Response of Vegetation and Soil Carbon and Nitrogen Storage to Grazing Intensity in Semi-Arid Grasslands in the Agro-Pastoral Zone of Northern China

Overgrazing has been the primary cause of grassland degradation in the semi-arid grasslands of the agro-pastoral transition zone in northern China. However, there has been little evidence regarding grazing intensity impacts on vegetation change and soil C and N dynamics in this region. This paper reports the effects of four grazing intensities namely un-grazed (UG), lightly grazed (LG), moderately grazed (MG) and heavily grazed (HG) on vegetation characteristics and soil properties of grasslands in the Guyuan county in the agro-pastoral transition region, Hebei province, northern China. Our study showed that the vegetation height, canopy cover, plant species abundance and aboveground biomass decreased significantly with increased grazing intensity. Similarly, soil organic carbon (SOC) and total nitrogen (STN) in the 0–50 cm were highest under UG (13.3 kg C m⁻² and 1.69 kg N m⁻²) and lowest under HG (9.8 kg C m⁻² and 1.22 kg N m⁻²). Soil available nitrogen (SAN) was significantly lower under HG (644 kg N hm⁻²) than under other treatments (725–731 kg N hm⁻²) in the 0–50 cm. Our results indicate that the pasture management of “take half-leave half” has potential benefits for primary production and livestock grazing in this region. However, grazing exclusion was perhaps the most effective choice for restoring degraded grasslands in this region. Therefore, flexible rangeland management should be adopted in this region.     

Predicting the Effects of Woody Encroachment on Mammal Communities,Grazing Biomass and Fire Frequency in African Savannas

With grasslands and savannas covering 20% of the world’s land surface, accounting for 30–35% of worldwide Net Primary Productivity and supporting hundreds of millions of people, predicting changes in tree/grass systems is priority. Inappropriate land management and rising atmospheric CO₂ levels result in increased woody cover in savannas. Although woody encroachment occurs world-wide, Africa’s tourism and livestock grazing industries may be particularly vulnerable. Forecasts of responses of African wildlife and available grazing biomass to increases in woody cover are thus urgently needed. These predictions are hard to make due to non-linear responses and poorly understood feedback mechanisms between woody cover and other ecological responders, problems further amplified by the lack of long-term and large-scale datasets. We propose that a space-for-time analysis along an existing woody cover gradient overcomes some of these forecasting problems. Here we show, using an existing woody cover gradient (0–65%) across the Kruger National Park, South Africa, that increased woody cover is associated with (i) changed herbivore assemblage composition, (ii) reduced grass biomass, and (iii) reduced fire frequency. Furthermore, although increased woody cover is associated with reduced livestock production, we found indigenous herbivore biomass (excluding elephants) remains unchanged between 20–65% woody cover. This is due to a significant reorganization in the herbivore assemblage composition, mostly as a result of meso-grazers being substituted by browsers at increasing woody cover. Our results suggest that woody encroachment will have cascading consequences for Africa’s grazing systems, fire regimes and iconic wildlife. These effects will pose challenges and require adaptation of livelihoods and industries dependent on conditions currently prevailing.     

Soil Carbon and Nitrogen Storage in Upper Montane Riparian Meadows

Though typically limited in aerial extent, soils of high-elevation riparian wetlands have among the highest density of soil carbon (C) and nitrogen (N) of terrestrial ecosystems and therefore contribute disproportionally to ecosystem services such as water retention, forage production, wildlife habitat, and reactive N removal. Because much soil C and N is stored in labile forms in anaerobic conditions, management activities or environmental changes that lead to drying cause mineralization of labile soil organic matter, and loss of C and N. Meadows are focal points of human activities in mountain regions, often with incised stream channels from historically heavy grazing exacerbated by extreme runoff events. To quantify soil C and N stores in montane riparian meadows across hydrologic conditions, 17 meadows between 1950- and 2675-m elevation were selected in the central Sierra Nevada Range, California, that were classified using the proper functioning condition (PFC) system. Results indicate that C and N density in whole-solum soil cores were equivalent at forest edge positions of properly functioning, functioning at-risk, and nonfunctioning condition. Soils under more moist meadow positions in properly functioning meadows have at least twice the C, N, dissolved organic C, and dissolved organic N (DON) than those under nonfunctioning meadows. Densities of total N and DON, but not C, of functioning at-risk meadows are significantly lower (P < 0.05) than those of properly functioning meadows at mid-slope and stream-bank positions, suggesting accelerated loss of N early in degradation processes. Though variable, the soil attributes measured correspond well to the PFC riparian wetland classification system.     

Estimating Ecosystem Nitrogen Addition by a Leguminous Tree: A Mass Balance Approach Using a Woody Encroachment Chronosequence

Difficulty in quantifying rates of biological N fixation (BNF), especially over long time scales, remains a major impediment to defining N budgets in many ecosystems. To estimate N additions from BNF, we applied a tree-scale N mass balance approach to a well-characterized chronosequence of woody legume (Prosopis glandulosa) encroachment into subtropical grasslands. We defined spatially discrete single Prosopis clusters (aged 28–99 years), and for each calculated BNF as the residual of: soil N (0–30 cm), above- and below-ground biomass N, wet and dry atmospheric N deposition, N trace gas and N₂ loss, leaching loss, and baseline grassland soil N at time of establishment. Contemporary BNF for upland savanna woodland was estimated at 10.9 ± 1.8 kg N ha^?1 y^?1, equal to a total of 249 ± 60 kg N ha^?1 over about 130 years of encroachment at the site. Though these BNF values are lower than previous estimates for P. glandulosa, this likely reflects lower plant density as well as low water availability at this site. Uncertainty in soil and biomass parameters affected BNF estimates by 6–11%, with additional sensitivity of up to 18% to uncertainty in other scaling parameters. Differential N deposition (higher rates of dry N deposition to Prosopis canopies versus open grasslands) did not explain N accrual beneath trees; iterations that represented this scenario reduced estimated BNF estimates by a maximum of 1.5 kg N ha^?1 y^?1. We conclude that in this relatively well-constrained system, small-scale mass balance provides a reasonable method of estimating BNF and could provide an opportunity to cross-calibrate alternative estimation approaches.     

土地利用变化对草原生态系统土壤碳贮量的影响

本文概述了长期非持续性土地利用（草原开垦和过度放牧）对草原生态系统土壤碳贮量影响的有关研究成果和进展,指出了今后这方面研究的重点内容和对策。     

Arbuscular Mycorrhizal Fungi Mediate Grazing Effects on Seasonal Soil Nitrogen Fluxes in a Steppe Ecosystem

Ecosystems - Grazing and arbuscular mycorrhizal fungi (AMF) influence soil nitrogen (N) cycling in grassland ecosystems. However, it remains unclear whether AMF mediate grazing effects on soil N...     

Sheep Grazing Decreases Organic Carbon and Nitrogen Pools in the Patagonian Steppe: Combination of Direct and Indirect Effects

We explored the net effects of grazing on soil C and N pools in a Patagonian shrub–grass steppe (temperate South America). Net effects result from the combination of direct impacts of grazing on biogeochemical characteristics of microsites with indirect effects on relative cover of vegetated and unvegetated microsites. Within five independent areas, we sampled surface soils in sites subjected to three grazing intensities: (1) ungrazed sites inside grazing exclosures, (2) moderately grazed sites adjacent to them, and (3) intensely grazed sites within the same paddock. Grazing significantly reduced soil C and N pools, although this pattern was clearest in intensely grazed sites. This net effect was due to the combination of a direct reduction of soil N content in bare soil patches, and indirect effects mediated by the increase of the cover of bare soil microsites, with lower C and N content than either grass or shrub microsites. This increase in bare soil cover was accompanied by a reduction in cover of preferred grass species and standing dead material. Finally, stable isotope signatures varied significantly among grazed and ungrazed sites, with δ¹⁵N and δ¹³C significantly depleted in intensely grazed sites, suggesting reduced mineralization with increased grazing intensity. In the Patagonian steppe, grazing appears to exert a negative effect on soil C and N cycles; sound management practices must incorporate the importance of species shifts within life form, and the critical role of standing dead material in maintaining soil C and N stocks and biogeochemical processes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions  RAG designed study, performed research, analyzed data, wrote the paper; ATA designed study, wrote the paper; CGGM designed study, performed research, analyzed data; MGP performed research; OES designed study; RBJ designed study, contributed new methods.     

Grazing and Ecosystem Carbon Storage in the North American Great Plains

Isotopic signatures of ¹³C were used to quantify the relative contributions of C₃ and C₄ plants to whole-ecosystem C storage (soil+plant) in grazed and ungrazed sites at three distinct locations (short-, mid- and tallgrass communities) along an east–west environmental gradient in the North American Great Plains. Functional group composition of plant communities, the source and magnitude of carbon inputs, and total ecosystem carbon storage displayed inconsistent responses to long-term livestock grazing along this gradient. C₄ plants [primarily Bouteloua gracilis (H.B.K.) Lag ex Steud.] dominated the long-term grazed site in the shortgrass community, whereas the ungrazed site was co-dominated by C₃ and C₄ species; functional group composition did not differ between grazed and ungrazed sites in the mid- and tallgrass communities. Above-ground biomass was lower, but the relative proportion of fine root biomass was greater, in grazed compared to ungrazed sites at all three locations. The grazed site of the shortgrass community had 24% more whole-ecosystem carbon storage compared to the ungrazed site (4022 vs. 3236 g C m⁻²). In contrast, grazed sites at the mid- and tallgrass communities had slightly lower (8%) whole-ecosystem carbon storage compared to ungrazed sites (midgrass: 7970 vs. 8683 g C m⁻²; tallgrass: 8273 vs. 8997 g C m⁻²). Differential responses between the shortgrass and the mid- and tallgrass communities with respect to grazing and whole-ecosystem carbon storage are likely a result of: (1) maintenance of larger soil organic carbon (SOC) pools in the mid- and tallgrass communities (7476–8280 g C m⁻²) than the shortgrass community (2517–3307 g C m⁻²) that could potentially buffer ecosystem carbon fluxes, (2) lower root carbon/soil carbon ratios in the mid- and tallgrass communities (0.06–0.10) compared to the shortgrass community (0.20–0.27) suggesting that variation in root organic matter inputs would have relatively smaller effects on the size of the SOC pool, and (3) the absence of grazing-induced variation in the relative proportion of C₃ and C₄ functional groups in the mid- and tallgrass communities. We hypothesize that the magnitude and proportion of fine root mass within the upper soil profile is a principal driver mediating the effect of community composition on the biogeochemistry of these grassland ecosystems.     

Soil Carbon Storage by Switchgrass Grown for Bioenergy

Life-cycle assessments (LCAs) of switchgrass (Panicum virgatum L.) grown for bioenergy production require data on soil organic carbon (SOC) change and harvested C yields to accurately estimate net greenhouse gas (GHG) emissions. To date, nearly all information on SOC change under switchgrass has been based on modeled assumptions or small plot research, both of which do not take into account spatial variability within or across sites for an agro-ecoregion. To address this need, we measured change in SOC and harvested C yield for switchgrass fields on ten farms in the central and northern Great Plains, USA (930 km latitudinal range). Change in SOC was determined by collecting multiple soil samples in transects across the fields prior to planting switchgrass and again 5 years later after switchgrass had been grown and managed as a bioenergy crop. Harvested aboveground C averaged 2.5?±?0.7 Mg C ha^?1 over the 5 year study. Across sites, SOC increased significantly at 0–30 cm (P?=?0.03) and 0–120 cm (P?=?0.07), with accrual rates of 1.1 and 2.9 Mg C ha^?1 year^?1 (4.0 and 10.6 Mg CO₂ ha^?1 year^?1), respectively. Change in SOC across sites varied considerably, however, ranging from ?0.6 to 4.3 Mg C ha^?1 year^?1 for the 0–30 cm depth. Such variation in SOC change must be taken into consideration in LCAs. Net GHG emissions from bioenergy crops vary in space and time. Such variation, coupled with an increased reliance on agriculture for energy production, underscores the need for long-term environmental monitoring sites in major agro-ecoregions.     

The Effects of Manure and Nitrogen Fertilizer Applications on Soil Organic Carbon and Nitrogen in a High-Input Cropping System

With the goal of improving N fertilizer management to maximize soil organic carbon (SOC) storage and minimize N losses in high-intensity cropping system, a 6-years greenhouse vegetable experiment was conducted from 2004 to 2010 in Shouguang, northern China. Treatment tested the effects of organic manure and N fertilizer on SOC, total N (TN) pool and annual apparent N losses. The results demonstrated that SOC and TN concentrations in the 0-10cm soil layer decreased significantly without organic manure and mineral N applications, primarily because of the decomposition of stable C. Increasing C inputs through wheat straw and chicken manure incorporation couldn''t increase SOC pools over the 4 year duration of the experiment. In contrast to the organic manure treatment, the SOC and TN pools were not increased with the combination of organic manure and N fertilizer. However, the soil labile carbon fractions increased significantly when both chicken manure and N fertilizer were applied together. Additionally, lower optimized N fertilizer inputs did not decrease SOC and TN accumulation compared with conventional N applications. Despite the annual apparent N losses for the optimized N treatment were significantly lower than that for the conventional N treatment, the unchanged SOC over the past 6 years might limit N storage in the soil and more surplus N were lost to the environment. Consequently, optimized N fertilizer inputs according to root-zone N management did not influence the accumulation of SOC and TN in soil; but beneficial in reducing apparent N losses. N fertilizer management in a greenhouse cropping system should not only identify how to reduce N fertilizer input but should also be more attentive to improving soil fertility with better management of organic manure.     

Woody Plant Encroachment by <Emphasis Type="Italic">Juniperus virginiana</Emphasis> in a Mesic Native Grassland Promotes Rapid Carbon and Nitrogen Accrual

The cover and abundance of Juniperus virginiana L. in the U.S. Central Plains are rapidly increasing, largely as a result of changing land-use practices that alter fire regimes in native grassland communities. Little is known about how conversion of native grasslands to Juniperus-dominated forests alters soil nutrient availability and ecosystem storage of carbon (C) and nitrogen (N), although such land-cover changes have important implications for local ecosystem dynamics, as well as regional C and N budgets. Four replicate native grasslands and adjacent areas of recent J. virginiana encroachment were selected to assess potential changes in soil N availability, leaf-level photosynthesis, and major ecosystem C and N pools. Net N mineralization rates were assessed in situ over two years, and changes in labile soil organic pools (potential C and N mineralization rates and microbial biomass C and N) were determined. Photosynthetic nitrogen use efficiencies (PNUE) were used to examine differences in instantaneous leaf-level N use in C uptake. Comparisons of ecosystem C and N stocks revealed significant C and N accrual in both plant biomass and soils in these newly established forests, without changes in labile soil N pools. There were few differences in monthly in situ net N mineralization rates, although cumulative annual net N mineralization was greater in forest soils compared to grasslands. Conversely, potential C mineralization was significantly reduced in forest soils. Encroachment by J. virginiana into grasslands results in rapid accretion of ecosystem C and N in plant and soil pools with little apparent change in N availability. Widespread increases in the cover of woody plants, like J. virginiana, in areas formerly dominated by graminoid species suggest an increasing role of expanding woodlands and forests as regional C sinks in the central U.S.     

Soil Carbon Storage Response to Temperature: an Hypothesis

Recently, global and some regional observations of soil carbonstocks and turnover times have implied that warming may notdeplete soil carbon as much as predicted by ecosystem models.The proposed explanation is that microbial respiration of carbonin ‘old’ mineral pools is accelerated less by warmingthan ecosystem models currently assume. Data on the sensitivityof soil respiration to temperature are currently conflicting.An alternative or additional explanation is that warming increasesthe rate of physico-chemical processes which transfer organiccarbon to ‘protected’, more stable, soil carbonpools. These processes include adsorption reactions, some ofwhich are known to have positive activation energies. Theoretically,physico-chemical reactions may be expected to respond more towarming than enzyme-mediated microbial reactions. A simple analyticalmodel and a complex multi-pool soil carbon model are presented,which separate transfers between pools due to physico-chemicalreactions from those associated with microbial respiration.In the short-term, warming depletes soil carbon. But in thelong-term, carbon losses by accelerated microbial respirationare offset by increases in carbon input to the soil (net production)and any acceleration of soil physico-chemical ‘stabilization’reactions. In the models, if net production rates are increasedin response to notional warming by a factor of 1.3, and microbialrespiration (in all pools) by 1.5, then soil carbon at equilibriumremains unchanged if physico-chemical reactions are acceleratedby a factor of about 2.2 (50% more than microbial reactions).Equilibrium soil carbon increases if physico-chemical reactionsare over 50% more sensitive to warming than soil respiration.Copyright 2001 Annals of Botany Company Soil organic matter, carbon, respiration, temperature, stabilization, decomposition, model     

Woody Plant Encroachment into Grasslands: Spatial Patterns of Functional Group Distribution and Community Development

Woody plant encroachment into grasslands has been globally widespread. The woody species invading grasslands represent a variety of contrasting plant functional groups and growth forms. Are some woody plant functional types (PFTs) better suited to invade grasslands than others? To what extent do local patterns of distribution and abundance of woody PFTs invading grasslands reflect intrinsic topoedaphic properties versus plant-induced changes in soil properties? We addressed these questions in the Southern Great Plains, United States at a subtropical grassland known to have been encroached upon by woody species over the past 50-100 years. A total of 20 woody species (9 tree-statured; 11 shrub-statured) were encountered along a transect extending from an upland into a playa basin. About half of the encroaching woody plants were potential N₂-fixers (55% of species), but they contributed only 7% to 16 % of the total basal area. Most species and the PFTs they represent were ubiquitously distributed along the topoedaphic gradient, but with varying abundances. Overstory-understory comparisons suggest that while future species composition of these woody communities is likely to change, PFT composition is not. Canonical correspondence analysis (CCA) ordination and variance partitioning (Partial CCA) indicated that woody species and PFT composition in developing woody communities was primarily influenced by intrinsic landscape location variables (e.g., soil texture) and secondarily by plant-induced changes in soil organic carbon and total nitrogen content. The ubiquitous distribution of species and PFTs suggests that woody plants are generally well-suited to a broad range of grassland topoedaphic settings. However, here we only examined categorical and non-quantitative functional traits. Although intrinsic soil properties exerted more control over the floristics of grassland-to-woodland succession did plant modifications of soil carbon and nitrogen concentrations, the latter are likely to influence productivity and nutrient cycling and may, over longer time-frames, feed back to influence PFT distributions.     

Long-Term Nitrogen Storage and Soil Nitrogen Availability in Post-Fire Lodgepole Pine Ecosystems

Long-term, landscape patterns in inorganic nitrogen (N) availability and N stocks following infrequent, stand-replacing fire are unknown but are important for interpreting the effect of disturbances on ecosystem function. Here, we present results from a replicated chronosequence study in the Greater Yellowstone Ecosystem (Wyoming, USA) directed at measuring inorganic N availability (ion-exchange resin bags) and ecosystem N pools among 77 lodgepole pine stands that varied in age and density. Inorganic N availability ranged from 0.07 to 3.20 μN bag⁻¹ d⁻¹ and nitrate (NO₃⁻) was, on average, 65% of total resin-sorbed N. Total ecosystem N stocks (live + detrital + soil) averaged 109.9 ± 3.0 g N m⁻² (range = 63.7–185.8 g N m⁻²). Live N was 14%, detrital N was 29%, and soil N was 57% of total stocks. Soil NO₃⁻, total ecosystem N, live N, and detrital N generally increased with stand age, but soil N stocks decreased. Models (AIC_c) to predict soil N availability and N stocks included soil P, soil Ca, bulk density, and pH in addition to age (adj R ² ranged from 0.18 to 0.53) and density was included only for live N stocks. Patterns of N stocks and N availability with density were strongest for young stands (<20 years) regenerating from extensive fire in 1988; for example, litterfall N stocks increased with density (adj R ² = 0.86, P < 0.001) but inorganic N availability declined (adj R ² = 0.47, P < 0.003). Across the complex Yellowstone landscape, we conclude that N stocks and N availability are best predicted by a combination of local soil characteristics in addition to factors that vary at landscape scales (stand density and age). Overall, total ecosystem N stocks were recovered quickly following stand-replacing fire, suggesting that moderate increases in fire frequency will not affect long-term landscape N storage in Greater Yellowstone. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author contributions   EAHS, MGT, and MGR conceived the study; DMK performed field research; EAHS and DMK oversaw laboratory analyses and analyzed data; EAHS wrote the paper.     

Effects of Soil Texture on Belowground Carbon and Nutrient Storage in a Lowland Amazonian Forest Ecosystem

Soil texture plays a key role in belowground C storage in forest ecosystems and strongly influences nutrient availability and retention, particularly in highly weathered soils. We used field data and the Century ecosystem model to explore the role of soil texture in belowground C storage, nutrient pool sizes, and N fluxes in highly weathered soils in an Amazonian forest ecosystem. Our field results showed that sandy soils stored approximately 113 Mg C ha^-1 to a 1-m depth versus 101 Mg C ha^-1 in clay soils. Coarse root C represented a large and significant ecosystem C pool, amounting to 62% and 48% of the surface soil C pool on sands and clays, respectively, and 34% and 22% of the soil C pool on sands and clays to 1-m depth. The quantity of labile soil P, the soil C:N ratio, and live and dead fine root biomass in the 0–10-cm soil depth decreased along a gradient from sands to clays, whereas the opposite trend was observed for total P, mineral N, potential N mineralization, and denitrification enzyme activity. The Century model was able to predict the observed trends in surface soil C and N in loams and sands but underestimated C and N pools in the sands by approximately 45%. The model predicted that total belowground C (0–20 cm depth) in sands would be approximately half that of the clays, in contrast to the 89% we measured. This discrepancy is likely to be due to an underestimation of the role of belowground C allocation with low litter quality in sands, as well as an overestimation of the role of physical C protection by clays in this ecosystem. Changes in P and water availability had little effect on model outputs, whereas adding N greatly increased soil organic matter pools and productivity, illustrating the need for further integration of model structure and tropical forest biogeochemical cycling. Received 3 March 1999; accepted 27 August 1999.     

Effects of Nitrogen and Phosphorus Fertilization on Soil Carbon Fractions in Alpine Meadows on the Qinghai-Tibetan Plateau

In grassland ecosystems, N and P fertilization often increase plant productivity, but there is no concensus if fertilization affects soil C fractions. We tested effects of N, P and N+P fertilization at 5, 10, 15 g m⁻² yr⁻¹ (N₅, N₁₀, N₁₅, P₅, P₁₀, P₁₅, N₅P₅, N₁₀P₁₀, and N₁₅P₁₅) compared to unfertilized control on soil C, soil microbial biomass and functional diversity at the 0–20 cm and 20–40 cm depth in an alpine meadow after 5 years of continuous fertilization. Fertilization increased total aboveground biomass of community and grass but decreased legume and forb biomass compared to no fertilization. All fertilization treatments decreased the C:N ratios of legumes and roots compared to control, however fertilization at rates of 5 and 15 g m⁻² yr⁻¹ decreased the C:N ratios of the grasses. Compared to the control, soil microbial biomass C increased in N₅, N₁₀, P₅, and P₁₀ in 0–20 cm, and increased in N₁₀ and P₅ while decreased in other treatments in 20–40 cm. Most of the fertilization treatments decreased the respiratory quotient (qCO₂) in 0–20 cm but increased qCO₂ in 20–40 cm. Fertilization increased soil microbial functional diversity (except N₁₅) but decreased cumulative C mineralization (except in N₁₅ in 0–20 cm and N₅ in 20–40 cm). Soil organic C (SOC) decreased in P₅ and P₁₅ in 0–20 cm and for most of the fertilization treatments (except N₁₅P₁₅) in 20–40 cm. Overall, these results suggested that soils will not be a C sink (except N₁₅P₁₅). Nitrogen and phosphorus fertilization may lower the SOC pool by altering the plant biomass composition, especially the C:N ratios of different plant functional groups, and modifying C substrate utilization patterns of soil microbial communities. The N+P fertilization at 15 g m⁻² yr⁻¹ may be used in increasing plant aboveground biomass and soil C accumulation under these meadows.     

Effects of Straw Return in Deep Soils with Urea Addition on the Soil Organic Carbon Fractions in a Semi-Arid Temperate Cornfield

Returning straw to deep soil layers by using a deep-ditching-ridge-ploughing method is an innovative management practice that improves soil quality by increasing the soil organic carbon (SOC) content. However, the optimum quantity of straw return has not been determined. To solve this practical production problem, the following treatments with different amounts of corn straw were investigated: no straw return, CK; 400 kg ha^-1 straw, S₄₀₀; 800 kg ha^-1 straw, S₈₀₀; 1200 kg ha^-1 straw, S₁₂₀₀; and 1600 kg ha^-1 straw, S₁₆₀₀. After straw was returned to the soil for two years, the microbial biomass C (MBC), easily oxidized organic C (EOC), dissolved organic C (DOC) and light fraction organic C (LFOC) content were measured at three soil depths (0–10, 10–20, and 20–40 cm). The results showed that the combined application of 800 kg ha^-1 straw significantly increased the EOC, MBC, and LFOC contents and was a suitable agricultural practice for this region. Moreover, our results demonstrated that returning straw to deep soil layers was effective for increasing the SOC content.     

Regional Estimate of Nitric Oxide Emissions Following Woody Encroachment: Linking Imaging Spectroscopy and Field Studies

Woody encroachment, a spatially explicit process of land-cover change, is known to affect the biophysical and biogeochemical properties of ecosystems. However, little information is available on the impacts of woody encroachment on N oxide emissions from savanna regions. We combined hyperspectral remote sensing and field measurements to quantify spatial patterns and estimate regional fluxes of soil N oxide emissions as they covary with vegetation cover and soil type across a semiarid rangeland in north Texas. Soil nitric oxide (NO) emissions were highly correlated with Prosopis canopy cover, allowing the extrapolation of NO fluxes from hyperspectral observations of woody cover. NO emissions were highly variable, ranging from 0 to 550 kg NO-N km^–2 y^–1 across the region, with the lowest emissions from shallow clay soils and highest from deeper upland clay loams. An estimate of annual NO emissions based on remotely derived Prosopis cover, temperature, and precipitation was 160 kg NO-N km^–2 y^–1, almost twice that of the value derived from traditional averaging of field measurements. We conclude that relationships between NO emissions and remotely sensed structure and composition are advantageous for quantifying NO emissions at the regional scale. This study also provides new insight into the role of woody encroachment on biogeochemical processes that are highly variable and otherwise difficult to measure at the regional scale.     

Effects of Soil Carbon Amendment on Nitrogen Availability and Plant Growth in an Experimental Tallgrass Prairie Restoration

Restoration of tallgrass prairie on former agricultural land is often impeded by failure to establish a diverse native species assemblage and by difficulties with nonprairie, exotic species. High levels of available soil nitrogen (N) on such sites may favor fast‐growing exotics at the expense of more slowly growing prairie species characteristic of low‐N soils. We tested whether reducing N availability through soil carbon (C) amendments could be a useful tool in facilitating successful tallgrass prairie restoration. We added 6 kg/m² hardwood sawdust to experimental plots on an abandoned agricultural field in the Sandusky Plains of central Ohio, United States, increasing soil C by 67% in the upper 15 cm. This C amendment caused a 94% reduction in annual net N mineralization and a 27% increase in soil moisture but had no effect on total N or pH. Overall, plant mass after one growing season was reduced by 64% on amended compared with unamended soil, but this effect was less for prairie forbs (?34%) than for prairie grasses (?67%) or exotics (?62%). After the second growing season, only exotics responded significantly to the soil C amendment, with a 40% reduction in mass. The N concentration of green‐leaf tissue and of senescent leaf litter was also reduced by the soil C treatment in most cases. We conclude that soil C amendment imparts several immediate benefits for tallgrass prairie restoration––notably reduced N availability, slower plant growth, and lower competition from exotic species.