Adjacent Native Grassland Research Articles

Soil Bacterial Community Shifts Are Driven by Soil Nutrient Availability along a Teak Plantation Chronosequence in Tropical Forests in China.

Simple SummaryTropical forests play an important role in the global carbon cycle, especially in the context of global climate change. Soil microorganisms are essential to the functions, services, and productivity of terrestrial ecosystems as a link to maintain the connections and interactions between the aboveground and belowground ecosystems. The interactions between plants and the soil microbiome are crucial for plant growth, health, and resistance to stressors. However, information on the response of soil microbial communities to a chronosequence of woody plants is lacking, especially in tropical forests. This study compares the soil properties, diversity, composition, and co-occurrence patterns of bacterial communities in the rhizosphere and bulk soils along a teak plantation chronosequence. The results show that the composition and co-occurrence patterns of the bacterial communities are statistically different among the plantations, while stand age has no significant impact on soil bacterial alpha diversity. The results further show that soil nutrients play a key role in shaping the soil bacterial community. The study also provides information about the dynamics and characteristics of these soil bacterial communities and adds valuable information that may underpin new strategies for the management of teak plantations.Soil bacterial communities play crucial roles in ecosystem functions and biogeochemical cycles of fundamental elements and are sensitive to environmental changes. However, the response of soil bacterial communities to chronosequence in tropical ecosystems is still poorly understood. This study characterized the structures and co-occurrence patterns of soil bacterial communities in rhizosphere and bulk soils along a chronosequence of teak plantations and adjacent native grassland as control. Stand ages significantly shifted the structure of soil bacterial communities but had no significant impact on bacterial community diversity. Bacterial community diversity in bulk soils was significantly higher than that in rhizosphere soils. The number of nodes and edges in the bacterial co-occurrence network first increased and then decreased with the chronosequence. The number of strongly positive correlations per network was much higher than negative correlations. Available potassium, total potassium, and available phosphorus were significant factors influencing the structure of the bacterial community in bulk soils. In contrast, urease, total potassium, pH, and total phosphorus were significant factors affecting the structure of the bacterial community in the rhizosphere soils. These results indicate that available nutrients in the soil are the main drivers regulating soil bacterial community variation along a teak plantation chronosequence.

Alterations in Arbuscular Mycorrhizal Community Along a Chronosequence of Teak (Tectona grandis) Plantations in Tropical Forests of China.

Arbuscular mycorrhizal (AM) fungi play a crucial role in promoting plant growth, enhancing plant stress resistance, and sustaining a healthy ecosystem. However, little is known about the mycorrhizal status of teak plantations. Here, we evaluated how the AM fungal communities of rhizosphere soils and roots respond to different stand ages of teak: 22, 35, 45, and 55-year-old from the adjacent native grassland (CK). A high-throughput sequencing method was used to compare the differences in soil and root AM fungal community structures. In combination with soil parameters, mechanisms driving the AM fungal community were revealed by redundancy analysis and the Mantel test. Additionally, spore density and colonization rates were analyzed. With increasing stand age, the AM fungal colonization rates and spore density increased linearly. Catalase activity and ammonium nitrogen content also increased, and soil organic carbon, total phosphorous, acid phosphatase activity, available potassium, and available phosphorus first increased and then decreased. Stand age significantly changed the structure of the AM fungal community but had no significant impact on the diversity of the AM fungal community. However, the diversity of the AM fungal community in soils was statistically higher than that in the roots. In total, nine and seven AM fungal genera were detected in the soil and root samples, respectively. The majority of sequences in soils and roots belonged to Glomus. Age-induced changes in soil properties could largely explain the alterations in the structure of the AM fungal community along a chronosequence, which included total potassium, carbon-nitrogen ratio, ammonium nitrogen, catalase, and acid phosphatase levels in soils and catalase, acid phosphatase, pH, and total potassium levels in roots. Soil nutrient availability and enzyme activity were the main driving factors regulating the shift in the AM fungal community structure along a chronosequence of the teak plantations.

Land Use Effects on Soil Fertility and Nutrient Cycling in the Peruvian High‐Andean Puna Grasslands

Core Ideas A short‐term increase in fertility and greater N mineralization were found in agricultural lands. Low plant recovery and soil degradation were found in fallow soils correlated with erosion indexes. In grass‐legume mixture pastures, improved soil structure and N build up increased soil fertility. P was found as the limiting nutrient in the studied agroecosystem. Expansion of crop production into high‐altitude native grasslands is occurring in the Peruvian High‐Andes due to climate change and agricultural intensification, with little understanding of the consequences to the ecosystem. The objective of this study was to determine the impact of land‐use changes on soil fertility and nutrient cycling to guide future land management. Comparisons were made between native grasslands and two alternative farming systems; the first was a system in which native grassland was replaced by an annual cash crop, maca (Lepidium meyenii Walp.), followed by a long fallow and the second was a long‐standing perennial cultivated pasture. There was greater N mineralization, extractable Bray‐1 P, and K+ in recently converted maca soils relative to adjacent native grassland soils most likely caused by tillage‐induced nutrient mineralization and by incorporation of manure and native grassland residue during the first year of land preparation prior to planting maca. Soil fertility, as determined by an ex situ pot trial to measure plant yield, was correlated with plant cover, soil total organic carbon, and water stable aggregates (WSA) under fallow following maca, suggesting that soil erosion might be an issue in the long term. In cultivated pastures, we found an increase in long‐term fertility driven primarily by the build‐up of soil N and improved soil structure in the perennial grass‐legume mixture compared with adjacent native grassland. Responsible management of the Puna agroecosystem requires restoration of plant cover after annual cropping of maca or the establishment of mixed perennial cultivated pasture.

Sorghum and Switchgrass as Biofuel Feedstocks on Marginal Lands in Northern China

To avoid food versus fuel conflicts in China, the production of bioenergy crops requires the evaluation of marginal lands under arid and semiarid conditions. We conducted field experiments with sorghum (Sorghum bicolor (L.) Moench) and switchgrass (Panicum virgatum L.) in Northern China. The biomass yield and nutrient (N, P, and K) accumulation of sorghum (leaves and stalks) and switchgrass (aboveground portion) were examined in 2012 and 2013. The soil organic carbon (SOC) content at depths of 0–30 cm was quantified at sorghum, switchgrass, and native grassland sites. The aboveground biomass yield of sorghum averaged 13.9 t ha−1 (14.2 and 13.6 t ha−1 in 2012 and 2013, respectively), while the switchgrass yield had a higher variability (14.8 versus 8.1 t ha−1 in 2012 and 2013, respectively) and averaged 11.5 t ha−1. The removals of N, P, and K averaged 56, 10, and 160 kg ha−1, respectively, for sorghum and 41, 11, and 89 kg ha−1, respectively, for switchgrass. To attain maximum biomass yield with minimum nutrient removal and the lowest transportation costs (due to the moisture content), the switchgrass harvest should be delayed until approximately 160 days after regrowth (DAR) in this region, whereas sorghum, which matures earlier, can be harvested as early as 120 days after sowing (DAS). Earlier harvests would increase the flexibility for large-scale ethanol production facilities. The SOC content within the 0–15-cm layer averaged 15.8 and 11.6 g C kg−1 before and 3 years after the establishment of the switchgrass culture, respectively, although the SOC increased with the culture age. In contrast, the SOC content of the 15–30-cm layer did not differ between the adjacent native grassland and the switchgrass planting. Under sorghum, the SOC content was reduced although it did not differ significantly from that of the native grassland at a significance level of p < 0.05. We conclude that high biomass yields can be obtained from the two crops under natural rainfall in the arid and semiarid conditions of Northern China. To address nutrient removal and SOC reduction, the leaves of sorghum should be returned to the field to increase the input of organic materials into the soil and reduce nutrient removal, which enhances soil fertility and sustainable production.

The impact of black wattle encroachment of indigenous grasslands on soil carbon, Eastern Cape, South Africa

Black wattle (Acacia mearnsii, De Wild.) is a fast growing tree species introduced into South Africa in the nineteenth century for commercial purposes. While being an important source of timber and firewood for local communities, black wattle is an aggressive invasive species and has pervasive adverse environmental impacts in South Africa. Little is known about the effects of black wattle encroachment on soil carbon, therefore the aim of this study was to investigate the impact of black wattle encroachment of natural grassland on soil carbon stocks and dynamics. Focussing on two sites in the Eastern Cape, South Africa, the study analysed carbon stocks in soil and litter on a chronosequence of black wattle stands of varying ages (up to >50 years) and compared these with adjacent native grassland. The study found that woody encroachment of grassland at one site had an insignificant effect on soil and litter carbon stocks. The second site showed a clear decline in combined soil and litter carbon stocks following wattle encroachment. The lowest stock was in the oldest wattle stand, meaning that carbon stocks are still declining after 50 years of encroachment. The results from the two sites demonstrate the importance of considering changes in soil carbon when evaluating ecosystem effects of invasive species.

Trace gas fluxes from a Northern mixed-grass prairie interseeded with Alfalfa

That legumes improve soil fertility is well established, however the extent that N fixed by legumes may increase trace gas emissions is poorly understood. Our aim was to determine if greater soil N in native prairies interseeded with alfalfa would increase trace gas emissions of nitrous oxide (N2O) and methane (CH4). A chronosequence study to investigate interseeded alfalfa on the fluxes of N2O and CH4 was undertaken on three prairie sites. At each site, native grasslands interseeded with alfalfa for 38, 16 and 5 years, along with adjacent native grassland controls and native grasslands fertilized with a one-time addition of N fertilizer, were measured for NO2 and CH4 using static chambers. Across interseeding years there were few significant differences between the three treatments (Native, Alfalfa, Fertilized). Despite increased soil N, the interseeded treatment released no more N2O than native grasslands. Methane uptake was generally greatest on interseeded grasslands and least on fertilized grasslands. Our results suggest that while alfalfa can increase soil N and improve productivity, it did not increase trace gas production relative to native grasslands. This is a win-win situation for ranchers wishing to increase soil fertility and thus their overall productivity and forage quality without increasing trace gas emissions.

Litter quality, decomposition rates and saprotrophic mycoflora in Fallopia japonica (Houtt.) Ronse Decraene and in adjacent native grassland vegetation

Fallopia japonica succeeds in invading different ecosystems likely because of its huge biomass production. This biomass is characterized by low nutritional quality and low decomposition rates but knowledge on whether these features are correlated to microbial decomposers is still lacking. The aims of this work were: i) to determine litter decomposition rates of native grassland vegetation and F. japonica under different conditions in a year-round experiment; ii) to evaluate litter quality and/or site effect on the decomposition of the invader and native vegetation and iii) to characterize mycoflora isolated from F. japonica and native vegetation litter. The results showed that F. japonica litter decomposes 3–4 times slower than that of native grassland, mainly due to its low N content and consequently high C/N ratio both in leaves and stems. As decomposition proceeds C/N in F. japonica litter decreases to values approaching those of the grassland litter. Site had no effect on the decomposition rates of F. japonica and grassland litter. Total fungal load and composition differed between F. japonica and native litter, and also varied across sites. These results indicate that the successful invasive plant F. japonica affects the structure and functions of the invaded ecosystem through a huge production of low quality, slow-decomposing litter that selects saprotrophic fungi.

Phenology of fine roots and leaves in forest and grassland

Summary 1 The phenology of temperate vegetation is advancing in association with climate warming. Most phenology data, however, comes from flowers and tree leaves. We tested the generality of results from shoot phenology by expanding data collection in two new directions. We related forest leaf phenology to root phenology, and to phenology in a second habitat, grassland. 2 We measured leaf and root phenology simultaneously in aspen forest and adjacent native grassland. Root growth accounts for 80–90% of productivity in these habitats. Seasonal variation in soil moisture and temperature were also measured. 3 Forest leaf production was greatest about 45 days before peak root production, resulting in a significant negative correlation between leaf and root production in forest. Grassland leaf production was greatest about 15 days before peak root production, and grassland leaf and root production were significantly positively correlated. The duration of root production was 40% greater than that of shoot production. 4 Forest leaf production increased significantly with increasing soil moisture, but not temperature. In contrast, the production of forest roots, grassland roots and grassland leaves increased significantly with soil temperature. 5 Synthesis. The most commonly measured aspect of phenology, forest leaves, is out of step with the majority of production in forest, as well as phenology in grassland. The invasion of grassland by woody vegetation is characterized by a decoupling of root and shoot phenology, a result that has not been reported previously. Given the global nature of woody plant encroachment, this decoupling may influence our general understanding of productivity and carbon sequestration in response to warming.

Carbon management index based on physical fractionation of soil organic matter in an Acrisol under long-term no-till cropping systems

The carbon management index (CMI) is derived from the total soil organic C pool and C lability and is useful to evaluate the capacity of management systems to promote soil quality. However, the CMI has not been commonly used for this purpose, possible due to some limitations of the 333 mM KMnO 4-chemical oxidation method conventionally employed to determine the labile C fraction. We hypothesized, however, that physical fractionation of organic matter is an alternative approach to determine the labile C. The objectives of this study were (i) to assess the physical fractionation with density (NaI 1.8 Mg m −3) and particle-size separation (53 μm mesh) as alternative methods to the KMnO 4-chemical oxidation (60 and 333 mM) in determining the labile C and thus the CMI, and (ii) to evaluate the capacity of long-term (19 years) no-till cropping systems (oat/maize: O/M, oat + vetch/maize: O + V/M, oat + vetch/maize + cowpea: O + V/M + C, and pigeon pea + maize: P + M) and N fertilization (0 and 180 kg N ha −1) to promote the soil quality of a Southern Brazilian Acrisol, using the CMI as the main assessment parameter. Soil samples were collected from 0 to 12.5 cm layer, and the soil of an adjacent native grassland was taken as reference. The mean annual C input of the cropping systems varied from 3.4 to 6.0 Mg ha −1 and the highest amounts occurred in legume-based cropping systems and N fertilized treatments. The C pool index was positively related to the annual C input ( r 2 = 0.93, P < 0.002). The labile C determined by density (4.4–10.4% of C pool) and particle-size separation (9.5–17.7% of C pool) had a close relationship ( r = 0.60 and 0.85, respectively) with the labile C determined using 60 mM KMnO 4 (7.3–10.5% of C pool). The labile C resulting from the three methods was related to the annual C input imparted by the cropping systems ( r 2 = 0.67–0.88), reinforcing the possibility of using physical fractionation as an alternative approach to determine labile C. In contrast, the chemical method using 333 mM KMnO 4 was not sensitive to different cropping systems and resulted in too high percentage of labile C, varying from 16.8 to 35.2% of the C pool. The CMI based on physical fractionation was a sensitive tool for assessing the capacity of management systems to promote soil quality, as evidenced by its close correlation ( r = 0.88, at average) with soil physical, chemical, and biological attributes. The introduction of winter (vetch) and, especially, summer legume cover crops (cowpea and pigeon pea), or application of fertilizer-N, improved the capacity of the management system into promote soil quality in this subtropical Acrisol.

The invasive plant species Centaurea maculosa alters arbuscular mycorrhizal fungal communities in the field

While several recent studies have described changes in microbial communities associated with exotic plant invasion, how arbuscular mycorrhizal fungi (AMF) communities respond to exotic plant invasion is not well known, despite the salient role of this group in plant interactions. Here, we use molecular methods (terminal restriction fragment length polymorphism analyses based on the large subunit of the rRNA gene) to examine AMF community structure in sites dominated by the invasive mycorrhizal forb, Centaurea maculosa Lam. (spotted knapweed), and in adjacent native grassland sites. Our results indicate that significant AMF community alteration occurs following C. maculosa invasion. Moreover, a significant reduction in the number of restriction fragment sizes was found for samples collected in C. maculosa-dominated areas, suggesting reduced AMF diversity. Extraradical hyphal lengths exhibited a significant, on average 24%, reduction in C. maculosa-versus native grass-dominated sites. As both AMF community composition and abundance were altered by C.maculosa invasion, these data are strongly suggestive of potential impacts on AMF-mediated ecosystem processes. Given that the composition of AMF communities has the potential to differentially influence different plant species, our results may have important implications for site restoration after weed invasion.

Carbon and nitrogen stocks in physical fractions of a subtropical Acrisol as influenced by long-term no-till cropping systems and N fertilisation

The long-term soil management effects on C and N stocks of soil physical fractions are still poorly understood for South American subtropical soils. This study aimed (i) to evaluate the influence of cereal- and legume-based cropping systems and N fertilisation on C and N stocks of the sand-, silt- and clay-size fractions of a no-tilled subtropical Acrisol in southern Brazil, (ii) to compute the Carbon Management Index (CMI) for those cropping systems using physical fractionation data, and (iii) to investigate the possible existence of finite capacity of those soil physical fractions to store C and N. Soil samples of a long-term experiment were collected from the 0–2.5 and 2.5–7.5 cm layers of three no-till cropping systems [fallow bare soil, oat/maize (O/M) and pigeon pea+maize (P+M)] under two N fertilisation levels (0 and 180 kg N ha−1). However, for fallow bare soil, only the non-fertilised sub-plot was sampled. An adjacent native grassland soil was sampled as a reference. The C and N stocks of the three soil physical fractions were higher in the legume-based cropping system (P+M) than in O/M and bare soil, because of the higher residue input in the former. The P+M cropping system restored the C and N stocks in sand- and silt-size fractions to the same levels found in grassland soil. Higher C and N stocks in all physical fractions were also obtained with N fertilisation. The C and N stocks and the C:N ratio were most affected by cropping systems in the sand- and least in the clay-size fraction. Particulate organic matter was found in the silt-size fraction, showing this fraction is not only constituted by mineral-associated organic mater, as commonly believed. Taking grassland soil as reference (CMI = 100), the CMI ranged from 46, in O/M no N, to 517, in P+M with N, pointing to a better soil management in the latter. The clay-size fraction tended to show a finite capacity to store C and N (48.8 g C kg−1 and 4.9 g N kg−1 of clay), which was not verified in sand- and silt-size fractions. The adoption of no-tillage and legume-based cropping systems with high residue input are adequate soil management strategies to improve soil quality and make the agricultural production systems more sustainable in subtropical regions.

Soil hydraulic properties of cropland compared with reestablished and native grassland

Conversion of cropland to perennial grasses will, over time, produce changes in soil hydraulic properties. The objective of this study was to characterize and compare hydraulic properties of fine-textured soils on adjacent native grassland, recently tilled cropland, and reestablished grassland in the Conservation Reserve Program (CRP) at three locations in the Southern Great Plains. A tension infiltrometer was used to measure unconfined, unsaturated infiltration over a range of supply pressure heads (nominally, h=−150, −100, −50, and −5 mm H 2O) at the soil surface. Intact soil cores were sampled within the Ap and Bt horizons to determine bulk density and water desorption curves, θ( h), at potentials ranging from −0.15 to −100 kPa. Unsaturated hydraulic conductivity K( h) over the range in supply pressure heads was estimated using Wooding's equation for steady-state flow from a disc source. The van Genuchten water retention model was fitted to θ( h) data to estimate parameter values. Soils in CRP had greater surface bulk densities than their grassland and cropland counterparts. The shape of the soil water retention curve for grassland and CRP land were similar, suggesting that converted croplands had fully reconsolidated. Mean near-saturated hydraulic conductivities of cropland at h=−5 mm were not significantly different from grassland. However, at −150 mm supply pressure head, cropped soils had a mean unsaturated conductivity 2.3 and 4.1 times greater than CRP land and grassland, respectively. Sites in CRP had the lowest ( P<0.05) near-saturated hydraulic conductivities ( h=−5 mm), which suggest that after 10 years, grasses had not fully ameliorated changes in pore structure caused by tillage. Comparison of unsaturated conductivities for grassland and CRP land suggest that long-term structural development on native grasslands was principally confined to effective pore radii greater than 300 μm. Land use practices had a greater effect on water movement than did soil series, indicating that the modifying effects of tillage, reconsolidation, and pore structure evolution on hydraulic properties are important processes governing water movement in these fine-textured soils.

Restoration of Microbial Residues in Soils of the Conservation Reserve Program

To elucidate the role of microorganisms for C and N sequestration in arable soils converted to grassland (sites of the Conservation Reserve Program; CRP), we determined amino sugars as indicators for microbial residues in surface samples (0–5 cm) obtained from each of 10 adjacent native grassland, CRP, and cropland sites across the U.S. Great Plains. The CRP sites were 6 to 10 yr and the cropland sites were &gt;80 yr old. Compared with native grasslands, the CRP sites had lost between 17 and 50% and the cropland sites between 32 and 94% of their surface soil organic matter (SOM). The C/N ratio was not significantly different among the three land‐use systems, indicating that C and N losses occurred at similar intensity. The mean amino sugar concentrations decreased in the order native grassland (70 g kg−1 C; 750 g kg−1 N) &gt; CRP (53 g kg−1 C; 570 g kg−1 N) &gt; cropland (47 g kg−1 C; 450 g kg−1 N). This decrease in the element‐normalized concentrations of amino sugars indicated that they responded faster to management than other C or N containing compounds. The response of individual amino sugars related to soil compaction and the temperature regime. We suggest that the resequestration of C and N into the residues of bacteria and fungi requires several years, but as it depends on land use it could be manipulated using, for example, soil decompacting techniques to improve CRP efficiency.

Modeling soil organic matter dynamics after conversion of native grassland to long-term continuous fallow using the CENTURY model

A long-term continuous fallow experiment provides a unique opportunity to test current approximations of soil organic matter dynamics, which are the basis of many models including the CENTURY model. This study was conducted to validate the CENTURY model by comparing the simulated results for a continuous fallow scenario to long-term measurements of soil organic carbon (SOC) in a 50-year continuous fallow field, located in the V.V. Alekhin Central-Chernozem Biosphere State Reserve in the Kursk region of Russia. Experimental data were obtained for soil physical properties, SOC and nitrogen (N) contents for the top 20 cm of soil at the 50-year continuous fallow field and an adjacent native grassland field. All sampled soils were classified as fine-silty, mixed, frigid Pachic Hapludolls. Live and dead plant biomass, and plant roots collected at the native site were analyzed for C, N and lignin contents. These new experimental data were incorporated into the CENTURY model parameters. Following conversion of measured SOC values to dry-combustion equivalent and correction of simulation output for changes in bulk density, the CENTURY model successfully simulated SOC dynamics in the 50-year continuous fallow field. Comparison of observed and modeled SOC concentrations yielded an R 2 value of 0.96.

Properties of soil particle size separates after 40 years of continuous corn

Changes in chemical and mineralogical characteristics associated with different particle size fractions in soil after 40 years of continuous production of corn by the conventional tillage method (CC) as compared with those of an adjacent native grassland site (NG) are investigated. Results indicate that corn cropping in a soil previously supporting native vegetation produces a decline in total and humified organic matter, phenolic compounds, enzymatic activities, cation exchange capacity (CEC), and hydrosoluble ions, both in the whole soil and in its particle‐size separates. The’ largest losses in organic carbon (C) and nitrogen (N) contents of the cultivated soil were observed in the sandy fractions, the lowest in the silt+clay separates. The humification index (HI) indicates a higher degree of humification of the organic matter in NG than in CC samples. For both NG and CC sites the finest fraction (silt+clay) resulted to be enriched in organic C, total N, humus, phenolic compounds, enzyme activity, CEC, and hydrosoluble ions with the only exception of mineral N forms and sulphates (SO4). Slight differences were observed in the mineralogical composition of NG and CC soils. The sandy fractions of NG showed greater amounts of phyllosilicates while a lower content was found in the silt+clay fraction of CC as a consequence of a crumbling of parent rock into small pieces induced by repeated tillage practices.