days and those of huisache 9.8 mg N kg^–1. Twenty-six percent of N from mesquite leaves and 11% of huisache was mineralized, if no priming effect was considered. Nitrogen release from the leaves was greater when the soil organic matter content was lower. It was found that soil under the canopy of mesquite and huisache effectively accumulated organic material, micro-organisms and valuable nutrients. In an alley cropping system huisache might be a better choice than mesquite as huisache grows faster than mesquite and sheds its leaves twice a year while mesquite only once, although the amount of N mineralized was larger from mesquite leaves than from those of huisache.     

青藏高原灌丛土壤碳氮含量及其相关功能基因分布特征

在微生物的代谢活动下,土壤中有机态碳氮化合物矿化分解释放矿质养分和二氧化碳,深刻影响着自然生态系统土壤碳、氮等元素的循环转化、土壤的养分供应和有机质的更新,并对地上植被的演替和分布有极为重要的意义。青藏高原灌丛面积分布广泛,地形和气候条件复杂,但目前对灌丛分布地区土壤碳氮含量、矿化作用强度及其影响因素等的认识较少。研究结合土壤理化分析和高通量定量PCR(quantitative microbial element cycling, QMEC)技术研究了青藏高原喜马拉雅山-冈底斯山地区不同类型灌丛土壤碳氮含量、碳氮矿化速率和相关功能基因的分布特征及其与植被、气候和土壤因子间的耦联关系。结果表明,不同类型灌丛土壤的有机碳、全氮含量、CO₂释放速率、净氮矿化速率、碳氮矿化基因的丰度有显著差异。其中,位于青藏高原东南部的雪层杜鹃和香柏灌丛分布区土壤有机碳和全氮含量、CO₂释放速率、净氮矿化速率显著高于位于中西部的变色锦鸡儿、金露梅和砂生槐灌丛地区,并与年平均降雨量显著正相关。然而,碳、氮矿化基因丰度分布趋势与之相反,在雪层杜鹃和香柏灌丛分布区丰度显著...     

Gross mineralization and plant N uptake from animal manures under non-N limiting conditions, measured using 15N isotope dilution techniques

To utilise wisely the manure resource, a better understanding of the processes that control the breakdown of organic N to inorganic N (mineralization) is required. ¹⁵N isotope dilution techniques should allow estimates of plant N uptake and gross mineralization from organic manures under non-N limiting conditions to be made. In natural systems the study of organic nitrogen breakdown to inorganic nitrogen, mineralization, is confounded by the processes of nitrification, nitrate leaching, gaseous N losses and plant N uptake. The ¹⁵N isotope dilution approach allows measurement of gross mineralization independently of these processes. Greenhouse experiments were conducted to determine plant N uptake from organic manures under non-N limiting conditions using the soil pre-labelling isotope dilution approach. The soil was pre-labelled with ¹⁵N and maize plants were then grown on the control treatments (no organic amendment) or on the manure treatments. The principle is thus that the control crop has a ¹⁵N abundance which reflects the ¹⁵N status of the soil and the treatment crop has a ¹⁵N enrichment diluted by the contribution of mineralized unlabelled manure N. Using this technique, it was estimated that maize plants derived 17 and 34% of their N from sewage sludge and turkey manure, respectively. The soil pre-labelling isotope dilution approach allowed yield-independent estimation of nitrogen derived from manures under non-N limiting conditions. Estimates of gross N mineralization were made to determine the breakdown of manure under field conditions. Results suggested that there was a rapid mineralization of turkey manure N in the initial weeks after application, in the order of 50 kg N ha^?1, which tailed off in the following weeks. The technique suggested that the soil used in the study had an extremely low basal mineralization rate, and a high nitrification rate.     

Rainfall and labile carbon availability control litter nitrogen dynamics in a tropical dry forest

N cycling in tropical dry forests is driven by rainfall seasonality but the mechanisms involved are not well understood. We studied the seasonal variation in N dynamics and microbial biomass in the surface litter of a tropical dry forest ecosystem in Mexico over a 2-year period. Litter was collected at 4 different times of the year to determine changes in total, soluble, and microbial C and N concentrations. Additionally, litter from each sampling date was incubated under laboratory conditions to determine potential C mineralization rate, net N mineralization, net C and N microbial immobilization, and net nitrification. Litter C concentrations were highest in the early-dry season and lowest in the rainy season, while the seasonal changes in N concentrations varied between years. Litter P was higher in the rainy than in the early-dry season. Water-soluble organic C (WSOC) and water-soluble N concentrations were highest during the early- and late-dry seasons and represented up to 4.1 and 5.9% of the total C and N, respectively. NH₄⁺ and NO₃⁻ showed different seasonal and annual variations. They represented an average 23% of soluble N. Microbial C was generally higher in the dry than in the wet seasons, while microbial N was lowest in the late-dry and highest in the early-rainy seasons. Incubations showed that lowest potential C mineralization rates and C and N microbial immobilization occurred in rainy season litter, and were positively correlated to WSOC. Net nitrification was highest in rainy season litter. Our results showed that the seasonal pattern in N dynamics was influenced by rainfall seasonality and labile C availability, and not by microbial biomass. We propose a conceptual model to hypothesize how N dynamics in the litter layer of the Chamela tropical dry forest respond to the seasonal variation in rainfall.     

Litter species traits, but not richness, contribute to carbon and nitrogen dynamics in an alpine meadow on the Tibetan Plateau

Aims

Litter, as afterlife of plants, plays an important role in driving belowground decomposition processes. Here we tested effects of litter species identity and diversity on carbon (C) and nitrogen (N) dynamics during litter decomposition in N-limited alpine meadow soil from the Qinghai–Tibet Plateau.

Methods

We incubated litters of four meadow species, a sedge (“S”, Kobresia humilis), a grass (“G”, Elymus nutans), a herb (“H”, Saussurea superba), and a legume (“L”, Oxytropis falcata), in monoculture and in mixture with meadow soil. CO₂ release was measured 21 times during the incubation, and soil available N and microbial biomass C and N were measured before and after the experiment.

Results

The organic C decay rate did not differ much among soils amended with monocultures or mixtures of litter, except in the H, S, L, and S+H treatments, which had much higher decay rates. Potential decomposable C pools were lowest in the control, highest in the L treatment, and intermediate in the S treatment. Mineralized N was completely immobilized by soil microbes in all treatments except the control, S+L, and S+G+L treatments. Litter mixtures had both additive and non-additive effects on CO₂-C emission (mainly antagonistic effects), net N mineralization (mainly synergistic), and microbial biomass C and N (both). Overall, these parameters were not significantly correlated with litter species richness. Similarly, microbial C or N was not significantly correlated with litter N content or C/N. However, cumulative CO₂-C emission and net N mineralization were positively correlated with litter N content and negatively correlated with litter C/N.

Conclusions

Litter N content and C/N rather than litter species richness drove the release of CO₂-C and net available N in this ecosystem. The antagonistic effects of litter mixtures contributed to a modest release of CO₂-C, but their synergistic effects enhanced net available N. We suggest that in alpine meadow communities, balancing species with high and low N contents will benefit soil carbon sequestration and plant competition for available N with soil microbes.     

Decomposition and nutrient release by green manures in a tropical hillside agroecosystem

The decomposition and nutrient release of 12 plant materials were assessed in a 20-week litterbag field study in hillsides from Cauca, Colombia. Leaves of Tithonia diversifolia (TTH) and Indigofera constricta (IND) decomposed quickly (k=0.035±0.002 d^–1), while those of Cratylia argentea (CRA) and the stems evaluated decomposed slowly (k=0.007±0.002 d^–1). Potassium presented the highest release rates (k>0.085 d^–1). Rates of N and P release were high for all leaf materials evaluated (k>0.028 d^–1) with the exception of CRA (N and P), TTH and IND (P). While Mg release rates ranged from 0.013 to 0.122 d^–1, Ca release was generally slower (k=0.008–0.041 d^–1). Initial quality parameters that best correlated with decomposition (P>0.001) were neutral detergent fibre, NDF (r=–0.96) and in vitro dry matter digestibility, IVDMD (r=0.87). It is argued that NDF or IVDMD could be useful lab-based tests during screening of plant materials as green manures. Significant correlations (P>0.05) were also found for initial quality parameters and nutrient release, being most important the lignin/N ratio (r=–0.71) and (lignin+polyphenol)/N ratios (r=–0.70) for N release, the C/N (r=0.70) and N/P ratios (r=–0.66) for P release, the hemicellulose content (r=–0.75) for K release, the Ca content (r=0.82) for Ca release, and the C/P ratio (r=0.65) for Mg release. After 20 weeks, the leaves of Mucuna deerengianum released the highest amounts of N and P (144.5 and 11.4 kg ha^–1, respectively), while TTH released the highest amounts of K, Ca and Mg (129.3, 112.6 and 25.9 kg ha^–1, respectively). These results show the potential of some plant materials studied as sources of nutrients in tropical hillside agroecosystems.     

Soil C and N dynamics within a precipitation gradient in Mediterranean eucalypt plantations

Soil C and N dynamics were evaluated in five eucalypt plantations within a precipitation gradient (500–2,000 mm) in Portugal. Soil physical and chemical properties, total and labile (particulate organic matter, hydrolyzable, hot water soluble and microbial) soil C and N pools, and C and N mineralization were measured to characterize the C and N dynamics and their controlling factors within this gradient. Contents of total and labile soil organic C and N were positively correlated with the mean annual precipitation. A similar relationship was observed for net N mineralization (anaerobic and long-term aerobic incubation), gross N mineralization (¹⁵N isotope dilution technique) and C mineralization. In contrast, rates of C and N mineralization (per unit of C and N) were higher in the driest sites due to their higher proportion of particulate organic matter C. Net and gross N mineralization were strongly correlated and showed similar controlling factors (mean annual precipitation, total and labile C and N and extractable P contents), suggesting that net N mineralization during long-term aerobic incubation reflects gross N transformations. Although, gross NO₃–N production and gross NO₃–N immobilization were observed in all sites, net nitrification in the drier sites was not observed in the first weeks of the study. Our results suggest that, under Mediterranean conditions, mean annual precipitation is the major factor determining the C and N dynamics in soils with Eucalyptus plantations.