Influence of earthworm activity on aggregate-associated carbon and nitrogen dynamics differs with agroecosystem management

Earthworms are known to be important regulators of soil structure and soil organic matter (SOM) dynamics, however, quantifying their influence on carbon (C) and nitrogen (N) stabilization in agroecosystems remains a pertinent task. We manipulated population densities of the earthworm Aporrectodea rosea in three maize-tomato cropping systems [conventional (i.e., mineral fertilizer), organic (i.e., composted manure and legume cover crop), and an intermediate low-input system (i.e., alternating years of legume cover crop and mineral fertilizer)] to examine their influence on C and N incorporation into soil aggregates. Two treatments, no-earthworm versus the addition of five A. rosea adults, were established in paired microcosms using electro-shocking. A ¹³C and ¹⁵N labeled cover crop was incorporated into the soil of the organic and low-input systems, while ¹⁵N mineral fertilizer was applied in the conventional system. Soil samples were collected during the growing season and wet-sieved to obtain three aggregate size classes: macroaggregates (>250 μm), microaggregates (53-250 μm) and silt and clay fraction (<53 μm). Macroaggregates were further separated into coarse particulate organic matter (cPOM), microaggregates and the silt and clay fraction. Total C, ¹³C, total N and ¹⁵N were measured for all fractions and the bulk soil. Significant earthworm influences were restricted to the low-input and conventional systems on the final sampling date. In the low-input system, earthworms increased the incorporation of new C into microaggregates within macroaggregates by 35% (2.8 g m⁻² increase; P=0.03), compared to the no-earthworm treatment. Within this same cropping system, earthworms increased new N in the cPOM and the silt and clay fractions within macroaggregates, by 49% (0.21 g m⁻²; P<0.01) and 38% (0.19 g m⁻²; P=0.02), respectively. In the conventional system, earthworms appeared to decrease the incorporation of new N into free microaggregates and macroaggregates by 49% (1.38 g m⁻²; P=0.04) and 41% (0.51 g m⁻²; P=0.057), respectively. These results indicate that earthworms can play an important role in C and N dynamics and that agroecosystem management greatly influences the magnitude and direction of their effect.     

pH regulation of carbon and nitrogen dynamics in two agricultural soils

Soil pH is often hypothesized to be a major factor regulating organic matter turnover and inorganic nitrogen production in agricultural soils. The aim of this study was to critically test the relationship between soil pH and rates of C and N cycling, and dissolved organic nitrogen (DON), in two long-term field experiments in which pH had been manipulated (Rothamsted silty clay loam, pH 3.5-6.8; Woburn sandy loam, pH 3.4-6.3). While alteration of pH for 37 years significantly affected crop production, it had no significant effect on total soil C and N or indigenous mineral N levels. This implies that at steady state, increased organic matter inputs to the soil are balanced by increased outputs of CO₂. This is supported by the positive correlation between both plant productivity and intrinsic microbial respiration with soil pH. In addition, soil microbial biomass C and N, and nitrification were also significantly positively correlated with soil pH. Measurements of respiration following addition of urea and amino acids showed a significant decline in CO₂ evolution with increasing soil acidity, whilst glucose mineralization showed no response to pH. In conclusion, it appears that changes in soil pH significantly affect soil microbial activity and the rate of soil C and N cycling. The evidence suggests that this response is partially indirect, being primarily linked to pH induced changes in net primary production and the availability of substrates. In addition, enhanced soil acidity may also act directly on the functioning of the microbial community itself.     

Land-use impacts on surface runoff and soil detachment within agricultural sloping lands in Northern Vietnam

Two consecutive years of investigation on soil surface features, surface runoff and soil detachment within 1-m² microplots on 40% slope highlighted the effects of land-use change, vegetation cover and biological activity on the water pathways in Northern Vietnam. Three replicate plots were set up on each of five land-uses: cassava (CAS), grass fodder of Bracharia ruziziensis (BRA), a 3-year old fallow (FAL), tree stands of Acacia mangium and Venicia montana (FOR), and a fallow with regrowth of Eucalyptus regularly cut (EUC). The second year, two of the microplots under FAL and EUC were treated with herbicide (FALh, EUCh), one of them was burnt (FALh+b, EUCh+b). The highest yearly surface runoff coefficient of 16%, and soil detachment rate of 700 g m^− 2 yr^− 1 in average with a maximum of 1305 g m^− 2 yr^− 1 have been recorded under CAS. On FALh and FALh+b, runoff ratios were 8.7 and 13.5%, respectively and detachment rates were 86 and 389 g m^− 2. On FAL and BRA the yearly runoff ratio varied from 5.9 to 9.8% but the detachment rate was limited at 24 to 35 g m^− 2. FOR and EUC annual runoff was ≤ 3.1% and annual soil detachment ≤ 71 g m^− 2. These values were very low compared to the values reported on steep slopes in Laos within similar climate and vegetation cover.The runoff and detachment rates underlined the importance of rainfall intensities, soil physical properties, soil surface features, soil vegetation cover and biological activity. The annual surface runoff was highly correlated to the soil surface crusting. CAS and BRA plots were prone to crusting especially after weeding at the onset of the rainy season, when the soil surface was still uncovered. Soil bioturbation (earthworm casting activity) was the second factor that explains local variation of surface runoff and soil detachment. The continuous production of earthworms casts on soil surface, especially on FOR and EUC microplots, induced a marked surface roughness and reduced the surface runoff. The production of casts was very limited in FAL and completely absent in CAS microplots. So it is evident that our results confirm the deleterious effects of cassava on soil and water conservation.     

Dissolved soil organic nitrogen and carbon in a Norway spruce stand and an adjacent clear-cut

 The aims of this study were to characterize dissolved soil organic N (DON) and C (DOC) in a coniferous stand and an adjacent clear-cut, and to evaluate the importance of DON in N leaching. The study was carried out in a Norway spruce stand and a clear-cutting treatment in the same forest stand. Concentrations of DON in soil solution were monitored for 5 years after clear-cutting with gravity lysimeters. In the Norway spruce stand DON comprised 62–83% of the total N in soil solution over the 5-year period. The concentrations of DON in the clear-cut were higher than in the forest stand, but the proportion of total N was lower. To characterize dissolved organic matter, soil samples were aerobically incubated for 6 weeks in the laboratory, and the quantity, molecular size distribution and chemical nature of both DON and DOC were determined from water extracts made before and after the incubation. In the soil samples from the Norway spruce stand, C-rich compounds with a high C/N ratio and large molecular size were formed. In contrast, after the incubation the major carriers of DON in soil samples from the clear-cut were N-rich organic compounds with a low C/N ratio and a small molecular size. The distribution of different chemical fractions of DOC in soil did not differ much whether recovered from the Norway spruce stand or the clear-cut. It was (from highest to lowest concentration): hydrophobic acids>hydrophilic acids>phenols>hydrophilic neutrals. A major part of DON was also carried by these fractions. During incubation the concentration of N-containing hydrophilic acids increased, especially in the soil from the clearcut. In soil samples from the Norway spruce stand, the rate of net N mineralization was low and no NO₃ was formed, whilst the rate of net N mineralization was high and net nitrification was intensive in soil from the clear-cut. Received: 12 June 2000     

Direct effects of litter decomposition on soil dissolved organic carbon and nitrogen in a tropical rainforest

To clarify how litter decomposition processes affect soil dissolved organic carbon (DOC) and soil dissolved nitrogen (DN) dynamics, we conducted a field experiment on leaf litter and collected DOC and DN from the underlying soil in a tropical rainforest in Xishuangbanna, southwest China. Principal components analysis (PCA) showed the first PCA axis (corresponding to degraded litter quantity and quality) explained 61.3% and 71.2% of variation in DOC and DN concentrations, respectively. Stepwise linear regression analysis indicated that litter carbon mass controlled DOC and hemicellulose mass controlled DN concentrations. Litter decomposition was the predominant factor controlling surface-soil DOC and DN dynamics in this tropical rainforest.     

土地利用方式对土壤溶解性有机碳组成的影响

选取河北曲周、北京顺义和山东寿光的成对的农田(小麦-玉米轮作)和菜田土壤为研究对象,比较两种土地利用方式下DOC的含量、剖面分布以及组成特征。结果表明:与农田相比,菜田表层有机碳(SOC)与全氮(TN)含量上升。其中菜田有机碳含量较农田增加3.16%~47.4%,全氮含量增加3.09%~64.1%。土壤C/N与pH降低。无论是在农田还是菜田,DOC含量均随着土壤深度增加而显著降低;三地区菜田表层DOC含量平均为农田DOC含量的1.70倍;DOC与SOC成显著正相关关系。两种土地利用方式下,DOC组成中亲水组分(H IM)比例最高,其次为憎水酸性组分(HOA),憎水中性物质(HON)和憎水碱性物质(HOB)的含量最低;菜田中憎水酸性物质比例为19.0%~26.7%,农田憎水酸比例为14.83%~16.42%;寿光地区农田与菜田憎水酸含量差异显著(p<0.05),可能与寿光菜田土壤较高的SOC含量和较低的pH有关。三地区中菜田中憎水性物质与亲水性物质比例分别为0.30、0.39、0.41,反映了菜田中存在着不同程度的腐殖化现象。总之,农田转变为菜田后,显著影响了DOC的数量和质量。     

水稻品种和砷污染对土壤溶解性有机碳氮的影响

选取有机质含量和pH不同的2种水稻土(黄泥田和红泥田),通过盆栽实验研究砷(As)污染条件下,种植9个水稻品种对土壤溶解性有机碳(DOC)和溶解性有机氮(DON)含量的影响,分析水稻品种、As污染和土壤类型的相对影响与交互作用.结果表明,水稻品种显著影响了土壤DOC和DON的变化,在水稻收获后,DOC平均含量的大小顺序为杂交稻(41.09±0.92 mg kg-1)＞籼稻(38.10±1.53 mg kg-1)＞粳稻(37.74± 1.37 mg kg-1);DON平均含量的大小顺序为粳稻(2.94± 0.40 mg kg-1)＞杂交稻(2.61±0.42 mg kg-1)＞籼稻(1.45± 0.17 mg kg-1).As污染降低了土壤DOC和DON的含量,但不同品种水稻的响应不同.与对照相比,As污染条件下,黄泥田和红泥田中DOC平均含量分别下降了14.4％和11.1％,DON平均含量分别下降了65.0％和44.7％;DOC在种植杂交稻后降幅最小,而DON在种植籼稻后降幅最小.在两种水稻土中,黄泥田的DOC和DON平均含量高于红泥田,在没有As污染条件下,分别高22.4％和45.8％,这与黄泥田有机质含量和pH高有关.水稻品种、As污染和土壤类型对DOC和DON变化的影响不同,3个因子对DOC变化的相对贡献率分别为7.7％、15.5％和27.6％,对DON变化的相对贡献率分别为14.7％、24.2％和2.0％.     

Carbon and trace element fluxes in the pore water of an urban soil following greenwaste compost, woody and biochar amendments, inoculated with the earthworm Lumbricus terrestris

The benefits of adding composted organic materials to soils to enhance carbon storage could be countered by the mobilisation of some harmful pollutants commonly found in frequently degraded urban soils. Therefore non-composted materials could be a safer option. In the present study, carbon and trace element fluxes in soil pore water were studied in response to the surface mulch addition and the incorporation into an urban soil of greenwaste compost versus two non-composted amendments; a woody oversize material and biochar following inoculation with the vertical burrowing earthworm Lumbricus terrestris. The aim was to establish (i) to what extent the non-composted amendments impacted on mobility of soluble trace elements in the soil, compared to the composted amendment, and (ii) if/how this was regulated by earthworm activity.Both composted and non-composted amendments enhanced dissolved organic carbon (DOC) in soil pore water to ∼100-300 mg l⁻¹ in the upper depth of the soil profile above which they were applied as a mulch and similarly within the mesocosms in which they were mixed. Dissolved organic carbon, dissolved nitrogen (DTN) and trace metals, especially Cu and Pb, where enhanced to the greatest extent by greenwaste compost, because of strong co-mobilisation of metals by DOC. Biochar enhanced As and Cu mobility in the field profile and, additionally Pb in the mesocosms, with no effect on Cd. The woody, oversize amendment neither greatly increased DOC nor As, Cu, Pb or Zn mobility although, unlike the other amendments, earthworms increased DOC and Cd mobility when soils were amended with this material.This study concludes that non-composted amendments had a lower impact on DOC and thus trace element co-mobility than the composted greenwaste in this urban soil, whilst the general influence of earthworms was to reduce DOC and hence associated trace element mobility. In wider environmental terms the addition of non-composted materials to some urban soils, versus composted greenwaste could reduce the risk of mobilising potentially harmful elements, whilst usefully improving soil quality.     

Soluble organic nitrogen pools in adjacent native and plantation forests of subtropical Australia

Soil soluble organic nitrogen (SON) can play an important role in soil nitrogen (N) cycling in forest ecosystems. This study examined the effect of land-use change from a native forest (NF) to a first rotation (1R) and subsequent second rotation (2R) hoop pine (Araucaria cunninghamii) plantation on soil SON pools. The impact of residue management on SON pools was also investigated in the 2R forest, where SON was measured in tree rows (2R-T) and windrows (2R-W). Various extraction techniques were used to measure SON pool size in the 0-10, 10-20 and 20-30 cm layers of soil. The results showed that land-use change had a significant impact on soil SON pools. In the 0-10 cm layer, 3.2-8.7, 14-23, 20-28, 60-160 and 127-340 mg SON kg⁻¹ were extracted by water, 0.5 M K₂SO₄, 2 M KCl, hot water and hot 2 M KCl, respectively. The size of the SON pools and the potential production of SON (PPSON) were generally highest in the NF soil and lowest in the 2R-T soil, and in all forest types decreased with soil depth. The larger SON pools in the NF soil coincided with lower soil, litter and root C:N ratios, suggesting that the difference in the size of SON pools between the NF and 1R soil may be related to differences in the quality of organic matter input under the different forest ecosystems. Differences in the size of SON pools between the 1R soil and the 2R soils and between the 2R-T soil and the 2R-W soil may be related to the quantity of organic matter input and time since disturbance. Significant relationships were found between the SON extracted by 0.5 M K₂SO₄ (SON_ps) and 2 M KCl (SON_KCl), and also among the SON extracted by hot 2 M KCl (SON_hKCl), hot water (SON_hw) and water (SON_w), suggesting that the organic N released by these groups of extracts may be at least partly from similar pools.     

Controls on soil nitrogen cycling and microbial community composition across land use and incubation temperature

We conducted a laboratory incubation of forest (Scots pine (Pinus sylvestris) or beech (Fagus sylvatica)), grassland (Trifolium repens/Lolium perenne) and arable (organic and conventional) soils at 5 and 25 °C. We aimed to clarify the mechanisms of short-term (2-weeks) nitrogen (N) cycling processes and microbial community composition in relation to dissolved organic carbon (DOC) and N (DON) availability and selected soil properties. N cycling was measured by ¹⁵N pool dilution and microbial community composition by denaturing gradient gel electrophoresis (DGGE), phospholipid fatty acid (PLFA) and community level physiological profiles (CLPP). Soil DOC increased in the order of arable<grassland<forest soil while DON and gross N fluxes increased in the order of forest<arable<grassland soil; land use had no affect on respiration rate. Soil DOC was lower, while respiration, DON and gross N fluxes were higher at 25 than 5 °C. Gross N fluxes, respiration and bacterial biomass were all positively correlated with each other. Gross N fluxes were positively correlated with pH and DON, and negatively correlated with organic matter, fungal biomass, DOC and DOC/DON ratio. Respiration rate was positively correlated with bacterial biomass, DON and DOC/DON ratio. Multiple linear modelling indicated that soil pH, organic matter, bacterial biomass, DON and DOC/DON ratio were important in predicting gross N mineralization. Incubation temperature, pH and total-C were important in predicting gross nitrification, while gross N mineralization, gross nitrification and pH were important in predicting gross N immobilization. Permutation multivariate analysis of variance indicated that DGGE, CLPP and PLFA profiles were all significantly (P<0.05) affected by land use and incubation temperature. Multivariate regressions indicated that incubation temperature, pH and organic matter content were important in predicting DGGE, CLPP and PLFA profiles. PLFA and CLPP were also related to DON, DOC, ammonium and nitrate contents. Canonical correlation analysis showed that PLFA and CLPP were related to differences in the rates of gross N mineralization, gross nitrification and soil respiration. Our study indicates that vegetation type and/or management practices which control soil pH and mediate dissolved organic matter availability were important predictors of gross N fluxes and microbial composition in this short-term experiment.     

Microbial immobilization and mineralization of dissolved organic nitrogen from forest floors

Dissolved organic nitrogen (DON) plays a key role in the N cycle of many ecosystems, as DON availability and biodegradation are important for plant growth, microbial metabolism and N transport in soils. However, biodegradation of DON (defined as the sum of mineralization and microbial immobilization) is only poorly understood. In laboratory incubations, biodegradation of DON and dissolved organic carbon (DOC) from Oi and Oa horizons of spruce, beech and cypress forests ranged from 6 to 72%. Biodegradation of DON and DOC was similar in most samples, and mineralization of DON was more important than microbial immobilization. Nitrate additions (0-10 mg N L⁻¹) never influenced either DON immobilization by microorganisms or mineralization. We conclude that soil microorganisms do not necessarily prefer mineral N over DON for meeting their N demand, and that biodegradation of DON seems to be driven by the microbial demand for C rather than N. Quantifying the dynamics of DON in soils should include consideration of both C and N demands by microbes.     

Short-term dynamics of carbon and nitrogen after tillage in a freshwater marsh of northeast China

The Sanjiang Plain, one of the largest freshwater marshes in China, has experienced intensive cultivation over the past 50 years. However, there were few reports of short-term dynamics of soil carbon and nitrogen and CO₂ emission after tillage. In this paper, we studied the short-term dynamics of carbon and nitrogen after tillage in a freshwater marsh of northeast China. The results showed that response of carbon and nitrogen dynamic to tillage was different for intact wetland and soil cultivated for 10 years. Tillage was followed by immediate and significant increases in CO₂ efflux, which peaked at 0.25 h after tillage, four times higher than control in the wetland soils; while, only 2.5 times higher than control in the cultivated soils. Although, dissolved organic C (DOC) increased, the relative stability of microbial biomass C (MBC) pools together with the decreased respiration in the wetland soil suggested that the tillage did not lead to a burst in microbial activity and growth. Other factors such as moisture content before and after tillage may play an important role in determining microbial activity in the intact wetland. On the contrary, although dissolved organic C did not change, MBC pools, and soil respiration increase after tillage, suggesting tillage led to an increase in microbial activity and growth in the cultivated soil. Tillage initiated changes in soil aeration that was an important factor affecting soil microbiology in the long history of cultivation. Net N mineralization and nitrification occurred in both wetland and cultivated soils, but at different rates after tillage that in the intact wetland soil was higher than cultivated soil. Macroaggregates in the wetland soil would be expected to contain larger amounts of organic matter, and thus release a larger source of newly available substrate for microbes after tillage. In the intact wetland soil, ammonium, nitrate, and dissolved organic N (DON) concentrations were significantly negatively correlated to soil moisture (p < 0.01), suggesting high soil moisture in the natural wetland was not in favor of N mineralization.     

Carbon and nitrogen dynamics in ageing earthworm casts in grasslands of the eastern plains of Colombia

 The effects of a large species of anecic earthworm, Martiodrilus carimaguensis Jiménez and Moreno, on soil C and N dynamics were investigated in a native savanna and a man-made pasture of the eastern plains of Colombia. We compared, across time (11 months), the total C, total N, NH⁺ ₄ and NO^– ₃ contents in the earthworm casts, the underlying soil and the adjacent soil. Additional sampling of root biomass and macrofauna was performed. In the two management systems, the total C and N contents were higher in casts (4.33–7.50%) than in the bulk soil (2.81–4.08%), showing that the earthworms selected food substrates with high organic contents. In general, C contents significantly increased during cast ageing (+100%), possibly because of CO₂ fixation processes, dead root accumulation and/or macrofaunal activities in casts. In fresh casts, NH⁺ ₄ levels were very high (294.20–233.98 μg g^–1 dry cast) when compared to the soil (26.96–73.95 μg g^–1 dry soil), due to the intense mineralisation processes that occurred during the transit of soil and organic matter through the earthworm gut. During the first week of cast ageing, NH⁺ ₄ levels sharply decreased, while NH^– ₃ levels showed successive peaks in the casts, the underlying soil and the adjacent soil. These results suggested the rapid production of NO^– ₃ by nitrification processes in the fresh casts, followed by diffusion to the nearby soil, first vertically, then horizontally. After 2 weeks of cast ageing, NH⁺ ₄ and NO^– ₃ levels only showed slight variations, likely because of organic matter protection in stable dry casts. The root biomass was higher (1.6–4.7 times) below the old earthworm casts. The ecological significance of these results is discussed. Received: 22 October 1998     

Changes in particulate and mineral-associated organic carbon with land use in contrasting soils

Soil organic carbon(SOC) is the largest terrestrial carbon(C) stock, and the capacity of soils to preserve organic C(OC) varies with many factors,including land use, soil type, and soil depth. We investigated the effect of land use change on soil particulate organic matter(POM) and mineral-associated organic matter(MOM). Surface(0–10 cm) and subsurface(60–70 cm) samples were collected from paired sites(native and cropped) of four contrasting soils.Bulk soils were separated into POM and MOM fract...     

Crop nitrogen recovery and soil nitrogen dynamics in a 10‐year field experiment with biowaste compost

When fertilizing with compost, the fate of the nitrogen applied via compost (mineralization, plant uptake, leaching, soil accumulation) is relevant both from a plant‐production and an environmental point of view. In a 10‐year crop‐rotation field experiment with biowaste‐compost application rates of 9, 16, and 23 t ha^–1 y^–1 (f. m.), the N recovery by crops was 7%, 4%, and 3% of the total N applied via compost. Due to the high inherent fertility of the site, N recovery from mineral fertilizer was also low. In the minerally fertilized treatments, which received 25, 40, and 56 kg N ha^–1 y^–1 on average, N recovery from mineral fertilizer was 15%, 13%, and 11%, respectively. Although total N loads in the compost treatments were much higher than the N loads applied with mineral fertilizer (89–225 kg N_tot ha^–1 y^–1 vs. 25–56 kg N_tot ha^–1 y^–1; both on a 10‐year mean) and the N recovery was lower than in the treatments receiving mineral N fertilizer, soil NO ‐N contents measured three times a year (spring, post‐harvest, autumn) showed no higher increase through compost fertilization than through mineral fertilization at the rates applied in the experiment. Soil contents of N_org and C_org in the plowed layer (0–30 cm depth) increased significantly with compost fertilization, while with mineral fertilization, N_org contents were not significantly higher. Taking into account the decrease in soil N_org contents in the unfertilized control during the 10 years of the experiment, 16 t compost (f. m.) ha^–1 y^–1 just sufficed to keep the N_org content of the soil at the initial level.     

不同土地利用方式下三江平原东北部土壤有机碳和全氮分布规律

利用在全国采集的1400个典型农田剖面数据(其中在此研究区域有53个剖面),系统分析了不同土地利用方式下三江平原东北部土壤有机C和全N的分布规律。三江平原东北部典型农田0～20cm土壤有机C和全N的平均含量是22.7g/kg和1.84g/kg,20～100cm平均含量为13.0g/kg和1.08g/kg,不同土壤类型和种植作物下土壤有机C和全N分布没有显著性差异(p0.05)。与第二次土壤普查时三江平原东北部典型剖面土壤有机C和全N数据进行比较发现,0～20cm有机C和全N含量呈现下降趋势,而20～100cm呈现增加趋势。     

Impact of land use changes on soil carbon, nitrogen and phosphorus and water pollution in an arid region of northwest China

Abstract. Physical, chemical and environmental consequences of land use change from cultivated land to desert grassland and vice-versa were monitored in the middle reaches of the Heihe River basin, which is one of the largest inland basins of arid northwest China. Levels of N and P in soils and surface waters and soil organic carbon were measured. After the first 3–5 years of cultivation the N and P contents of various former grassland soils, including mountain-meadow and plains-meadow grasslands, decreased significantly. After some 13 years of cultivation, soil nutrient content in former mountain meadow grasslands gradually stabilized, whereas those of desertified grassland, where cultivation had simply been abandoned, showed a notable decrease. Under these latter conditions, soil N and P were lost at a rate of 276 kg ha⁻¹ and 360 kg ha⁻¹, respectively, over the 13-year period. The transformation of grassland into cultivated land and that of cultivated land into desert grassland resulted in organic carbon emissions of 1.68 Tg C and 0.55 Tg C, respectively, over 13 years. Land use changes in the arid inland region clearly have a significant influence on the soil organic carbon pool and carbon cycle. Falls in soil N and P led to 63% and 34% mean enrichment of N and P, respectively, in downstream waters, thus posing a future environmental problem for the arid region of northwest China.     

Seasonal variation in soluble soil carbon and nitrogen across a grassland productivity gradient

Understanding the fate and turnover of the pools that comprise dissolved organic nitrogen (DON) in soil is key to determining its role in ecosystem functioning. We investigated seasonal changes of dissolved organic carbon (DOC) and nitrogen (DON) concentrations within four molecular weight (MW) size fractions across an altitudinal gradient (from lowland to montane systems), and quantified individual amino acids and amino acid constituents of oligopeptidic-N, as well as nitrate and ammonium. We tested two ideas: first, that DON is more abundant than DIN in low-productivity relative to high-productivity grassland ecosystems; and second, that the abundance of peptides and amino acids is likewise greater in low- than high-productivity grassland. The most productive site had a history of inorganic fertiliser application, and hence in this site alone DIN was more abundant than DON. Plant productivity varied 3-fold between the other sites, and DON was generally at higher concentrations in the sites of lower productivity both in absolute terms as well as relative to DIN, with a large increase observed in spring. The fraction containing the highest concentration of the DON had a MW of >100 kDa, and in summer and autumn this fraction was more abundant at the lowest productivity site. We conclude that relationships between the abundance of DON relative to DIN and ecosystem productivity is dependent on season, and hence more complex than previously suggested, and that peptides are a dynamic and potentially nutritionally significant component of DON.