Normal Precipitation Years Research Articles

Landscape-scale hydrologic response of plant invasion relative to native vegetation in urban forests

Large-scale invasion modifies watershed hydrology by changing surface runoff and lowering the seasonal availability of water to native plants. Due to costly field-based evapotranspiration (ET) measurements, which are highly localized and occasionally subject to instrument failure, landscape-scale water use assessments of invasive plants are infrequent. Therefore, the extent to which plant invaders alter water allocation between native and non-native vegetation in a given landscape is rarely assessed. We used a remote sensing-based ET modeling approach to measure the hydrologic response of an invasive shrub, Ligustrum sinense, across forests of the Charlotte Metropolitan Area, North Carolina. We hypothesized that this invader's widespread occurrence and dominant plant physiology significantly competes with native forests for water resources. We tested this hypothesis by comparing inter- and intra-annual variations in ET from invaded and uninvaded sites estimated using the surface-energy-balance system (SEBS) model and cloud-free Landsat images for the wettest (2003), driest (2007), and normal (2005 and 2011) water years. Our findings suggest that the water demand of L. sinense is higher than native forests (deciduous and evergreen) for most of the year except during the early spring and after high precipitation events. The daily ET flux of L. sinense was significantly different than evergreen vegetation during the driest year (2007) that, five years later (2011 - normal water year), was significantly different than both deciduous and evergreen vegetation types. This suggests that L. sinense consumes more water than native forest types, particularly during dry and normal precipitation years with increasing canopy cover over time making it a strong competitor with native vegetation for water resources in urban forests. Therefore, accounting for the hydrologic response of invasive plants and potential water savings from their removal from forests, particularly in water-scarce regions, may enable land managers and decision-makers to prioritize areas for monitoring and control efforts.

Nonlinear plant-plant interactions modulate impact of extreme drought and recovery on a Mediterranean ecosystem.

Interaction effects of different stressors, such as extreme drought and plant invasion, can have detrimental effects on ecosystem functioning and recovery after drought. With ongoing climate change and increasing plant invasion, there is an urgent need to predict the short- and long-term interaction impacts of these stressors on ecosystems. We established a combined precipitation exclusion and shrub invasion (Cistus ladanifer) experiment in a Mediterranean cork oak (Quercus suber) ecosystem with four treatments: (1) Q.suber control; (2) Q.suber with rain exclusion; (3) Q.suber invaded by shrubs; and (4) Q.suber with rain exclusion and shrub invasion. As key parameter, we continuously measured ecosystem water fluxes. In an average precipitation year, the interaction effects of both stressors were neutral. However, the combination of imposed drought and shrub invasion led to amplifying interaction effects during an extreme drought by strongly reducing tree transpiration. Contrarily, the imposed drought reduced the competitiveness of the shrubs in the following recovery period, which buffered the negative effects of shrub invasion on Q.suber. Our results demonstrate the highly dynamic and nonlinear effects of interacting stressors on ecosystems and urges for further investigations on biotic interactions in a context of climate change pressures.

Representing methane emissions from wet tropical forest soils using microbial functional groups constrained by soil diffusivity

Abstract. Tropical ecosystems contribute significantly to global emissions of methane (CH4), and landscape topography influences the rate of CH4 emissions from wet tropical forest soils. However, extreme events such as drought can alter normal topographic patterns of emissions. Here we explain the dynamics of CH4 emissions during normal and drought conditions across a catena in the Luquillo Experimental Forest, Puerto Rico. Valley soils served as the major source of CH4 emissions in a normal precipitation year (2016), but drought recovery in 2015 resulted in dramatic pulses in CH4 emissions from all topographic positions. Geochemical parameters including (i) dissolved organic carbon (C), acetate, and soil pH and (ii) hydrological parameters like soil moisture and oxygen (O2) concentrations varied across the catena. During the drought, soil moisture decreased in the slope and ridge, and O2 concentrations increased in the valley. We simulated the dynamics of CH4 emissions with the Microbial Model for Methane Dynamics-Dual Arrhenius and Michaelis–Menten (M3D-DAMM), which couples a microbial functional group CH4 model with a diffusivity module for solute and gas transport within soil microsites. Contrasting patterns of soil moisture, O2, acetate, and associated changes in soil pH with topography regulated simulated CH4 emissions, but emissions were also altered by rate-limited diffusion in soil microsites. Changes in simulated available substrate for CH4 production (acetate, CO2, and H2) and oxidation (O2 and CH4) increased the predicted biomass of methanotrophs during the drought event and methanogens during drought recovery, which in turn affected net emissions of CH4. A variance-based sensitivity analysis suggested that parameters related to aceticlastic methanogenesis and methanotrophy were most critical to simulate net CH4 emissions. This study enhanced the predictive capability for CH4 emissions associated with complex topography and drought in wet tropical forest soils.

Mulching Measures Improve Soil Moisture in Rain-Fed Jujube (Ziziphus jujuba Mill.) Orchards in the Loess Hilly Region of China

Water shortage is the main bottleneck restricting the sustainable development of rain-fed jujube (Ziziphus jujuba Mill.) orchards in the loess hilly region of China. Given the effect of mulching on soil moisture conservation, straw mulching (SM) and jujube branch mulching (BM) were applied to a rain-fed jujube orchard in this study. Soil moisture dynamics, soil water storage, water consumption, and soil moisture attenuation after typical rainfall under SM, BM, and clean tillage (CT) were studied. The results showed the following: (1) The 0–60 cm soil layer was the seasonal fluctuation layer of soil moisture under SM, BM, and CT in both the normal precipitation year and the dry year studied. The moisture contents of the 0–60, 60–160, and 160–280 cm soil layers under SM and BM were higher than that under CT in the three experimental years studied, and SM showed the most obvious effect of increasing soil moisture. (2) SM and BM showed a significant soil water storage effect in all of the jujube growth stages, and SM had a better water storage effect than BM. (3) SM reduced the amount of water consumption by 94.3, 60.8, and 121.3 mm compared to CT in the whole jujube growth period in 2014, 2015, and 2016, respectively. The amount of water consumption of BM decreased by 34.8 and 31.0 mm compared to that of CT in the whole growth period in 2014 and 2015, respectively. (4) CT had the maximum soil moisture loss rate under continuous drought after rainfall. The soil moisture loss rate of CT was above 37.3% on the eleventh day after typical rainfall in 2014, 2015, and 2016. With the extension of drought, the soil moisture loss rate under SM increased slowly. This study suggests that SM is the best mulching measure for rain-fed jujube orchards, and pruned jujube branches can also be used for in situ mulching to obtain a certain moisture conservation effect.

Crustal Strain Patterns Associated With Normal, Drought, and Heavy Precipitation Years in California.

We invert continuously operating Global Positioning System (cGPS) data obtained between 2007 and 2019 to quantify non steady‐state horizontal strain anomalies in California. Our long‐wavelength transient strain model shows seasonal and multiannual variations in horizontal strain anomalies within the plate boundary zone. During the summer, in general, a zone of extensional dilatation develops along the San Andreas Fault zone and Sierra Nevada, whereas contractional dilatation develops along the Eastern California Shear Zone (ECSZ) north of 36.5°N. The patterns of dilatational strain are opposite during the winter. We find that these seasonal strain anomaly patterns vary in magnitude, depending on precipitation intensity in California. Investigating hydrologic loading models and their horizontal elastic responses reveal that water mass loads on the surface from the precipitation in California are the major sources of the observed long‐wavelength horizontal transient strains. We show, however, that a heavy damping in the inversion of the cGPS data is required for the long‐wavelength horizontal strain solutions to best match with the expected elastic response from hydrologic loading. Appropriate fitting of the horizontal cGPS yields amplified horizontal strain signals in the Sierra Nevada, along regions adjacent to the San Andreas Fault, and within the ECSZ. The larger‐than‐expected amplitudes may be associated with poroelastic responses or thermoelastic changes that are superimposed on the hydrologic response. We demonstrate that there is a persistent sharp boundary of horizontal dilatational strain domains at the transition between the High Sierra and Basin and Range Province, caused by the sharp gradient in hydrologic loading there.

Investigation on flood event variations at space and time scales in the Huaihe River Basin of China using flood behavior classification

Flood is one of the severest natural disasters in the world and has caused enormous causalities and property losses. Previous studies usually focus on flood magnitude and occurrence time at event scale, which are insufficient to contain entire behavior characteristics of flood events. In our study, nine behavior metrics in five categories (e.g., magnitude, duration, timing, rates of changes and variability) are adopted to fully describe a flood event. Regional and interannual variations of representative flood classes are investigated based on behavior similarity classification of numerous events. Contributions of geography, land use, hydrometeorology and human regulation on these variations are explored by rank analysis method. Results show that: five representative classes are identified, namely, conventional events (Class 1, 61.7% of the total), low discharge events with multiple peaks (Class 2, 5.3%), low discharge events with low rates of changes (Class 3, 18.1%), low discharge events with high rates of changes (Class 4, 10.8%) and high discharge events with long durations (Class 5, 4.1%). Classes 1 and 3 are the major flood events and distributed across the whole region. Class 4 is mainly distributed in river sources, while Classes 2 and 5 are in the middle and down streams. Moreover, the flood class is most diverse in normal precipitation years (2006, 2008–2010 and 2015), followed by wet years (2007, 2013–2014), and dry years (2011 and 2012). All the impact factor categories explain 34.0%–84.1% of individual flood class variations. The hydrometeorological category (7.2%–56.9%) is the most important, followed by geographical (1.0%–6.3%), regulation (1.7%–5.1%) and land use (0.9%–2.2%) categories. This study could provide new insights into flood event variations in a comprehensive manner, and provide decision-making basis for flood control and resource utilization at basin scale.

Responses of forest carbon and water coupling to thinning treatments from leaf to stand scales in a young montane pine forest

BackgroundWater-use efficiency (WUE) represents the coupling of forest carbon and water. Little is known about the responses of WUE to thinning at multiple spatial scales. The objective of this research was to use field measurements to understand short-term effects of two thinning treatments (T1: 4500 stems ha−1; and T2: 1100 stems ha−1) and the control (NT: 27,000 stems ha−1) on WUE at the three spatial scales (leaf level: the ratio of leaf photosynthesis to leaf transpiration; tree-level: tree growth to tree transpiration; and stand level: net primary production (NPP) to stand transpiration) and intrinsic WUEi (the ratio of leaf photosynthesis to stomatal conductance at leaf-level; and NPP to canopy conductance at stand-level) in a 16-year old natural lodgepole pine forest. Leaf-level measurements were conducted in 2017, while tree- and stand-level measurements were conducted in both 2016 (the normal precipitation year) and 2017 (the drought year).ResultsThe thinning treatments did not significantly affect the tree- and stand-level WUE in the normal year of 2016. However, the thinning significantly affected WUE in the drought year of 2017: T2 exhibited significantly higher tree-level WUE (0.49 mm2 kg−1) than NT (0.08 mm2 kg−1), and compared to NT, the stand-level WUE values in the thinned stands (T1 and T2) were significantly higher, with means of 0.31, 0.56 and 0.70 kg m−3, respectively. However, the leaf-level and stand-level WUEi in the thinned stands in the drought year were significantly lower than those in the unthinned stands. No significant differences in the leaf-level WUE were found among the treatments in 2017. In addition, the thinning did not significantly change the WUE-VPD relationships at any studied spatial scale.ConclusionsThe thinning treatments did not cause significant changes in all studied WUE metrics in a normal year. However, their effects were significantly promoted under the drought conditions probably due to the decrease in soil water availability, demonstrating that thinning can improve WUE and consequently support forests to cope with the drought effects. The inconsistent results on the effects of the thinning on forest carbon and water coupling at the spatial scales and the lack of the consistent WUE metrics constraint across-scale comparison and transferring of WUE.

Measurements and modeling of hydrological responses to summer pruning in dryland apple orchards

Long-term imbalance of water demand over supply in drylands can result in serious ecological issues such as severe soil desiccation, posing a great challenge to sustainable development of dryland orchards. This study examined the effects of multi-intensity summer pruning on water balance, production and water use efficiency of apple orchards on China’s Loess Plateau using in situ measurements and the process-based MAESPA model. A field experiment with four intensities representing no (0% or control), light (10%), moderate (25%) and heavy (40%) pruning of branches was carried out in three replicates under each treatment. The pruning intensities dictated transpiration, soil moisture and drainage. Although light pruning had weak impacts on these factors, moderate and heavy pruning significantly reduced growing-season transpiration by an average 10.4% and 28.5%, respectively. However, the effects progressively faded away with time at both diurnal and seasonal scales. Moderate and heavy pruning also increased soil water content in the 20–180 cm by an average 18.3% and 20.8%, respectively, and thus enhanced drainage below 180 cm (>220 mm), relative to light pruning and control (<170 mm). Furthermore, light and moderate pruning clearly promoted apple yield and water use efficiency whereas heavy pruning significantly reduced (24.7%) apple yield. Thus, moderate pruning can be a good choice for apple orchards in normal precipitation years while heavy pruning may be an alternative in severe drought years. The MAESPA model reliably simulated seasonal variations of transpiration (R2: 0.799–0.910, root mean square error (RMSE): 0.111–0.207 mm), soil evaporation (R2: 0.368–0.628, RMSE: 0.468–1.076 mm), and profile soil water content (R2: 0.400–0.894, RMSE: 0.005–0.024 cm3 cm−3) for all treatments and can be used as a tool to quantify the effects of pruning on hydrological processes in dryland apple orchards. This study provides insights into the development of a sustainable management strategy in dryland orchards.

Tree growth, transpiration, and water-use efficiency between shoreline and upland red maple (Acer rubrum) trees in a coastal forest

Coastal shoreline forests are vulnerable to seawater exposure, the impacts of which will increase due to sea-level rise, but the long-term adaptation strategies and vulnerability of coastal forests are not well understood. We used whole-tree transpiration, leaf water potential, tree-ring width, and tree-ring δ13C (a proxy for intrinsic water use efficiency, iWUE) to examine the long-term adaptation strategies of red maple (Acer rubrum) trees at the coastal interface (i.e., shoreline) and nearby upland in Maryland, USA. Red maple trees that grew along the shoreline and were exposed to slightly saline water (up to two PSU) had higher transpiration rates than those growing in the nearby upland forest during a wet year, but these differences disappeared during a normal precipitation year. Shoreline trees grew more slowly than upland trees over the last four decades, but these growth differences have disappeared in the last six years. Shoreline and upland red maple trees had similar variation in iWUE, indicating that higher transpiration rates of the seawater-exposed trees did not translate into differences in water use efficiency. There were no differences in predawn and midday water potential between upland and shoreline trees, suggesting no additional water stress occurs in shoreline trees. These findings indicate that mature red maple in our coastal study site maintains gas exchange and growth at a consistent or homeostatic level under slight soil salinity.

Impact of planting time soil moisture on cereal crop yield in the Upper Blue Nile Basin: A novel insight towards agricultural water management

Soil moisture prior to the planting time can influence crop yield through affecting seedling rooting and emergence, making it an important factor in agronomic management. However, the degree to which initial soil moisture influences crop yield has not been quantified, especially in emerging regions where food security is a major challenge and field data on soil moisture and meteorological conditions are sparse. Here, we quantify the sensitivity of crop yield and production to initial soil moisture conditions in the Upper Blue Nile Basin (UBNB) of Ethiopia making use of a process-based spatially distributed crop model (DSSAT) linked to a hydrologic model (CREST), and assess the potential of soil moisture management in promoting agricultural outcome. Modeling results suggest that the cereal crop yield and production increase with initial soil moisture, and show a higher sensitivity in eastern UBNB than western UBNB and during dry years than wet years. In some areas of eastern UBNB, compared to normal soil moisture conditions, extremely wet initial soil can double the crop yield; in extremely dry years, wetting the soil at planting could prevent crop failure and improve crop yield to a level at or even above that of an average precipitation year. Increasing the initial soil moisture from extremely dry to extremely wet conditions leads to an additional production of 2.3 × 105, 7.7 × 105, 4.5 × 105, and 2.7 × 105 tons for barley, maize, sorghum, and wheat in UBNB respectively. The resulting improvement in total production is especially effective over eastern UBNB. These findings can provide guidance for farmers and decision makers in water allocation and optimization, and thus help improve Ethiopia’s food security.

Roots and aboveground carbon and nitrogen inputs by black oats (Avena strigosa Schreb.) as affected by grazing and nitrogen in integrated crop-livestock system in subtropical Brazil

Roots are important contributors to soil organic matter in pasturelands, yet in integrated crop-livestock systems (ICLs), the effects of grazing intensities are poorly understood. We investigated the effects of grazing intensities and nitrogen application to black oats pasture (Avena strigosa) on carbon and nitrogen additions, mainly by roots, into soil. The ICL experiment in southern Brazil comprised no, moderate and intensive grazing of black oats pasture receiving 0, 75 and 150 kg N ha−1. Root (0–30 cm) and aboveground biomass of black oats were assessed over two years, in a sequence of maize (Zea mays)–black oats–soybean (Glycine max)–black oats. Moderate grazing did not change the carbon and nitrogen additions by roots, but reduced the aboveground additions by 45–54%, compared to no grazing. Generally, the intensive grazing did not affect root or aboveground additions relative to moderate grazing, but in year of normal precipitation and supplied with 150 kg N ha−1, it reduced root carbon and nitrogen addition by 11–22%. Nitrogen application had no clear effect on root additions, but increased the aboveground additions. In ICL system, moderate grazing maintained carbon and nitrogen additions by roots of black oats, compared to no grazing, while nitrogen fertilization can be used to increase carbon and nitrogen aboveground additions.

Emergence of unprecedented climate change in projected future precipitation

The future time of emergence when precipitation changes due to anthropogenic influences begins to continuously exceed the previous maximum value is defined as the ‘tipping year’ Historical experiments and future experiments simulated by state-of-the-art climate models were utilized. A total of 510,000 time series from year 1856 to 2095 were generated by sampling the natural internal variability in precipitation. The time evolutions of internal variability in the whole time period were estimated from the combination of past and future experiments with preindustrial control experiments. A large ensemble size enabled an estimation of the probability density function of the tipping year at each grid point, providing precise information on the uncertainty of the projection. The tipping year of average precipitation emerges earlier in high latitudes than in lower latitudes. In some regions in lower latitudes and mid-latitudes, the tipping year of intense precipitation emerges faster than that of average precipitation. The tipping years of average and intense precipitation are earlier for higher anthropogenic forcing scenarios than for lower scenarios. The global average of the tipping year for intense precipitation might be attributed to the enhancement of the thermodynamic effect (moisture) rather than the dynamic effect (vertical motion).

Plant Selection by Bumble Bees (Hymenoptera: Apidae) in Montane Riparian Habitat of California.

Many bumble bee species (Bombus Latreille) have declined dramatically across North America and the globe, highlighting the need for a greater understanding of the habitat required to sustain or recover populations. Determining bumble bee species' plant selection is important for retaining and promoting high-quality plant resources that will help populations persist. We used nonlethal methods to sample 413 plots within riparian corridors and meadows in the Sierra Nevada of California for bumble bees during two summers following extremely low and normal precipitation years, respectively. We assessed the five most abundant bumble bee species' plant selection by comparing their floral use to availability. Additionally, we described the shift in plant selection between years for the most abundant species, Bombus vosnesenskii Radoszkowski. Bumble bee species richness was constant between years (13 species) but abundance nearly tripled from 2015 to 2016 (from 1243 to 3612 captures), driven largely by a dramatic increase in B. vosnesenskii. We captured bumble bees on 104 plant species or complexes, but only 14 were significantly selected by at least one bumble bee species. Each of the five most frequently captured bumble bee species selected at least one unique plant species. Plant blooming phenology, relative availability of flowers of individual plant species, and plant selection by B. vosnesenkii remained fairly constant between the two study years, suggesting that maintaining, seeding, or planting with these 'bumble bee plants' may benefit these five bumble bee species.

Nitrogen addition gradient can regulate the environmental filtering of soil potassium or phosphorus in shaping the community assembly of alpine meadow

Abstract Nitrogen (N) deposition and warming leads to environmental gradients that shape plant communities and ecological diversity, space is treated as an equally important variable as the environmental variables in the environmental gradients. The Qinghai-Tibetan Plateau (QTP) is very sensitive to large increases in N deposition rates and warming. Environmental gradients caused by N deposition and spatial factors act as environmental filtering in community assembly by affecting plant functional traits and phylogeny. To investigate the effects of environmental filtering caused by N deposition on the community assembly in alpine meadows, we randomly placed and fertilized at 6 levels of N addition (no N addition, 8 kg N ha-1year−1, 24 kg N ha−1 year−1, 40 kg N ha−1 year−1, 56 kg N ha−1 year−1 and 72 kg N ha−1 year−1) both in a normal (normal precipitation and normal temperature) year and a warm (normal precipitation and warm) year. We used the fourth-corner analysis and extended RLQ (R stands for a matrix of environmental variables by samples, L stands for a species-cover-by-samples matrix and Q stands for a species-by-traits matrix) through multiple factors (soil factors, spatial heterogeneity, species traits, phylogenetic factor) to examine the effects of environmental filtering on the community assembly in alpine meadows. The results demonstrated that all soil variables showed a clear gradient and were affected by simulated N deposition; N addition at 8 kg N ha-1year−1 can filtrate the species of the gramineae, but N addition at 72 kg N ha−1 year−1 can filtrate the species of non-gramineae, the main factors of environmental filtering were potassium in normal year and phosphorus in warm year. With the increase of N deposition rate in alpine meadow of QTP, non-gramineous plants will be favored first, but eventually gramineous plants will occupy the dominant position in the QTP. Rational mitigation strategies should be developed for different climate change scenarios of alpine grasslands on the QTP according to their responses to the N addition gradients and climate change scenarios in the future.

Climate, Landforms, and Geology Affect Baseflow Sources in a Mountain Catchment

AbstractBaseflow is essential for stream ecosystems and human water uses, particularly in areas with Mediterranean climates. Yet the factors controlling the temporal and spatial variability of baseflow and its sources are poorly understood. Measurements of oxygen and hydrogen isotopic composition (δ18O and δ2H) were used to evaluate controls on baseflow in the stream network of a 64‐km2 catchment in western Oregon. A total of 607 water samples were collected to contrast baseflow in a year of near average precipitation (2016) to a year with low winter snowpack and subsequent summer drought conditions (2015). Spatial autocorrelation structures and relationships between surface water isotopic signatures and geologic and topographic metrics throughout the network were determined using Spatial Stream Network models. Isotope values varied widely in space and between years, indicating disparate baseflow water sources. During average flow conditions, the spatial variation in δ18O was primarily related to elevation, reflecting the influence of prior precipitation and input of water from snowmelt at higher elevation. In contrast, during drought conditions, the spatial variation in δ18O was also related to terrain slope and roughness—proxies for local water storage in deep‐seated earthflows and other Quaternary deposits. A prominent spring‐fed tributary with high unit baseflow discharge illustrated the importance of subsurface water storage in porous volcanic bedrock. As drought increases in a warming climate, baseflow in mountain catchments may become more dependent on storage in geologic and geomorphic features.

Response of soil water movement and groundwater recharge to extreme precipitation in a headwater catchment in the North China Plain

Soil water storage and movement are highly heterogeneous across landscapes and their response to spatiotemporal variations in meteorological forcing is complex. While different pools of soil water (including bound and mobile water) are observed, the mechanisms of soil water movement in semi-arid and sub-humid regions are not well understood due to high variation in soil water storage conditions. The Taihang Mountain is a headwater region that recharges both groundwater and surface water systems of the North China Plain, where groundwater levels have been declining and water storage loss is serious. Increasing land cultivation in the Taihang Mountain areas has increased evapotranspiration and reduced both surface runoff and groundwater recharge. Although extreme precipitation is critical for groundwater recharge in the headwater regions, the response mechanism of soil water movement and groundwater recharge remains unclear. In this study, soil water movement and groundwater recharge mechanisms in a cultivated farmland (FL) and land under natural vegetation (NV) were determined for a normal and an extreme precipitation year through the combined use of soil water content and stable isotopes of water (18O and 2H). Soil water got enriched in δ18O and δ2H (δ18O changed from −11.2 to −7.0‰ at NV and from −11.1 to −4.4‰ at FL; δ2H changed from −71 to −49‰ at NV and from −73 to −30% at FL) with increasing soil depth during the growing season suggesting that winter precipitation was generally transported via advection dispersion flow mechanism. However, this process was accompanied by the mixing of previously enriched soil water after large rain events (20–50 mm/day) during the rainy season in a normal precipitation year. Water movement changed from translatory flow to preferential flow after extreme precipitation in a wet precipitation year. Cultivation intensified water evaporation in the top soil layer (upper 10–20 cm), and induced preferential flow down to 50 cm soil depth under FL relative to land under NV. Thus, cropping significantly reduced groundwater recharge. Excessive storm during a wet year produced bypass flow after the first rainstorm, which rapidly recharged deep soil layers (50–100 cm depth). Bypass flow induced by excessive precipitation and contributed the most to groundwater in FL. The observed rapid response of soil water and groundwater to extreme precipitation events is critical for soil and water management to mitigate problems such as nitrate leaching and groundwater contamination in headwater regions of semi-arid and sub-humid areas.

Carbon and evapotranspiration dynamics of a non-native perennial grass with biofuel potential in the southern U.S. Great Plains

Old world bluestem cultivar WW-B Dahl [Bothriochloa bladhii (Retz.), S. T. Blake] is a non-native perennial C4 bunch grass with biofuel production potential grown predominantly in the Southern U.S. Great Plains. Although this is a popular introduced grass cultivar, data on carbon fluxes and evapotranspiration (ET) from this warm-season grass is rare. In this study, the eddy covariance method was used to measure CO2 and ET from an established stand of bluestem for three years (2013–2015). Year 2015 had the highest gross primary production (GPP; 1358 ± 143 g C m−2) followed by 2014 (1250 ± 31 g C m−2) and 2013 (1024 ± 91 g C m−2). The average loss of GPP as ecosystem respiration (Reco) was 76%. Annual NEE sums were -302 ± 15 g C m−2 in 2013, -265 ± 41 g C m-2 in 2014, and -287 ± 32 g C m−2 in 2015. Results from this study show that the NEE in grasslands in years with normal precipitation that is well distributed may not vary from years with above-normal precipitation. This is because precipitation enhances Reco along with carbon uptake, which may result in lower net carbon uptake in perennial grasslands in higher precipitation years than normal precipitation years. Gross primary production showed a linear relationship with ET (R2 = 0.90) and above ground biomass (R2 = 0.74). Only 26% of the GPP was allocated to above ground biomass indicating a higher allocation of carbon to below ground biomass. The water use efficiency of bluestem (2.9 g C kg-1 of water) matched well with that of native prairies and other dedicated biomass crops grown in the Southern Great Plains. As the demand for cellulosic biofuels is increasing, results from field experiments quantifying seasonal changes in carbon fluxes and ET could be important in understanding the contributions of large-scale production of novel biomass crops to regional carbon and hydrologic cycles.

Do Tillage, Cover Crops, and Compost Management within Organic Grain Cropping Affect Greenhouse Gas Emissions?

Core Ideas Drought decreased the CO2 and N2O 2 out of 3 yr of the study but in a normal precipitation year, GHG emissions increased significantly for corn and wheat but stayed relatively low for soybean. Increasing compost from 1× to 1.5× the recommended rate showed no increase in N2O emissions. NTCC and 1×+ compost rates typically resulted in the greatest GWP. Organic crop production is increasing with consumer demand, but research is needed on the best management practices that minimize greenhouse gas (GHG) emissions. This study was conducted in central Missouri from 2012 to 2014 using a 3‐yr rotation, consisting of corn (Zea mays L.), cereal rye cover crop (Secale cereale L.), soybean (Glycine max L.), winter wheat (Triticum aestivum L.), summer cover crop mix [Buckwheat (Fagopyrum esculentum Moench), sunn hemp (Crotalaria juncea L.), cowpea (Vigna unguiculata (L.) Walp.)], and cereal rye/hairy vetch cover crop mix (Vicia villosa L.). Treatments included tillage/cover crop practice (TCCP) combinations: (i) tillage with no cover crop (TNCC), (ii) tillage with cover crop (TCC), and (iii) no‐till with cover crop (NTCC) and compost additions relative to the recommended rate (0×, 1×, 1.5×). Carbon dioxide (CO2) and nitrous oxide (N2O) emission measurements followed GRACEnet protocol. GHG emissions were not influenced by management practices in 2012 due to severe drought. TCCP had direct (2013 corn, 2014 soybean) or interactive (2014 wheat) effects on CO2 emissions; compost had a direct effect (2014 corn) on N2O emissions. Drought in 2012 and 2013 contributed to generally low CO2 and N2O emissions (99–2924 kg CO2–C ha−1 and 0–0.8 kg N2O–N ha−1, respectively) while emissions increased in 2014 (527–7740 kg CO2–C ha−1 and &lt;0.1–10.1 kg N2O–N ha−1, respectively). Across all years and crops, NTCC showed greater CO2 emissions than TNCC (1579 and 893 kg CO2–C ha−1, respectively). While few treatment differences were observed, TCCP resulted in more GHG emission differences than compost rate.

Comparison of Two Tillage Practices in a Semi‐Arid Cotton–Grain Sorghum Rotation

Cotton and sorghum yield were more dependent on precipitation than tillage management. No‐till produced a greater cotton and sorghum crop yield in some years. Economic analyses indicate a reduction of costs with no‐till versus conventional practices. Conventional tillage (CT) cropping systems in semiarid regions of south Texas are associated with soil degradation and increasing fuel costs. The objective of the study was to evaluate the effects of no‐till (NT) compared with CT on a dryland cotton (Gossypium hirsutum L.)–grain sorghum [Sorghum bicolor (L.) Moench] rotation system with respect to crop yields, yield stability, soil moisture and structure, and economics. The experiment (four replicated plots of each crop and tillage treatment annually) was conducted on a Vertisol at Corpus Christi, TX, from 2011 to 2015. Cotton yield was similar among tillage treatments, except for the drought year of 2013 when 86% greater yield occurred with NT and above average precipitation year of 2015 (15% greater yield for NT). Sorghum yield was impacted by tillage in 2012 when NT produced 33% greater yield than CT, whereas, in 2015, CT produced 28% greater yield. Yields were stable across crops and tillage treatment. Soil moisture was not impacted by tillage treatment; however, tillage caused bulk density to increase at 15‐ to 30‐cm depth compared with 0‐ to 15‐cm depth. All soil chemical properties remained constant, except NT had greater total soil N than CT in 2015. Cash costs for CT were greater than NT for both crops, and net cash farm income was positive only for NT cotton. No‐till is a viable alternative for dryland farmers in this region because of stable yields and increased income.

Playa‐Wetlands Effects on Dryland Biogeochemistry: Space and Time Interactions

AbstractPlayas are ephemerally flooded wetlands found in hydrologically closed dryland catchments. Recent research showed how upland‐catchment biophysical characteristics control the amount of organic carbon and nutrients that accumulates in playas from upland grasslands and shrublands. Here we further explored the role of allochthonous and autochthonous processes in fixing carbon and accruing soil nutrients in playas across a desert basin. We assessed whether playas contribute soil nutrient, organic‐carbon stocks, and primary productivity rates in amounts disproportionately higher or lower than upland grasslands and shrublands. We found that playas had higher soil carbon and nutrient storage but no difference in mean annual primary production. Playas, on average, had similar primary productivity than upland systems, because they exhibited lower productivity during extremely wet years that offset the larger productivity during average or below average precipitation years. These results suggest that playas contain higher carbon and soil nutrient stocks because of runoff‐mediated allochthonous inputs, rather than autochthonous sources, since long‐term average productivity was similar to upland ecosystems. We also used the precipitation‐aboveground net primary production (ANPP) relationships to accurately measure run‐on in playas using differences between playa and upland grassland ANPP. The dependence of playa ANPP on surface‐water inputs from upland catchments makes them susceptible to future changes in extreme precipitation events.