藏北高原牧区人工草地建设布局的适宜性分析

人工草地是缓解天然草地退化和提升草地生产力的一种有效途径,但人工草地建设发展需注重区域布局、种植区划、经营管理等战略问题,尤其是在高寒牧区建立人工草地,目前还存在诸多值得探讨的科学问题。为此,选取藏北高原高寒牧区为研究区,基于遥感数据,土壤数据,气象数据,地形和土地利用数据,结合野外实地调查,从可利用土地资源角度考虑,通过分析藏北现有人工草地建设的立地条件,识别出区域适宜人工草地建设的潜在分布区,并与现有人工草地分布位置及其面积进行对比分析和验证,以期为区域未来人工草地建设布局提供科学指导。研究结果表明:在各种约束因子的限制下,藏北满足人工草地建设条件的适宜区域极其有限,难以发展大面积的人工草地建设工程。水热条件和海拔是限制区域人工草地建设的主要地理因素,尤其是那曲地区,绝大部分区域无法满足人工牧草生长活动的积温需求。因此,区域牧草种植规划中需重点考虑牧草品种的生物学特性。另一方面,在藏北高寒牧区建立人工草地必须慎重,今后人工草地种植规划还需要加强牧草的抗寒性和抗旱性研究,在人工草地的管理方面要特别关注已建人工草地的可持续性和稳定性,防止出现草地退化和沙化等问题。     

脉冲式降水对不同类型草地土壤微生物呼吸碳释放量的影响

降水事件引起土壤短时间内释放大量CO₂的现象常称为降水脉冲效应。降水事件发生后,由于水分和养分可获得性快速提升使土壤微生物呼吸速率快速升高至正常水分状况的数倍,从而导致土壤CO₂大量释放并一定程度上影响着生态系统碳循环过程和土壤碳平衡,尤其在干旱或半干旱地区。利用自主研发的能快速测定土壤微生物呼吸速率的装置,对内蒙古三类典型草原(草甸草原、典型草原和荒漠草原)土壤分别开展土壤复湿实验(60%饱和含水量),并采用高频测定(48 h测定288次)。在土壤复湿后在所有温带草地类型中均发生了明显的脉冲效应,降水脉冲过程中单位有机质(土壤有机碳,SOC)最大呼吸速率(R_SOC-max)整体表现为荒漠草原(1.59 mg C g^-1 SOC h^-1)>草甸草原(0.73 mg C g^-1 SOC h^-1)>典型草原(0.50 mg C g^-1 SOC h^-1);而脉冲效应的持续时间(Dura...     

Different Response Patterns of Soil Respiration to a Nitrogen Addition Gradient in Four Types of Land-Use on an Alluvial Island in China

It has been well documented that nitrogen (N) additions significantly affect soil respiration (R _s) and its components [that is, autotrophic (R _a) and heterotrophic respiration (R _h)] in terrestrial ecosystems. These N-induced effects largely result from changes in plant growth, soil properties (for example, pH), and/ or microbial community. However, how R _s and its components respond to N addition gradients from low to high fertilizer application rates and what the differences are in diverse land-use types remain unclear. In our study, a field experiment was conducted to examine response patterns of R _s to a N addition gradient at four levels (0, 15, 30, and 45 g N m^?2 y^?1) in four types of land-use (paddy rice–wheat and maize–wheat croplands, an abandoned field grassland, and a Metasequoia plantation) from December 2012 to September 2014 in eastern China. Our results showed that N addition significantly stimulated R _s in all four land-use types and R _h in croplands (paddy rice–wheat and maize–wheat). R _s increased linearly with N addition rates in croplands and the plantation, whereas in grassland, it exhibited a parabolic response to N addition rates with the highest values at the moderate N level in spite of the homogeneous matrix for all four land-use types. This suggested higher response thresholds of R _s to the N addition gradient in croplands and the plantation. During the wheat-growing season in the two croplands, R _h also displayed linear increases with rising N addition rates. Interestingly, N addition significantly decreased the apparent temperature sensitivity of R _s and increased basal R _s. The different response patterns of R _s to the N addition gradient in diverse land-use types with a similar soil matrix indicate that vegetation type is very important in regulating terrestrial C cycle feedback to climate change under N deposition.     

Environmental controls and the influence of tree species on temporal variation in soil respiration in subtropical China

Background and aims

The knowledge of individual tree species impacts on soil respiration based on rigorous experimental designs is limited, but is crucial to help guide selection of species for reforestation and carbon (C) management purposes.

Methods

We assessed monthly soil respiration and its components, litterfall input, fine root production and mortality under 19-year-old native coniferous Cunninghamia lanceolata and broadleaved Mytilaria laosensis plantations in sub-tropical China.

Results

Total soil respiration from October 2011 to March 2013 was significantly lower under the C. lanceolata than the M. laosensis plantation. The difference in respiration rates derived from fine roots and the litter layer explained much of the variation of total soil respiration between the two tree species. We used an exponential equation and base temperature (10 °C) to normalize soil respiration rate and its components (R₁₀) and determined the correlation between R₁₀ and soil moisture. Although soil moisture had a positive relationship with R₁₀ derived from roots or litter under both C. lanceolata and M. laosensis forests, these positive correlations were masked by negative relationships between soil moisture and R₁₀ derived from root-free soil, which resulted in a neutral correlation between total R₁₀ and soil moisture under C. lanceolata forests. Monthly litterfall input was associated with variation in concurrent total soil respiration rate under the M. laosensis plantation and respiration rate lagging 3 months behind under the C. lanceolata plantation, which may suggest that litterfall input from M. laosensis can more rapidly produce C substrates for microbial respiration than litterfall from C. lanceolata.

Conclusions

This study highlighted that tree species-induced variation in the quality and quantity of fine roots and litterfall can impact not only the soil respiration rate but also the seasonal variation model of forest soil respiration.     

Differential effects of grazing,water, and nitrogen addition on soil respiration and its components in a meadow steppe

Background and aims

Understanding the influences of environmental variation and anthropogenic disturbance on soil respiration (R_S) is critical for accurate prediction of ecosystem C uptake and release. However, surprisingly, little is known about how soil respiration and its components respond to grazing in the context of global climate change (i.e., precipitation or nitrogen deposition increase).

Methods

We conducted a field manipulative grazing experiment with water and nitrogen addition treatments in a meadow grassland on the Songnen Plain, China, and assessed the combined influences of grazing and global change factors on R_S, autotrophic respiration (R_A), and heterotrophic respiration (R_H).

Results

Compared with the control plots, R_S, R_A and R_H all exhibited positive responses to water or nitrogen addition in the wet year, while a similar effect occurred only for R_H in the dry year. The responses of R_S to precipitation regimes were dominated by both frequency and amount. However, grazing significantly inhibited both soil respiration and its components in all subplots. Further analysis demonstrated that the plant root/shoot ratio, belowground biomass and microbial biomass played dominant roles in shaping these C exchange processes.

Conclusion

These findings suggest that changes in precipitation regimes, nitrogen deposition, and land utilization may significantly alter soil respiration and its component processes by affecting local carbon users (roots and soil microorganism) and carbon substrate supply in meadow steppe grasslands. The future soil carbon sequestration in the studied meadow steppe will be benefited more by the moderate grazing disturbance.

     

Roots affect the response of heterotrophic soil respiration to temperature in tussock grass microcosms

Aims and Background

While the temperature response of soil respiration (R_S) has been well studied, the partitioning of heterotrophic respiration (R_H) by soil microbes from autotrophic respiration (R_A) by roots, known to have distinct temperature sensitivities, has been problematic. Further complexity stems from the presence of roots affecting R_H, the rhizosphere priming effect. In this study the short-term temperature responses of R_A and R_H in relation to rhizosphere priming are investigated.

Methods

Temperature responses of R_A, R_H and rhizosphere priming were assessed in microcosms of Poa cita using a natural abundance δ¹³C discrimination approach.

Results

The temperature response of R_S was found to be regulated primarily by R_A, which accounted for 70 % of total soil respiration. Heterotrophic respiration was less sensitive to temperature in the presence of plant roots, resulting in negative priming effects with increasing temperature.

Conclusions

The results emphasize the importance of roots in regulating the temperature response of R_S, and a framework is presented for further investigation into temperature effects on heterotrophic respiration and rhizosphere priming, which could be applied to other soil and vegetation types to improve models of soil carbon turnover.     

Soil CO2 efflux and soil carbon balance of a tropical rubber plantation

Natural rubber is a valuable source of income in many tropical countries and rubber trees are increasingly planted in tropical areas, where they contribute to land-use changes that impact the global carbon cycle. However, little is known about the carbon balance of these plantations. We studied the soil carbon balance of a 15-year-old rubber plantation in Thailand and we specifically explored the seasonal dynamic of soil CO₂ efflux (F _S) in relation to seasonal changes in soil water content (W _S) and soil temperature (T _S), assessed the partitioning of F _S between autotrophic (R _A) and heterotrophic (R _H) sources in a root trenching experiment and estimated the contribution of aboveground and belowground carbon inputs to the soil carbon budget. A multiplicative model combining both T _S and W _S explained 58 % of the seasonal variation of F _S. Annual soil CO₂ efflux averaged 1.88 kg C m^?2 year^?1 between May 2009 and April 2011 and R _A and R _H accounted for respectively 63 and 37 % of F _S, after corrections of F _S measured on trenched plots for root decomposition and for difference in soil water content. The 4-year average annual aboveground litterfall was 0.53 kg C m^?2 year^?1 while a conservative estimate of belowground carbon input into the soil was much lower (0.17 kg C m^?2 year^?1). Our results highlighted that belowground processes (root and rhizomicrobial respiration and the heterotrophic respiration related to belowground carbon input into the soil) have a larger contribution to soil CO₂ efflux (72 %) than aboveground litter decomposition.     

亚热带沟叶结缕草草坪土壤呼吸

随城市化进程加速,城市草坪生态系统释放CO2将对区域碳循环产生重要影响。采用LI-8100开路式土壤碳通量测量系统对亚热带沟叶结缕草草坪（Zoysia matrella）土壤呼吸进行为期1a的定位研究,结果表明：草坪土壤呼吸季节动态呈现为单峰曲线,全年土壤呼吸速率的变化范围在38.99—368.50 mg C?m-2?h-1之间,年通量为1684 g C?m-2?a-1。土壤温度、总生物量、以及二者的交互作用对土壤呼吸季节变化的解释程度接近,分别为89%、88%和90%,但仅二者的交互作用进入土壤呼吸的逐步回归方程,表明草坪土壤呼吸的季节变化主要受土壤温度与总生物量共同驱动。春末修剪草坪对土壤呼吸速率没有显著影响。在秋末无雨时期,浇水后1—2d土壤湿度对土壤呼吸的促进作用可掩盖同期降温的影响,使土壤呼吸速率显著升高。     

Responses of Soil,Heterotrophic, and Autotrophic Respiration to Experimental Open-Field Soil Warming in a Cool-Temperate Deciduous Forest

How global warming will affect soil respiration (R _S) and its source components is poorly understood despite its importance for accurate prediction of global carbon (C) cycles. We examined the responses of R _S, heterotrophic respiration (R _H), autotrophic respiration (R _A), nitrogen (N) availability, and fine-root biomass to increased temperature in an open-field soil warming experiment. The experiment was conducted in a cool-temperate deciduous forest ecosystem in northern Japan. As this forest is subjected to strong temporal variation in temperature, on scales ranging from daily to seasonal, we also investigated the temporal variation in the effects of soil warming on R _S, R _H, and R _A. Soil temperature was continuously elevated by about 4.0°C from 2007 to 2014 using heating wires buried in the soil, and we measured soil respiratory processes in all four seasons from 2012 to 2014. Soil warming increased annual R _S by 32–45%, but the magnitude of the increase was different between the components: R _H and R _A were also stimulated, and increased by 39–41 and 17–18%, respectively. Soil N availability during the growing season and fine-root biomass were not remarkably affected by the warming treatment. We found that the warming effects varied seasonally. R _H increased significantly throughout the year, but the warming effect showed remarkable seasonal differences, with the maximum stimulation in the spring. This suggests that warmer spring temperature will produce a greater increase in CO₂ release than warmer summer temperatures. In addition, we found that soil warming reduced the temperature sensitivity (Q ₁₀) of R _S. Although the Q ₁₀ of both R _H and R _A tended to be reduced, the decrease in the Q ₁₀ of R _S was caused mainly by a decrease in the response of R _A to warming. These long-term results indicate that a balance between the rapid and large response of soil microbes and the acclimation of plant roots both play important roles in determining the response of R _S to soil warming, and must be carefully considered to predict the responses of soil C dynamics under future temperature conditions.     

Effects of Spartina alterniflora Invasion on Soil Respiration in the Yangtze River Estuary,China

Many studies have found that plant invasion can enhance soil organic carbon (SOC) pools, by increasing net primary production (NPP) and/or decreased soil respiration. While most studies have focused on C input, little attention has been paid to plant invasion effects on soil respiration, especially in wetland ecosystems. Our study examined the effects of Spartina alterniflora invasion on soil respiration and C dynamics in the Yangtze River estuary. The estuary was originally occupied by two native plant species: Phragmites australis in the high tide zone and Scirpus mariqueter in the low tide zone. Mean soil respiration rates were 185.8 and 142.3 mg CO₂ m⁻² h⁻¹ in S. alterniflora and P. australis stands in the high tide zone, and 159.7 and 112.0 mg CO₂ m⁻² h⁻¹ in S. alterniflora and S. mariqueter stands in the low tide zone, respectively. Aboveground NPP (ANPP), SOC, and microbial biomass were also significantly higher in the S. alterniflora stands than in the two native plant stands. S. alterniflora invasion did not significantly change soil inorganic carbon or pH. Our results indicated that enhanced ANPP by S. alterniflora exceeded invasion-induced C loss through soil respiration. This suggests that S. alterniflora invasion into the Yangtze River estuary could strengthen the net C sink of wetlands in the context of global climate change.     

京津风沙源治理工程对草地土壤有机碳库的影响——以内蒙古锡林郭勒盟为例

研究草地生态建设工程对土壤有机碳库的影响,对于草地生态建设工程成效评估及草地碳循环研究具有重要意义.本文以内蒙古锡林郭勒盟京津风沙源治理工程区为例,采用IPCC推荐的碳收支清单法,分析了2000-2006年京津风沙源治理工程对草地土壤有机碳库的影响,并对3种管理措施下（人工种草、飞播牧草和围栏封育）草地土壤达到最大有机碳密度的时间进行估算.结果表明： 2000-2006年,京津风沙源治理工程对草地土壤碳汇具有极大的促进作用,工程区整体表现为碳汇,碳汇量为59.26×10⁴ t C;不同草地管理方式下草地土壤固碳速率和效益差异显著,其中,人工种草的土壤固碳速率较快,为0.25 t C·hm^-2·a^-1,而围栏封育导致土壤固碳效益较高（0.63亿元）;与其他草地类型相比,低地草甸和温性草甸草原的土壤固碳速率较快,均为0.14 t C·hm^-2·a^-1.通过管理措施使草地达到潜在最大土壤有机碳密度是一个长期的过程,相对而言,人工种草所需的时间较短（57.75年）.     

Responses of soil respiration to soil management changes in an agropastoral ecotone in Inner Mongolia,China

Studying the responses of soil respiration (R_s) to soil management changes is critical for enhancing our understanding of the global carbon cycle and has practical implications for grassland management. Therefore, the objectives of this study were (1) quantify daily and seasonal patterns of R_s, (2) evaluate the influence of abiotic factors on R_s, and (3) detect the effects of soil management changes on R_s. We hypothesized that (1) most of daily and seasonal variation in R_s could be explained by soil temperature (T_s) and soil water content (S_w), (2) soil management changes could significantly affect R_s, and (3) soil management changes affected R_s via the significant change in abiotic and biotic factors. In situ R_s values were monitored in an agropastoral ecotone in Inner Mongolia, China, during the growing seasons in 2009 (August to October) and 2010 (May to October). The soil management changes sequences included free grazing grassland (FG), cropland (CL), grazing enclosure grassland (GE), and abandoned cultivated grassland (AC). During the growing season in 2010, cumulative R_s for FG, CL, GE, and AC averaged 265.97, 344.74, 236.70, and 226.42 gC m^?2 year^?1, respectively. The T_s and S_w significantly influenced R_s and explained 66%–86% of the variability in daily R_s. Monthly mean temperature and precipitation explained 78%–96% of the variability in monthly R_s. The results clearly showed that R_s was increased by 29% with the conversion of FG to CL and decreased by 35% and 11% with the conversion of CL to AC and FG to GE. The factors impacting the change in R_s under different soil management changes sequences varied. Our results confirm the tested hypotheses. The increase in Q₁₀ and litter biomass induced by conversion of FG to GE could lead to increased R_s if the climate warming. We suggest that after proper natural restoration period, grasslands should be utilized properly to decrease R_s.     

Pathways regulating decreased soil respiration with warming in a biocrust‐dominated dryland

A positive soil carbon (C)‐climate feedback is embedded into the climatic models of the IPCC. However, recent global syntheses indicate that the temperature sensitivity of soil respiration (R_S) in drylands, the largest biome on Earth, is actually lower in warmed than in control plots. Consequently, soil C losses with future warming are expected to be low compared with other biomes. Nevertheless, the empirical basis for these global extrapolations is still poor in drylands, due to the low number of field experiments testing the pathways behind the long‐term responses of soil respiration (R_S) to warming. Importantly, global drylands are covered with biocrusts (communities formed by bryophytes, lichens, cyanobacteria, fungi, and bacteria), and thus, R_S responses to warming may be driven by both autotrophic and heterotrophic pathways. Here, we evaluated the effects of 8‐year experimental warming on R_S, and the different pathways involved, in a biocrust‐dominated dryland in southern Spain. We also assessed the overall impacts on soil organic C (SOC) accumulation over time. Across the years and biocrust cover levels, warming reduced R_S by 0.30 μmol CO₂ m^?2 s^?1 (95% CI = ?0.24 to 0.84), although the negative warming effects were only significant after 3 years of elevated temperatures in areas with low initial biocrust cover. We found support for different pathways regulating the warming‐induced reduction in R_S at areas with low (microbial thermal acclimation via reduced soil mass‐specific respiration and β‐glucosidase enzymatic activity) vs. high (microbial thermal acclimation jointly with a reduction in autotrophic respiration from decreased lichen cover) initial biocrust cover. Our 8‐year experimental study shows a reduction in soil respiration with warming and highlights that biocrusts should be explicitly included in modeling efforts aimed to quantify the soil C–climate feedback in drylands.     

Soil carbon stocks and fluxes in a warm-temperate oak chronosequence in China

Soil respiration (R_S) and soil carbon stocks, as well as stand properties were investigated in a warm-temperate oak chronosequence in order to understand the age effect on soil CO₂ efflux. The chronosequence consisted of three 40-year-old, 48-year-old, 80-year-old, and 143-year-old oak stands, respectively. R_S measurements were conducted using a Li-8100 soil CO₂ flux system from October 2008 to October 2009. Temporal variations of R_S of all the four forests largely depended on soil temperature of 5 cm depth (T₅) (R²?=?0.738?C0.825). The mean R_S for 40-year-old, 48-year-old, 80-year-old, and 143-year-old forests were 2.37, 2.59, 2.99, and 3.32 ??mol CO₂ m^-2 s^-1 respectively. Both top soil organic carbon (SOC) and light fraction organic carbon (LFOC) stocks were significantly correlated to R_S variation, while only significant different LFOC among stands was found. This indicated that cumulated labile organic carbon was a better indicator on R_S variation, which was further illustrated by a better relationship between R ₁₀ and LFOC than that of R₁₀ and SOC. We found that the variation of mean R_S among stands was well correlated with basal area (BA). Marginal correlation between R_S and fine root biomass (FR) demonstrated the relationship between R_S and belowground metabolism. We also found total porosity (TP) negatively influenced the mean R_S and this negative effect may mainly be attributed to the capillary porosity (CP). Forest growth and yield could be contributed to R_S variation among stands. Forest succession also changed soil labile carbon stock and soil physical properties that influenced the CO₂ efflux.     

Patterns of wood carbon dioxide efflux across a 2,000-m elevation transect in an Andean moist forest

During a 1-year measurement period, we recorded the CO₂ efflux from stems (R _S) and coarse woody roots (R _R) of 13–20 common tree species at three study sites at 1,050, 1,890 and 3,050 m a.s.l. in an Andean moist forest. The objective of this work was to study elevation changes of woody tissue CO₂ efflux and the relationship to climate variation, site characteristics and growth. Furthermore, we aim to provide insights into important respiration–productivity relationships of a little studied tropical vegetation type. We expected R _S and R _R to vary with dry and humid season conditions. We further expected R _S to vary more than R _R due to a more stable soil than air temperature regime. Seasonal variation in woody tissue CO₂ efflux was indeed mainly attributable to stems. At the same time, temperature played only a small role in triggering variations in R _S. At stand level, the ratio of C release (g C m^?2 ground area year^?1) between stems and roots varied from 4:1 at 1,050 m to 1:1 at 3,050 m, indicating the increasing prevalence of root activity at high elevations. The fraction of growth respiration from total respiration varied between 10 (3,050 m) and 14% (1,050 m) for stems and between 5 (1,050 m) and 30% (3,050 m) for roots. Our results show that respiratory activity and hence productivity is not driven by low temperatures towards higher elevations in this tropical montane forest. We suggest that future studies should examine the limitation of carbohydrate supply from leaves as a driver for the changes in respiratory activity with elevation.     

Drought and recovery effects on belowground respiration dynamics and the partitioning of recent carbon in managed and abandoned grassland

The supply of soil respiration with recent photoassimilates is an important and fast pathway for respiratory loss of carbon (C). To date it is unknown how drought and land‐use change interactively influence the dynamics of recent C in soil‐respired CO₂. In an in situ common‐garden experiment, we exposed soil‐vegetation monoliths from a managed and a nearby abandoned mountain grassland to an experimental drought. Based on two ¹³CO₂ pulse‐labelling campaigns, we traced recently assimilated C in soil respiration during drought, rewetting and early recovery. Independent of grassland management, drought reduced the absolute allocation of recent C to soil respiration. Rewetting triggered a respiration pulse, which was strongly fuelled by C assimilated during drought. In comparison to the managed grassland, the abandoned grassland partitioned more recent C to belowground respiration than to root C storage under ample water supply. Interestingly, this pattern was reversed under drought. We suggest that these different response patterns reflect strategies of the managed and the abandoned grassland to enhance their respective resilience to drought, by fostering their resistance and recovery respectively. We conclude that while severe drought can override the effects of abandonment of grassland management on the respiratory dynamics of recent C, abandonment alters strategies of belowground assimilate investment, with consequences for soil‐CO₂ fluxes during drought and drought‐recovery.     

Subalpine grassland carbon dioxide fluxes indicate substantial carbon losses under increased nitrogen deposition,but not at elevated ozone concentration

Ozone (O₃) and nitrogen (N) deposition affect plant carbon (C) dynamics and may change ecosystem C‐sink/‐source properties. We studied effects of increased background [O₃] (up to [ambient] × 2) and increased N deposition (up to +50 kg ha^?1 a^?1) on mature, subalpine grassland during the third treatment year. During 10 days and 13 nights, distributed evenly over the growth period of 2006, we measured ecosystem‐level CO₂ exchange using a static cuvette. Light dependency of gross primary production (GPP) and temperature dependency of ecosystem respiration rates (R_eco) were established. Soil temperature, soil water content, and solar radiation were monitored. Using R_eco and GPP values, we calculated seasonal net ecosystem production (NEP), based on hourly averages of global radiation and soil temperature. Differences in NEP were compared with differences in soil organic C after 5 years of treatment. The high [O₃] had no effect on aboveground dry matter productivity (DM), but seasonal mean rates of both R_eco and GPP decreased ca. 8%. NEP indicated an unaltered growing season CO₂–C balance. High N treatment, with a +31% increase in DM, mean R_eco increased ca. 3%, but GPP decreased ca. 4%. Consequently, seasonal NEP yielded a 53.9 g C m^?2 (±22.05) C loss compared with control. Independent of treatment, we observed a negative NEP of 146.4 g C m^?2 (±15.3). Carbon loss was likely due to a transient management effect, equivalent to a shift from pasture to hay meadow and a drought effect, specific to the 2006 summer climate. We argue that this resulted from strongly intensified soil microbial respiration, following mitigation of nutrient limitation. There was no interaction between O₃ and N treatments. Thus, during the 2006 growing season, the subalpine grassland lost >2% of total topsoil organic C as respired CO₂, with increased N deposition responsible for one‐third of that loss.     

Soil warming accelerates decomposition of fine woody debris

Aims

There is evidence that increased N inputs to boreal forests, via atmospheric deposition or intentional fertilization, may impact negatively on ectomycorrhizal (ECM) fungi leading to a reduced flux of plant-derived carbon (C) back to the atmosphere via ECM. Our aim was to investigate the impact of N fertilization of a Pinus sylvestris (L.) forest stand on the return of recently photoassimilated C via the ECM component of soil respiration.

Methods

We used an in situ, large-scale, ¹³C-CO₂ isotopic pulse labelling approach and monitored the ¹³C label return using soil gas efflux chambers placed over three different types of soil collar to distinguish between heterotrophic (R_H), autotrophic (R_A; partitioned further into contributions from ECM hyphae and total R_A) and total (R_S) soil respiration.

Results

The impact of N fertilization was to significantly reduce R_A, particularly respiration via extramatrical ECM hyphae. ECM hyphal flux in control plots showed substantial spatial variability, resulting in mean flux estimates exceeding estimates of total R_A, while ECM contributions to R_A in N treated plots were estimated at around 30%.

Conclusion

Significant impacts on soil C cycling may be caused by reduced plant C allocation to ECM fungi in response to increased N inputs to boreal forests; ecosystem models so far lack this detail.     

Soil Biological and Chemical Properties in Restored Perennial Grassland in California

Restoration of California native perennial grassland is often initiated with cultivation to reduce the density and cover of non‐native annual grasses before seeding with native perennials. Tillage is known to adversely impact agriculturally cultivated land; thus changes in soil biological functions, as indicated by carbon (C) turnover and C retention, may also be negatively affected by these restoration techniques. We investigated a restored perennial grassland in the fourth year after planting Nassella pulchra, Elymus glaucus, and Hordeum brachyantherum ssp. californicum for total soil C and nitrogen (N), microbial biomass C, microbial respiration, CO₂ concentrations in the soil atmosphere, surface efflux of CO₂, and root distribution (0‐ to 15‐, 15‐ to 30‐, 30‐ to 60‐, and 60‐ to 80‐cm depths). A comparison was made between untreated annual grassland and plots without plant cover still maintained by tillage and herbicide. In the uppermost layer (0‐ to 15‐cm depth), total C, microbial biomass C, and respiration were lower in the tilled, bare soil than in the grassland soils, as was CO₂ efflux from the soil surface. Root length near perennial bunchgrasses was lower at the surface and greater at lower depths than in the annual grass–dominated areas; a similar but less pronounced trend was observed for root biomass. Few differences in soil biological or chemical properties occurred below 15‐cm depth, except that at lower depths, the CO₂ concentration in the soil atmosphere was lower in the plots without vegetation, possibly from reduced production of CO₂ due to the lack of root respiration. Similar microbiological properties in soil layers below 15‐cm depth suggest that deeper microbiota rely on more recalcitrant C sources and are less affected by plant removal than in the surface layer, even after 6 years. Without primary production, restoration procedures with extended periods of tillage and herbicide applications led to net losses of C during the plant‐free periods. However, at 4 years after planting native grasses, soil microbial biomass and activity were nearly the same as the former conditions represented by annual grassland, suggesting high resilience to the temporary disturbance caused by tillage.     

Reducing nutrient loss from farms through silvopastoral practices in coarse-textured soils of Florida,USA

Trees integrated into the range- and pasturelands of Florida could remove nutrients from deeper soil profiles that would otherwise be transported to water bodies and cause pollution. Soil nitrogen (N) and phosphorus (P) concentrations were monitored in three pastures: a treeless pasture of bahiagrass (Paspalum notatum); a pasture of bahiagrass under 20-year-old slash pine (Pinus elliotti) trees (silvopasture); and a pasture of native vegetation under pine trees (native silvopasture). Soil analysis from 10 profiles within each pasture showed that P concentrations were higher in treeless pasture (mean: 9.11 mg kg⁻¹ in the surface to 0.23 mg kg⁻¹ at 1.0 m depth) compared to silvopastures (mean: 2.51 and 0.087 mg kg⁻¹, respectively), and ammonium–N and nitrate–N concentrations were higher in the surface horizon of treeless pasture. The more extensive rooting zones of the combined stand of tree + forage may have caused higher nutrient uptake from silvopastures than treeless system. Further, compared to treeless system, soils under silvopasture showed higher P storage capacity. The results suggest that, compared to treeless pasture, silvopastoral association enhances nutrient retention in the system and thus reduces chances for nutrient transport to surface water. The study reflects the scope for applying ecological-engineering and ecosystem-restoration principles to silvopastoral-system design.