Sandy Grassland Soils Research Articles

Effects of litter placement, soil moisture and temperature on soil carbon dioxide emissions in a sandy grassland soil

AbstractSemiarid grasslands contribute significantly to global soil carbon (C) storage. Carbon loss from these systems via microbial decomposition is controlled by abiotic and biotic factors such as soil moisture and temperature and C input. Plant litter in these systems can be present above the soil surface or mixed with surface soil by trampling, especially in intensively grazed areas. A quantitative understanding of abiotic factors' interactive effect is critical for predicting soil C dynamics in response to grassland management and environmental conditions changes. Therefore, we conducted a 3‐month laboratory incubation experiment to quantify the impact of litter placement and soil moisture on soil carbon dioxide (CO2) emissions under three controlled temperatures. The treatments including three litter placements (no litter, litter on top and litter mixed with surface soil) and three soil moisture levels (23%, 37% and 50% water‐filled pore space [WFPS]) were incubated at three temperatures (10°C, 20°C and 30°C). Carbon dioxide fluxes were measured every 2 weeks. Soil CO2 fluxes were higher for all moisture and litter treatments initially and declined overtime at 20°C and 30°C. Mixing litter with soil increased the cumulative CO2 emissions by 24% and 19.5% at 20°C and 30°C, respectively, compared to no litter. Also, soil‐litter mixing compared with litter on top showed a 14.3% and 21.6% increase in cumulative CO2 emissions at temperatures of 20°C and 30°C, respectively. At all temperatures, 37% and 50% WFPS resulted in similar cumulative CO2 emissions. The results from this study indicate that rising temperatures from 10°C to 30°C accelerate the effect of soil litter mixing on increasing CO2 emissions compared to litter on top and no litter.

Repeated drying and rewetting differently affect abiotic and biotic soil phosphorus (P) dynamics in a sandy soil: A 33P soil incubation study

Soil drying and rewetting (DRW) events are expected to occur at higher frequencies because of alterations in climate patterns. Readily extractable inorganic and microbial soil phosphorus (P) pools may be affected due to rapid changes in soil water availability. We aimed to determine how soil P dynamics are affected by repeated soil DRW using a sandy grassland soil that regularly experiences DRW. In a laboratory soil incubation study, the soil was exposed to three DRW cycles, with each cycle consisting of a two-day drying phase, a three-day dryness phase and a four-day moist phase after rapid rewetting. The indicators of abiotic processes (P sorption) and biotic processes (respiration, microbial abundance, potential phosphatase enzyme activities) were regularly determined together with water-extractable P, resin-extractable P and microbial P in a 33P-labelled soil.During the first DRW cycle, microbial P was reduced by half and accompanied by a concomitant but not equivalent increase in water-extractable P and a slight as well as delayed increase in resin-extractable P. Thus, increases in water-extractable P were explained by microbial P released during drying but also by microbial P occupying soil P sorption sites, thereby decreasing soil P sorption. Changes in the 33P-isotopic composition of microbial P at the same time suggested that microorganisms did not respond homogenously to the DRW treatment and indicated an increased mineralisation of previously unavailable organic P compounds. However, during the second and third DRW cycles, only water-extractable P, soil P sorption and potential phosphatase activities were affected by the DRW treatment, whereas all other parameters remained similar in values to the constant moist treatment. The effects of DRW on soil P dynamics appeared to affect water-extractable P more long-lastingly, whereas microbial P and most of the biotic indicators quickly adjusted to the DRW treatment. We conclude that the current concepts suggesting an increased mobility of soil P towards other environmental compartments due to soil DRW should consider that abiotic and biotic soil P dynamics are not equally affected in the case of short repetition of DRW incidences.

Reproductive phenology of two co‐occurring Neotropical mountain grasslands

AbstractAimClimate tends to explain phenological variations in tropical ecosystems. However, water availability and nutrient content in soil strongly affect plant communities, especially those on old, climatically buffered, infertile landscapes (OCBILs), and may impact these ecosystems’ plant reproductive phenology over time. Here, we compare the reproductive phenology of sandy and stony tropical grasslands, two co‐occurring herbaceous communities of the campo rupestreOCBILs. We asked whether flowering, fruiting and dispersal are seasonal in both grasslands, and whether these phenophases differ due to variations in soil properties. We also asked whether the phenological strategies and the number of flowers and fruits differ between these two grasslands as soil conditions vary.LocationSerra do Cipó, Minas Gerais, Brazil.MethodsThe phenology of herbaceous species of sandy and stony grasslands was monitored monthly over two consecutive years.ResultsPlants on sandy and stony grasslands flowered and fruited throughout the year. We did not find a distinct seasonal pattern at the community level of either studied grassland. However, flowering, fruiting and seed dissemination occurred in stony grasslands mainly during the rainy season, while sandy grassland species flowered in both seasons and fruited and disseminated seed mainly during the dry season, as observed in other savanna vegetation types in the Cerrado. Flower and fruit production was higher in sandy grasslands than in stony grasslands, which may be linked to higher water retention in sandy grassland soils. In both communities, species of Cyperaceae, Eriocaulaceae and Xyridaceae contributed most to overall production, whereas Poaceae and Velloziaceae, two important families in campo rupestre, barely participated in the reproductive phenology during our 2‐yr survey.ConclusionsDespite a strong seasonal climate, there was no reproductive seasonal pattern at the community level in campo rupestre. This first investigation of Neotropical grassland phenology indicates that the differences in soil content may constrain the grassland reproductive phenology and restrict reproduction of stony grassland species to the most favourable season. Further studies of grassland phenology are necessary to disentangle the relative importance of soil, climate and other triggers, especially fire.

Sensitivity of water stress in a two-layered sandy grassland soil to variations in groundwater depth and soil hydraulic parameters

Abstract. Monitoring and modelling tools may improve irrigation strategies in precision agriculture. We used non-invasive soil moisture monitoring, a crop growth and a soil hydrological model to predict soil water content fluctuations and crop yield in a heterogeneous sandy grassland soil under supplementary irrigation. The sensitivity of the soil hydrological model to hydraulic parameters, water stress, crop yield and lower boundary conditions was assessed after integrating models. Free drainage and incremental constant head conditions were implemented in a lower boundary sensitivity analysis. A time-dependent sensitivity analysis of the hydraulic parameters showed that changes in soil water content are mainly affected by the soil saturated hydraulic conductivity Ks and the Mualem–van Genuchten retention curve shape parameters n and α. Results further showed that different parameter optimization strategies (two-, three-, four- or six-parameter optimizations) did not affect the calculated water stress and water content as significantly as does the bottom boundary. In this case, a two-parameter scenario, where Ks was optimized for each layer under the condition of a constant groundwater depth at 135–140 cm, performed best. A larger yield reduction, and a larger number and longer duration of stress conditions occurred in the free drainage condition as compared to constant boundary conditions. Numerical results showed that optimal irrigation scheduling using the aforementioned water stress calculations can save up to 12–22 % irrigation water as compared to the current irrigation regime. This resulted in a yield increase of 4.5–6.5 %, simulated by the crop growth model.

Re-aggregation dynamics of degraded cropland soils with prolonged secondary pasture management in the South African Highveld

Prolonged arable cropping of subtropical sandy grassland soils resulted in substantial losses of water-stable aggregates. The objective of this study was to evaluate how rapidly and to which degree soil aggregates may be restored when degraded cropland soils are converted to secondary pasture lands. Hence, chronosequences of degraded cropland soils (more than 20years of arable cropping) were sampled that had been converted to secondary pastures between 1 and 52years ago in three agro-ecosystems of the South African Highveld. Primary grasslands as well as sites under long-term cropping served as references. The surface soils (Plinthusthalfs; 0–10cm) were fractionated to aggregates of different sizes by wet sieving (8000–2800μm, 2800–2000μm, 2000–500μm, 500–250μm, 250–53μm and<53μm). All fractions were analyzed for their respective content of soil organic carbon and total nitrogen and corrected for sand content. The results showed that reconversion of cropland into secondary pasture restored the soil structure after 9.5–18years of pasture management almost completely: the amount of sand-corrected large macroaggregates (>2000μm) approached 85–94% of the levels of the primary grasslands. The stocks of organic matter bound to large macroaggregates recovered more slowly and reached only 50% of that of the primary grassland, which, however, exhibited also a slightly elevated clay content. The carbon concentration in the aggregates did not change significantly. Thus, the increase in C stocks in the secondary pasture soils was mainly due to a rebuilding of large macroaggregates, which contained more C than the smaller-sized aggregate classes, but not more C than the respective aggregates in the degraded cropland. We conclude that only the amount of large macroaggregates was restored upon land conversion, while their protective capacity was obviously not restored and thus not strong enough to account for a full sequestration of soil organic matter.

Biodiversity, Nitrogen Deposition, and CO2 Affect Grassland Soil Carbon Cycling but not Storage

Grasslands are globally widespread and capable of storing large amounts of carbon (C) in soils, and are generally experiencing increasing atmospheric CO2, nitrogen (N) deposition, and biodiversity losses. To better understand whether grasslands will act as C sources or sinks in the future we measured microbial respiration in long-term laboratory incubations of soils collected from a grassland field experiment after 9 years of factorial treatment of atmospheric CO2, N deposition, and plant species richness on a deep and uniformly sandy soil. We fit microbial soil respiration rates to three-pool models of soil C cycling to separate treatment effects on decomposition and pool sizes of fast, slow, and resistant C pools. Elevated CO2 decreased the mean residence time (MRT) of slow C pools without affecting their pool size. Decreasing diversity reduced the size and MRT of fast C pools (comparing monocultures to plots planted with 16 species), but increased the slow pool MRT. N additions increased the size of the resistant pool. These effects of CO2, N, and species-richness treatments were largely due to plant biomass differences between the treatments. We found no significant interactions among treatments. These results suggest that C sequestration in sandy grassland soils may not be strongly influenced by elevated CO2 or species losses. However, high N deposition may increase the amount of resistant C in these grasslands, which could contribute to increased C sequestration.

Determining soil organic carbon for agricultural soils: a comparison between the Walkley &amp; Black and the dry combustion methods (north Belgium)

AbstractMapping soil organic carbon (SOC) and establishing any change over time are important because of CO2 fluxes between soil and atmosphere and cropland decreases in SOC. The latter is one of the main causes of soil fertility decline and increased erodibility. As most analytical methods underestimate total SOC content, correction factors are needed to avoid methodological bias when comparing SOC data from sampling campaigns using different analytical procedures. The traditional method for SOC analysis used to be, and in most cases still is wet oxidation in potassium dichromate, better known as the Walkley &amp; Black method. In this study, we aim to estimate correction factors for the classic and modified version of the Walkley &amp; Black method for different land use and soil type combinations for agricultural soils in north Belgium. General correction factors of 1.47 for the classic Walkley &amp; Black method and 1.20 for the modified Walkley &amp; Black method are proposed. The results show that sandy grassland soils are characterised by lower recoveries than silt loam grassland soils. Furthermore, the correction factor appears to increase with soil wetness.

Host resistance reverses the outcome of competition between microparasites

Predators and parasites can control the abundance or biomass of herbivores with indirect effects on producer communities and ecosystems, but the interplay of multiple natural enemies may yield unexpected dynamics. We experimentally examined interactions between two microparasites (entomopathogenic nematodes) isolated from sandy grassland soils of coastal California: Heterorhabditis marelatus (Heterorhabditidae) and Steinernema feltiae (Steinernematidae). Heterorhabditis marelatus drives trophic cascades by attacking root- and stem-boring ghost moth caterpillars (Hepialus californicus, Hepialidae), thereby indirectly protecting bush lupine shrubs (Lupinus arboreus, Fabaceae). Extensive field surveys demonstrated sympatric overlap in microhabitat use under lupine canopies and similar mean prevalence of the two nematode species. Using a response-surface design in the laboratory, we varied relative and absolute microparasite densities to test for competitive outcomes within an evolutionary naive host, larvae of the greater wax moth Galleria mellonella (Pyralidae), and within the native host Hepialus californicus. Independent of conspecific or interspecific density, S. feltiae dominated as expected over H. marelatus within the naive Galleria, but S. feltiae infected hosts at low frequency and showed lower reproductive fitness than H. marelatus within native Hepialus hosts. Contrary to studies that demonstrate the pairwise dominance of steinernematid over heterorhabditid species in laboratory hosts, host resistance to S. feltiae may provide a mechanism for coexistence of multiple microparasite species. We hypothesize that the ubiquitous field prevalence and rapid life history of S. feltiae imply its use of widespread, abundant but small-bodied hosts and indicate the lack of direct competition with H. marelatus in the Hepialus-Lupinus trophic cascade.

Nitrous oxide emissions and dynamics of soil nitrogen under 15N‐labeled cow urine and dung patches on a sandy grassland soil

AbstractGrazing animals highly influence the nutrient cycle by a direct return of 80% of the consumed N in form of dung and urine. In the autumn‐winter period, N uptake by the sward is low and rates of seepage water in sandy soils are high, hence high mineral‐N contents in soil and in seepage water as well as large losses of N2O are expected after cattle grazing in autumn. The objective of this study was the quanitfication of N loss deriving from urine and dung leaching and by N2O emission. Therefore the deposition of urine and dung patches was simulated in maximum rates excreted by cows by application of 15N‐labeled cow urine and dung (equivalent to 1030 kg N ha–1 and 1052 kg N ha–1, respectively) on a sandy pasture soil in N Germany. Leachate was collected in weekly intervals from free‐draining lysimeters, and 15N‐NO$ _3^- $, 15N‐NH$ _4^+ $, and 15N‐DON (dissolved organic N) were monitored over 171 d. Furthermore, the 15N‐N2O emission rates and the dynamics of inorganic 15N in the upper soil layer were monitored in a field trial, adjacent to the lysimeters. After 10 d following the urine application, the urea was completely hydrolyzed, shown by a 100% recovery of urine‐N in the soil NH$ _4^+ $. The following decrease of 15N‐NH$ _4^+ $ in the soil was higher than the increase of 15N‐NO$ _3^- $, and some N loss was explained by leaching. Amounts of 51% and 2.5% of the applied 15N were found in leachate as inorganic N, 2.4% and 0.7% as DON derived from urine and dung, respectively. Release of N2O from urine and dung patches applied to the pasture was low, with losses of 0.05% and 0.33% of the applied 15N, respectively. Overall loss of dung‐derived N was very low, but as the bulk dung N remained in the soil, N loss after mineralization of the dung needs to be investigated.

Monitoring microbial biomass and respiration in different soils from the Czech Republic—a summary of results

The microbial biomass (C bio), respiration (basal respiration (BR) and potential respiration (PR)), and derived indices for 520 independent soil samples of 117 different soils from the Czech Republic were statistically analysed. The broad range of soil samples allowed the stepwise breakdown of the database into six reasonable categories of soil: arable soils, loamy grassland soils, sandy grassland soils with weak organic matter content, sandy grassland soils with moderate organic matter content, forest soils with moderate organic matter content, and forest organic soils with rich organic matter content. Because soil microbiology lacks benchmarking values, the ranges of the microbial characteristics for these categories were stated and are presented here. The separation into soil groups narrowed the ranges enough to be useful for comparative purposes. The groups displayed significant differences in basal microbial parameters. The lowest microbial biomass was found in arable soils and grassland sandy soils with weak organic matter content. The highest microbial biomass was shown by loamy grassland soils and organic forest soils. Respiration displayed similar results to the microbial biomass. The derived indices revealed less significant differences confirming their inner-standard nature. The relationships between the soil contamination and microbial parameters were not explored because of the confounding effect of soil organic matter. However, it was not shown by the category of grassland sandy soils with weak organic matter content suggesting they could be especially suitable for the biomonitoring of harmful effects of chemicals on soil microorganisms.

Soil microbial community of abandoned sand fields.

Microbiological evaluation of sandy grassland soils from two different stages of secondary succession on abandoned fields (4 and 8 years old fallow) was carried out as a part of research focused on restoration of semi-natural vegetation communities in Kiskunság National Park in Hungary. There was an apparent total N and organic C enrichment, stimulation of microbial growth and microbial community structure change on fields abandoned by agricultural practice (small family farm) in comparison with native undisturbed grassland. A successional trend of the microbial community was found after 4 and 8 years of fallow-lying soil. It consisted in a shift of r-survival strategy to more efficient C economy, in a decrease of specific respiration and metabolic activity, forced accumulation of storage bacterial compounds and increased fungal distribution. The composition of microbial phospholipid fatty acids mixture of soils abandoned at various times was significantly different.

Modelling belowground processes to explain field-scale emissions of nitrous oxide

Globally, soils are the most important source of nitrous oxide (N 2O). In several recent studies, N 2O fluxes from soils have been monitored. Regression relationships between N 2O fluxes and variables like average soil moisture content, nitrogen content and temperature did in general not explain more than 50% of the flux variance. The quantitative insight into the relation between measured fluxes on the one hand, and the underlying basic processes and their determining factors on the other, is still limited. This study aims to contribute to this insight by modelling underground processes to explain N 2O profiles in the soil. On sandy grassland soils in the Wageningen Rhizolab, we monitored aboveground N 2O fluxes as well as underground profiles of several variables. A process-based model was adapted to simulate the relevant processes. In the model we assumed that the sandy soil profile consisted of homogeneous soil layers. The modelled processes included biological and physical processes. Nitrous oxide is produced during denitrification and is transported via diffusion. The simulated O 2 and CO 2 profiles were satisfactory, indicating that the (bulk) respiration rates used in the model were realistic. The N 2O profiles, however, were less well simulated. With respect to N 2O production, the assumption of homogeneity within the soil layers was probably not correct for the sandy soil. We discuss modelling approaches to explain observed N 2O profiles by describing partial anaerobicity in soil layers. Of these, the so-called ‘randomly distributed-pore model’ [Eur. J. Soil Sci. 46 (1995) 507] and an adapted version for regularly distributed pores appear good candidates for incorporation in field-scale models for a layered sandy soil. Such models would describe processes like bulk respiration and the transport and plant uptake of water and nutrients. The randomly distributed-pore model is a relatively simple model for a steady state situation. For sandy soil layers, the model predicts that a substantial part of the volume can be anoxic under a range of conditions including conditions that often occurred in our experiment: a ratio (volumetric moisture content)/(saturated volumetric moisture content)≥0.7 and oxygen percentages in the air-filled pores between 5 and 21%. The regularly distributed-pore model also predicts the occurrence of anoxic fractions under common experimental conditions, but these fractions are significantly smaller than those according to the randomly distributed-pore model.

Denitrification in grassland soils in The Netherlands in relation to irrigation, N-application rate, soil water content and soil temperature

Denitrification rates in agricultural grassland systems are variable, mainly due to soil type, management and weather conditions. The soil water status in such systems was often found to be the most important factor controlling denitrification. The individual and combined effects of irrigation, N-application rate and temperature on denitrification were studied in a field-experiment on a sandy grassland soil in The Netherlands. The relationship between denitrification and soil water status was studied in a laboratory experiment using sand, loam and peat soil, in which four soil water contents were established. Denitrification rates were measured in soil cores kept in a C 2H 2-enriched atmosphere (10% v/v). The results of the field experiment showed that: (1) denitrification rates in the soil under study were primarily affected by soil water conditions; (2) the effect of temperature on denitrification rate was stronger in non-irrigated plots; and (3) N-application rate had no effect on denitrification rates because NO 3 − was not limiting. The results of the laboratory experiment indicated that denitrification rates were very low when the soil water-filled porosity had decreased below a threshold value, which varied with soil texture. However, when this threshold value was expressed as soil water tension, the threshold for all soil types was pF 2, or field capacity.