Timing patterns of nitrogen application alter plant production and CO2 efflux in an alpine meadow on the Tibetan Plateau,China

Nitrogen (N) availability is an important factor that determines ecosystem productivity and respiration, especially in N-limited alpine ecosystems. However, the magnitude of this response depends on the timing and amounts of N input. Moreover, we have only a limited understanding of the potential effects of the timing of N fertilization on ecosystem carbon (C) and N processes, and activities of the soil microbes. A nitrogen fertilization experiment was conducted in an alpine meadow on the Tibetan Plateau to determine how plant productivity and ecosystem respiration (RE) respond to the timing and amount of N application. In this study, half of the N was added either in the early spring (ES), before the growing season, or in the late fall (LF), after the growing season. All treatments received the other half of the N in mid-July. Three N levels (10, 20, 40 kg N hm⁻² yr⁻¹) were used for each of two N treatments, with no N addition used as a control. Plant aboveground biomass, ecosystem respiration (RE) and soil respiration (RS) were measured for the 2011 and 2012 growing seasons. The LF treatment enhanced ecosystem CO₂ efflux compared with the ES treatment at high N addition levels, resulting from an increase of soil dissolved organic C (DOC) and soil microbial activity. The ES treatment resulted in increased plant aboveground biomass when compared with LF during both growing seasons, although this increase accounted for little variation in ecosystem and soil respiration. Overall, the ES treatment is likely to increase the ecosystem C pool, while the LF treatment could accelerate ecosystem C cycling, especially for the high N treatment. Our results suggest that supplying N during the early stage of the growing season benefits both forage production and soil C sequestration in this alpine ecosystem.     

Effects of warming and grazing on N2O fluxes in an alpine meadow ecosystem on the Tibetan plateau

A great deal of uncertainty is associated with estimates of global nitrous oxide (N₂O) emissions because emissions from arid and polar climates were not included in the estimates due to a lack of available data. In particular, very few studies have assessed the response of N₂O flux to grazing under future warming conditions. This experiment was conducted to determine the effects of warming and grazing on N₂O flux at different time scales for three years under a controlled warming-grazing system. A free-air temperature enhancement system (FATE) using infrared heaters and grazing significantly increased soil temperatures for both of growing (average 1.8 °C in 2008) and no-growing seasons (average 3.0 °C for 3-years) within 20-cm depth, but only warming reduced soil moisture at 10-cm soil depth during the growing season during the drought year of 2008. Generally, the effects of warming and grazing on N₂O flux varied with sampling date, season, and year. No interactive effect between warming and grazing was found. Warming did not affect annual N₂O flux when grazing was moderate during the growing season because the tradeoff of the effect of warming on N₂O flux was observed between the growing season and no-growing season. No-warming with grazing (NWG) and warming with grazing (WG) significantly increased the average annual N₂O flux (57.8 and 31.0%) compared with no-warming with no-grazing (NWNG) and warming with no-grazing (WNG), respectively, indicating that warming reduced the response of N₂O flux to grazing in the region. Winter accounted for 36-57% of annual N₂O flux for NWNG and NWG, whereas only for 5-8% of annual N₂O flux for WNG and WG. Soil temperature could explain 5-35% of annual N₂O flux variation.     

Effects of tree harvesting, forest floor removal, and compaction on soil microbial biomass, microbial respiration, and N availability in a boreal aspen forest in British Columbia

The effects of timber harvesting and the resultant soil disturbances (compaction and forest floor removal) on relative soil water content, microbial biomass C and N contents (C_mic and N_mic), microbial biomass C:N ratio (C_mic-to-N_mic), microbial respiration, metabolic quotient (qCO₂), and available N content in the forest floor and the uppermost mineral soil (0-3 cm) were assessed in a long-term soil productivity (LTSP) site and adjacent mature forest stands in northeastern British Columbia (Canada). A combination of principal component analysis and redundancy analysis was used to test the effects of stem-only harvest, whole tree harvest plus forest floor removal, and soil compaction on the studied variables. Those properties in the forest floor were not affected by timber harvesting or soil compaction. In the mineral soil, compaction increased soil total C and N contents, relative water content, and N_mic by 45%, 40%, 34% and 72%, respectively, and decreased C_mic-to-N_mic ratio by 29%. However, these parameters were not affected by stem only harvesting or whole tree harvesting plus forest floor removal, contrasting the reduction of white spruce and aspen growth following forest floor removal and soil compaction reported in an earlier study. Those results suggest that at the study site the short-term effects of timber harvesting, forest floor removal, and soil compaction are rather complex and that microbial populations might not be affected by the perturbations in the same way as trees, at least not in the short term.     

The adverse effects of soil salinization on the growth of Trifolium alexandrinum L. and associated microbial and biochemical properties in a soil from Iran

The aim of this study was to determine the effects of increasing concentrations of salt solutions (including 0.12, 2, 6, and 10 dS m⁻¹) on the growth of berseem clover (Trifolium alexandrinum L.) and related soil microbial activity, biomass and enzyme activities. Results showed that the dry weights of root and shoot decreased with an increase in the concentrations of salt solutions. Soil salinization depressed the microbiological activities including soil respiration and enzyme activities. Substrate-induced respiration was consistently lower in salinized soils, whereas microbial biomass C did not vary among salinity levels. Higher metabolic quotients (qCO₂) and unaffected microbial biomass C at high EC values may indicate that salinity is a stressful factor, inducing either a shift in the microbial community with less catabolic activity or reduced efficiency of substrate utilization. Acid phosphatase and alkaline phosphatase activities decreased with increasing soil salinity. We found significant, positive correlations between the activities of phosphatase enzymes and plant's root mass, suggesting that any decrease in the activities of the two enzymes could be attributed to the reduced root biomass under saline conditions.     

Effect of land‐use on soil nutrients and microbial biomass of an alpine region on the northeastern Tibetan plateau,China

Land‐use patterns affect the quantity and quality of soil nutrients as well as microbial biomass and respiration in soil. However, few studies have been done to assess the influence of land‐use on soil and microbial characteristics of the alpine region on the northeastern Tibetan plateau. In order to understand the effect of land‐use management, we examined the chemical properties and microbial biomass of soils under three land‐use types including natural grassland, crop‐field (50 + y of biennial cropping and fallow) and abandoned old‐field (10 y) in the area. The results showed that the losses of soil organic carbon (SOC) and total nitrogen (TN) were about 45 and 43 per cent, respectively, due to cultivation for more than 50 y comparing with natural grassland. Because of the abandonment of cultivation for about a decade, SOC and TN were increased by 27 and 23 per cent, respectively, in comparison with the crop field. Microbial carbon (ranging from 357·5 to 761·6 mg kg⁻¹ soil) in the old‐field was intermediate between the crop field and grassland. Microbial nitrogen (ranging from 29·9 to 106·7 mg kg⁻¹ soil) and respiration (ranging from 60·4 to 96·4 mg CO₂‐C g⁻¹ C_mic d⁻¹) were not significantly lower in the old‐field than those in the grassland. Thus it could be concluded that cultivation decreased the organic matter and microbial biomass in soils, while the adoption of abandonment has achieved some targets of grassland restoration in the alpine region of Gansu Province on the northeastern Tibetan plateau. Copyright © 2010 John Wiley & Sons, Ltd.     

Seasonal effects of liming,irrigation, and acid precipitation on microbial biomass N in a spruce (Picea abies L.) forest soil

Summary Seasonal effects of liming, irrigation, and acid precipitation on microbial biomass N and some physicochemical properties of different topsoil horizons in a spruce forest (Picea abies L.) were measured throughout one growing season. The highest biomass N was recorded in autumn and spring in the upper soil horizons, while the lowest values were obtained in summer and in deeper horizons. The clearest differences between the different soil treatments were apparent in autumn and in the upper horizons. Liming increased the microbial biomass N from 1.7% of the total N content to 6.8% (Olf₁ layer) and from 1% to 2% of the total N content in the Of₂ layer. The main inorganic-N fraction in the deeper horizons was NO _inf3 ^sup- . An increase in cation exchange capacity was observed down to the Oh layer, while soil pH was only slightly higher in the Olf₁ and Of₂ layers after liming. The effects of irrigation were less marked. The microbial biomass N increased from 1.7% of total N to 4.8% in the Olf₁ layer and from 1% to 2% of total N in the Of₂ layer. In the Olf₁ layer an increase in C mineralization was observed. Acid precipitation decreased the microbial biomass N in the upper horizons from 4.8% of total N to 1.8% in the Olf₁ layer and from 2% to 0.5% in the Of₂ layer. No significant changes in soil pH were observed, but the decrease in cation exchange capacity may result in a decrease in the proton buffering capacity in the near future.     

施氮水平和打顶后涂抹NAA 对盐渍土壤上长绒棉铃脱落和产量的影响

为探讨减少盐碱地棉花铃脱落的途径, 田间条件下研究了土壤供应低氮(N 226.5 kg·hm^-2)和高氮(N 346.5 kg·hm^-2)条件下, 不打顶、打顶和打顶后涂抹生长素对棉花铃脱落和产量构成因素的影响。结果表明, 低氮和高氮水平下, 打顶、打顶后涂抹NAA 处理果枝落铃率都低于不打顶处理。打顶后涂抹浓度为0.3 mmol·L^-1 的NAA 处理在低氮水平下的成铃率比传统打顶处理高1.3 个百分点, 而铃脱落率比传统打顶处理低0.8 个百分点, 皮棉产量比传统打顶处理高23.7%。高氮水平下, 与传统打顶处理相比, 打顶后涂抹浓度为3 mmol·L^-1 的NAA 单株成铃率、铃脱落率、皮棉产量之间差异不显著。在同一处理中, 高氮水平的单株成铃数、单铃重和吐絮数均低于低氮水平, 而衣分无明显差异, 从而导致皮棉产量下降。上述结果说明, 打顶后涂抹一定浓度的NAA 可增加棉花单株现铃、成铃、吐絮数, 增加单铃重, 降低落铃率, 从而提高产量, 而过量施用氮肥会导致产量下降。     

Temporal and spatial dynamics of soil solution C and N concentrations during Lolium perenne L. sward establishment and the effects of elevated CO2 and N additions

There is now clear evidence for a prolonged increase in atmospheric CO₂ concentrations and enrichment of the biosphere with N. Understanding the fate of C in the plant-soil system under different CO₂ and N regimes is therefore of considerable importance in predicting the environmental effects of climate change and in predicting the sustainability of ecosystems. Swards of Lolium perenne were grown from seed in a Eutric Cambisol at either ambient (ca. 350 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric pCO₂ and subjected to two inorganic N fertilizer regimes (no added N and 70 kg N ha⁻¹ month⁻¹). After germination, soil solution concentrations of dissolved organic C (DOC), dissolved inorganic N (DIN), dissolved organic N (DON), phenolics and H⁺ were measured at five depths down the soil profile over 3 months. The exploration of soil layers down the soil profile by roots caused transient increases in soil solution DOC, DON and phenolic concentrations, which then subsequently returned to lower quasi-stable concentrations. In general, the addition of N tended to increase DOC and DON concentrations while exposure to elevated pCO₂ had the opposite effect. These treatment effects, however, gradually diminished over the duration of the experiment from the top of the soil profile downwards. The ambient pCO₂ plus added N regime was the only treatment to maintain a notable difference in soil solution solute concentration, relative to other treatments. This effect on soil solution chemistry appeared to be largely indirect resulting from increased plant growth and a decrease in soil moisture content. Our results show that although plant growth responses to elevated pCO₂ are critically dependent upon N availability, the organic chemistry of the soil solution is relatively insensitive to changes in plant growth once the plants have become established.     

The influence of water stress on biomass and N accumulation, N partitioning between above and below ground parts and on N rhizodeposition during reproductive growth of pea (Pisum sativum L.)

In the next few years, grain legumes should be used as a mean of N acquisition in cropping systems due to the depletion of non-renewable sources of energy. However, this requires improvements in the accuracy with which biological N₂ fixation, N balances and the N benefit for following crops are estimated. Moreover, grain legume crops are largely influenced by water stress while the world area exposed to drought periods may increase in the coming years due to global warming. This work aims to quantify biomass and N accumulation, N partitioning between above and below ground parts and N rhizodeposition by a pea (Pisum sativum L.) when influenced by water stress. In a controlled environment, pea plants were exposed to a severe drought or not stressed, either at flowering or during pod filling. N rhizodeposition was measured using the split root method and plants were harvested at the end of flowering (59 days after sowing, DAS 59), at the end of the drought period applied during pod filling (DAS 74) and at maturity (DAS 101). Water stress strongly affected pea dry weight and N accumulation. In both stressed treatments, nodule biomass and N content were reduced by about 65% in the absence of stress. Regardless of the treatment, total below ground plant N (root N + N rhizodeposition; BGN) and N rhizodeposition were correlated with total plant N content and the proportion of BGN to total plant N was similar among treatments at each sampling date. At DAS 59 and 74, the N contained in rhizodeposits represented around 30% of the total BGN and increased to around 60% at maturity though BGN decreased from around 20 to 13% of the total plant N between DAS 74 and maturity. The results suggest that water stress has no specific effect on N partitioning between above and below ground parts.     

Partitioning soil respiration and assessing the carbon balance in a Setaria italica (L.) Beauv. Cropland on the Loess Plateau, Northern China

A study was conducted in a Setaria italica (L.) Beauv. cropland on the Loess Plateau in order to partition total soil respiration (Rt) into microbial respiration (Rm) and root respiration (Rr) and to determine the carbon balance of the cropland ecosystem. A trenching method with micro-pore mesh was used to create root-free soil cores. Differences between mesh and non-mesh treatments were used to determine root respiration. Similar pattern was found in the diurnal variation of Rt and Rm with the minimum values at 3:00-6:00 h and the maximum at 13:00-15:00 h. The diurnal pattern of Rr was completely different, the minimum values appeared at 11:00-13:00 h and the maximum at 0:00-3:00 h. Soil temperature exerted predominant control over the diurnal variations of Rt and Rm. The daily mean values of Rt, Rm and Rr were close to the measurements taken at 9:00 h. On the seasonal scale, Rm was strongly dependent on soil temperature, with higher correlation with 2-cm-depth temperature (r² = 0.79, P < 0.001) than with 5-cm-depth temperature. When the effects of both soil temperature and moisture were considered, a linear model provided more accurate prediction of Rm (r² = 0.83, P < 0.0001). Root respiration (Rr) exhibited pronounced daily variation corresponding to changes in photosynthesis and seasonal variation related to crop phenological development. The seasonal variation in Rr was strongly correlated with leaf area index (LAI) (r² = 0.85, P < 0.05), and also positively, but marginally correlated with root biomass (RB, P = 0.073). Contribution of root respiration to total soil respiration (Rr/Rt ratio) showed pronounced diurnal and seasonal variations. The daily mean values of Rr/Rt ratios were close to the values obtained at 9:00 h. In different phenological stages, Rr/Rt ratios ranged from 22.3% to 86.6%; over the entire growing season, the mean Rr/Rt ratio was 67.3%.Total annual loss of C due to Rm in 2007 was estimated to be 121.3 g C m⁻² at the study site, while the annual NPP (net primary production) was 262.1 g C m⁻². The cropland system thus showed net carbon input of 140.8 g C m⁻².